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Hi
How does the starter attach to N2 during flight, when N2 is rotating but not enough to proceed with a windmilling start?
I am talking about restarting an engine with pneumatic air from the APU.
I thought the starter would fail to attach to the high pressure section, because N2 is still rotating. (At low speed, but still rotating)
Do we have the risk to break the starter by doing this procedure?
Thanks!
How does the starter attach to N2 during flight, when N2 is rotating but not enough to proceed with a windmilling start?
I am talking about restarting an engine with pneumatic air from the APU.
I thought the starter would fail to attach to the high pressure section, because N2 is still rotating. (At low speed, but still rotating)
Do we have the risk to break the starter by doing this procedure?
Thanks!
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The starter is disengaged, detached from the N2 rotor after starter cutout as confirmed by the start valve open light extinguished, that means no air is passing thru (the valve is pneumatically actuated).
Whenever a motoring is required after an aborted engine start, you must wait until the N2 reads 0% (jurassics), 20% (CL and NG) before re engaging the starter using the engine start switch. This is precisely to prevent damage to the starter.
Also, when in-flight starting an engine in a CL, using starter assist, the starter must not be engaged unless N2 reads less than 15%.
If your speed isnīt appropriate for a windmill start or if the engine's been shut down for the last hour (NG), a cross bleed start is going to be needed, which implies engagement of the starter.
Given the chance I'd go for a windmilling as it means a lot less workload and no packs need to be switched off.
hope it helps.
SW
Whenever a motoring is required after an aborted engine start, you must wait until the N2 reads 0% (jurassics), 20% (CL and NG) before re engaging the starter using the engine start switch. This is precisely to prevent damage to the starter.
Also, when in-flight starting an engine in a CL, using starter assist, the starter must not be engaged unless N2 reads less than 15%.
If your speed isnīt appropriate for a windmill start or if the engine's been shut down for the last hour (NG), a cross bleed start is going to be needed, which implies engagement of the starter.
Given the chance I'd go for a windmilling as it means a lot less workload and no packs need to be switched off.
hope it helps.
SW
Last edited by sudden Winds; 30th Aug 2011 at 19:59. Reason: personal preference added
follow the checklist , if the windmill start doesnt work then and your parameters are right then you've nothing to lose by trying after all who cares about a damaged starter on a dead engine !
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The starter motor drives through a clutch, could be a 'sprag clutch' or a ratchet/pawl assembly. With the starter off the pawls just ride over the ratchet, as the N2 turns the gearbox, engaging the starter, as it starts to spin up the pawls engage and drive the ratchet gear and gearbox. The problems that could arise if the engine is turning the starter could overspeed before it engages (due to no load) damaging the starter and/or gearbox. Thats why on a failed start always wait until the N2 is zero before re-engaging. However if there should be a tail pipe fire or an attempt be required to restart a flamed out engine for instance that precaution could be ignored but it would probably (depending on aircraft/engine type) require to be written up as there may be additional checks required. 
It depends on the airplane. On the 727 the only way you could do an inflight start was by windmilling. On the 747-400, it will tell you if it wants a starter assist. How it keeps the gears from crashing, I have no idea, I just read the checklist.
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Just to add to what has been said. The starter is engaged to the N2 gearbox via a ratchet pawl Assy on most aircraft.
Think of the starter in two parts, the connection into the gearbox once N2 speed is high enough disengages the sprung pawls via centrifugal force disconnecting the turbine/planetary gear section.
When this section is disconnected and you send pneumatics to the turbine it spins up blowing it to pieces. Starters can be crash engaged at certain speeds, but this can damage the starter, post checks are actioned.
QRH re-start envelope N2 speeds are pretty much spot on.
Think of the starter in two parts, the connection into the gearbox once N2 speed is high enough disengages the sprung pawls via centrifugal force disconnecting the turbine/planetary gear section.
When this section is disconnected and you send pneumatics to the turbine it spins up blowing it to pieces. Starters can be crash engaged at certain speeds, but this can damage the starter, post checks are actioned.
QRH re-start envelope N2 speeds are pretty much spot on.
