A320 Starter Engagement Limitations

[flyingforeternity]

Hi all,

The A320 Limitations for the Pneumatic Starter on the Engine mentions -

1) No Starter Engagement above 10% N2 on ground and 18% N2 inflight - for IAE Engines
2)No Starter Engagement above 20% N2 - for CFM and NEO Engines.

Mu questions are :
1) Why is there such a limitation for the starter in the first place ?
2) Why is the limitation different for on ground and inflight condition on IAE Engines ?

I did find out that the starter for IAE uses a ratchet type clutch with leaf springs and the CFM uses a centrifugal clutch (I might be wrong). Lets say the engine is spooling down due to some reason and you engage the starter...the clutch pawls will initially rub down on the teeth, but they will eventually engage and the starter will drive the HP compressor. Where does the limitation come in then ?

Thanks in advance ! Inputs are greatly appreciated.

[Fursty Ferret]

1) Why is there such a limitation for the starter in the first place ?

The starter mechanically disengages from the engine above a certain speed. If you re-engage it above this speed then it won't spin the engine. More critically, as the engine slows down the starter will re-engage while running at its maximum speed and shear out.

You have the higher N2 in flight probably because at certain speeds you'd find that you wouldn't be able to attempt a start, but the engine isn't running fast enough for an in-flight start.

[Goldenrivett]

Mu questions are :
1) Why is there such a limitation for the starter in the first place ?
2) Why is the limitation different for on ground and inflight condition on IAE Engines ?

I think the starter motor uses a "sprag type clutch" see Sprag Clutch (https://www.google.com/search?sxsrf=ALeKk01qw4i4YIbEj3teOPJBuYEqoErNyA:158773059550 1&q=sprag+clutch+animation&sa=X&ved=2ahUKEwjt8ZKxhYHpAhWuRBUIHeSDC-8Q1QIoAHoECAwQAQ&biw=1280&bih=564&dpr=2)

When you mechanically engage any rotating unit (starter motor) with a very massive lump of engine (N2 assembly on your engine), then you want the minimum kinetic energy in the starter motor to avoid breaking bits. Hence normal engine starts on the ground specify a very low N2. The sprag clutch will engage with zero kinetic energy when N2 = 0 hence long service life.

In flight, N2 will always be >0, so a higher maximum N2 for assisted start is published and acceptable because they occur far less often.

[flyingforeternity]

Thank you for the amazing answers folks !!!

[Dave Therhino]

The axial input flow air turbine starters I'm familiar with on all the Boeing airplanes other than the 787 use a pawl and ratchet clutch, not a sprag clutch. The pawls are attached to the outer part of the clutch, which is the output that rotates with the engine gearbox. When the N2 shaft is at very low speed, the pawls are held against the ratchet teeth on the starter turbine shaft by spring force. At higher N2 speed when the starter is not driving the N2 shaft, that spring force is overcome by the centrifugal force of the output shaft rotation. If the pawls are not loaded, that disengagement speed is in the ballpark of 25% to 30% N2 as I recall (it could be somewhat different). What keeps the starter engaged during starting as the engine accelerates to starter cutout speed (typically 50% N2) is the friction from the loading of the pawls as the starter provides torque to accelerate the N2 shaft. As soon as the starter torque drops off due to the air valve closing at cutout speed, the pawls retract from the centrifugal force.

If the N2 shaft is rotating above the centrifugal pawl disengagement speed with the starter stopped, there is no contact between the pawls and the ratchet teeth. If you were to turn on the starter air supply under that condition, the starter would free run with no load. What the procedures are trying to avoid is someone turning on the starter with N2 above this centrifugal pawl engagement speed as the engine is running down, and having the starter spool up to high speed, then the engine decelerating to the point where the pawls under the spring force move to engage the ratchet, which is now rotating at the starter's free run speed (in the ballpark of 100,000 rpm turbine speed). This results in what's called a crash re-engagement, which can destroy the starter.

A true sprag clutch, such as that used in helicopter drive systems, does not have this issue, so I don't know what the reason for the limitation on that type of starter is.

[flyingforeternity]

Thanks for the amazing answer sir !