b737 inflight inadvertent GND start selection with engine running

[togatogatoga]

just out of curiosity!

what would happen if during flight the pilot would inadvertently select GND start with the engine running?
would it break the starter?
is it inhibited if N2 is above a certain value?

[STBYRUD]

I could imagine that it could damage the starter drive before it could disengage by itself, just a guess though :confused:

[captplaystation]

Could result in severe bruising to your left cheek ! (surely only a very new FO could do this for chrissakes ? )

[STBYRUD]

Stranger things have happened... This would be a nice combo: "Engine Anti-Ice Off please" - Hyd A pumps off, starters in GND :D

[Checkboard]

Go outside and start your car. Now turn the key again - same thing.

Grinding of gears with a clash engage of the starter - in flight it would only grind away, the torque difference would be too much for any engagement.

[NSEU]

(EDIT) Had a look at the wiring schematics for the 737NG, and revised my original opinion. It seems it is possible to manually hold the switch in the start position and open the start valve. This may cause some damage.

Apologies.

[aviatorhi]

I believe on the -100 and -200 you'd cause the starter to experience a bit of self destruction, when the JT8D starters do that parts fly out fast enough to make multiple holes in the aircraft.

[MarkerInbound]

NSEU,

You're assuming there is automatic starter cutout. I don't knows about all Boeings but some 727s had and some didn't. Our NTSB has a nice report with pretty pictures about a MD-80, JT8D-217s, the start valve either didn't get fully closed or opened up on T/O. Shortly after T/O it decided it had had enough and left the engine. Easy to see where the starter was located because there is a large section of cowling that went with it. Since there was fuel and air and spark left behind, there were all the ingredients for a fire outside the burner cans.

[NSEU]

You're assuming there is automatic starter cutout.

Agreed ) I've already revised my original message.

Even on the automatic starter cutout types, I see now it's only the switch latching circuit which will be inhibited with the N2s above (circa) 50%. This will not stop you holding the switch in the start position (and damaging something)

Cheers
NSEU

[aviatorhi]

737-300 QRH says:

Condition: The START VALVE OPEN light illuminated or a lack of
rapid rise in the duct pressure associated with starter cutout
indicates the start valve has opened or remains open after engine
start.
ENGINE START SWITCH……………………………….………….OFF
If the Engine Start Switch fails to automatically return to the OFF position by 50% N2, manually move the switch to the OFF position.

If no starter cutout or if START VALVE OPEN light remains
illuminated:
ISOLATION VALVE SWITCH………………………......... CLOSE
PACK SWITCH (affected side)… ………………………….. OFF
[Causes the operating pack to regulate to high flow in-flight with flaps up.]
ENGINE BLEED AIR SWITCH (affected engine)…..…..….. OFF
APU BLEED AIR SWITCH (engine no. 1 only)……….….... OFF

During ground operations:
GROUND AIR SOURCE (if in use)……………DISCONNECT
ENGINE START LEVER (affected side)……….……CUTOFF

Must be pretty serious if they want you to shut off the air to the starter and/or shut down the engine.

Based on my 727 experience I can safely tell you that a situation like this warrants an engine shut down in that aircraft in flight or on the ground. The starter will continue to run until mechanical failure (read as "fragments moving outward at high speed") occurs. This is due to the location of the engine bleed shutoff valve, with the valve closed the 8th stage bleed will continue to supply air to a starter valve which is stuck open. On the 737 closing the bleed and isolation valve should be sufficient to remove air from the starter. With the "old" JT8Ds you can and will experience failure if it is opened for even a small amount of time (part of the reason FEs were always taught not to attempt to reengage the starter if their finger slipped off the ignition switch). The CFMs, on the other hand, allow for the starter to be engaged while the N2 spool is still spinning (a bit no no with the JT8), however, I would still venture to say that you will experience a failure of the unit (given enough time with the switch in GRD), I'm uncertain as to whether it would cause other damage since I've only flown a CFM in a simulator.

[NSEU]

They gave me a copy of the symatic's of the starter gearing and even I could clearly see that it would be a non event.

It was my understanding that the starter should not be re-engaged during a tailpipe fire (for example) until the rpms had gone below a certain value, to prevent damage to the starter/gearbox.

However, I see your point. There is a centrifugal clutch which should stop starter-gearbox reengagement at engine operating speeds. The engagement pawls are thrown outwards by increasing engine speed, disengaging them from ratchet ring drive. To be honest, I didn't think of a mechanical solution.

The first statement still holds true, however. At sub-idle speeds, the pawls are not thrown outwards so far and the engagement with the ratchet ring drive may not be so good, causing damage to the starter.

Cheers
NSEU

[mono]

The damage is not caused by the starter re-engaging with the engine gearbox (although this indeed could happen at sub-idle speeds).

It is caused because, as has been said, the centrifugal clutch has thrown the pawls out so the starter just spins up with no load what so ever. It overspeeds and will eventually self destruct causing damage, possibly catastrophic to the engine and other nearby components.

You would have to hold the switch in the start position for sure. But only for about 30-45 seconds, maybe less, IMO.

[Hunter S Thompson]

This situation has actually occurred to me, with an F/O selecting a start switch to start instead of off as part of the after t/o scan. Fortunately he didn't hold it in the ground position and the switch immediately cutout without the starter engaging.

[isaneng]

As per Mono's answer at post 14. On a previous type the starter valve closure was confirmed by a pressure rise in the bleed manifold. Unfortunately a partial closure one day gave almost exactly the right indications, but left the starter spinning under no load. 3 hours later there was a big bang, and the starter exploded under it's own overheated/overspeed condition. The resultant damage included a 10-12 inch hole in the gearbox casting (turboprop reduction gearbox, a strong bit of kit).