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Prop going into ground idle in flight
- Thread starter chperplt
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It didn't happen to me, but it happened to a different crew at my company where there was a problem with the P3 pneumatic line (I can't recall what the exact fault was) but it caused the PT6 turboprop to go to "minimum governing", which is basically ground idle.
Marko Ramius
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I was going to ask you if was on approach. I haven't flown the plane in a while, but as I remember that seems like the most likely time, because that's when the primary governor is no longer capable of maintaining RPM at the value the props are set at-1550 RPM at my former company, probably 1700 everywhere else. This is the reason for the low pitch stop system, to prevent the primary governor from seeking negative blade angles in flight in an attempt to maintain an onspeed condition. I think the only way to reset the low pitch stop to ground idle occurs when either the right main squat switch in closed or by the throttle quadrant switch when the levers are lifted over the gate into ground fine. Could there have been some sort of intermittent problem with the right squat switch or elsewhere in the low pitch stop system. If the squat was sending intermittent signals on approach where the prop has already reached the flight idle stop, I guess you could get the blade angle dropping towards ground idle. I don't know whether or not it would get all the way to the five degree ground idle low pitch stop or somewhere in between that and flight idle. Perhaps you could try to hold the low pitch stop test switch down to bypass the squat and see if that keeps it in flight idle. That's my best guess on what could have happened FWIW, I'm sure it's full of holes.
The last reply was good, but I have a small correction. When you're on approach with the props at 1550, they are indeed still on the governors. This is due in most part to the fact that you still have sufficient airflow over the props and that the blades do not have to go to the flight low pitch stop in order to maintain 1550. In fact, the props are on the governors up until the final power reduction to idle and flare before touchdown. If you look at the schematic for the prop system (BE1900D), the blades need to be at the low pitch stop before the solenoid can move the beta valve in order to increase oil pressure to the prop dome. So, since the props are not at the stops until the flare, the squat switch (which activates the solenoid) will have no effect on the props until that time. In our 1900's we've had a few that were misrigged and in the flare, one prop would drop off faster than the other, resulting in a yaw (sometimes very significant) just before touchdown.
Keep in mind that there is a lot more to this system and I am about as far from a mechanic as one could get, but this has been my experiences in the 1900. A few years back, Colgan pancaked a 1900 by inadvertantly lifting the levers over the gate in the flare. At that time, the props were off the governors so that is why they could go into beta.
Can any 1900 mechanics shed some more light on this?
SuperD
Keep in mind that there is a lot more to this system and I am about as far from a mechanic as one could get, but this has been my experiences in the 1900. A few years back, Colgan pancaked a 1900 by inadvertantly lifting the levers over the gate in the flare. At that time, the props were off the governors so that is why they could go into beta.
Can any 1900 mechanics shed some more light on this?
SuperD
chperplt
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From the reports I've read on that accident, both props went into reverse before it pancaked in HYA. The levers were not lifted over the gate. Beech actually admitted a little while back that they have had similar problems with other 1900s going into reverse on its own....
Marko Ramius
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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?
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
SuperD
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.
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
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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.
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.
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.
Marko Ramius said:
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
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ifly4food said:
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?
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