Jump to content
aldowns

Hobbs vs Tach time

Recommended Posts

aldowns   

I am getting conflicting opinions on maintenance intervals. Do we use tach or hobbs on the 912? Called 3 Rotax Centers and was told different things. Lockwood says Hobbs, Leading edge says tach and Flight Design West says tach. I really would like the correct answer and to know where to find it in writing. I have been using Hobbs from new but I just got two planes that have been using tach since new. Can anyone help? I have been unable to find it in the manuals. They just say for example "oil change 50 hr".

 

Thanks

Share this post


Link to post
Share on other sites
gbigs   

I am getting conflicting opinions on maintenance intervals. Do we use tach or hobbs on the 912? Called 3 Rotax Centers and was told different things. Lockwood says Hobbs, Leading edge says tach and Flight Design West says tach. I really would like the correct answer and to know where to find it in writing. I have been using Hobbs from new but I just got two planes that have been using tach since new. Can anyone help? I have been unable to find it in the manuals. They just say for example "oil change 50 hr".

 

Thanks

 

Tach time is recorded in the engine's log books and is supposed to be used to determine oil change intervals and TBO.   Hobbs will show higher because it records every second the engine is on based on oil pressure, the tach of course measures crank RPM.

 

We use Hobbs in the Pilot Log book, tach for the engine maint.

Share this post


Link to post
Share on other sites
Anticept   

The regulatory answer is "time in service." What does that mean? Well, you'll have to make that decision. I'll explain:

 

Time in service, in the strictest definition, is the time when the wheels leave the ground, to when they touch down again (this is when an aircraft is in "service"). However, in the early years, only airliners were regularly equipped with flight timers, and small aircraft didn't have anything. Even today, there are vintage aircraft that still do not have timers. Time is service is tracked by what the owner says it is. "I flew 1.5 hours today, so TIS is incremented by 1.5 hours".

 

When timers were first being installed in GA, it was back when electrical systems were still pretty darn basic if not non-existent, and tachs were cable driven (many still are, even in new aircraft). These tachs were calibrated for a cruising speed. If the engine ran slower, they would run very slowly. If the engine was run at high RPM a lot, the tachs would run faster. This was found to be an acceptable approximation of TIS.

 

Later, as electrical systems developed, hobbs meters were installed. These were primarily used by FBOs originally and are now often installed standard, and are actually somewhat deceptive. I say somewhat deceptive because they would run as soon as the master was turned on (later, by an oil pressure switch to curb "cheating", flight design however uses a very weird setup linked to the alternator), yet maintenance was done to tach, which would run very slowly during ground operations. If you spent a lot of time on the ground at a busy airport waiting for clearance, you still paid the same rate, despite very little wear on the aircraft or fuel use. Since proper installations of hobbs meters ALWAYS runs equal or faster than "time in service", these are also acceptable means to record time in service. Rarely will you ever find any place that still prices by tach, because tach pricing is markedly higher. Due to this misunderstanding of tach vs hobbs pricing, customers went elsewhere to people advertising the seemingly lower hobbs rates, thinking it was "cheaper".

 

Anyways, Rotax wants mainteannce done in accordance with a timer, which operates in real time as long as the engine is operating. This reference can be found in the 912 maintenance manual, edition 3, revision 2, chapter 5-10-00, page 3, paragraph 1.1. The closest thing to this type of operation, is the hobbs meter. HOWEVER: You do not need to track TIS in accordance to hobbs (be it engine, prop, or airframe). You could still do maintenance in accordance to hobbs, but TIS in accordance with tach. This is highly recommended, otherwise you will be unnecessarily and excessively devaluing your aircraft. Aircraft stresses occur during flight, very little happens during ground operation, so why use a hobbs for TIS?

 

Anyways, if you have already been tracking TIS by hobbs all this time, you CAN switch, there is nothing wrong with that! BUT, to be safe about it, don't go rolling back your TIS time. That is certain to raise eyebrows and probably get you in trouble. What I do is take one final meter reading, record it in the maintenance log book with the TIS calculation, and made a statement that TIS is now tracked by the tachometer. This is a lot like replacing a meter. I still do maintenance in accordance with the hobbs, but my aircraft TIS will be more realistic now.

Share this post


Link to post
Share on other sites

I believe Maintenance time and TIS are pretty much the same thing for most private operators.  Only commercial ops that may have limits on what they can do on condition would really need the distinction, am I correct?

 

I'm kind of stuck with Hobbs time for now anyway, since my simple tach has no timer.  I don't find Hobbs time to be inconvenient or maintenance intervals too short.

