Prop overspeed at high altitude

[whizzer58]

Can anyone confirm that we are to not practice single engine work at flight levels (i.e. 25K, etc.) because I was told that when you retard the prop to right at feather and then bring the prop lever back up to cruise setting (1700 RPM) that the governor can not engage quickly in the thin air and a prop overspeed situation can develop.

And, are we to not pull throttles back to idle at altitude during this maneuver?

(PW PT6-52 turbo props with only 450 hours on a King Air)

Pilot was practicing single engine work. Pulled throttle and prop levers back to idle. When everything was stabilized he pushed prop lever back forward and all heck broke loose in that blown engine. It had to be shut down and metal and 'stuff' was thrown all over the Gulf.

Also read that 'zero thrust' imitation was to only pull prop back to 1600 RPM, not back to the idle detent.

Any thoughts on what happened?

Thanks

[aerobat77]

hey ! never heard this, can you tell more details ?

we practise single engine airwork as well single engine landing on the cheyenne with pt6a without any issues. of course always first bring throttle back to idle since when you feather under load the big resistance from the gearbox may truly screw up something.

when you pull the props forward again ( eg. at touchdown with one prop feathered) they will kick in quickly , but never observed an overspeed.

my only quess would be that he had not the power lever at idle when he unfeathered the prop again and the power output of the turbine created such a massive spool up of the prop/free turbine stages that the governor was not able to stabilize and a destructive overspeed happened.

can you find out what exactly failed on the engine ? the gearbox? the power section?

cheers !

[de facto]

Can anyone confirm that we are to not practice single engine work at flight levels (i.e. 25K, etc.) because I was told that when you retard the prop to right at feather and then bring the prop lever back up to cruise setting (1700 RPM) that the governor can not engage quickly in the thin air and a prop overspeed situation can develop.

Having done recurrents on SA227 and be 1900 in the actual aircraft,i never went higher than 10 000 ft.
Steep turns ,approach to stall were all done around 7-8000 feet,rest of training circuit altitude.

Why go at 25 000 ft to do such exercise?is it because your aircraft is limited to 25 000 due to oxygen mask not being quick donning type and your boss wants to demonstrate the aircraft handling at max sop altitude?
If so, i think it is unnecessary risks.
The higher the altitude,the smoother the inputs the better

[whizzer58]

He was at 27,000 feet and was bored so he decided to pull an engine on the young pilot flying. Engine only had 450 hours on it.
We don't know acutal damage yet. Will post it later this week.

[de facto]

He was at 27000 in a king air?
Were they wearing oxygen masks?
I remember flying at FL 250,light weight,winter time,the speed margin was quite low on the SA227.
I think they are lucky of the outcome.

[whizzer58]

King Air can fly at 35,000. It's pressurized.

[de facto]

35000ft, i remember the SA227 ceiling being 31000ft,but 25000 ft limitation was due to pilot oxygen not being quick donning.

[Type1106]

It's a very long time since I had to think about basic propellor theory but I'm surprised you prop drivers don't know why this happens.

I think you'll find it has to do with "TAS/IAS relationship". You'll need to go and look this up as i've forgotten the detailed explanation.

In practical terms, at high altitude for a given IAS an uncontrolled (ungoverned) prop will over speed and the result can be the separation of prop and engine with disastrous results for the airframe.

1106

[aerobat77]

He was at 27,000 feet and was bored so he decided to pull an engine on the young pilot flying

it depends what it means " he decided to pull an engine " . when he suddenly pulled back the prop lever to feather on a turbine producing cruise power ( so without first retarding the power lever to idle) it would be no surprise he cracked the reduction gearbox- and it maybe finally failed when he reved up the prop again. the exaxtly kind of damage would be interesting.

beyond that... he was bored and just "pulled the engine"??? with the result melting a 400000$ turbine ???

He was at 27000 in a king air?
Were they wearing oxygen masks?

???

best regards

[aerobat77]

He was at 27,000 feet and was bored so he decided to pull an engine on the young pilot flying

it depends what it means " he decided to pull an engine " . when he suddenly pulled back the prop lever to feather on a turbine producing cruise power ( so without first retarding the power lever to idle) it would be no surprise he cracked the reduction gearbox- and it maybe finally failed when he reved up the prop again. the exaxtly kind of damage would be interesting.

beyond that... he was bored and just "pulled the engine"??? with the result melting a 400000$ turbine ???

He was at 27000 in a king air?
Were they wearing oxygen masks?

???

In practical terms, at high altitude for a given IAS an uncontrolled (ungoverned) prop will over speed and the result can be the separation of prop and engine with disastrous results for the airframe.

hey! the only condition where the prop is not governed is the beta range/ground idle ( flat pitch) with the power lever on the pt6a.


best regards

[de facto]

Quote:
He was at 27000 in a king air?
Were they wearing oxygen masks?

In a company i worked for,the operation was single pilot and the oxygen masks were not quick donning type,ops were therefore restricted to FL250.(type SA227/BE1900c).

[whizzer58]

He would have pulled the throttle back to idle, initially. Then prop and condition lever back to the idle detent.

He says that when he began to bring the prop back forward is when he heard a 'pop' and the engine spit metal out the back.

[aerobat77]

something is wrong here .

there is no "idle" on the prop lever- you set the governed prop rpm or- pulling fully back , feather the prop.

the condition lever is the whole time at "idle" ( in some application low or hi idle) . pulling it fully back means shutting the turbine.

like written above most probably he pulled the prop back to feather without first pulling the power back and screwed the gearbox .

the governor has very little in common with thin air - and feather the prop on a really idling engine is not an event for the pt6 since its a free turbine where the power section is not mechanically connected to the gas generator .

cheers