Common Causes and Fixes
I get many phone calls on vibration in owner’s aircraft. Here we’ll talk about some of the common causes, where to look and what you can do to help mitigate these vibration issues.
Here is a common list, but is not all inclusive;
1. Carbs not synced properly.
2. Carb vent hose improperly placed or removed.
3. Carbs not opening equally or fully.
4. Prop blades not the same pitch or out of track.
5. Prop out of balance.
6. Aircraft wheels not balanced.
7. Old rubber engine mounts.
8. Mag drop difference too wide between ignition modules.
9. Trigger coil air gaps too wide.
10. Gearbox worn, damaged or in need of maintenance.
So let’s address each of these.
The carb sync (#1) should be fairly obvious to most now. The carbs should be synced at each annual / 100 hour inspection or anytime they have been removed for maintenance or you suspect a problem like vibration and you need to rule this in or out as the problem. Carb sync is vital to a good smooth long lasting running engine. You don’t want one side trying to run at 5100 rpm while the other may be trying to run at 5200 rpm. Sync those carbs. Once done it’s easy to keep them there.
The carb vent hose (#2) that may be attached to the standard Rotax air box, a small clear plastic tube on the side of the carb under the carb bowl bale or some others have them routed to different places. These hoses should be as close to equal length as possible and be routed to the same area of pressure. If one hose has fallen off the side of the carb and the other is still attached it will cause the carbs to become unbalanced which will cause your vibration. Do not place these hose ends in the air stream outside the cowl. These only take a minute to confirm their attachment and placement.
Check to see if the carbs open equally (#3) by moving the throttle from idle to wide open when the engine is off. You may see some signs of this during a carb sync, but most people don’t go above 3500 rpm for a carb sync so you need to double check this while the engine is off to see if they do in fact reach WOT at the same time or if one hangs up slightly.
The prop blades all too often are not the same pitch (#4) from blade to blade. This is easy to double check and can be done with either a prop protractor and or a 12” digital level. Measure back from each tip 8”- 9” and put a mark on each blade. Make the blade out to your right level with the floor and then put the level on the back of the blade where you made the line from the tip. The blades should be no more than one tenth of a degree out from each other. That measurement seems small, but it is quite easy to accomplish. The Sensenich prop gauge pins are not accurate enough. Check them by hand with a prop gauge or level once you are close. To check tracking place a box underneath the bottom tip of a blade pointing straight down. Put a line on the box where that tip just barely touches the box. Then swing the other blade(s) around and see if they all cross at the exact same mark. If they don’t you’ll need to loosen the prop flange bolts and
re-torque them to get the blades to all track over your line on the box.
Prop blades now days are much better in balance (#5) than they were decades ago, but all props should still be dynamically balanced. All wood blades in humid climates can change due to moisture absorption. With all the new composites that aren’t susceptible to this anymore I’m not a fan of all wood blades. Even the main bolts change torque with humidity changes. A dynamic balance will not only help vibration, but will help save your gearbox from wear or damage. The heavier the blades i.e. long Warp Drive props the more important this becomes.
I have never found an aircraft wheel (#6) in balance. Most do not ever think about the smaller aircraft wheel being out of balance as a vibration cause, but over the years I have cured many a vibration just by balancing the wheels. I always balance all new wheels I install. I see some occasionally that would need up to 20 x ¼ oz. weights to bring them in balance. If you failed to balance your wheels you would never find this huge disparity. These come off and go back to the distributor. What I normally see is 2 – 8 x ¼ oz. weights per wheel. It usually takes me about 3-5 minutes to balance a wheel after it’s off the plane. Don’t disregard this when you are looking for a vibration cure.
Old rubber engine mounts (#7) are a common problem. Rotax wants a 5 year rubber replacement which I’m a fan of. This includes the rubber engine mounts. Rubber can get hard or soft from repeated heating and cooling cycles plus chemical exposure and just the ozone in the air. I replace these every time I do a rubber replacement on an aircraft. It usually isn’t hard or expensive.
The mag drop vibration (#8) should be obvious when you do your mag drop check. Most see anywhere from 40 rpm – 100 rpm as a normal drop and usually both mags are within about 10 rpm – 30 rpm of each other. If you experience
300-1000 rpm drop then it’s time to troubleshoot your ignition system. There are documents out there that tell you how and where to look for ignition issues.
It could just be a bad plug, too wide a plug gap, a bad plug boot, a bad connection at the plug boot where the wire screws in. If it is a large drop like 800+ rpm it may be a bad ignition module. These are all items you need to rule in or out. Always start with the most common, easiest and cheapest first. Do not just throw money at everything hoping to hit the jackpot. Most ignition issues are simple common issues.
The trigger coils (#9) in the flywheel compartment can at times have too wide an air gap between the pick-up and flywheel trigger point. These are checked by using a feeler gauge and checking the gap tolerances listed in the Heavy Maintenance manual and setting them to the proper gap. These can even be off from the factory so check them before installing a new engine when they are easy to get to. You not only are checking the gap, but the screw torque for tightness.
Gearbox (#10) care is important. As you look for your vibration issue consider the gearbox. It has maintenance service times at either 600 or 1000 hours. Using an automotive oil over a motorcycle oil can cause premature wear and damage. At your 100 and annual inspections you should be doing a gearbox friction torque check. Normal measurements that I usually see in the field is between 425-490 in. lbs. There is a low limit, but I personally don’t like to see anything in the 300 in. lb. numbers. It only takes a few minutes to perform. Checking the magnetic oil plug for debris at every oil change is another check for gearbox wear and damage. Prop strikes should have the gearbox removed and sent to a distributor for a special inspection. Gearbox’s when taken care of tend to last a long time, but there have been a few with excessive wear in early run hours. There have been some with the 912iS engine.
These are the 10 common causes for unwanted vibration. Most are easy to fix and find. When trouble shooting start with the cheapest and easiest to rule in or out and progress to the harder least common when you do your checks. Whatever you do be methodical and don’t jump all around to exotic areas to check. Most Rotax issues are easy to find when you start at “A” and then work to B, then C and so on.
I hope this helps some reduce any frustration in locating an unwanted vibration.
Signed your friendly,
Chasing The Perfect Idle RPM
The highs and lows
Is there a perfect idle rpm for the 912 series engine? This seems to be a widely discussed and at times a hotly debated item on many forums. The simple answer is NO. Set the idle where you want and or need it, but do it for the right reason. A big discussion seems to be that if the idle isn’t set low enough I can’t land or I’ll float way down the runway. This answer may surprise you, but it shouldn’t and I’ll address this later in the article. The Rotax operator’s manual says the idle rpm should be no less than 1400 rpm. While you think about this remember that the manuals are written with different engines and configurations in mind and that you will need to adjust your final thinking along those lines that fit your personal aircraft and engine setup for the idle rpm.
