PDA

View Full Version : 777 Auto Pilot


Global Warrior
16th Feb 2010, 23:40
Hi Lads and Lasses

this post is about me learning and not wanting to hijack the other threads about this issue. There's enough momentum on the NZ and AF aborts, i believe, for me to start a new thread, so for those of you that think i should have posted in the aformentioned threads........ sorry.

Im interested to know and learn about how an inadvertant auto pilot engagement on the ground can happen on a 777. Ive just had dinner with one of my other captains and assumed that it could happen to us and if it did would we know?

The reason im going down this road is because we are all trained for an engine failure at V1 but statistically we are more likely to encounter a tyre burst or (fill in your emergency) .......... (....)

With regards to the threads about the high speed aborts and possible inadvertant auto pilot engagement, i really want to hear from the chaps that fly the plane.... not "experts" that dont.

Can you please talk me through a possible inadvertant auto pilot engagement scenario.........

i'm assuming the following might happen but i know nothing about the 777 systems, ergonomics or sop's.

Please allow me some licence here......... In our aircraft, a Global Express, our conrorl movement check would surely annunciate a control issue if the auto pilot was enageged at the time. So for us, an inadvertant auto pilot engagement issue would happen because of

a Flight Control Panel button selection which inadvertantly selected the autopilot to engage.........when the intended button/knob selection may have been a heading..... altitude........ or a radial as part of a clearance for example........ after the control check.

Is the auto pilot engage knob/button/lever on a 777 on the flight guidnce panel or somewhere else such as the center pedestal? There seems to be severall (10 maybe) instances of this happening on a 777. I would like to hear from the guys that fly the plane.... is this an sop, ergonomic or training issue? Subjectively, could it happen on any aircraft?

Would a full control check on line up, for example, reveal the problem or is there a software issue that allows the AP to engage without a positive button selection?

Also i believe the 777 is equipped with Boeing's version of fly by wire. At rotate on this aircraft........if the autopilot was engaged....... does the control column move but without any corresponding flight control movement, or does it feel "jammed" ?

Thanks for the education.

Regards

GW

fdr
17th Feb 2010, 06:09
Hi.

The specific problem with the B777 events is being addressed by a SB, which our fleet will complete mods by the end of this month. (We have a modest fleet of these aircraft, about 25 at present with about another 15 coming). the engagement logic will preclude the autopilot being engaged on takeoff until above 50'RA. Normal minimum engagement altitude is 200'RA, by the AFM limitations.

Your suggested causation is pretty good, that any MCP modification post the flight control check, which is completed after engine start has the possibility of accidentally selecting the AP switch. The AP/FD mode will change and be annunciated correctly above the attitude display, and will have an attention getting box for 10 seconds following mode change, but could be missed in practice. For the AF case, the possibility is that the failure of the TOGA switches to engage the Auto throttle resulted from not arming the ATR switches (L&R) on the top LHS of the MCP, and proximate to the Captains side AP engage switch (2 fitted, one at each end of the MCP) Various actions could be taken by a flight crew member that may place their hands near the AP switch, more likely on taxi or on a takeoff if human nature exceeds SOP's.

A flight control check prior to takeoff would identify an engaged AP, as would the annunciation on the PFD, and of somewhat less value, the lit up lower section of the switch.

On the B777, the handling pilot would be confronted with a control system that appears jammed in roll and pitch channels, as the AP back drives through the flight controls.

The failure to observe an annunciation change is not uncommon, and there are few clues other than a control check that would otherwise indicate an inadvertent engagement.

On a normal takeoff, there is little input in the elevator channel or ailerons unless there is a very light weight takeoff, slippery runways, or crosswind condition.

AFAIK, the ground engagement has been operator related not a failure of the system such as internal shorting of the Korry switches. It may be that this is not the case..... that would be a surprise.

Control checks are like flossing, only floss those you want to keep.

The control check changes on the Airbus were for a different reason, but do appear prudent, as would a rigorous review of modes selection prior to takeoff. Any such procedure relies on human performance, so there is room for failure.

regards,

FDR

M2dude
17th Feb 2010, 09:30
Hi guys, but here lies the problem. The 777 autopilot CAN be engaged while the aircraft is static on the ground; the T/O inhibit is only active above 80 KTS up to 50'RA, if engaged before the T/O roll commences, the A/P will remain engaged. Unlike all previous Boeings, there are no autopilot servos, but commands are directed to the Primary Flight Computers, which also send the backdrive commands. With the A/P engaged, the backdrives will make it appear, as was rightly stated, that there is a control jam. (Of course pilot direct input can over-ride and disengage the A/P). The presence of two symetrical A/P engage buttons each side of the MCP does not help the scenario either, but the BIG clue here is of course the PFD master mode annunciation. Shorting of Korry switches is extremely unlikely, as there are dual contacts feeding dual processor channels within the MCP. I don't think that you will find any recorded history of this occuring. Another major difference over previous designs, is that you do not engage the L, C or R autopilots individually, the press of the one engage switch selects and engages all available autopilots (all three unless there is a problem in one A/P).
I hope my blurb is of some help. :)

kijangnim
17th Feb 2010, 12:10
Greetings

If the takeoff is continued the autopilot will disengage when flight crew manually overrides the excessive control column forces.

M2dude
17th Feb 2010, 17:18
errrrrrr... not exactly the point here I'm afraid. At rotation, the pilot notices excessive control forces. (It is not THAT easy to break out any A/P) and no pilot in his right mind would want this scenario in even their worst nightmare, as was the case of AF in LOS. (In this case, due to a nice long runway, we had a coincident Vr/V1 , and the crew RTO'd !!!!!). No way would relying on breakout forces be acceptable to anyone in their right mind as a way of checking A/P engagement. As was posted by many in a related topic, don't mess with the MCP or anything else during the T/O, and 'live and die by your (E)ADI. (PFD in a 777).:ugh:

kijangnim
17th Feb 2010, 17:54
Greetings,
The Effort on the Controls is equals to twice normal, I agree that not expecting any resistance at all leads to the wrong conclusion such as jammed ... however... I am not debating :} I am just providing the figures, and facts :ok:

Flaperon777
18th Feb 2010, 05:39
If I knew the damn thing was engaged I would simply use the disengage buttons...:cool: