I've just been told that our QRH does mention the possibility of A/T disconnect during Alt Flap extension (this was found under "Flap Asymmetry").
It still doesn't make much sense though.
The A/T disconnect circuits are looking for position transmitter "fail" logic. The diagrams I have with me at the moment suggest that the A/T is receiving TE flap position data directly from the Left Flap/Slat Position Module (FSPM). No input from the FSEU regarding asymmetry.
Loss of TE position transmitter signal(s) may disconnect the A/T, but if there are any time delays or other prerequisites concerning the TE flaps, I don't know. The trip circuitry may only look at TE flap position when the flaps are in a specific range, hence the trip around 5 units. There may be some kind of fault trigger threshold around Flaps 5 (or the threshold may be set lower than this, but there are inbuilt time delays for trips). Normally (non-ALT ops), the TE flaps begin to move when you select Flaps 5 (No movement at Flaps 1). Perhaps, between Flap 1 and Flap 5 the TE flap position fault detection system wakes up in the A/T computer(?)
There is no apparent Flap Slat Electronics Unit (FSEU) input to the A/T. The FSEU does, however, look at the position transmitters (so that it can generate transmitter failure or asymmetry warnings/messages). I see that Asymmetry warnings are generated when the asymmetry reaches 12% of full TE travel: Where 12% is in relation to Flaps 5, I don't know. If it is before Flap 5 and the flaps are frozen automatically at that 12% point, would you be allowed to force the flaps to 5 units via the ALT flaps system? Is 5 units the maximum asymmetry tolerable? (IS roll correction possible via trim up to this point, without losing lift because of spoiler actuation?). Using Alt control, you may be able to drive the flaps to the A/T computer flap fault detection point/threshold (if there is one).
Our latest 767 (FADEC) has more complex/sophisticated Flap Asymmetry circuits, but I can't see anything to suggest that the A/T logic is any different from the older aircraft (with regard to flap fault detection) .
Anyway, I'll let you know if I come up anytthing.
Cheers.
Q.