Tech question

Dave, I'm not an instrumentation expert but you are right on about gyros. Airplanes have been using gyros in their instruments since the early twenties. If you look at photos of earlier airplanes you can see the venturi sometimes on the side of the fuselage. These produced a pressure differential (suction)between the instrument and ambient pressure and it drove a little turbine that spun a gyro. The artificial horizon, turn and bank, are two of them. The modern instrumentation is a bit beyond me now and Don V. and J. Curry would have much better knowledge than I do. Airplanes have become so large that pilots don't feel what the airplane is doing and rely on the instruments and the instruments rely on the elements that sometimes cause them to fail, as you pointed out. The heavy reliance on computerization has shunted too much of the piloting off to the side and the boxes try to constantly fix things when they gate an abnormal reading or input. I had better shut up before I paint myself into a stall.

 

Hey Bob, leave me out of this, I'm a structures guy.

In the steam gage era the instruments (6-pack) were more or less independent. In the event of a failure (blocked pitot or static, failed vacuum pump, etc) there were methods to rectify the problem (break the glass on the VSI) or operate on partial panel. Glass panels which combine a six-pack into a single display do not have the same redundancy. Even display of AHRS (electronic gyro) data is somewhat dependent on the pitot-static system and gps input. Most glass panel installations require independent back-up systems, but true redundancy may be misleading. I have two aspen units running off completely separate AHRS units, however both of them and the back-up steam gage ASI are both connected to the same pitot system. The recent thread on the Buddy Holly accident did not have any mention of the theory that the attitude indicator, a mechanical gyro, had a role in the crash, and not due to a malfunction. To the OP, the vast majority of aircraft accidents are due to pilot error. Recurrent training and currency, or lack of, often play a big role in accident scenarios.

 

That's happened elsewhere. Two 757s were lost because of clogged pitot tubes feeding incorrect data to the autopilot. Both flights were at night so the pilots couldn't figure out what was wrong until it was too late. In one case a wasp nest had been built inside the tube (the aircraft had been sitting idle for some time); in the other case, mechanics had taped over the pitot tube and neglected to remove the tape.

 

In my many years of messing around with aviation I can't count the times when accidents (which are never accidental) were caused by sloppy maintenance or lax preflight checks or both. The bugs in the pitot tube will always be a safety issue if the operator is sloppy and leaves the tube unprotected. I was guilty of this ONCE and I disconnected the tube in the airplane and blew out the tube and a stream of green and orange larvae flew out. I have seen large bird's nests in the cylinders spaces in the cowling and in this country one should check and clear the seaplane drains on fabric covered airplanes because water has to have a way out. A lot of airplanes sit outside and all kinds of things can make a home in them. And then, every spring there is a spate of "engine failures" because the fuel tanks weren't checked for water. It'll never change. Masking tape left on pitot static systems or pressure ports often is missed after maintenance or repainting and this happened in the Boeing plant in the 60's when the pressure sensing port was covered with tape on a pressure test bench that had just been refurbished. They were testing a center section for leaks and blew it up.

 

I'm not a Bus driver, but I'll give this a shot. The AOA indication only measures your angle of attack, or the distance between the wing's cord line and the relative wind. The degree of pitch at which the aircraft will stall will change with a variety of factors, so you couldn't use your attitude indicator as a substitute. The problem with the Bus is that the fly-by-wire system will not let the aircraft stall. In these accident cases the air data was corrupted by inaccurate reading of the AOA indicators and the aircraft thought it was in a different condition than it was, therefore it over-rode pilot control inputs. On that peticular accident, They initiated a stall maneuver (part of the acceptance check) at a much lower altitude than required. When it went south they had no altitude to figure it out and recover.

 

I think I read that it was this incident that cause AB to come up with a way to by pass the "flight law" computers for pilots having control problems.

Of course this relies on the pilots knowing the instrument's are at fault and the aircraft is still perfectly flyable.

