Turboprop Question

Profile picture for user Mark L

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16 years 6 months

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I fly in Turboprops far more than I fly in jets, and one thing has always puzzled me. After pulling onto the stand, the engine condition levers are retarded to idle, or feathered (there is a reduction in power in any case) and the props are just idling. The pilots keep the engine in this situation for about 10 seconds, before then turning them off completely. At first I thought this was so that ground power can be inersted, however having watched numerous Q400s at Belfast City, this does not appear to be the case. Not having much of a knowledge about the operation of turboprops, can someone please explain why this procedure is carried out? Its rather annoying, not that I have to wait an extra 10 seconds (I'm not that pushed for time usually) but its the fact that when the engines are spooled down to idle, everyone assumes they have been switched off, and starts standing up, at which point cabin crew have to tell them to sit down, in fact wasting more time than if they had just stayed sat down in the first place!
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Profile picture for user tenthije

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19 years 9 months

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My guess would be to limit the stress on the engine. Compare it with shifting down in your car from 5th to 1st gear. Not recommendable, and usually preceded by shifting down to 2nd or 3rd before going into 1st. Again, just a guess.
Profile picture for user andrewm

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19 years 9 months

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My guess would be to limit the stress on the engine. Compare it with shifting down in your car from 5th to 1st gear. Not recommendable, and usually preceded by shifting down to 2nd or 3rd before going into 1st. Again, just a guess.
I just use the clutch!

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19 years 9 months

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probably to do with how fast the crew can do their engine off checks!
Profile picture for user wysiwyg

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In a free turbine the props would be brought out of the beta range (where they would be turning at typically 1000 -1200 rpm) to the feathered position (rotation at something like 300 rpm and the blades at 90 degrees to their rotated direction, ie head on to the airflow when there is no rotation). It is advisable to let the temperatures and pressures in both the prop and the engine stabilise after performing the transition before shutting off the fuel supply to close everything down. *edited for apalling spelling for a man of my age*

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thanks wys
Profile picture for user Mark L

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Thanks a lot mate!

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I actually queried this with Pratt & Whitney some time ago as it is only the PW100 series engines that this seems to occur - I was told that the procedure was followed to allow gear oil to drain back into the gearbox from the prop spinner before shut down. Has anyone noticed that ATRs have an engine brake (on the port side, I think) - I have seen this happen where the engine appears to be at a slow idle and the prop suddenly stops. I'm not sure why this happens although I think the APU on ATRs is located in the port engine nacelle which may have something to do with it. Hope this helps.
Profile picture for user Mark L

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16 years 6 months

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The ATR doesnt have an APU at all, so the prop brake function is used to replicate this.
Profile picture for user wysiwyg

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19 years 9 months

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Mark's right, there is no APU on the ATR. The prop brake is on the right hand engine and when it is operation it is referred to as Hotel Mode. The Saab 340 has the same arrangement however most operators remove it as the weight advantage is in the order of 2 adult passengers! By the way, it's not just the P&W engines that need to do this but any free turbine engine (a turboprop where there is no mechanical linkage between the engine and the propeller).