Boeing differential elevators

Factual Questions
1

I’ve been reading all kinds of reports that the right and left elevators on EgyptAir 990 were in different positions, possibly because the pilot in the left seat was pulling back on the yoke raising the left elevator and the co-pilot was pushing forward lowering the right one. That just doesn’t make sense.

I’ve worked on navy aircraft that have “elevons” that can move in different directions from each other but that’s a different function, they act as ailerons when the stick is moved side to side.

I am not familiar with flight controls on heavies so can someone send me a clue? Is this how fly by wire handles conflicting control input on a 767?

2

To the best of my knowledge, the B767 is NOT fly-by-wire.

Cables, pulleys, and hydraulic actuators are probably what controls the elevators. Could you give us a reference for where you read/heard the “differential elevators” theory?

Now, if there was a hydraulic system failure, well, that’s another story. I’d have to research a little more, though.

Saludos.

3

I saw the same thing on the news, and based on everything I’ve learned (quite a bit considering my field of study) this makes no sense. Differential evelators have been used on prototyipical aircraft, but its has never been implimented on commercial aircraft let alone retrofitted to an old 767. So the entire theory is ricockulous. Also, the pilot and copilot pulling in opposite directions on the yoke wouldn’t create this effect if it were installed. There are so many flaws with this theory I can’t begin to explain.

I do however give the media a very small slice of credit in these theories. I don’t think they’d come up with this stuff on their own, and create elaborate simulations of it unless it was purported by a “expert”. I suppose there is some new technology that I haven’t been exposed to, and that some breakdown in equipment could create a differential effect, but I don’t see how a pilot/copilot tug-o-war would be responsible.

4

I heard the same thing as the OP on the news. National public radio. The idea was that the pilot pulled, while the co-pilot pushed, and that the data recorder recorded it all.

This is weird, for several reasons. First, I thought the 767 was a fly-by-wire. That has already been refuted by another poster; I could very well be wrong, in any case.
Second, even if the plane were purely hydraulic, it surprises me that the elevators would work in this way. It sounds like some goofy explanation I would dream up tongue-in-cheek. In any case, I WAG that if the plane is hydraulic, it might follow that the pilot’s yoke, being slightly closer to the left elevator, might have yanked it up, while the co-pilot’s countervailing push influenced the left elevator more strongly.

Anyway, NPR didn’t assert any facts, they just repeated Federal (NTSB?) theories. And I don’t know if the Feds are saying, “Here’s what we’re pretty sure happened” or “This is so weird we’re going to have to track down even the remotest possibilities, since it’s all we’ve got.”

Nothing I write about any person or group should be applied to a larger group.

  • Boris Badenov
5

Omniscient The idea I got wasn’t that the elevators functioned that way, but rather that they malfunctioned that way. This still presents problems, since I don’t know enough hydraulic pressures to know if the pilot would really have slightly more control over the left elevator.

6

I got the information from CNN, CNN.com and MSNBC.com There was a CNN report on TV last night with a computer graphic simulation of the tail surfaces, left one up, right one down.

I’m still stumped. I agree with Boris that this sounds like a malfunction rather than an intentional control mode - fly by wire or not. I’m sure the NTSB would have corrected the news media by now if this was in error but it still doesn’t make sense. There is never an aircraft engineer around when you need one.

Sorry about such an arcane post. :wink: Next time I promise to make one that lends itself to more humorous followups.

7

My roommate has some design information books on various commercial aircraft at home. When I get home, I’ll do 3 things:

  1. See if he has a book for the 767
  2. If so, see if it provides information, schematics, etc. on the control systems
  3. If so, interpret it to the best of my abilities and report back here.
8

From a Reuters report on Yahoo:

This also sounds surreal to me, but one other report I read (sorry, couldn’t find it) said that “Boeing says that it would take 50 pounds of force difference between the pilot’s yoke and the co-pilot’s yoke to result in the control setting…” implying that someone at Boeing described this as a possibility. Weird.

9

The way I heard it on the news was that the two elevators could go different directions (after the 50 lbs of force was applied) so that in case one jammed, you’d still have one working elevator.

10

The NTSB has consistently and repeatedly said there is no evidence of any mechanical malfunction in the aircraft. This includes the conflicting elevator deployment; Boeing designed the plane that way for a reason, to assist the crew in controlling the aircraft during situations of engine/hydraulic systems failure requiring this abnormal configuration. What those special circumstances are, I have no idea - but the controls worked the way they were supposed to.

11

Perhaps we’re in a gray area between pilot error and mechanical failure here. I simply can’t figure out why Boeing would design its elevators to turn into aelerons (ailerons? spelling?) whenever the flight crew decides to yoke-wrestle.

The real problem, if this theory is true, was that one fella was trying to save the plane and the other was trying the opposite. I’m not blaming Boeing - a 50-pound pressure differential on a pair of yokes isn’t an everyday hazard. Nor am I saying that something busted on the plane. I’m just saying - if the “CNN theory” (for want of a better term) is true, then design features came directly into play. Had the plane had some sort of “pilot override” feature - probably workable only in fly-by-wire, a responsible pilot could have saved the plane from a sui/homicidal co-pilot. It’s not a big deal, since flight crews are not picked casually by any airline I know of. I just think the Boeing elevators might be pretty weird.

