PT6A surging

[islandguy]

I was riding with a collegue who flies a King Air 200.

On approach I was watching ahead through the cockpit and somewhere probably 4-5 miles final, one of the engine torques started surging. Lasted about 5 seconds then stopped.

After shutdown I talked to the guys to ask what happened and they said they were set up for landing and after bringing the props full forward with torque around 500 ft lbs, the right torque just started fluctuating back and forth between 500-300 ft lbs. They mentioned the autofeather and autoignition annunciators were flickering back and forth at the same time. It only returned to normal after they increased the torque slightly. Rest of the flight was normal. They did a short ground run after landing but nothing out of the ordinary.

I haven't been able to chat with them since then so I don't know what their maintenance came up with. Anyone have this happen before or any ideas what would cause this? I've been trying to think through the systems to get a better understanding, always wlecome to input.

Thanks.

[hillberg]

:Dworn out compressor, run it too hard or pull back on the power it will pop pop pop :eek:compressor stalling.:ouch: FOD or lots of hours.:rolleyes:

[islandguy]

Compressor stall?

There were no audible sounds coming from the engine like those you hear with compressor stalls. Is it possible to have a compressor stall with virtually no noticable sound?

You reckon the bleed valve at the P2.5 station is sticking closed at the high power settings of cruise, then failing to open properly at reduced power settings like this occasion on approach?

Thanks for the info!

[blackhand]

CSU may be the problem

[Beech_Boy]

If the bleed valve is sticking closed, this will cause compressor stall because the engine produces more air than it needs to run

[ivor toolbox]

No popping / banging = no surge, as it cleared with increase in torque
(and consequent increase in gearbox torque pressure) suspect one of autofeather px switches on way out, as evidenced by autofeather / ignition lights flashing.

Been a while since I worked PT6, but it was an occasional fault on 'Sheds' with PT6-45

ttfn