Piper Turbo Arrow III Not responding to throttle after long decent at idle
I was out doing commercial maneuvers today in my 1978 Arrow III turbo. Did some lazy eights, steep turns, chandells, and lastly a steep spiral from 8k feet to 2500 feet at a low throttle position. After this was complete, I advanced the throttle to gain power and climb but there was no response from the engine with regards to manifold pressure or RPM. Manifold pressure was at 20 and fuel flow was around 6-8 tried swapping tanks and fuel pump no changes. All gauges read normal. I could advance the throttle to the firewall observing the overboost light with no changes at all to manifold or anything else so I brought it back and left it centered. After about 20 seconds the manifold pressure started to slowly increase along with fuel flow and RPM. After about 30-45 seconds I had full control of the throttle and for the entire flight back everything was normal. I landed called my mechanic to get suggestions and went to go run it up to fly back to my home airport. One initial runup I observed the same behavior as before, got the rpm to 15-1800 to do a mag check brought back to idle then advanced to a high manifold setting but it wouldn't advance past 1500 rpm regardless of throttle setting. Brought it back to and did it again and it worked just fine. Repeated four more times with normal operation observed so I flew back to my home airport with no issues.
My throttle cable has been getting really tight last few weeks and I am having that replaced. Not sure if there could be something slipping with the cable or a fuel/engine issue. Thoughts?
Comments
Acbruno,
You must be doing something right as the Universe approves and got you home without issue.
I would not fly again until a definitive cause is identified. If the power rolled back just after rotation you would have been in a tight spot. It is possible the cable is snapped and only friction is allowing adjustment....
One puzzle is the overboost sensor coming on at high throttle settings. I had thought this was a sensor of manifold pressure and completely independent of the throttle position. If manifold pressure was actually high to the sensor but low afterwards in the induction system this suggests a significant obstruction in the intake system.
Any recent work to the induction system? Everything points to the throttle and/or cable - except the overboost light. If you had a rag sucked up there that was restricting flow it might be unresponsive to throttle and if the airflow is low the fuel flow will be as well.
This is a great example of a problem that needs a solid root cause. Keep us updated!
Eric Panning
1981 Seneca III
Hillsboro, OR (KHIO)
went to go fly over to my mechanic but couldn't even start the plane. Temps were below freezing and I didn't have my heater on. Anyway, the plane isn't going anywhere. The throttle cable is definitely busted. I ordered one from BAS part number 455-322 (Alt: PS50146-13-10) 71 inches long. Hopefully this is the right one. Same part number for prop and throttle.
Here's a video. I was moving the throttle arm at the servo open and closed.
https://youtu.be/4ZXN7HRNSPE
I still don't get how the overboost light came on.
Well after you get the junk throttle cable changed out you got a bunch of other things to check. One make sure the turbo rotates and is not locked. Two making an assumption that you had the stock bypass screw and not aftermarket wastegate. Three you need to verify you have no blockage in fuel system up to the fuel pump Make sure fuel screen clear. Disconnect line at the flow divider and see what kind of fuel flow you get with the boost pump on. you have an altitude compensating fuel pump so there's a couple failure modes with that pump to prevent good pump pressure, so saying that you're going to need to put a set of gauges on it and verify what the pump is doing at idle up through whatever power settings you're able to get.
You didn't indicate that you tried the boost pump.
Without actually being at your airplane this kind of problem is kind of hard to troubleshoot so you will have to kind of start at the beginning which sounds like you have a fuel supply issue so you need to figure out what's going on with the boost pump first
Carl
48 yrs A/P IA DAL aircraft inspector. 172N
The torque nut on the throttle valve has likely slipped (loosened) on the throttle shaft… The safety wire that holds the throttle arm to the stop cam is also probably missing. That’s what happened on my Archer III, with a similar issue to what you described.
There should be no gap between the cam and the stop when the throttle is set to full-throttle. Note that the safety wire is missing in the attached photo. Realign the cams, tighten the cam nut until there is no slippage between the two cams, and put the safety wire on. The safety wire is NOT installed at the factory… The installer of the carb/engine is supposed to put it on.
A URL link to the Precision carb service manual is attached.
https://wiki.ad7zj.net/wiki/images/8/85/MSA_Aricraft_Carburetor_Service_Manual_Models_MA3A_MA3PA_MA3SPA_MA4SPA.pdf
SMR551, thanks for sharing your experience.
Eric
Eric Panning
1981 Seneca III
Hillsboro, OR (KHIO)
Sm well that's good information The turbo aero 3 has a injected Continental engine. Carl
48 yrs A/P IA DAL aircraft inspector. 172N