12

FWIW, no Boeing commercial jets are fly-by-wire. Some fighter aircraft are and, IIRC, some of the newer Airbus models are but nothing in the Boeing commercial product line.

“Vandelay!! Say Vandelay!!”

13

Newsweek said that the elevators work together BUT there is a maneuver that will let them function apart. Apparently it is a rare maneuver, and this was a rare crash…

i am special. i am cool. i am doper 3000!

14

So are they saying the crash was caused by the differential elevators or that’s just an interesting bit of trivia?

How do they explain the drop, the climb, and then the crash? 1) Copilot tries to crash plane. 2) Pilot comes back, tries to save plane. 3) Plane flips upside down? Wings fall off?

Rav

15

Please check my last post (Nov-18, at 11:24pm) on this thread: http://www.straightdope.com/ubb/Forum7/HTML/000636.html

Might be helpful.

Saludos.

16

Well, E1, your post doesn’t enlighten anything, it just preaches one-line rhetoric from a anti-media/skeptical standpoint. Now I don’t support the media, but the simple fact they reported it doesn’t make it wrong.

That said, here’s my understanding of the senario as gleaned from a composite of sources, and deciphered using an educated guess. By the way, Padeye, I am an Aeronautical Engineer, but that isn’t to say I know the specifics of a 767-300. So even the presence of an aircraft engineer doesn’t automatically create any better explanation ;).

First, the traditional understanding of an elevators control linkage to a pair of yokes doesn’t allow any possibility for this differential action. That theory of a pilot/copilot tug of war is about as logical as saying a driver and passenger in the family car fighting over the wheel could cause the front wheels to go in opposite directions. It just doesn’t make sense.

Now, the news stories seem to be claiming, with some slight discrepancies, that the pilot was away from the cockpit in the main cabin. The plane suddenly pitched forward, the pilot ran to the cockpit and began attempting to bring the plane to a climb. They theorize that the pilot and copilot were pushing and pulling in opposite directions. The radar and flight data say that the plane began a steep dive, then a short climb, followed by a decent and a roll to inverted. It accelerated, and the wings shreared off and the plane then crashed nose first into the sea. This is the theory as I understand it.

The basis that I’ve heard is a supposed uttering, and prayer from the copilot before the initial desent. A yell, and then a series of pleas for help from the pilot to the copilot as the plane is coming out of the desent and into the final plunge. The radar data indicates a controled, but erratic pattern of flight. The flight data indicates an intact plane, and the differential position of the aileron. So all this crap, and we get a movie of the week type senario. I suppose its possible, and I don’t see any other possibilities.

The problems with it is the arguement that the copilot is a happy, successful family man, with no apparent cause for this. The airlines, and defensive pilots are getting protective and claiming that this isn’t possible. The concept of a differential elevator is unheard of by most, and they claim the plane was not mechanically flawed at the time. So, is the elevator supposd to work that way, or do they mean the plane was not malfunctioning with the exception of the elevators?

The 50 pound differential in force sounds plausible, but if the explanation is to have an override in the case of a jammed yoke, why have the elevators move seperately? I understand having one yoke break loose so the other is controlable, but I can’t see any good coming from the elevators operating seperately. If this is the case in design, then Boeing may be open for critisism, but they need to clarify if this is a design feature or not. Otherwise, I can’t see any other explanations other thna the suicidal copilot.

17

First, I heard it was the “tail elevators” that were acting in a differential manner. I really have no clue, but I think they’re controlled by pedals whereas the primary elevators on the wings are controlled by the yokes which are mechanically tied together.

Second, if one elevator was up and one was down, wouldn’t the plane…uhh…pitch (more pilot terms–by this I mean rotate around the axis from the nose to the tail) and basically spiral instead of smoothly dive.

To me, both these things make sense–I could see pilots fighting on pedals and each getting control of one tail elevator. All speculation on my part, though.

Hey, aren’t you supposed to be at work?

18

This is very elementary, but accurate:
http://wings.ucdavis.edu/Book/Controls/intermediate/basics-01.html

19

This link explaines the reason for the ability to split the elevators on the Boeing 767.
( I hope this works ) http://www.newsweek.com/nw-srv/printed/us/na/a55096-1999nov14.htm

Peace
t lion

20

Well, here is a novel thought for us: Why not wait until ALL the evidence is available before speculating on the cause of the crash?
As to the post from NeedAHobby, first, the elevators splitting would not lead to pitch. Pitch is movement around the lateral axis; roll is movement around the longitudinal axis. If anything, there might be some affect on roll.

Second, I suggest learning some basic airplane terminology, especially about control surfaces. The control surfaces on the wings are the ailerons, which cause the plane to roll, or bank (there are also flaps called speed brakes that help slow a plane down). The flaps on the horizontal part of the tail assembly are the elevators, which affect the pitch. Both of these are controlled by the yoke, or steering column of the plane. The rudder in a standard airplane is what is affected by the pedals, and it affects yaw, the movement around the vertical axis. I don’t even know if 767’s have pedals - perhaps you should find a good mock-up of a 767 cockpit to see.

As to the elevator split likelihood, all I can say is that Boeing itself has verified to the press that differential pressure on the two steering columns can cause a split in the elevator action. Whether that makes sense or not I suppose could be debated by someone who has an in depth knowledge of the troubles that a modern passenger aircraft can have, but it does little good at this point to doubt that it is an accurate explanation. Again, let’s wait and see…