What do I mean about different considerations? You may have a 912UL 80 HP @ 9:1 compression or a 912ULS 100HP @ 11:1 compression and either one could be with or without a gearbox overload clutch. The 912UL without the overload clutch and a lower compression ratio can handle lower idle rpm’s better than the 912ULS with the higher compression with an overload clutch. Low idle rpm with the 912ULS just causes excessive wear from vibration and pulsation in the gearbox. Warm ups and long idling times in the 912 and 914 series engines should be above 2000 rpm.
So what should we consider when picking an idle rpm? First think of being nice to your gearbox parts and second is setting the idle rpm so you don’t have to ride the brakes all the time. We have all been told time and time again not to warm up at low idle rpm due to excessive gearbox wear which is usually under 2000 rpm. You also don’t want too high an idle rpm for starting as this will make your starts much tougher. The choke and ASM system work best at starts with a lower idle rpm around 1600-1800 rpm. Landing rpm should be a last consideration and I’ll explain shortly. So what is the perfect rpm? There isn’t one, but there is an idle rpm range for each to consider. A reasonably good place to be would be between 1600-1800 rpm at operational oil temp and some like the twin 912 Tecnam can be higher due to its overall higher aircraft weight and tendency not to roll as easy as a lighter aircraft. The things that may come into play here is what engine you have and how heavy a plane do you own. This would take into consideration as to how much rpm it takes to make it roll for taxi. Heavy planes would need more and light aircraft less. This part is about saving the brakes or slowing for turns. (Just a quick note; Carbs should be synced at idle along with the higher rpms)
Here’s the biggy!
The next question is how much does the idle rpm affect my landing?
Not as much as many want to believe and it’s more about piloting skills.
No one usually sets their idle up excessively high and a 100 rpm difference at normal idle rpms
(1600-1800) shouldn't make any big difference in a landing especially if you we're talking the difference between 1650 and 1750. Most set their idle rpms around 1600-1900 rpm for the ULS. High idle rpms makes you ride the brakes more on the ground. Being able to achieve a lower idle rpm usually reduces taxi brake use over the higher idle settings.
I will have people swear that an 1800 idle rpm over a 1650 idle rpm makes them float way down a runway or they almost can’t land. We have to go back to Flying 101 in the student flight manuals. What controls the aircraft speed? Is it the throttle or the stick? Way too many tell me it’s the throttle. For the sake of this singular discussion let’s just talk about idle rpm and maybe up to 2500 rpm.
The stick controls your speed not the throttle. If you have more rpm (or speed) on landing then pulling the stick back a tad farther will land you at the same speed as if the idle rpm were lower. Try it yourself. Pulling the stick back will increase angle of attack which will increase drag which reduces speed regardless of throttle setting and the plane will settle. This can be done in any phase of flight and is done during landings. We use this during slow flight practice as we pull back on the stick to reduce speed. It's the flare in landing. The plane at idle rpm will lose altitude and settle when the angle of attack and drag are increased. This is normal for jet landings and they leave throttle in to touch. I have many friends that leave a small amount of throttle in their 912’s to touch for better tail and directional control with the prop wash. I’m one of them. It’s not outlandish it’s just a different way and there is and always has been different configurations to land. You could if you wanted land at full stall at idle or 2500 rpm. The difference would be just a higher nose up at 2500 rpm because you had to increase the drag and angle of attack which helps reduce some lift to get the speed down to stall.
Try a landing at idle rpm then try one at let's say 2500 rpm. The main difference should be that the stick is farther back with 2500 rpm to help slow the plane, but the touch speed should be the same. If you fail to pull the stick back farther will you glide down the runway and maintain a bit more speed, of course you will, but that’s piloting skills. On approaches and round out the idle speed will never get to 1600 if it was set there. Depending on the plane, pilot and approach speed the rpm will always be higher and what affects that more than any single item is your angle of attack which either increases the speed and the loading on the prop or decreases it. The more landing speed the more the prop turns due to some wind milling added to the idle rpm. So even at idle with a 1650 rpm the engine speed may still be around 2100 and decreasing after you round out or flare to land. You can actually turn the engine off if gliding at a safe altitude and unless you slow down the prop can still turn with the engine off.
Last year this vary scenario was presented to a large fly-in group of pilots. Many knew the correct answer, but I was stunned at the number that thought idle rpm controlled their speed. They have had a change of mind after some demos that showed landings with an idle rpm set at 1600 and rpm left right to touch down at 2700 rpm. The only difference was the 2700 rpm group had to have the stick back a bit farther to land at the same speed as the ones set at 1650 rpm. 7 years ago some friends in the UK said they couldn’t land on a 300m grass strip unless the idle was set at 1500 rpm with an aircraft with a 14:1 glide ratio. I challenged them that it can be done at 2700 rpm and on an asphalt runway. We measured off 1000’ and I made 8 landings with 2700 rpm in half that distance. The key points are leave no runway behind you at touch and use the stick to control speed and to land at almost stall. It was quite easy.
What is the perfect idle rpm? Well that’s up to you, but most will be in the 1600-1800 rpm range depending on which 912 engine and aircraft they have. Does having an idle rpm set at 1650 rpm versus 1750 rpm make a difference in landing? Not if you realize it’s the stick that controls the aircraft speed (and prop loading) and not a 100 rpm difference in idle rpm. Lower idle rpm does help for taxi on the ground and if you decide to have a low idle rpm for landing just keep in mind you have the throttle and shouldn’t let it just idle at low rpms when just sitting on the ground.
PROCEDURES FOR MECHANICAL AND PNEUMATIC SYNCHRONIZATION OF THE 912 CARBS:
This will get you through the basics and after you do a sync or two it will get easier and you will get better. This is not an official document and is only meant for your edification and will get you through a carb sync. Some experienced people that do carb syncs on a 912’s may vary some, but the basics ideas are the same.
Mechanical Synch of the Carburetors – Do this First
Bowden cables adjustment screws (cable housing adjuster) should be centered, but not mandatory. By having the Bowden cable adjustments in the middle of their adjustment range will give you a little more adjustment when you get to that part of the procedure.
Adjust throttle cables so that both carb throttle arms move simultaneously to both extremes (open and closed).
Ensure Idle Mixture Control Screw is properly set (on the bottom of each carb). Screw them all the way in then back out 1½ turns.
Reverse the throttle arm springs so they pull toward “closed” throttle just to facilitate mechanical sync.
Unscrew the Idle Stop Screw. Make sure the throttle is closed in the cockpit. With a .004” feeler gauge between the screw and the arm tighten the screw until it just touches. There should be minimal friction on the feeler gauge.
Remove the feeler gauge and screw the Idle Stop Screw in 1 turn.
Return springs to original configuration (i.e., pulling the throttle arms toward to the “open position”).
Connect the Pneumatic Synch Tool
It is imperative to have the engine up to the operating temperature. Remove one end of the rubber crossover compensating tube between the two intake manifolds or connect to the top of the intake manifolds or use the cross over tube rubber hose pinch off method shown in Rotax owner videos. All these attachment methods will work.