I pose this question to Bob and the other Boeing guys here, Given the same set of circumstances (AOA fault) can you push the nose down to break the stall and over come the AP and other flight control systems manually ??

 

All of these aircraft had back-up instruments that were functioning fine when they lost control. When your primary instruments look squirrelly, get on the back-ups and you can control the aircraft until you figure out what is wrong. What is apparently missing are emergency procedure sims that screw with the primary instruments enough times to where the crew immediately goes "get on the back-ups" when something looks wrong. Simple gyro attitude indicators have been working for a long time.

 

I watched this same documentary last night.
When the flight computers experienced conflicting data due to the iced up angle-of-attack indicators a warning message lit up above one of the glass instrument panels to "USE MAN HORIZ TRIM", or something to that effect.

If they had seen this warning right after it came on they could have manually inputted nose-down trim using the horizontal stabilizer and possibly recovered from the stall (or possibly prevented it).

 

I think that this is what I was trying to say. The computer's influence is destroying the mental abilities of the the pilots to figure out what they must do to FLY THE AIRPLANE. There has to be better communication between the computer and the " Pilot" and pilots need better training in FLYING THE AIRPLANE instead of watching what a computer thinks is happening.

 

Gyroscopic instruments wear out and need maintenance. The plane I started my training in was so bad I had to adjust the heading indicator every 15-20 minutes when navigating.

 

Yup, they sure do, but did it tell you which way was up and down reliably?

 

I saw that as well, So your saying that the trim "wheel" (or what ever AB use) can over ride the flight computers but the side sticks cannot ?? That's strange, (to me at least).


If that's the case one would think that the flight training and sim course's would have that operation as one of the "memory" items....I'm sure there is a reason they don't.

 

Certain flight controls have mechanical (cables) actuation means on FBW aircraft. Its a backup/safety feature. Stab trim and one set of spoilers is typical, thus retaining some pitch and roll authority.

 

The computer on the A320 was getting information from the AOA sensors to pull the nose up and the pilot was telling it to put the nose down creating control conflict so the computer gave up and flashed the warning to use manual trim adjustment.

My understanding is that the manual adjustment of the horizontal trim moves the entire horizontal stabilizer with a big jackscrew, not just an adjustment of the horizontal elevators (flaps). The jackscrew is driven with a hydraulic motor. On a B777 there is fly-by wire AND control cables back there.

 

The details are dim but I remember that a memo was released early in the 777 program that there would be a manual connection from the cockpit to the horizontal stab trim and to the outboard spoilers. Stabilizer trim means HORIZONTAL STABILIZER not elevators. I believe that you are correct.

 

Didn't the B2 that crashed at take off have the same problem? Also when I read about the Airbus that crashed in the Atlantic they were critical of it's computer control systems as compared to Boeing.

 

Water in the pitot static system

 

Ok that makes sense.

So the side sticks and their associated trim affect the elevators and their trim tabs via the computers, but there is a manual back up that moves the entire Horizontal section of the empennage?
If the computers see the manual input (I would assume it has position sensors) would it still try to command a nose up attitude from the elevators themselves, or "switch off" and leave the pilot in comand ?

 

Yes.

In the B777. Not sure how it is set up in the A320

I have the same question but from info in another thread it appears that enough deflection of the horizontal stabilizer can overcome elevator inputs.
Again, a question best answered by Airbus. (or one with better knowledge than I)

 

From my rudimentary knowledge of physics and aerodynamics, I would imagine that moving the entire horizontal surface of the tail will overcome any input from the much smaller elevators and trim tabs...cant imagine the opposing loads that would put on all the joints.

It would be an interesting experiment to watch..

 

I'm going to go out on a limb on this one...old memory. Stab trim was originally used for Mach trim and it is a powerful control surface that would overcome any force of the elevators. The area of the the stabilizer is greater than the elevator and the travel is greater.

 

it would seem that the pilot should be able to take full manual control, which seems not to be the case... reports indicate pilots are either still retaining some AP functions or not able or are unwilling to do a full disconnect...