Fly by Wire questio0n

[Chris Scott]

Airbus FBW afterthoughts

Milt,
I'm probably suffering senile dementure, but still can't work out what PantLoad means by "SA" Airbus - there used to be a Toulouse-Blagnac (Caravelle) manufacturer called Sud Aviation, a predecessor of Aerospatiale, but I don't suppose that's what you've got in mind, PantLoad?

Anyway, following on from PantLoad's excellent description of the A320 family (the 330/340 are supposed to be similar), you can find a bit more on handling matters at the following link on PPRuNe:
http://www.pprune.org/forums/showthr...=308926&page=3
Being biased, I suggest #56, #65 - 68, and #71.

Re the sidestick, it's worth mentioning that there is a special armrest, adjustable in height and rake, for each pilot. The idea is to rest the flat of your forearm on it, wrist slightly raised. You do not touch the stick until you need to make an input.

For simple roll-rate demands, you can use your thumb and 1 - 3 fingers. For up demands, use your fingers; for down demands, your thumb. To do these different tasks, your wrist will twist to the necessary angle, but will not move up or down, or significantly sideways. Inputs are normally the most successful when small, short-duration, and frequent. The obvious exceptions are rotation on take-off, and a simple roll from one side to the other.

The stick can be moved in any direction, of course. I believe you are right: the stick does take account of your hook being stronger than your slice. Although the pitch and roll-rate resistances are well harmonised, combinations of the two are the easiest to get wrong. The sticks do not move to reflect autopilot inputs. Normally the signal to the EFCS is the algebraic sum of the 2 sticks. But if one pilot wants to take control, he/she can press a red button on the stick (eliciting a 'gringo' announcement and warning lights). The red button is also used for AP disconnect.
PantLoad has answered many of your other questions.

With controller free does the system freeze the attitude?
Generally speaking, yes, but it will not return the aircraft to that attitude after it has been disturbed by a gust – you have to do it.

How can you cope with electrical power failures and computer malfunctions?
There are 5 flight control computers, plus 2 augmentation computers (FACs) that do a lot of useful calculations for the EFCS. of the first 5, there are 2 ELACs (elevator-aileron) and 3 SECs (spoiler-elevator). In the early days, we often used to have failures of a SEC, but rarely of an ELAC. Each has a PB on the overhead panel, but – if cycling that didn't work – when convenient we used to reboot by tripping the CB, waiting 10 seconds, and resetting it... I think they may have made many of the CBs inaccessible on the latest models! I had about 5 lightning strikes in 13 years and never lost a flight computer (or any EFIS).

Loss of more than one computer, I THINK, causes the FBW to degrade from Normal Law to Alternate Law. There is still auto-pitch-trim, but some of the protections are lost. [Forgotten, ask someone current.] The next degradation is to Direct Law, using manual pitch-trim. On the A320 family, I think Alternate Law always degrades to Direct Law when the L/G goes down. [Something to do with functions being switched to battery power, because the hydro-electric RAT can be shielded by the nose gear in sideslip]. Finally, if you are having a good day, you should be able to stuff it on the runway using stab trim and rudder only (hydraulics permitting), if you lose all the computers. [Useful, that static stability...] But the idea is to get one or two computers restored first.

Presumably also all of your auto stabs and limitations are auto but maybe with some variations available?
Very few variations, I'd say. You can exceed the normal max bank (36 degrees?), if you hold the stick over and keep holding it, to the 67-degree limit (2.5G). You can NEVER exceed +2.5G or -1G. You can NEVER stall the aeroplane, but you can reach and hold alpha-max if you keep the stick back. [That's why someone was able to do that perfect wings-level crash at Habsheim in our early days. They had relied on the alpha-floor function of the A/Thr to apply TOGA thrust automatically, forgetting that it never does that below 100 ft.]

As PantLoad says, it's easy to land. The Normal Law modifies in pitch in (roughly) the last 100ft, to simulate the increasing elevator control load of a conventional aeroplane in the flare. After the flare, a slight de-rotation helps, as on so many types.

Just as I immediately dispensed with the conventional electro-mechanical ASI during my first simulator session on the A310 (1984), I found the sidestick was second nature within an hour on my first A320 session (1988). Wish I could say the same for the non-driven throttle levers... Bernard Ziegler (Airbus chief design engineer) wouldn't budge on that one. But they are a conventional joy in manual thrust, which I almost invariably used for manual landings. Trouble is, all the airlines are banning manual thrust now.

DISCLAIMER ! [Airbus pilots] Don't believe any of the above. It's 6 years since I last flew. Read your FCOM. Good flying.