Share this post


Link to post
Share on other sites

Rotax wants Hobbs. Hobbs is recording anytime the oil pressure is over 15 psi so that means the engine is running and Hobbs is not affected by rpm. Anytime the engine runs that's wear and tear on parts and oil contamination. See Line maintenance manual 05-10-00 sec 1.2 Operating hours

Tach time is not the same or setup the same on all planes. Some Tach's run when just the electric is energised. SWo if these people just sit and play with their instruments it's ticking time off. Some have a switch that doesn't allow  it to start until the oil pressure gets over 15 psi, but both of these tach setups still record time according to the rpm. Low rpms has the tach turning slower than real time and high rpms have it turning faster. A tach for many of us is 15%-20% off.

This was discussed here many years ago. We are to use Hobbs for all our maint. If you only have a tack like some with only analog gauges and you do not have a pressure switch one should be added. Tach's tend to show higher times than the Hobbs because you usually spend more time at higher rpms than at idle. 

 

Many FD users that had Dynon in the earlier days had tach setup to read on the Dynon. This was changed many years ago and was supposed to have been switched to Hobbs by the user.

post-3-0-76092800-1447164951_thumb.png

Share this post


Link to post
Share on other sites
Anticept   

Tach's tend to show higher times than the Hobbs because you usually spend more time at higher rpms than at idle. 

 

If your cruise RPM is properly set up properly on tach, it will actually run slower. We spend a considerable amount of time in descent, landing, taxiing in and out, and run up; far more than climb RPM, and there's a significant spread when you compare idle to cruise vs cruise to climb RPMs. I've never, ever, seen a tach run faster than a hobbs meter.

Share this post


Link to post
Share on other sites

I have a CTSW with analog gauges at my field and the Tach is 15% higher than the Hobbs.

Remember too if the Tach has no pressure switch then it records with just power.

Share this post


Link to post
Share on other sites
Anticept   

I have a CTSW with analog gauges at my field and the Tach is 15% higher than the Hobbs.

Remember too if the Tach has no pressure switch then it records with just power.

 

Your tach has something wrong then, it's not supposed to do that. If it's just a power driven tach, then that's stupid, it's not a tach at all, it's just a hobbs meter on an RPM gauge.

 

There are two types of tach that I am familiar with. Cable driven tachs, which have a spinning cable attached to the crank or gears, and pulse-driven tachs, which use the electrical pulses from a pickup.

 

Tachs should be configured for typical cruise RPM. which makes it run 1:1 real-time at that RPM. If it's running 15% faster, then you're running at a very redline often, or the cruise speed setting is far too low, or you have a silly tachometer. 15% is considerably higher than what it should be. It's hard to even get my mooney to run close to hobbs, and I would have to try very hard to get it to run faster than that.

Share this post


Link to post
Share on other sites

Hi Cecil,

 

The Hobbs in any CT with a Dynon or a Rotax Fly Dat will only run when the engine runs with the oil pressure 15 psi or above. 

 

Lone Mnt. is wrong and needs to read the manuals and SI's.

 

Rotax and FD say to use the Hobbs as do many other LSA Mfgs.

 

 

Hi Corey,

 

Our Tach's aren't adjustable. Maybe in certified aircraft, but most if not all LSA can't time the tach and even if we could the time changes with the rpm.

Share this post


Link to post
Share on other sites
Anticept   

Our Tach's aren't adjustable. Maybe in certified aircraft, but most if not all LSA can't time the tach and even if we could the time changes with the rpm.

 

 

They should have come preconfigured for cruise RPM. My point still stands though, your tach isn't working right :P

Share this post


Link to post
Share on other sites
Anticept   

Doug: In most electronic systems, you can set the cruise RPM. This includes dynons. Read the manual to see how to set it. Also, dynons record flight timers too, triggered by a minimum airspeed of 25 knots or so.

 

But what is cruise rpm? Different for people with the Rotax worldwide. If we use Hobbs it's all the same.

 

Right, but you shouldn't be getting 15% higher tach, that's very excessive. If we assumed you ran at 5500 RPM constantly, that means it would be set for 4,800 RPM. If I had to venture a guess, you probably run 5000ish. That would mean the tach's cruise RPM is set at around 4300, which is wayyyyy too low. It's probably lower than that once we factor in taxiing, decent, and landings.

Share this post


Link to post
Share on other sites

Is there a difference between what my Hobbs and my Dynon measure? They show different totals on my CTLS.

 

The Hobbs meter will only run with the alternator switch on. The Dynon runs at engine start up.

Share this post


Link to post
Share on other sites

The Hobbs meter will only run with the alternator switch on. The Dynon runs at engine start up.

