B767 Autothrottle - C Hyd Sys

[Grann]

I am wondering whether any B767 pilots may know why on some older airplanes there is a requirement to turn off the Autothrottle (otherwise it will trip off by itself) with Flap extension during a loss of the C Hydraulic System.

Thank you in advance.

[QAVION]

I was thinking that the A/T computer may have been sensitive to power fluctuations and that using the Alternate Flap system, drawing large amounts of current, would affect the A/T. However, it seems that most, if not all flap groups, seem to be powered by the opposite AC bus to the bus powering the A/T computer (Left AC Bus).

However, another possibility exists. The A/T computer has a Flap Lever input. According to my notes, the computer estimates the slats will move to 1 unit within 5 seconds (linearly) when the flap lever is moved to the 1 detent. It doesn't say why, but I'm guessing that the A/T commands an automatic thrust increase when this happens in anticipation of the extra drag(?). What is the normal procedure for using ALT flaps? Is the flap lever moved to match the ALT flap setting? If so, you may have a sudden unwanted increase in thrust (?) when you move the lever.

Regards.
Q.

[Golden Rivet]

Hi Q

FSEU input to TMC has something to do with alpha protection.

Regards

[Grann]

Thank you Qavion and Golden Rivet.

In our procedures we do not move the Flap Lever during Alternate Flap extension. There seems to be a difference in the procedures between older 200 and newer 300 series aircraft. On earlier 200 and 300 series aircraft the Authrottle will disconect once you select the Alternate Flaps to Flap 5. On newer aircraft there is no mention in the QRH to disconnect the Autothrottle and the Autothrottle will not disconnect with Alternate Flap selection. Could it have any thing to do with the FADEC computer?

Regards

[QAVION]

"On earlier 200 and 300 series aircraft the Authrottle will disconect once you select the Alternate Flaps to Flap 5. On newer aircraft there is no mention in the QRH to disconnect the Autothrottle and the Autothrottle will not disconnect with Alternate Flap selection."

Interesting. I've just been told that in our -200/-300 QRH, no mention is made of disconnecting the A/T. It must have something to do with specific harware/software P/N's & S/N's.

As I recall, (older?) 767's have less-than-accurate cockpit flap position indications during Alt Flap extension (particularly noticeable at larger flap settings).

Also, according to my notes, the A/T will disconnect if no position information is available from the FSEU(Flap/Slat Position Module) when in SPD mode, N1 mode in flight, or VNAV SPD mode.

I'm trying hard to link the two, but not having much success.

If the A/T is using the same position data as the cockpit flap position indicator, then I could see how alpha computations would be affected, but.... I don't see why the A/T should disconnect, especially at 5 units.

Regards.
Q.

[QAVION]

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.

[Grann]

Well done Quavion.

For your info we have three 767 simulators, on the latest one the A/T does not disconnect during Alternate Flap extension. Interesting enough we are told that whilst our older aircraft have no trailing edge flap assymmetry protection in the alternate mode - our newer aircraft do have flap assymmetry protection in the alternate mode.

It would be nice if you could post any new findings.

Regards