Hook up gauges or electronic sync tool. Secure one end of a gauge or electronic device to the rubber end of the compensating tube and the other to the gauge at the 90 degree fitting coming out of the intake manifold where the compensating tube hose was removed. Secure both attachments with a hose clamps to prevent leaks. Leaks will affect the results of your sync process. You can hook up your sync tool at the small screw on top of the intake manifold, but then you have to pinch off the rubber hose between the carbs on the cross over compensating line. I find this is just more work and you need to make sure the rubber hose on the compensating tubes are long enough (usually newer engines) and you don’t allow any leaks between the carbs. Many people have to cut the tube shorter and make the rubber hose longer to pinch it if you have an older engine with short rubber connecting tubes. I think it is easier and faster by just pulling the hose off as originally described and it facilitates complete carb separation.
Make sure you have good brakes, use wheel chocks and or if you have not so good brakes or no wheel chocks then tie your plane down and always have someone in the cockpit. Never do this alone while running an engine and no one in the cockpit..
“On Idle” Synch
NOTE* Many people like to do the high rpm or off idle adjustment first and then do the idle set.
I would do the high rpm sync first because it will affect the idle sync.
The gauge or carb with the lower vacuum number is getting more air/fuel and the gauge or carb with the higher vacuum number is getting less air/fuel.
Bring the cockpit throttle lever to idle stop. Ideal idle RPM is approximately 1700-1800 rpm depending on your specific aircraft and needs. Adjust idle setting to this value. The 912UL 80 HP can handle a little lower idle rpm over the 912ULS 100 HP due to compression ratio.
If the idle is high (say, 1900), adjust the carb with the lower vacuum number on the gauge because that is the one getting more air/fuel. Retard the Idle Set Screw until the vacuum numbers are the same. Example: If one gauge is at 15” of vacuum and one at 14” of vacuum retard the lower 14” until it drops to 15” to equal the other carb which will now lower your idle rpm.
If idle is low (say, 1600), adjust the carb with the higher value, it is getting less air/fuel. Advance the Idle Set Screw until the vacuum readings are the same. Example: If one carb is at 15” and one at 14” turn the idle stop screw in on the carb with 15” of vacuum to match the one with 14”. This will raise the overall idle rpm.
Needles should now match.
“Off Idle” or high rpm Synch
If while running the engine the gauge needles shake then you will need to install or close down the inline needle valves until the gauge needles stop shaking and pulsating. Now the needles will likely show different vacuum readings. Do this sync before the idle sync because this sync usually will affect your idle sync set point.
Run engine at approximately 3500 RPM. Do the “Off Idle” synchronizations at this rpm. I prefer 3500 rpm to start since that is closer to where you run the engine. You may find that setting the balance at lower rpms doesn’t stay very accurate and that as you move up in rpm to 4000-4500 rpm they are out of sync again. Set them at the higher rpm or at least double check the higher rpm when you think you are done. There is a good chance they won’t be in sync if you used a low (<2500) rpm to set them.
These adjustments will be made only at the brass Bowden cable adjustment for the higher rpm sync, where the cable housing meets the carb adjuster on top of the carb, do not touch the idle set screws.
The gauge or carb with the lower reading is getting more air/fuel and the gauge or carb with the higher reading is getting less air/fuel.
Compare vacuum gauge readings. Normally, but not always, adjust the carb (retard) with the LOWER vacuum reading (getting more fuel) back down towards the carb with the higher vacuum reading. Adjust the Bowden cable screw outward (toward reducing the air/fuel) so the throttle arm backs off (decreased RPM) to bring vacuum reading to a higher vacuum number.
When vacuum readings are equal, lock it down.
If vacuum readings are the same, bring throttle lever back to idle stop then advance it again to 3500 rpm. Verify vacuum readings are still the same for both carbs. If readings are different adjust the carb’s Bowden cable with the lowest gauge reading to match the higher gauge. Once the gauges are aligned move the throttle back to idle and back to 3500 rpm a time or two to make sure the carbs stay equal. Throttle to idle. Re-adjust the Idle Stop Screw if necessary at this point. You are now done.
Remove the gauges and hook the carb compensating tube back up. The engine will run slightly smoother after the compensating tube is reconnected and sometimes the idle will be slightly higher after the cross over balance tube is reconnected. If you know this then while doing the sync set the idle 50 rpm lower so when the balance tube is reconnected you be right on your personal target.
I would like to take a moment and talk about good verses poor logbook entries and how they may affect you. I don’t know how to make this a short article because it has so many ramifications and implications for owners and mechanics. I get many questions about documentation for our logbooks and I get to see many logbook entries. This article is prejudiced toward good documentation and for those who say; “ if I don’t write it they can’t blame me or hold me liable”, you are sorely mistaken. I have watched many professionals bite the bullet legally and monetarily because they thought this was true. Doctors, nurses, lawyers, police, paramedics, mechanics, title companies, ect… the list goes on and on. Some mechanics and owners absolutely do a good job and then some are very poorly done. Some poor logbook entries are done out of a lack of education on making a good entry and some are from pure laziness.
So why the big deal some say so long as it meets the FAA definition of legal? The short answer is for the same reason that the medical or police community documents everything. Good documentation will help protect the mechanic and the owner from civil suits, to show the next owner or mechanic what has been done on the plane, warranty claims, to protect you from legal issues with enforcement from agencies like the FAA and to help you with possible ensuing insurance claims if the need arises. So let’s explore a few of these one by one and then talk about what makes a poor or good logbook entry.
The mechanic always has risk involved with any maintenance work. He/she should always strive to do whatever is necessary to protect the owner and himself from civil and legal enforcement and or liability. The one way to do this is to document well what he/she did during the maintenance. Poor logbook or illegal documentation can not only get a mechanic in trouble, but the owner too, even if the owner had nothing to do with the annual. Hand written illegible logbook entries doesn’t cut it. If no one can read it, it’s worthless, so print it or type it if your writing skills need help. Some mechanics will argue these 3-4 line annual logbook entries fit the legal definition. If you’re the owner you may want to look for another mechanic. I have many owners and mechanics ask what to put in the logbook or how to write it. The how to write it can certainly vary with different styles, but no matter what the style the content should be basically the same. You don’t have to make up any thing or be an expert writer. Just write down what you did. For instance; if you did an oil & filter change, changed plugs, re-torqued the engine mounts, replaced a hose clamp, performed a gearbox friction torque check, compression test, changed the coolant then log all these. As you do the inspection or general maintenance just jot down what you found wrong and what you did to correct the problem.
The logbook entry will then write itself, you are nothing, but a secretary writing down what you did. USE AN INSPECTION CHECK LIST, SIGN EACH ITEM OFF AND THE OWNER SHOULD KEEP IT. You can make notes in the margins.
