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Andrew Lane

Low Fuel Pressure Alert

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Hi guys,

twice I’ve had this happen, fuel pressure drops and sky view starts freaking me and my CFI out with alarms about fuel psi, it goes as low as 1.4psi and doesn’t recover to normal (over 2psi) for about 2 mins.

Approx 10 gallons of fuel, coming in for a landing. Using some forward slip.

first time was a somewhat hard touch and go

Second time was today (days later, many normal flights in between), come in for a landing, crosswind gusts so we go around. Full engine throttle and steep climb angle (~68kt) then fuel pump starts alarming again.

anyone else have this happen? Very stressful....

as soon as engine throttle is reduced or level off things are normal again.

does NOT happen every flight.

Thoughts?

 

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Previous owner of my airplane had this issue when running auto fuel with ethanol in it on hot days. I haven’t had it happen with ethanol free fuel or the occasional 100LL though. 

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Do you have, or can you borrow, a fuel pressure gauge?  Are you sure it isn’t a faulty sender and fuel pressure is actually OK?  I had similar issues on my Sportcruiser and replacing the fuel pressure sender solved the issue.  I believe the fuel pressure sender is a VDO unit and can be had from Aircraft Spruce.

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5 hours ago, rtk said:

Do you have, or can you borrow, a fuel pressure gauge?  Are you sure it isn’t a faulty sender and fuel pressure is actually OK?  I had similar issues on my Sportcruiser and replacing the fuel pressure sender solved the issue.  I believe the fuel pressure sender is a VDO unit and can be had from Aircraft Spruce.

thanks for the suggestion! I'll look into it.

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Hello Andrew: I had a similar serious issue today: Flying a CTSL 912is brand new (100 hours), 35°C OAT, Passing 4000 feet, oil temp = 112°C... and engine suddenly quit with low pressure warning and voice alert from Dynon.

5 minutes of fight to restart the engine: needed to let the AuxPump on for 40 sec. before attempting a new start. Turning off the aux pump= engine off again.

After a 2nd restart, Aux fuel pump on all the time, I climb to 8000", OAT = 16°c. Pressure was back at 2,99 bar without aux pump. I flow back to home (2 hours), but keeping always an airfield in the "cone".

Analysis: Both tank were filled with a poor Mogas 95 (suspected to be still winter fuel). I have never done this before on this 912is (usually using Mogas 98 from car filling station).

Lesson learn! Let see tomorrow  and how thing will develop: expected same temperatures, but will use back Mogas 98!

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1 hour ago, mbenoit02 said:

Hello Andrew: I had a similar serious issue today: Flying a CTSL 912is brand new (100 hours), 35°C OAT, Passing 4000 feet, oil temp = 112°C... and engine suddenly quit with low pressure warning and voice alert from Dynon.

5 minutes of fight to restart the engine: needed to let the AuxPump on for 40 sec. before attempting a new start. Turning off the aux pump= engine off again.

After a 2nd restart, Aux fuel pump on all the time, I climb to 8000", OAT = 16°c. Pressure was back at 2,99 bar without aux pump. I flow back to home (2 hours), but keeping always an airfield in the "cone”.

😮 Wow, that sounds pretty scary!  I’m glad to read that you were able to figure out how to keep the engine running!

The 912is should have the fuel under pressure at all times due to fuel injection.  I didn’t think the 912is would be susceptible to vapor lock or poor grade fuel.  Does the auxiliary pump increase fuel line pressure higher than the standard fuel injection fuel pump?  Please fly safe and let us know how it turns out.

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4 hours ago, rtk said:

😮 Wow, that sounds pretty scary!  I’m glad to read that you were able to figure out how to keep the engine running!

The 912is should have the fuel under pressure at all times due to fuel injection.  I didn’t think the 912is would be susceptible to vapor lock or poor grade fuel.  Does the auxiliary pump increase fuel line pressure higher than the standard fuel injection fuel pump?  Please fly safe and let us know how it turns out.

