B737-300/-500 engine start question

[Badente]

B737-300/-500 (CFM56-3) engine start, no starter cutout at 46% N2. Otherwise normal engine start. For some reason the CPT doesn't command the start switch to be moved to OFF but cuts the engine start lever.

In my understanding the starter connects via a centrifugal clutch to the engine shaft. When there is no starter cutout the starter disconnects from the shaft at 46% N2 but accelerates to max rpm as there is no load on the starter. When you now cut the engine start lever and the engine spools down below 46% N2, the centrifugal clutch reconnects the starter leading to mechanical desctruction of the starter.

Am I correct on this behaviour or am I fooled by my mind? I have a faint memory of reading about it during my typerating years ago but can't find it anymore. Can someone shed some light on this topic? Any clarification is appreciated.

Regards,
Badente

[A37575]

There is a difference between no starter cut-out and start valve remains open.
With no starter cut-out the VOL 1 states "At 46% N2, verify that the Engine Start switch moves to Off. If not, move the Engine Start switch to Off. End of drill except read the MEL.

Start Valve remains Open is an entirely separate problem and dealt with via the QRH

[Badente]

A37575 thank you for your answer. I'm more after the technical background. What happens if you don't stick to the book and simply move the start lever to cutoff (N2 above 46%) and the starter still running?

[contractor25]

Maybe this of help to you :

CFM 56 Specific Operating Instructions (http://www.b737.org.uk/cfm56_soi.htm)

[Badente]

Thank you as well contractor25, however I didn't find the answer I was looking for.

In the meantime I got an answer from our maint dept and they agree that it will kill either the centrifugal clutch or the starter if you don't stick to the book in this case.

[cirr737]

Thought about that situation a while ago - I think the only thing you could do as an F/O in that situation to save the engine is to kill bleed supply immediately - i.e. all bleed sources off, all packs to high to get the pressure out of the duct...

[BOAC]

Badente - it is all covered in the QRH. In the event of the start valve open light remaining illuminated (which I assume is your scenario?), the actions will sort it out. If your 'CPT' does not call for this action but closes the start lever or the F/O does not complete it.......................:ugh:- have a credit card ready?

[TheWanderer]

A37575 thank you for your answer. I'm more after the technical background. What happens if you don't stick to the book and simply move the start lever to cutoff (N2 above 46%) and the starter still running?

Technically, the starter valve is electrically controlled and pneumatically operated.
The starter operation is controlled by the start valve.
When selecting the start switch to GRD, a solenoid in the switch is energized and holds the switch in the GRD position until the N2 tachometer indicator switch opens at cutout speed of 46.33% N2. The start switch solenoid is de-energized and the start switch returns to the OFF position.
A sprag clutch disengages the clutch inner race from the clutch outer race after air to the starter is turned off.

In your scenario, when the N2 tachometer switch fails, manually override the solenoid and position the start switch manually to OFF.
As long as the start switch is in the GRD position, and the APU, external source or other engine provides air, the starter will still operate and turn the engine N2 shaft, the engine is "motored".
Once the GRD switch is in the OFF position, air supply to the starter stops and starter re-engagement above 20% N2 can result in starter damage.

Boeing recommends starter re-engagement at 0% N2, 0% - 10% N2 is allowable, 10% - 20% is permissable for purging fuel or to clear a fire but above 20% N2 it is not recommended for any reason.

Refer to your manuals and/or QRH, ask your maintenance for a printout of the AMM 80-11-00 for more details.