I always have a Discrepancy List that I jot my notes on and then type that up for the owner to keep. I always use the Rotax Line maintenance inspection check list and I print it out and sign off each item I inspected or corrected and give that as part of my documentation to the owner. I do the same with the fuselage check list. If you have taken a few notes and actually inspected what was on the Rotax list then you would have had to perform some task so then how can you document that with only 3 lines for the annual?
IAW = “In accordance with” too many times is a lazy persons way to try to use a catch all phrase to cover everything for a complete inspection if there was no other detailed content. I don’t have a problem with the term IAW, just how some use it to skip out on detailed documentation. How does the owner know what was done or the FAA, Rotax or the insurance company. Try to defend that in court when you write those 3-4 lines and 10 months later they ask you in court what the compression test results were or the gearbox friction torque was. If that friction torque contributed to a failure and an off field crash with injuries or a death, you may be held liable. The lawyer for the injured party will bring in five good documenting mechanics to compare your three line logbook entry and they will all agree you were remiss and the courts many times will conclude that if it isn’t written down it wasn’t done. Professionals over many decades has been beaten up in court for not only wrong doing, but more times than not, have been found liable because it wasn’t documented so it wasn’t done. Why on earth would anyone want to leave themselves open to that type of liability when all you had to do was take 5 minutes and just jot down what you really did. Half the time when I see IAW they missed half the items for the inspection when I quizzed them on what they did and those types rarely ever use an inspection check list. So IAW can be extremely misleading.
Another place documentation plays a part is the warranty. Rotax requires an oil purge on all new engine installations and that engine requires a 25 hour warranty inspection. If you fail to perform and or log these and you have a major issue during warranty you most likely will be denied and all it would have taken was 2-3 minutes of your time to log them.
I did a research project a few years back on re-sale value and the affects of documentation. Depending on the price of the aircraft poor logbooks will cost you money at the time of sale. For many of the LSA in the US that poor documentation cost sellers an average of $5K-$10K and that was right out of the owner’s pocket because of the lack of the extra 3-4 minutes of documentation during inspections. That’s an expensive few minutes for each annual.
What about documentation for the owner or next mechanic. If your last mechanic did a poor job and failed to document a service bulletin or a service alert that may have been done then you’ll pay the next mechanic again to see if it was done. What if the next mechanic finds a low compression on one cylinder was this a trend or was this a first time low compression?
Let’s look for a second about wording. Can this be a problem? Here is an example; If you live in the US and own an LSA you have an Annual Condition Inspection. The others in the general aviation community have an Annual inspection. The difference will be a little more defined in the mechanics last sentence in the logbook. If it was a certified aircraft then a mechanic can say he put it back in service. If it is an LSA aircraft then it has an annual condition inspection and it can’t just be put “back in service” with that phraseology. It should say something similar (doesn't have to be exact) to this; “I certify that this aircraft has been inspected within the scope and detail of the Flight Design and Rotax maintenance manuals and found to be in a safe condition for operation”. I used a Flight Design plane, but it could have been any LSA aircraft. You must put in there that it was found to be in a safe condition for operation.
Here is a pet peeve of mine and you can decide for yourself. You get an inspection done and a facility writes a nice computer printed label, but only a 3-4 line IAW logbook entry and states that the details are on file. You just spent
$1K-$5K for an inspection and they couldn’t even hit the print tab to print out a single piece of paper that they said they have typed out. You or no one else has a clue as to what was actually done unless you research this and have them send a copy. They give you no check list and held back on details that should be in the book. What if these records or servers get damage or they go out of business. How hard is it to hit the print tab and hand a piece of paper to the owner or just put it in the book where it really belongs. Is what they do legal, yes, but in my opinion a terrible practice. If it were me paying those dollars for an inspection it better pay for a piece of paper.
So what makes better inspection documentation? Well the sky’s the limit. I saw one mechanic not only write everything down, but took pictures of everything and put them in a 3 ring binder for the owner. That mechanic is certainly at the top of the class for documentation. I always use and give the owner a typed discrepancy. List, a signed off copy of the Rotax Line maintenance inspection check list and a signed off fuselage check list for that particular plane. This will help protect the owner and it will help protect you the mechanic which could be one in the same. Then I type out on a sticky label so it’s readable and it’s a complete detailed list of items I may have checked or actual issues I may have corrected. I’m just a secretary and just write down the things I did, nothing more and nothing less. Additionally I make sure the document has the date, TTSN hours, tail number, aircraft type, aircraft serial and Rotax serial number.
Logbook entries can be written many ways and we all have different writing styles and that’s okay, but content needs to be there. Here are a few examples of bad logbook entries and few better ones. You can certainly be as detailed as you want and many include the manual, paragraph and section in the manual where you got the information to correct a problem. Examples below are just that, examples and they can be more detailed if you so choose.
The first ones here are not very good. They lack a lot of detail and content.
(See attachments below for the poor shortened logbook entries, jpg's.)
Here are a couple of better examples, but these aren't necessarily all inclusive or top of the line entries, but they do include detail and important inspection information and at least shows that someone made a better effort to document and most likely did a better inspection.
(See attachments below for the poor shortened logbook entries. pdf's)
All these are just examples and yours may differ quite a bit and be worded differently. You may include more information than these and quote manuals, pages and paragraphs. No matter how you feel about what you have read I hope that you might take away a piece or two of this article to make your logbook entries more complete and to ask for better documentation from your mechanic.
Rotax 5 Year Rubber Replacement
The Installation Considerations
For those who have decided that they will do a Rotax 5 year rubber replacement we need to look at what it covers and how we can utilize good sterile maintenance practices to keep debris from our hose lines. I will admit that there is more than one way to accomplish this procedure and what will be discussed in this article is, but one way.
The Rotax 5 year rubber replacement covers all fuel, oil and coolant lines. It covers any V-belt, carburetor diaphragm and carburetor rubber intake sockets and any other air intake rubber hose or tubing. With the new maintenance manual just out the fuel pump has been added as a replacement item too. So now you need to decide what brand hose you are going to use. Should it be fuel injection hose or standard carburetor hose? Since we are dealing with a worldwide distribution for engines the hose selection can be vast, but by all means should be thought out. We need to decide what tools we are going to use to cut the hose and how we are going to secure it in place. These again will vary depending on your geographical location.
Since I live in the US and have access to many a mechanic and can relate to my own observations I’ll use this as a base for my comments and you can adapt and or re-think how it may pertain to you where you live.
I have access to the Rotax hose change practices, information and results from several mechanics including myself which easily covers a couple hundred Rotax engine hose change procedures.