Yes, the auxiliary pump was increasing fuel line pressure. The tricky part on the 912is is that you need a significant time of running  the pump when fuel lines are dry, knowing that primary fuel pump is only in when you activate the starter, or when the backup red switch is on. If this is happening again, And engine quit again, I believe the right procedure should be:

- fly and level the plane (looking at my Dynon log yersterday, I have let the speed drop at 54 knots before I recognise It, and gain speed again, and despite the AOA alarm)

- turn the auxiliary  pump on, turn the back up red switch on

- wait 20 sec.

- activate starter for max 10 s.

I will also correct my take off and landing procedure: before this event, I was not turning on the aux pump to avoid Dynon yellow alarm due to high fuel pressure.

This is not right, and I will know systematically turn this pump on during landing and take off phases.

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Analysis: Both tank were filled with a poor Mogas 95 (suspected to be still winter fuel). I have never done this before on this 912is (usually using Mogas 98 from car filling station).

MBenoit,

I too have a 912is, Hobbs is 73 hours.  I was taught (unofficially, not Rotax sanctioned) to run both fuel pumps before a Hot start... for about a minute.  Only as a way to start the engine quickly, not as a precautionary fuel starvation problem.  Did you contact Rotax and Flight Design???  You mentioned that you believe the root cause is the gas quality... are you certain?  This is a big deal, I am hoping Rotax and Flight Design are both following up with you to study your situation.

Since the plane is new, there may be FOD in the system (dust from sanding, etc.) and perhaps you should take a look at the fuel filter chain.  There may be something blocking or semi blocking the fuel lines back and forth between the  Header Tank, the Gas Tanks and the Engine itself.  

If I were in your situation, I would ground the plane until Flight Design (and possibly Rotax) performs a thorough analysis on your fuel system.  Good work on the re-start.

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After a couple of forced landings in my new ctlst, I make the following comments.

Most likely is that the filter is blocked by very fine carbon fibre dust. The gascolater screen is blocked. the inlet filter on the pumps could have carbon fibre dust.

I do not think it is a fuel problem as such but is very likely to be the residue from the tanks. 

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I think you are more right than wrong ct9000, I hope the guy raises the issue with the dealer/factory ASAP...  There is a rubbery "paint" that is applied to the inside of the gas tanks that sometimes flakes away during the early life of the plane (esp on the aluminum return line tube in the left gas tank -- the tube is fuel injection specific) , which could also be an issue at the fuel filter(s) level.  

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1 hour ago, AGLyme said:

. . . that sometimes flakes away during the early life of the plane (esp on the aluminum return line tube in the left gas tank -- the tube is fuel injection specific) . . . 

Who or where is the source of that information?

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I have done a test flight yesterday, after having replaced the fuel by Mogas 98 I usually use: fuel pressure remains stable all along the flight (1 + 1 hour), even after having let the plane few hours under a strong sun.

My current (and personnal!) bet: vapor-lock occurred following the addition of those root causes:

  • Poor fuel 95 Mogas: the filling station of Mogas 95 is located close to avgas filling station. To get mogas 95 on an airfield in France is very unusual, therefore not many pilot are using this pump=old fuel.
  • Very hot day > 37°C 
  • Plane standing on field for 4 hours under strong sun
  • Plane park not fully level: one wing was at 15 liters level, while the second one was at 60 liter: I believe that has cause the 15 liters fuel to warm up above  reasonable level for this poor mogas 95

Regarding dust in the filter, I had this issue as well and this was cleared during 25 hours maintenance after replacing the fuel filter and cleaning the gazcolator. Note that the fuel pressure was high (3,18 bars w/o aux pump) before the cleaning, and then back to 3.00 bar after the cleaning and filter replacement. This make sense looking at the pressure sensor location (after the pump, and before the filter).

All of the above are my personal understanding, which could be wrong: FD Germany has been informed today on this issue by the french distributor and will investigate further, I have also suggest that restart procedure when engine quit needs to be review and detailed when CT is deliver with 912is.

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cleared during 25 hours maintenance after replacing the fuel filter and cleaning the gazcolator. Note that the fuel pressure was high (3,18 bars w/o aux pump) before the cleaning, and then back to 3.00 bar after the cleaning and filter replacement.

