Rotax 914 loss of power during takeoff/flight

Check the pneumatic lines from the airbox to the three way valve and carbs.  Make sure no cracks.  These lines enrich the mixture above about 4000 RPM.

Des Howson wrote:

Just some thoughts.

You don’t mention the aircraft type, is it tightly cowled, is an intercooler installed?

I see you are in Florida, was it a hot day & did you have extended ground running - long taxi, holding for traffic, etc before take-off? The 914 Turbo Control Unit (TCU) has a function that will automatically reduce power by opening the waste gate if the air box temperature reaches a certain figure (72C or 88C depending on engine/TCU serial/part number). There will be no other indication of this function occurring (no lights flashing, etc). Unless the manufacturer of your aircraft has installed an additional sensor and readout of air box temperature, you will not know if the air box temperature (as sensed by the TCU) is approaching or has reached its intervention temperature.

Heat soak in the cast turbocharger during your circuit may account for the second occurrence.

Installing an air box temperature sensor with cockpit readout would be handy I think.

Service Letter SL-914-014 has some information that may be of interest to you.

https://rotax.my.salesforcesites.com/DocumentsSearch/sfc/servlet.shepherd/

version/download/0681H00000FTeDYQA1?asPdf=false

Here is some more reading material, note the air box temperature mentioned in this document is 72C, in later engines this increased to 88C.

https://www.rotax-owner.com/pdf/UNDERSTANDING%20THE%20914%20ROTAX.pdf

The 914 Operators Manual describes a technique for setting take-off power then temporarily deactivating the TCU motor but that removes the temperature and other protections.

If the power reduction you describe is due to the above, I would suggest your engine is operating as designed, you could try using the Rotax TLR software to look at the TCUs history and/or MAYBE, very cautiously, fly again in cooler temps as a test.

https://www.rotax-owner.com/en/videos-topmenu/elearning-videos/659

Hope this helps.

I flew again with a computer hooked up. Airbox temp seems to be the culprit here. Temps are exceeding 88C during takeoff and boost is being pulled. Now I've got to figure out why....

At the risk of stating the obvious, any leakage of under cowl air into the induction system before the turbo will be detrimental, even if only a little leakage at high power when suction is high.

I’d start by looking at the easy stuff, checking the induction hoses, clamps, cowl duct, air filter housing, etc. for condition and security. If there is a carby heat or alternate air system installed, ensure the mechanism is closing completely when it should and can’t be sucked open even just a little. Looking for ANY holes.

If that all checks out, perhaps something is causing high under cowls temps. Exhaust leak, induction hose too close to an exhaust pipe, lack of shielding?

Good luck & please let us all know what you find.