technical question on PT6

[acat]

Hi,
I was wondering if you can damage the generators on a pt6 by shutting down the engine before turning them off. I know the poh calls for feathering and then shutting the gens off and then pulling the condition levers. Just came to mind because when parking on an ice covered ramp the other day I just pulled the condition levers right away in order to keep the aircraft from sliding. Just curious whether or not that will somehow hurt the generators or if I need to make sure that I get them turned off ahead of time. On start up you can't bring them on line until 65% ng.

 

[tbmpilot]

On the TBM700 (dash 64) the checklist calls for the Generator switch off after condition levers to cut off.

 

[cvsfly]

B200, F90, and most other KA I can recall - Battery/Gen (Gang bar) is one of the last items on shutdown after Condition Levers to Cut-off. I don't think it should be an issue. On Start-up checklist calls for load below 50% for 2nd engine start (cross gen start) on these models above - on some others (older) after first engine start and battery recovered, Gen goes off for 2nd Eng start.

 

[JAFI]

It can be a bad mistake thinking all PT-6 powered aircraft are alike. First, the PT-6 has been around for a long time and the early ones don't always operate like the newer ones. The aircraft manufacturer can buy the engine, then hang on what ever prop and components they want. The XYZ aircraft can operate different than the ABC aircraft with the exact same engine but different components. The generator may not fry in your case but some other component may. Also it may not fry the first time but maybe the third time..... Be careful.

Maybe you should ask about a specific aircraft type, model, and which PT-6 you are asking about. It can change from one serial number to the next.

 

[avbug]

Whether a manufacturer recommends shutting off the generator before shutdown or not, one might remember that a number of PT6A engine failures have been traced back to damage caused by the starter-generator. A common thread to many of these occurences has been magnetizng of engine components, including bearing components due to shorting and transient voltages...the most common time for a transient voltage is during start-up and shut-down.

On start-up, the starter generator is being used as a starter and is not producing voltage, but absorbing it. Conversely, on shutdown, the starter-generator is producing voltage which destabilizes as the engine speed decays. During the shutdown process, as speed decays and voltage attempts to drop, field strength will increase in an attempt to maintain the rated voltage. When this is no longer possible, the field will be cut, which amounts to a voltage spike or surge, even an arc...and that can do damage. In all the engine failures caused by the starter-generators (which included Shorts 360's, PC-12's, and BE-1900's), arcing and burning internal in the generator was found.

So far there have been 17 such failures reported worldwide, in which electrical discharge damage has occured from internal shorting. In each case, the primary damage occured to the number one main bearing, resulting in metal failure.

To date, I don't believe a direct connection has been made with failure to shut off the generator prior to shutdown, and presently there's no reason to make that connection. This doesn't mean one should resist doing everything one can to prevent such damage, and that includes common sense practices such as shutting off the generator prior fuel chopping the engine.

True, the generator will drop off line as the engine dies, but as it gets there it will also try to spike or fluctuate. If you've returned from a flight and are running the engine to normalize the internal temps (which you should do), shut off the generators during that time to unload the engine and maximize your cooling.

As an aside, I believe most or all of the failures involved TRW startere generators (Also Lucas Aerospace, and BF Goodrich; same generators, same company, different names due to different owners at different times).

 

[acat]

The engines are pt6a-135. They are on a PA31T2 (cheyenne). Thanks for the info.

 

[avbug]

I just glanced at a Cheyenne QRH, and it called for generators off prior to engine shutdown.