Prop going into ground idle in flight

[Marko Ramius]

I told ya there'd be some holes in it. My thoughts went to the low pitch stop system because as Super D stated, the only way for the props to go below flight idle pitch(12 degrees?) is to have that beta valve readjusted. I still think it has something to do with either the squat switch or the throttle quadrant switch(es) because the problem was intermittent which means that the valve was likely adjusting it position back and forth. Beech says that's impossible in flight, but there's enough story that say otherwise. A couple friends of mine had a PT6 go to ground idle at about 1500 feet on a approach. I remember one time when I had a plane that wouldn't do the low pitch test on the runup. After a multitude of adjustments and replacements, the mechanic found that the entire throttle quadrant was loose. This was causing the backup low pitch switches on the throttles to become engaged which was screwing with the test. The problem was intermittent, and the plane had a lot of low pitch stop test writeups occur before this mechanic figured out the real problem. The question is what would happen if something like that happened in flight?

 

[SuperD]

Hmmm, well I can't understand why they would go into reverse by themselves, especially both at the same time. But I don't doubt for a second that it could happen. Thanks for the info, I guess I got some bum info about what really happened. All I know is that we bought that very airplane from Colgan and fixed it up. It's now a part of our fleet. However, it has a new mx problem every day and has earned the deserved nickname "Frankenbeech".

SuperD

 

[niteflyer]

Whoops. Disregard my replies, I just realized this question is about the propellor. The incident I was referring to was the engine (fuel control unit) going to "min governing".

 

[ifly4food]

I'm surprised it didn't cause an overspeed (versus min RPM). There have been several accidents and incidents on the E120 Brasilia due to the props going into Beta in flight either due to mechanical problems or from crews accidentally bringing the levers back too far (before the FIS locks were installed).
In all cases, they lost control of the prop with an Np of around 140% and engine destruction. There was at least one fatal crash (ASA in BQK).
The E120 has PW1800 engines and Hamilton 14RF-9 props.

 

[Marko Ramius]

Well Pratt did a better job on the 1900 engine than on the EMB-120 "ATF". For an overspeed to occur three governers would have to fail completely. The primary and secondary governors essentially work on the same principles that you see in pistons-using flyweights to move a valve that admits oil into or releases it from the hub. Should both of those fail, a fuel topping governor takes over limiting RPM to I believe 106% of that selected by the prop lever. It basically changes the bleed air input to the fuel control unit causing it to reduce fuel flow to the engine and subsequently slow the prop. Under certain situations this can cause the engine to fail, but that's a different story.

 

[flint4xx]

If a p3 line failed, the resulting fuel flow of 100 PPH would cause the engine to rollback to about 55%. That would activate the autofeather feature on the PT6. However at somepoint during this cycle, the governors and fuel control would start to argue about whether the engine was still operating causing the prop to try and cycle in and out of feather.

There is a limitation against beta range in flight, but we all know strories of pilots that have tried it. There is no lockout. The gear switch only allows props to go from flight idle to ground idle. Lifting the levers is much worse, and there is no way both props will do either without pilot input.

 

[ifly4food]

Well Pratt did a better job on the 1900 engine than on the EMB-120 "ATF". For an overspeed to occur three governers would have to fail completely. The primary and secondary governors essentially work on the same principles that you see in pistons-using flyweights to move a valve that admits oil into or releases it from the hub. Should both of those fail, a fuel topping governor takes over limiting RPM to I believe 106% of that selected by the prop lever. It basically changes the bleed air input to the fuel control unit causing it to reduce fuel flow to the engine and subsequently slow the prop. Under certain situations this can cause the engine to fail, but that's a different story.

Well, you pretty much described the system on the 120. We have a primary and secondary governor as well as p3 fuel topping to the HMU. The governors limit up to 103% and the fuel topping limits up to 109%.
Despite all 3 of these safeties, there have still been several overspeeds. The early ones were due to destruction of the transfer tube that sends the control input from the governor to the hub. Without it, the governors are useless.
Several others were due to crews lifting the gates and shifting to Beta. Locks were installed to prevent this in flight.
As recently as the last 6 months, ASA had a catastrophic overspeed which the crew managed to get on the ground through heroic airmanship. The prop reached 160%Np before the engine came apart. It was outside the marker on final at a very mountainous airport. Neither Embraer, the airline or the FAA have been able to figure this one out yet.
So don't say it's impossible. Be ready...

 

[Marko Ramius]

Well, you pretty much described the system on the 120. We have a primary and secondary governor as well as p3 fuel topping to the HMU. The governors limit up to 103% and the fuel topping limits up to 109%.
Despite all 3 of these safeties, there have still been several overspeeds. The early ones were due to destruction of the transfer tube that sends the control input from the governor to the hub. Without it, the governors are useless.
Several others were due to crews lifting the gates and shifting to Beta. Locks were installed to prevent this in flight.
As recently as the last 6 months, ASA had a catastrophic overspeed which the crew managed to get on the ground through heroic airmanship. The prop reached 160%Np before the engine came apart. It was outside the marker on final at a very mountainous airport. Neither Embraer, the airline or the FAA have been able to figure this one out yet.
So don't say it's impossible. Be ready...

It's impossible for me since I don't have a job!! But besides that the point I was trying to make wasn't so much that the governing system was better(I was bored and trying to see if I could remember it), but that I heard that the Brasilla's prop system requires oil or air pressure to move the prop towards high pitch/feather. I never flew the Brasillia and don't it's systems at all, does that setup introduce more potential failure modes in an overspeed situation?