Glad to hear you had the above work done, and. for clarifying the re-start procedure.  I too had an early fuel pressure issue and I don't have the bar vs psi conversion graph handy however we had similar issues and results (my pressure reading too high as well).  I am also glad you contacted FD Germany.  One quick check you can do yourself, open the left side gas tank cover and use a flashlight to look at the top of the aluminum tube and make certain it is clear of debris.  It is critical that the tube be allowed to breathe. I have added that check in my preflight procedure given my early experience with the aforementioned rubbery paint flaking.  

In America, we invented the fine art of suing each other and blame shifting..; )... seriously speaking, with a known event such as an engine quitting during flight (you deserve a medal for the restart), I would place the plane in the hands of the Dealer (and FD Germany) and demand a sign off that all is well with your plane.  You may be correct about the gas thing, but I am still of the mind that you had some form of blockage in the fuel return system.  The system relationship (fuel tanks, header tank, air venting, filters) is allegedly fool proof -- unless something artificially blocks it.  Fortunately, our North American Dealer here in the USA was accommodating and immediately reviewed my pressure spike issue.

What I do from time to time is to take a photo of my panel during climb out and cruise.  I take these photos because oftentimes when I "feel" that something just isn't quite right with RPM's, or pressures, I review the past photos and can determine a value Delta, if any.  

Thank you for posting this critical event and very much appreciate your follow up posts.

 

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ON THE TOPIC OF FUEL PRESSURE ... I noticed, more than a few times, my red engine warning light would go on for a second and my oil pressure would spike. The engine ran completely fine during this momentary situation.

Then tonight, while flying, the fuel pressure gauge went nuts - but only for 5-seconds. Pressure went up to 12 then down to 3, the back up and back down. It happened very quickly and erratically. The engine never missed a beat. It was rock-steady.

I landed and am trying to figure it out.

My gascolator (lowest point for fuel draining) does not have a screen in it - never did. The main fuel filter is behind the panel where the ignition switch is attached. It is a very small inline filter. I have 160 hours and it has never been changed. 

Another note ... the fuel pressure gauge does not technically have a red line on the high pressure end. But it does trigger the red engine light when it goes above 6.

Any ideas?

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The screen for the gascolator is on top above the gasket. If you don't have one I would order one and install it, 120 microns. I would order a new gasket while you are at it. https://www.aircraftspruce.com/catalog/appages/acsgascolator.php#

For the inline filter the inspection checklist says to inspect at least every 200 hours. The SB for exchange of the plastic fuel filter says follow the MM for inspection. Based on my experience providing maintenance it should be checked during the condition inspection, and this is something I do on aircraft I maintain.

On the fuel pressure, do a search on the forum, but if I remember correctly with the new style pump you can reset the high end of the pressure limit at 7.2. I don't have time to dig for the resource right now.

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The sending unit works on resistance. anything that makes the resistance go up will increase the fuel pressure. Likely a bad spot on the wiper in the sending unit.

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The entire fuel system checks out. I located the fuel pressure sending unit (pictured here). I figure the most expedient way to deal with momentary spikes and erratic  readings 

is to replace it. I don't think the actual pressure is deviating because the engine runs perfectly, throughout all of the abnormal readings.

UNADJUSTEDNONRAW_thumb_c64.thumb.jpg.05d3c412a8a8169567ceda9d39d9364e.jpg

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By the way ... that sender (shown in the photo) does not appear in any of the service, maintenance or parts manuals. There is no reference anywhere to that part or part number.

After searching Dynon and Rotax boards and various support forums, I found that erratic fuel pressure is a common issue with "VDO" sending units (like in the one in my plane). Apparently, the senders are unreliable.

Dynon (and Rotax) recommend switching to the Dynon Avionics P/N 101690-000 (AKA, Kavlico Fluid Pressure, 0-15 PSI – P/N 101690-000). Here's a pic:

sender.jpg.0745fc78f7f10d254ee4914ce058bd44.jpg 

You also can select this specific probe in the skyview setup menu.

My only concern now ... how to go from a two wire sender to a three wire?

 

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