 

I know of one CTSW with the Hobbs wired to the master, it will tick with the master on unless you pop the breaker.  

 

I voiced my dismay at this.  Could you imagine if you accidentally left the master on overnight?  Suddenly you'd gain a ton of hours on your hobbs meter!    :o

Share this post


Link to post
Share on other sites
gbigs   

On a Skyview equipped CTLSi both the Hobbs and the Tach Timer are graphically displayed on the Dynon MFD in 1/10th hour increments. 

 

The Tach Timer cruise RPM setting is a system setup parameter.  The timer runs as an increment of the setup parameter setting.

 

The Hobbs Timer runs when the oil pressure is above 15psi or the engine RPM is above 200.

 

The default setup does not display the Hobbs timer...so we added it using the Screen Layout Editor.

Dynon-Skyview-Pilots-Guide1.pdf

Share this post


Link to post
Share on other sites
Anticept   

I know of one CTSW with the Hobbs wired to the master, it will tick with the master on unless you pop the breaker.  

 

I voiced my dismay at this.  Could you imagine if you accidentally left the master on overnight?  Suddenly you'd gain a ton of hours on your hobbs meter!    :o

 

Fortunately, with a log entry, you can fix that. Just make a note that you left the master on for x hours.

Share this post


Link to post
Share on other sites

I know of one CTSW with the Hobbs wired to the master, it will tick with the master on unless you pop the breaker.  

 

I voiced my dismay at this.  Could you imagine if you accidentally left the master on overnight?  Suddenly you'd gain a ton of hours on your hobbs meter!    :o

Both of the CT's I've had the Hobbs was wired to the alternator switch. I also know if the alternator/generator light bulb is burnt out the hour meter doesn't run.

Share this post


Link to post
Share on other sites

Allows,

SLSA have no legal maintenance requirements based on time in service.

There are two exceptions: 1, aircraft operated for hire, or 2, maintenance requirements brought about as a result of a Safety Directive.

In the first exception, time in service is as was previously stated; time from wheels off to wheels on ground. In the case of a Safety Directive, time is defined however the directive says it is to be.

The owner is required to keep certain aircraft records to be transferred when the aircraft is sold (permanent records). Aircraft, engine, propeller total time in service is one of these requirements (ref. 14 CFR part 91.417).

Time in service is defined in 14 CFR part 1.

As a practical matter, almost any installed meter whether Hobbs, tach or other, will be more conservative on time than the legal requirement, so one is not in danger of running afoul of the rules unless the meter fails, or is disabled..........which is not uncommon in my experience. If you replace a tach or Hobbs, be prepared to explain how you were tracking TIS. The FAA loves to ask about this. I wouldn't worry about Rotax.

Doug Hereford

Share this post


Link to post
Share on other sites

So we now have two CTLS aircraft for our tiny flight school.  

On our 2009 with Dynon D-100, the Dynon says Hobbs: 423.6.  Then there is a quartz clock near the top of the panel that says 417.6.

Our 2012 has the Dynon Skyview Classic.  It says Hobbs: 246.1.  Then there is a quartz clock that ALSO says HOBBS and it reads 251.2.

So what's up?  which one is Hobbs and which one is tach?  Why is the Dynon higher than "quartz" on the 2009, and the Dynon is LOWER than the "quartz" on the 2012?

Puzzled in AZ

thanks,

Sid

Share this post


Link to post
Share on other sites

Hi Sid,

Use Hobbs. This only records when the oil pressure is 15 psi or above which means the engine is running. Hobbs is not affected by rpm and Tack time usually is unless it is setup right. Rotax wants Hobbs. Some tachs are off as much as 15% - 20%. Use the Hobbs time on the D120

Share this post


Link to post
Share on other sites
Anticept   

I echo the same feeling. Maintain to hobbs. But I still reiterate that you can still track TIS by tach or, most preferable, flight timer.

Share this post


Link to post
Share on other sites
ct9000   

My CTSW had the normal meter on the panel and also had an other meter mounted on the left side of the mushroom wired through an airswitch (just a simple pressure switch) set to about 30kt. The difference in reading was about 800hrs airswitch and about 1000hrs on the panel after six years. We call it VDO time but call it clock time or hobbs tine does not matter, what matters is that when the engine is running the pilot logs this time. As far as time in service goes airswitch is the time that the plane is actually in service. I agree with Roger the Rotax manual says to use engine running time which seems a bit silly to me because the engine is not really working during taxi etc. If Rotax really wanted to get picky we could use cycles - the number of landings - to indicate start up numbers.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×