Crash Report Shows Confused Cockpit

**Chogy** · 03 Jun 11,, 15:56

Did you read the exerpt I posted in post #12 on stall identification? That explains what was going on wrt the yoke pressure. They really don't want you pitching up

I did read it, and I am probably going to embarrass myself, but to the best of my knowledge, there is no mechanism or system that applies pressure to the yoke, counter to what the pilot wants, with the autopilot off... with the exception of the annoying "Speed Trim" system, which is N/A here. For the 737-800 computer to apply yoke pressure in pitch, it would have to use the stab trim system, and when it does, the pedestal wheel rotates and it is very obvious. It's much simpler and old-school when compared to the airbus system.

On downwind, I trimmed the airplane to a hands-off state in pitch. During configuration, all manual pitch trim changes were normal. So (in theory) there shouldn't be any force that would cause the heavy yoke. Weight and CG were all completely normal for landing.

**highsea** · 03 Jun 11,, 17:45

Welll I will probably embarass myself too.

I don't know how to explain this very well. The amount of force you have to apply to the column is set by the elevator feel computer via hydraulic pressure to the feel and centering unit.

In your situation, you had a stall indication and no inhibit condition. So the hydraulic pressure was increased in the forward (pitch down) direction. That should translate to less column force needed to pitch down, and more column force needed to pitch up.

This isn't the same as a direct force applied to the column- it's just a change to the feel of the column.

At least that's the way I am understanding it, which probably doesn't mean a hell of a lot...

**Chogy** · 04 Jun 11,, 14:24

You are correct, there IS artificial feel to the yoke - all big jets have this - and I was looking at anti-stall devices. I guess what's confusing me here is that the artificial feel isn't an advanced, dynamic programming, it is a pretty simple counterweight/spring setup, and if it can vary depending upon flight parameters, I was not aware of it. I'll look more into it. Something's going on here, and it's likely my own lack of system knowledge.

**highsea** · 04 Jun 11,, 18:28

Originally Posted by Chogy

...I guess what's confusing me here is that the artificial feel isn't an advanced, dynamic programming, it is a pretty simple counterweight/spring setup, and if it can vary depending upon flight parameters, I was not aware of it.

I have the schematic, but I can't post it- it's a protected document.

On the 737-NG it is computer controlled, and it adjusts dynamically to conditions including airspeed (and your stall warning). In fact, there are two pitot inputs to the elevator feel computer. So that as airspeed increases, it takes increasing column force to deflect the elevator the further you go from center.

The idea is that it should be more difficult to input maximum deflection of the control surface when you are at high speed, but should be much easier when you are taking off or landing. So it gives the pilot a progressive feel according to the airspeed inputs.

When you flared, you had heavy column to pitch up. But when you brought the nosewheel down it was easy, right? The computer was making it easy to go one direction and difficult to go the other.

**Chogy** · 05 Jun 11,, 13:41

When you flared, you had heavy column to pitch up. But when you brought the nosewheel down it was easy, right? The computer was making it easy to go one direction and difficult to go the other.

Very good, I didn't realize it was a computerized, dynamic system, I thought it was a lot more basic than that.

As far as lowering the nose, I don't even remember. With Weight-on-wheels, everything changes and there is a possibility that the system "shifted gears" so to say.

I guess what got me going on this whole thing was the fact that we had a cockpit full of bizarre and contradictory problems, and in the end, the QRH told us

"Use the correct set of airspeed indications"

and

"Ensure probe heat is ON. Avoid icing conditions."

That was all.

**highsea** · 05 Jun 11,, 17:41

Originally Posted by Chogy

Very good, I didn't realize it was a computerized, dynamic system, I thought it was a lot more basic than that.

The ailerons have the same system.

The toilets use a different one, lol.

I don't think there's anything that isn't computer controlled on that jet. Maybe the seat cushions...

Welcome to the 21st century, right?

Originally Posted by Chogy

I guess what got me going on this whole thing was the fact that we had a cockpit full of bizarre and contradictory problems, and in the end, the QRH told us

"Use the correct set of airspeed indications"

and

"Ensure probe heat is ON. Avoid icing conditions."

That was all.

I hear that. You had what you interpreted as two separate faults, the IAS mismatch and the AOA warning.

You did it exactly right. First things first- you flew the plane.

**Ararat** · 06 Jun 11,, 02:47

I don't think it is a good idea to have the artificial feel control unit dependent on the proper operational of AOA indication system and subject to its malfunctions....airspeed input I can understand.