TCASTESTOK

What exactly is it? According to a 767 sim checkride prep manual I read it doesn't exactly saw what it is but that it is caused by having pneumatic air being supplied to the plane while closing the shutoff lever. The manual says

rickair7777

It means the starter tries to engage while the engine and N2 gearbox are already turning. Likely to break the starter and if you're really unlucky the gearbox.

Kind of like trying to start your car when the motor's already running (old cars would let you do that).

TCASTESTOK

So would the engine have a catastrophic failure or would it just need to be overhauled?

rickair7777

It's designed so the starter should limit the damage to itself. Those are line replace-able in an hour or so (if you have a spare handy).

If the gearbox got damaged, that would be much more involved. All the accessories would have to come off, and then the gearbox.

If the damage included anything on the N2 spool, that might be an overhaul.

JohnBurke

Have you ever flown a light airplane with a manual landing gear extension that uses a crank? If so, were you warned not to let go of the crank while extending the gear, because the gear would freefall, and the spinning crank might break your fingers? That's crash engagement.

Ever try to shift gears on a manual vehicle without using the clutch? That's crash engagement.

To engage the starter, depending on the type of starter, the starter shaft must inserted into a splined drive on the engine (or in certain cases, an electromagnetic clutch engaged, much like an air conditioning compressor). If the engine is already turning too quickly, the starter shaft or shear coupling in the drive assembly can fail, or worse.

If this occurs in flight, it may negate the possibility of any attempts to perform a starter-assisted relight. It may also mean that the starter becomes engaged and stuck in that position, which means that if the engine is windmilled or run to a position higher than the starter limit, the starter can destruct. Whereas a generator will run on a constant speed unit (automatic transmission, governed to a particular speed), the starter motor is not; a stuck starter can burst or fail, which can lead to a number of subsequent failures, and catastrophic damage.

Adlerdriver

Not really. The gearing and manual extension handle are already engaged when you begin the extension process. What you describe is just an uncontrolled extension due to the air load on the gear as it extends (spinning the crank dangerously).

Your other examples are valid.

JohnBurke

You missed the point.

The crash engagement is the fingers and the handle, once the handle is already spinning.

Hence the broken fingers.

Perhaps you've never experienced that in a light twin...same principle, but in this case, the pilot's hand is the one crash engaging.

All the examples are valid.

Adlerdriver

Ok. Regardless of whether I've experienced this particular situation in a light twin, the concept is pretty basic. I simply thought you were referring to the mechanical gearing of the manual extension in your example. If you want to consider one's hand as the component experiencing the "crash engagement" , I suppose that works.

badflaps

Occasionally got a "start valve" light. F/O - Should we shut down the engine? Me- Naw, we would have known something by now.

JohnBurke

I like examples that involve personal tactile response; sticking the hand out the car window to explain AoA and drag and lift, bicycling up a hill or down such that a student understands what happens when going up or down in the airplane; really basic things that most have in common.

In the case of dusted knuckles with the gear, I'm the idiot that let go...