Rubber fuel hose was really not intended to push over barbed fittings as documented on several fuel hose manufacture websites. They prefer the flared or bulb end to slide the hose over. That said most of us have exactly that, barbed hose ends on many aircraft installations. What we have found is that fuel injection hose is less flexible than the standard carburetor hose. When you try to push fuel injection hose over a barbed fitting it has less give because of its pressure rating and tends to scrape the hose liner and small particles then float down stream. This is exacerbated by using a cutting tool with a serrated edge. Something like serrated edge scissors. Then add using the wrong style clamp and over tightening the clamp crushing the hose all adds to debris floating down stream towards your carburetors and a power loss when you absolutely don’t want one. So picking our hose type, tools and clamps are important ahead of time. As the old saying goes, “Failure to plan is planning to fail”.
Generally speaking, fuel injection hose has a working pressure normally around 100 psi and a burst pressure of around 900 psi. Standard carburetor fuel hose has a working pressure of 50 psi and a burst around 250 psi. The good thing with the standard carburetor hose is it has a little give in it since it isn't as stiff and rated for the higher fuel injection pressure. This hose will slide over the flared, bulb or barbed fittings much easier and there is far less chance of scraping the inner liner and causing unwanted debris. A smooth inner liner is more preferable than a raised criss-cross pattern on the inside of some fuel injected hose which is caused by the thread re-enforcement of the hose. This tends to scrape more with a barbed fitting. I know everyone remembers that our normal 912 fuel system operating pressure is up to 2.2 to 5.8 psi for the 912UL or ULS engine. So let’s just say the upward limit is 6 psi to round it off. So using the standard carburetor hose works just fine with plenty a pressure safety margin. Now who’s brand to use? Well that is up to you and where you live in the world. For me and many of my mechanic friends we have been using Gates Barricade standard carburetor hose with the green shield technology. It has 4 liners and is rated for any fuel and even 100% alcohol.
(See Gates Barricade picture below)
That should pretty much cover anything we will put in our 912 engine. We have not had any issues with scraping inside lining or hose degradation. The other big issue here is to use the proper cutting tool. I used to manufacture dive compressors and used 200’K of hose a year and needed perfectly smooth cuts. I use a Sears Craftsman Utility cutter Craftsman 3-7/8 in. Handi-Cut, item #00937301000, Mfr. model #37301.
(See Craftsman Utility Cutter picture below)
It has a 3 7/8” long straight and scalpel sharp edge. It leaves a perfectly smooth cut and will handle any hose on a 912 engine. Serrated edge cutters will leave very tiny particles on a cut edge which gets shoved into the hose line when pushed over a fitting.
Hose clamps. I’m sure everything gets used here. You really should not use the standard hose clamp with the serrated openings like you buy in a hardware store. Yes they do get used, yes they have been around a long time, but they don’t give a good solid 360 degree seal, they can be over tightened and easily stripped and they cut into the outside of the hose. If you absolutely want to use a screw type clamp then use a fuel injection clamp. It won't cut into the hose, you can’t strip it and it does a better job with a 360 degree seal. If you have to use a screw clamp then the one in the picture below with the raised ribs is a better choice and won't over cam and strip and won’t cut into the hose. These clamps are better for coolant hose and not fuel. The next picture shows a fuel injection clamp (on the left) verses the standard hardware store clamp. Fuel injection clamps are good for oil hose and if you don’t have access to good Oetiker clamps then it’s a good second choice for fuel.
(See serrated worm drive clamps, fuel injection clamps and worm drive raised rib clamp pictures below)
(See Oetiker clamps kit and pinch pliers picture below)
The better way to go and is preferable as an industry standard is something like an Oetiker band clamp. (Pictured below) I personally use a stainless steel one ear stepless Oetiker clamp on all fuel hose applications. They come in all sizes from the very tiny to the very large. It slides over the hose and you use a set of pinch pliers to squeeze it shut. I know then it won’t rust, it can’t come loose and provides an excellent 360 degree seal. One note for this clamp is to use the correct size and don’t use too small a clamp and crush it to death especially over a barbed fitting.
Now we should also mention that all your fuel and oil lines should be in fire sleeve. The best and industry standard is to use Band-It style clamps to secure the fire sleeve ends. These look like a little noose that gets pulled and cinched down to secure the fire sleeve. These can apply tremendous torque so only barely snug these down on any hose. If you over tighten these they will close off the hose underneath and restrict its flow. These clamps are already on the fire sleeve on the fuel pump from Rotax. The best way to remove these clamps and Oetiker clamps is with a Dremel tool and a re-enforced cut off wheel.
There is one last item to cover. We are supposed to seal the ends of the fire sleeve with something to keep fuel, oil or any liquid from wicking up into the fibers of the fire sleeve at its ends. You can use a couple of different methods. Some use the red silicone RTV and thin it slightly with a little toluene and brush it into the end and fibers of the fire sleeve. Some use a product called “End Dip” which comes in a quart can for $185-$225. You dip the fire sleeve ends and let it dry. I hate to sit and wait so there is one more good option. You can use fire sleeve self vulcanizing tape that is fire rated like the fire sleeve.
(See fire sleeve end tape picture below)
You as an owner can do one last safety check. After the hose has been installed and the engine is run and the carburetors synced you can run the engine for another 20-30 minutes on the ground at 3500-4000 rpm. Then head back to the hangar and pull the carburetor bowls off and look for any debris that may have escaped your careful planning and installation. It’s just a good safety check before your first flight and should help you cover all your bases in looking for any leaks and debris before your first flight.
So here we are at the end. The whole idea of this two part article is to make you think about keeping yourself and your passenger safe with a good solid maintenance practice and not being the test subject for hose time limits. Don’t worry there will still be the ones who, ”Won’t fix it until it breaks” that we can read about in the news with the off field landing or expensive engine tear down from a damaged engine. Hopefully you will look at a Rotax 5 year rubber change with a positive attitude and plan to think about what hose, tools and clamps you want to use and not use.
Everyone Fly Safe and Often!
The Task at Hand
Rotax 5 Year Rubber Replacement
This discussion is going to focus on a topic that will undoubtedly have Rotax engine owners on both sides of the fence, both for and against in a major discussion, but I hope to instill a sense of “Doing things right and for the right reason” and without all the worry some seem to have over this solid and sound maintenance practice. As you can see from the last few words in the last sentence this article will focus on the positives and good maintenance practice and hopefully get away from the all encompassing “Don’t fix it if it isn’t broke” concept. I do believe that some items are fine to be on a condition inspection or even wait until it wears out, but those are not flight safety issues or will they present a hazard when they fail. Planes in general and the hose change cost money , I’ll be the first to admit that, but you decided to fly and now you need to ask yourself what your life and your passenger’s life is worth and do you want to spend a little money now and keep safe and flying or spend a lot later and be grounded?
The Rotax Line Maintenance manual dated 09-20-12, Section 05-10-00, paragraph 2.1 and subject head “Time limit for rubber parts” is the section we will be addressing in this article. There are two important issues here to cover. First is the issue with hose time limits and how long will a rubber hose last? The second and possibly more important is hose selection and the right way to install a hose which is covered in part 2 of this article.
