Dw747400 From United States of America, joined Aug 2001, 1244 posts, RR: 1 Reply 2, posted (9 years 8 months 1 week 3 days 15 hours ago) and read 2516 times:
The current 747 variants do not have FBW technology, though the -400 series makes extensive use of some fairly modern avionics systems. The 747-500X and -600X were supposed to use FBW if my memory serves me, but both of them were canceled.
Cx flyboy From Hong Kong, joined Dec 1999, 6341 posts, RR: 56 Reply 3, posted (9 years 8 months 1 week 3 days 6 hours ago) and read 2441 times:
The current 747 development plan is for the 747EX, or 747Adv, as it seems to also be known. This is a slight stretch on the current 744 and will carry just over 400 pax in a 3 class configuration. It will have FBW technology and engines being developed for the 7E7. Cathay are looking at these but they won't be available till 2008 or later, and indeed hasn't even been formally launched yet.
Ikarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2 Reply 6, posted (9 years 8 months 1 week 2 days 20 hours ago) and read 2339 times:
Traditionally, the pilot controls are directly connected to all control surfaces, by hydraulics. FBW replaces this direct (and heavy) link with wires that merely transmit the information, not the mechanical movement, to actuators and motors at the relevant surfaces. The advantage is, the computer can be put in-the-loop. That means the computer can prevent the pilot from doing undesirable things (such as stalling, or pulling up in a stall). It also means, the computer can make the plane react to pilot inputs differently than it would if there was a direct link.
For example, if you use full deflection of some surface at high speed flight, the plane might react very violently, even tear off the surface in question, or produce structural damage. With a computer there, the deflection of the pilot control could be proportional to the rate of change of the flight path, as opposed to being proportional to the deflection. So moving the stick XX degrees to the left might produce the same result at any speed, whereas doing the same thing with a direct link would produce different results (less violent at low speeds, much more violent at high speeds). This is all theoretical - I do not know what exactly the computer does, it might well try and simulate the real thing very closely. Or it might not. One aim, apparently, is to make all planes feel the same to the pilot, so that an A320 is as sluggish as an A340 in its response, or vice versa, the 340 as lively as the 320. This reduces pilot type rating conversion times, along with similar cockpit layouts.
So, FBW allows the designer to let the plane response differently to pilot inputs than it would with direct links and hydraulics. It also allows the computer to have direct control and the final word (overruling pilot inputs) in controlling the plane.
Rick767 From United Kingdom, joined Jan 2000, 2662 posts, RR: 52 Reply 7, posted (9 years 8 months 1 week 2 days 20 hours ago) and read 2349 times:
"It also allows the computer to have direct control and the final word (overruling pilot inputs) in controlling the plane."
Strictly speaking that is a function of Flight Envelope Protection, rather than Fly By Wire itself.
It only protects the aircraft from entering a very dangerous flight condition, such as a stall. The protection system therefore very rarely activates, if ever, and so the final control of the aircraft does remain with the pilot in all normal operations.
I used to love the smell of Jet-A in the morning...
Ikarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2 Reply 8, posted (9 years 8 months 1 week 2 days 20 hours ago) and read 2337 times:
But without FBW, the flight Envelope Protection could not take control, right? The way I understood it is that FBW is an enabler - FBW could be a 100% replacement for hydraulics if designers wanted that, but they wanted more...
and so the final control of the aircraft does remain with the pilot in all normal operations.
True, but I always figured that the reason for keeping a human in the loop at all aren't the normal operations anyway, so having a computer that has the final word in exceptional situations is, in my personal opinion, a mistake. There've been 2 known incidents where the computer was wrong, so far. Admittedly, there've been many more incidents where humans were wrong - but as aeronautical engineer, I'd rather have 10 pilot errors than one error designed into the plane, which is why I can be rather critical towards the current applications of computer control.
N844AA From United States of America, joined Jul 2003, 1352 posts, RR: 1 Reply 10, posted (9 years 8 months 1 week 2 days 16 hours ago) and read 2296 times:
Ikarus, which crashes were caused by failure of the computers? I'm not saying I don't believe you; I'm just curious because don't recall having read about one in which a computer failure was the definitive cause.
The only possible case coming to mind is that airshow A320, and from what I understand of it, it sounded like the pilot had flown the plane too low and too slow.
New airplanes, new employees, low fares, all touchy-feely ... all of them are losers. -Gordon Bethune
Ikarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2 Reply 11, posted (9 years 8 months 1 week 2 days 15 hours ago) and read 2283 times:
The Lufthansa crash in Warsaw - the computer did not permit thrust reversers to be used because it thought the plane had not landed yet. The plane aquaplaned down the runway with minimal breaking action, and ended up in a hill at the end of the runway.
FredT From United Kingdom, joined Feb 2002, 2184 posts, RR: 26 Reply 12, posted (9 years 8 months 1 week 2 days 14 hours ago) and read 2270 times:
Plenty of aircraft, FBW or not, will not allow a number of things (reverse thrust, spoilers, braking) unless they are convinced the aircraft is on the ground. If wheel spin up is one of the parameters, aquaplaning can throw a spanner in the works.
IMO there should always be a way for the pilot to tell the aircraft that "yes, we HAVE landed and you better give me thrust reverse right now". Air-ground logic input sensors do fail. If there's not, it's not an FBW problem but a design problem. If there was a means of forcing the aircraft into ground condition, it was activated, the command reached the FBW computers and was ignored, THEN I'll blame it on FBW.
I'd like to read more about the latter accident before concluding anything. Got a link?
I thought I was doing good trying to avoid those airport hotels... and look at me now.
On another note - I did not state it was the FBW computer/software in particular that caused the LH accident, but a computer. I think the problem was that the wheel sensors registered a load, but not enough for the computer to accept the condition as landing. As consequence, the threshold was lowered. (Was it from 12 tons per main gear to 6 tons? Oh, dunno, I forgot the numbers...)