Prop going into ground idle in flight

[chperplt]

Anyone have any experience with this?

 

[niteflyer]

It could be the P3 line going to the fuel control unit. Just a thought.

 

[Otto]

Just curious...did this happen to you and if so, what aircraft? If it happened to me in the C-130, I probably wouldn't be around to tell you about it.

 

[niteflyer]

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.

 

[Albotross]

You Colgan guys...ask big jimmy in the Hanger.

 

[chperplt]

It happened in the mighty 1900.. It was popping in and out on short final.

Thanks for the info

 

[niteflyer]

I believe the crew also experienced fluctuations for several minutes before it went to "minimum governing".

 

[Marko Ramius]

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.

 

[SuperD]

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

 

[chperplt]

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.

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....