Many of the machines we use now days in a high risk environment have some type of maintenance program the manufacturer wants followed and some are mandatory. Even our automobiles have a recommended hose, serpentine belts and V-belt replacement program. Some owners are good about following these practices and of course we see some autos stranded on the side of the road or even a damaged plane in an off field landing and these folks may not have been as good about their maintenance practices. Even with a good maintenance program mechanical parts and hose can and do fail. The whole idea is to put the odds in our favor and not test the limits. If we are in our auto and have a hose failure we can pull over and call for assistance. If we are flying in our aircraft then a hose failure is probably going to bring you and possibly a loved one down in the worst possible area. Do aircraft hoses fail before their time? Yes they do and have caused many an aircraft emergency. With that in mind many aircraft engine and aircraft manufactures in general have recommended time tables to which they recommend a hose maintenance program which is usually backed up by 20-60 years worth of data and failures not to mention the recommendation right from the hose manufacturer themselves. The bottom line is Rotax and others are trying to error on the side of safety and not test the limits of each hose within the hostile environment in the engine compartment as you are flying over the Grand Canyon. If you are one that says don’t fix it until it breaks then you may be willing to switch sides as you glide down into the Grand Canyon that has no landing areas and a has the fast running Colorado river.
Many aircraft manufacturers now recommend that you follow the Rotax 5 year rubber replacement program. So how long is oil, fuel and coolant hose good for? The answer is, Who knows for sure. Could it last only one year before a failure, yes. Could it last ten years before a failure, yes. No one can ever tell you exactly when a hose may fail so we use decades of observance and factor in some safety and make our best guess for you to get to that point and not have an issue. Over the last several years we are seeing a huge increase in owner compliance with the hose replacement program and that’s good news, but too many have had hose particles reaching the carbs and causing a power reduction. The immediate response has been that it must be bad hose, but in 98% of the cases it has been mechanical damage from poor installation practices and possibly poor hose choices. There have been a few pieces of bad hose and Rotax issued a Service Bulletin for the fuel pump hose because of that very issue, but that is usually very rare compared to the amount of hose actually sold and used. Let’s look at the hose time table for replacement. Many want it to be a condition inspection replacement item. Okay so what are your replacement limits? Is it when the hose gets hard? That’s too late in the game. What about the fuel and oil hose in the fire sleeve? Do you dismantle all that hose and pull it all out of the fire sleeve to inspect it? I know of no one that does that. While you are looking at the outside what about the inside that begins to flake or degrade from time? How do you inspect that? How do you inspect the hose for cracking and separation under the hose clamp at the edge of the fitting on the inside and outside of the hose? (This is the most common problem area.) How many of you have been trained by a hose manufacturer to know even what to look for or were you just taking someone else’s word for your education?
So looking at it from a safety stand point, none of us are hose experts or have all the data the engine and hose manufacturer have so it just makes good common sense to error on the safe and practical side for you and your passenger’s safety. I would like to mention one other item here and look at it from a legal burden which none of us hopes to have to encounter. If you have someone in the plane with you and go down because of a hose failure and it is past the aircraft and engine manufacturers recommend rubber replacement time and the other person or other person’s family member takes you to court I would hope that you can back up all the good solid reasons that you didn’t do the recommended maintenance because it will be brought up and your hose expertise will come to the forefront. Family members and their lawyers are not very forgiving. That alone is enough to scare me because I have been to those types of court cases for over 30 years. If you error on the right and safe side it is much easier to defend from a legal standpoint.
Winning the Engine Lottery
Not one you want to win!
Here I was flying along with 3 other CT’s on our way for breakfast and a happy camper. I was the leader that day and decided to take a new route to our breakfast airport. What a good decision that was that day and I didn’t even know it. I was in mid sentence giving the ATIS information to the others when my engine burbled for 2 seconds then back to normal. It burbled again 2 more times, but after the third time it stayed and I tried to pull back to idle and it remained, then I tried WOT and it remained.The longer this went on the rougher then engine became. It felt like I was running on 3 cylinders or worse. I didn’t know what was up at that time, except landing was soon to come. All the temps and pressures were normal except for the right side EGT which instead of 1400F was only 700F. I was out in farm country with lots of roads for landing. The airport we were headed to was 9 miles further and I was entering nothing, but housing sub-divisions. I wanted to pick my landing spot and not have it picked for me and make the front page of the newspaper. You can keep that 15 minutes of fame!
Decision time, try for the airport as the engine got rougher and most likely land on someone’s street or land on a farm road? The farm road was looking pretty good, but one of our group jumped in and said I was right on top of an airport. It wasn't on the GPS, but later we found it on a paper map. I was only 1500’ AGL. I couldn’t see the airport because I was literally right over it. I tipped the wings and sure enough there it was a private strip. Who says miracles don’t happen! I was at idle and I swung way out to the left and tear dropped in at zero flaps and crossed the numbers at 80 knots with a nice long runway under me. No chance of coming up short on this approach since I was on top of the airport. A nice touchdown and I was able to taxi the very short distance to a tie down spot. This was a very nice and high dollar private strip. I pulled the plugs on #1 cyl. and they were black and wet. Well there was nothing more to do with what I had there. The engine would start, but knocked and banged so I shut it off and tied it down. Hopped in another CT and off for breakfast. That was March 23rd on a Friday. Sunday the 25th I went up to the airport with a 26’ U-Haul truck and I was very lucky to have 7-8 of my good friends that volunteered to come help. (Thank You All ) We drained the fuel, pulled the stab off and hung it along the wall in a strap sling with blankets in the truck. Then we pulled the wings and hung them wrapped in shipping blankets along the walls in some straps. A friend had made two wood ramps and we just pushed the CTSW up into the truck. I did have to pull the prop because of the sweeping arc of the rear door. It took maybe 30min. for 3 people to unload it all.
Back home I found this during the investigation. The plugs on cyl. #2, 3 and 4 all looked normal , but #1 cyl plugs were black. A cold compression test revealed cyl’s #2, 3 and 4 were 84 psi with an 87 psi test pressure and #1 was only 75 psi. I pulled the #1 valve cover and the outer facing (#1 exhaust) rocker arm was mushy and the inner intake rocker was rock solid. I then pulled the head and everything looked good. I pulled the cylinder and it looked okay. I pulled out the #1 intake lifter and it came out easily and looked in good condition. The #1 exhaust lifter would not come out. It would rotate freely and was shortened. They are about the size of a fat thumb.
