914 Fluctuating pressures

G'day Geoff,

Looking at the symptoms, I would break them down into 3 components.

Firstly the MAP fluctuations. Could be a sticking and/or poorly adjusted wastegate. I would ground run the engine with the B.U.D.S. software running on a P.C. connected to the serial interface of the TCU. This software accesses more data than the G3X can display and will assist in troubleshooting a TCU or sensor malfunction. It also allows engine parameter history files to be accessed. Instructions for the installation and use of the software is found in the MMH chapter 76-10-00 commencing on page 4. Here's a "how to" re the cable required Connecting ROTAX TLR software to the 914 TCU. I found the Sabrent cable on Amazon.AU and it works well. Personally I would perform an engine run without changing anything on the engine and record the data then perform a second run after having cleaned, lubricated and adjusted the wastegate servo cable "system" in accordance with the MML 12-20-00 page 65 (which also then refers to the MMH for further information). There is also a video on this site regarding adjusting the cable. Wastegate Adjustment - 914 Additionally I would check and adjust, if necessary, the throttle position. I notice also you had a wiring issue previously with the MAP indication. Although these fluctuations appear to me to be more than just an indication issue, perhaps another check of the engine wiring (TCU in particular) is in order to eliminate that as a culprit. 

Secondly the fuel pressure observations. The fuel pressure regulator on your engine is designed to maintain fuel pressure at 6.5psi (0.25 bar) above the airbox pressure. To observe this relationship requires a differential pressure sender be installed and connected to the indicator. With that installation, you would observe a fuel pressure of 6.5 psi (approximately) during all phases of engine operation. I notice your fuel pressure indication varies widely suggesting you have a sender installed that refers to atmospheric pressure and not the airbox. Your indicated fuel pressure will vary due to altitude as well as power setting (higher fuel pressure with more power/MAP &/or altitude and vice versa) At this stage I would rule fuel pressure out as a contributing factor.

Thirdly the major reduction in power on a hot day. I think your engine operated as designed. There is a function of the TCU that, in order to protect the engine from excessive temperatures, will reduce the boost pressure by opening the wastegate WITHOUT WARNING if the airbox air reaches a certain temperature (88C on later engines). Unless your aircraft has an airbox temperature sensor and indicator installed, you will have no way of knowing you are approaching this limit. It would be interesting to see the coolant (or CHT) temperature of the flight in question on the Savvy graph you posted. Hopefully the B.U.D.S. software can find the logs from that flight and by looking for elevated airbox temperature and a simultaneous servo position change opening the wastegate, you will be able to determine if this was indeed the case. Extended ground running, not doing runups into wind, etc. can allow under cowl temperatures to increase. Also please check the condition of the induction system for holes in the SCAT hose, loose clamps, air filter housing misaligned, etc. If the engine is ingesting under cowl air, that will raise the airbox temperature too.

I hope this is of some value to you.

Cheers,

Des.

Thanks Des, will check out what I can - some good tips in there.

Attached are the MAP vs Coolant Temp graphs for the same two flights looks liek I don't have the individual CHT's ... I'm not sure it's getting too hot in the induction side, but that could be something to check with the SCAT hoses... the bit that concerns me the most was the unexpected in-flight reductions.

Pretty sure we've met - are you the Des building a SLG2 TDR with the same paint scheme as my -4, former ZU-TAD ? If so I'm 'just' down the road in Canberra - not geting much response from my LAME's might have to come visit    :-)

Cheers,

Geoff

Hi Geoff, yes that’s me, I hope you are well.

I never quite know how to respond to posts, don’t want to sound like I’m teaching how to suck eggs.

Here’s a link to an old document that you may find of interest. Some of the numbers it quotes differ from the current manuals but the basics still apply. If you’ve seen it before please disregard.

http://www.europaowners.org/gallery2/d/81227-7/UNDERSTANDING+THE+914+ROTAX.pdf

G3X allows only one coolant temp input per engine, the only way to have both coolant probes active at the same time is to wire them to a different connector on the GEA24 and call them EGT, a bit of a pain.

I’d definitely plug the P.C. in and try to find the TCU logs. I’m trying to think what would cause the relatively high frequency/low amplitude MAP oscillations prior to the big drop. That’s why I suggested a wiring check, maybe power the TCU with the engine stopped and the BUDS program running then jiggle the wires/connectors trying to induce a reaction in a sensor. Maybe disconnect the sensor connectors and give them a spray with contact cleaner too.

A lube and adjustment of the wastegate cable wouldn’t hurt. Maybe drop the float bowls and check for debris.

There has been some history of servo motors failing over the years but I don’t know if your symptoms point to that, maybe?

The low power you described on the subsequent flight with takeoff power set could very well be related to and caused by the same “inflight” issue you experienced. However I’d ask you not to discount the possibility of TCU intervention due to air box temperature I mentioned. A 30c (in the shade) day, air box close to a relatively dark coloured top cowl perhaps in the sun with let’s say 15 minutes of engine running including highish power for mag & prop checks will have the undercowl temperature pretty warm. We need to consider the now heat soaked engine, plus the heat transfer from the turbo mass to the induction air as it passes through then add in the temp rise due to compression then we give it the beans introducing even more heat from burning more fuel. It won’t take long for the critical temp to be reached.

I had the great fortune and honour of knowing Sean Russell at Sling in Joh’burg and he once told me that if he could choose only one option on a 914 powered aircraft it would be an air box temperature indicator. My 914 powered ‘2 has airbox temperature monitoring. I still don’t understand the Rotax logic of having a function designed into their aircraft engine that can significantly reduce power at a time most critical to flight safety (and any other time for that matter) and not provide an indication of impending activation of said function to the operator.

Here’s a link to a video you may also find of interest.

https://youtu.be/lE6Y37GPQqY?si=f9FYXm5uKZMh-rBE

It would be great to catch up, my email is djh022430 at gmail.com if you’d like to get in touch.

Good luck and once again, cheers.

Des.