So what happened? Here is where I won the engine lottery. This only happens to 1-2 engines worldwide each year. The hardness coating on the #1 exhaust lifter face broke down. Now the lifter rubs against the cam, metal to metal and the cam just grinds it down into dust. No slivers or large chunks, just dust. When this happens you will get no prior warning and it will fail right now. The engine had 833 hrs. and the last inspection was at 808 hrs without any issues. The magnetic plug at this inspection and every inspection was clean. Now the magnetic plug looked like it had a dust beard, the oil filter had very fine dust in it, I drained the oil over 4 magnets and it had dust in it. When I finally pulled the oil tank it had a lot of metallic dust in the bottom. The engine case was eventually split and the dust was everywhere and in everything. It had ruined and was in all bearing surfaces, other lifter surfaces, piston pins, it was everywhere and that made this engine not
re-usable or cleanable. The lotto winner!!!! Wupeeeeee
I have preached about good detailed paperwork and documentation and here is where it paid off. I sent Rotax an email and then filled out a Rotax CSIR (Customer Service Information Report) and I had to send them a copy of my entire engine logbook. Because of all the detailed documentation since the aircraft was new, i.e.. on time scheduled maint complied with, oil analysis, all SB’s documented and complied with, hose change, oil changes on time, ect… Rotax stepped up and offered to replace the engine within about 48 hrs. Poor or little paperwork equates to good luck on help or how long it might take. This is a Dec. 2006 CTSW. I did have to pay for the 833 hrs of use ($7200), but that was nothing compared to $18,300 for a new engine. Yes I was way out of warranty, but Rotax stepped up to the plate with what they call a “good faith” offer to help out because bottom line this should never happen. Thank You Rotax.
New engines do not come with a radiator, exhaust pipes, muffler, fuel hoses or oil hoses. All the hoses needed to be replaced. The coolant reservoir needed to be relocated more forward on the engine which meant new hoses. The oil pressure sender is the new Honeywell. The starter casing needed to be rotated 180 degrees to get the cable lug facing outward. Better do this before you put the engine mount on because the starter mounting screws will not come off with the engine mount in place. The extra mounting brackets on the starter needed to be cut off. Some parts had to be removed and remounted to get the engine mount on. The new engine has different oil fittings over the old style. There were a few other minor parts that FD uses that Rotax does not. This new engine has the new soft start system. That’s good.
The engine is up and running with 5 hours of time and everything looks good. For now I’ll keep a few more feet of altitude under me until I get a little more time on.
Now I have a zero time engine so maybe all isn’t bad.
Someone was looking out for me that day. I was flying higher than normal, I picked a completely different route than normal, I was over an airport when it happened, I made a great landing, I had great friends to help me not only get home, but bring her back 2 days later.
Yes, I would rather have not spent $7200, but all things considered it all worked out in the end and the ending was a good one.
Manuals; Do I Really Need Them?
The simple answer is absolutely, YES! It doesn’t make any difference if you have a Rotax 2 stroke or a 4 stroke. If you own a Rotax engine and pick up a screwdriver or wrench and intend to touch your engine then you need the manuals on hand. Your neighbor is not Wikipedia and won’t always have correct advice. Even if you think you know what to do,something unexpected always comes up. An example would be; what to torque a bolt or nut, so you may need the Line Maintenance, the Heavy Maintenance or the Part manual. Another applicationfor the manual would be to see if you are supposed to have a space between certain parts and how much space, ect… This list can go on forever. All smart mechanics and owners will know that the path to easier maintenance and successful engine ownership requires manuals for references. Even if you are screwdriver challenged and never intend to touch your own engine then being up on the particulars of your engine will helpyou keep the mechanic on the correct path. If your mechanic doesn’t have your engine manuals then have him print them out or you can give him a set for his birthday. You need to have all the manuals that apply to your engine. For example the Rotax 912 series engine has 5 manuals that apply to your everyday needs and will answer just about any question that may come up pertaining to your engine. Many questions right here on the forum can have an answer ascertained from these manuals in a few minutes. I do agree that sometimes there could be a little more information or maybe just be a little better in its explanation, but the manuals are an absolute must. The reason you need them all is because information may not be in the book you are using on a specific task. An example of this is the torque settings on many nuts or bolts for a special application and it may have a special torque value and not the same as another nut or bolt somewhere else on the engine. You can buy these manuals or you can print them out on the
Rotax-Owners Forum. Get a three ring binder and start your printing and while you’re at it make sure to print any Service Bulletins, Alerts or Service Information that pertains to your particular engine and keep them on hand as well. I use the manuals all the time in the shop. You just can’t remember everything, besides you need to know when your neighbor’s advice was wrong. These manuals are your Bible, encyclopedia and teacher. You need them to make life easier for you and your engine. Follow the manuals inspection periods and maintenance schedules and you will stand a good chance of not having any major issues. I do realize that engines are mechanical things and do fail so this is another reason to keep your manuals on hand. All engines need maintenance to keep on functioning and neglected engines will usually cause its owner grief of some sort sooner or later if maintained improperly. Don’t wait for a problem to arise. Read through each manual or table of contents and get some idea of where things are or even how to accomplish a simple procedure. I always read the section I’m going to perform work on the night before the job.
I know this isn’t a technical article, but I get questions everyday about fairly simple things that are in the manual. Don’t get me wrong I really don’t mind the calls and I enjoy talking to people from all over the world, but I was just trying to have people get their answers right at their finger tips and be better equipped to deal with any issues right on the spot at home.
Safety Comes With Knowledge!
Understanding the Ground Adjustable Prop
Keeping it simple and effective for the common setup.
Let's discuss prop pitch and how it affects flight characteristics. It can help flight characteristics or it can hinder. I'm often asked; What's the correct prop pitch for a specific plane? There is no single answer as many props are available to us today for Rotax engines and for different fuselages. There are, however, some commonalities and that is where we are headed in this article.
Certain principles do apply in either the 2 stroke or 4 stroke engines, although the numbers will be different as with most ground adjustable props. This article won't get into all the designs, blade twists, angles, thrusts, shaft powers, etc...etc. Whoa, just thinking about it puts my brain in a tail-spin. We are going to keep it simple and easy to follow. I am going to use the Rotax 912ULS as an example.
First let's pick a few numbers to keep in the back of our minds for later. We are going to shoot for certain idle rpm, so let's pick 1700 +/- rpm and 5600-5650 rpm for wide open throttle (WOT), flat and level at your average cruise altitude. It wouldn't make sense to set a prop for sea level when you are at 9,000' msl all the time. Why 5600- 5650 rpm as a target? Through a lot of testing this was found to be the BEST balance point for climb, cruise, engine temps and fuel consumption. The "continuous run" rpm Rotax recommends for the Rotax 912ULS is 5500 and that rpm can be flown all the time if you chose to do so. Another good reason would be if you were to break a cable or had a throttle control failure. One carb would probably go wide open, as it's supposed to do and then you could advance your throttle and have the other carb go wide open. You could then fly to wherever you needed for a safe landing area; shut down and land. Anything over 5500 rpm (i.e. 5600-5800 rpm) would limit you to a 5 minute run time under normal flight situations for longevity of the engine, but in an emergency the engine can truly run for much longer times without fear of damage. A prop manufacturer will usually have some instructions for their prop and sometimes a suggested starting point for pitch depending on the engine. Another often asked question is “What should my static rpm be”? There is no specific or accurate answer for everyone's engine and prop. The static won't mean much if you only want to fine tune your existing setup. Static is more important for the first run owners or for new prop installation. The static rpm setting is just to get you in the ballpark and then you will need to fine tune it for your specific aircraft and needs while flying WOT at your average altitude. So in keeping it simple, you will want to set the pitch on most props to achieve a target with a beginning static (ground run) WOT rpm of around 4800-5000 rpm, but your static rpm may be slightly different depending on what you wanted for a final in flight WOT rpm outcome and different props can run different static rpms from one to another. The factors here are long vs shorter blades, two vs three blades and stiff vs flexible blades.
(Note: These next figures are general and yours may vary slightly) To do this properly, you will need to go fly at your average cruise altitude and fly flat and level at WOT for at least 1 minute. Now if your WOT rpm at this time is 5500 rpm and up to 5650 rpm you're probably set up fairly well for your engine, temperatures and fuel economy. If you are up at 5700+ rpm then you may want to land and add a little pitch (about .25-.75 degrees) back into the prop pitch, which will make it more coarse.
If you already have your prop setup is only turning 5200 rpm WOT flat and level you need to flatten or reduce the pitch approximately 1.5-2 degrees to achieve 5600-5650 rpm. Now you may have some special circumstance like a float equipped aircraft (heavy aircraft) that needs a little better climb, lots of high DA takeoffs or lots of tight short fields where better climb is more important so a climb pitch of 5650-5725 rpm WOT might be warranted. We need to tune our props for the type of flying that we do.
What else does my prop pitch do for me?
Setting the prop pitch excessively coarse (i.e. 5000-5300 rpm WOT) causes excessive stress on engine components and gearbox which may necessitate early maintenance and has been known to crack crankcases. Having the pitch too coarse will cause higher engine (CHT, EGT) and oil temperatures, excessive fuel consumption, poor climb and decreased cruise speed. Your engine doesn't have the horse power and torque to turn an excessively pitched prop. All piston engines have their limits and the props all have limits, too. So if your engine temps are up and your WOT engine rpm is below 5500 rpm try unloading the engine by reducing the prop pitch. If you have a prop that is too flat then it may climb well, but have a loss in cruise speed and of course engine temps and fuel are affected again.
Your exact numbers may vary some, but you now have a general idea on what to look for and how it may affect your flying and engine. We'll keep this discussion on the root topic of ground adjustable props. Special circumstance rpm settings and in flight adjustable props will warrant discussion in a future article.
One last parting comment: If adjusting prop pitch sounds complicated, it isn't; it usually will only take 30-40 minutes, a couple of wrenches, a prop protractor and/or a level. So take the time to fine tune, your engine will say thank you in improved performance.
Do I Have Enough Tension?
(Tension in this article doesn’t apply to work or home
Let’s talk about the tensionon your muffler exhaust springs. Too little?---Too much? Although there several types of exhaust and muffler setups out there, I’ll limit this article to the most common and that is where the exhaust spring holds the exhaust male outlet pipe into the female socket on top of the Rotax stock muffler. If you don’t have a stock Rotax muffler, but do use springs, then this article will still have significance for you.
What happens if there is not enough tension on our exhaust springs? The muffler will have excessive vibration and pulsation from the exhaust and cause the exhaust tube to hammer in its female socket. This will cause cracks, broken out pieces and the chaffing will eat away the edges of the exhaust pipe and socket and too little tension will cause hot exhaust gas leaks that might impinge on your hoses or wiring.
I just performed maintenance on a 912ULS with 375 hours where the female socket on top of the muffler was destroyed from too little spring tension which allowed the exhaust pulsations to hammer on the socket. Since we are talking about this exhaust socket, or joint, I should bring up that this needs periodic lubrication. Use copper anti seize on this joint to help prevent early failure.This is the same anti seize used in the gearbox assembly. (You may have even seen what you thought was brass metal particles in the oil the first time the engine oil was changed. It was, most likely, just this copper anti seize.) When there is too little spring tension the spring hooks will vibrate excessively and wear at the spring hook where it comes in contact with the loop welded on the exhaust tube and muffler. (See attached pictures)
Conversely, when there is too much tension; although usually less of a problem because you have to be Godzilla to put them on, it can cause the spring to break and wear excessively at the hooks. Breaking will be the most common problem with too much tension. Either too little, or too much tension can cause the metal loops that the exhaust spring attaches to wear out prematurely. These can be replaced by cutting them off and welding on another metal spring attachment loop.
So, let’s examine the solutions There are a few different types of springs available not to mention all the ones owners may buy at the local hardware store. I would recommend Rotax approved springs. So it may be hard toget the perfect tension addressed in this article so again, I will limit this discussion to just two types. (Pictures below are the two most common.) One isthe Rotax spring part number, 938-795. The other one commonly seen is astainless steel spring and is much heavier in construction. It isn’tnecessarily better, just different and the correct tension will prevent premature wear. The wrong tension can make any spring wear out or break in as little as 200 hours. If you use the heavy stainless steel spring an effective quick evaluation for the best tension is obtained when you can see a little daylight between the coils or you have enough room to insert your fingernail betweencoils easily. If you can’t see any space,then the spring may not have enough tension. At least you will know that there is some tension on the spring if you see a little daylight and it won’t be over stretched. The Rotax springs will have more daylight between coils, around 1/8”– 3/16”. The loops welded on the exhaust tube and on top of the muffler can be adjusted by bending to help obtain your correct spring tension.
Get back in your seats; we aren’t finished yet. We need to apply some high temperature RTV silicone. This helps prevent vibration from wearing on the spring and helps give it a little extra support. There are two ways to apply the RTV silicone. The least common and least desirable way is to fill the spring’s interior with the silicone. I don’t particularly like this because you need to safety wire the spring and remove it at times and then it becomes a problem trying to get all the RTV out.The best, and most common way, is to lay a bead of the silicone from top to bottom on the outside of the spring. Here is where it was important to see a little daylight between coils because you want to work the silicone in between the coils as you apply your bead down the side. The bead should be at least 3/8”-1/2 inch wide and 3/16” – 1/4 inches tall. This gives the silicone some body and strength. Applying a pencil thin bead, or a line down a fully collapsed shut spring is close to worthless. One last thing you should do is safety wire those springs. Too many have gone through props on pushers. You can run the wire down through the center from welded loop to loop, or through the spring and back around the outside, but still through the welded loops. Do not make the safety wire tight. Leave a little play in it so it can move. It isn’t there to hold the exhaust together, but only keep the spring in place in case it comes off or breaks. If you make the wire tight the vibration will just wear through the wire quickly. My personal choice is to use .041 wire and not .032.
I have attached a couple of pictures to show what excessive wear will do to a heavy, stainless steel spring hook and a muffler socket. The red RTV silicone was applied well, but isn’t between the coils.There is a picture of the Rotax spring connected to the exhaust with the RTV silicone and safety wire already applied.