This is a possible explanation of the AF447 pitch up from FL350 to FL380. This is theory or speculation at this point. I believe the pitch up was inadvertent, was caused by difficulty in managing roll angle which task overloaded the PF to the extent his scan broke down.
Before you exclaim "Poppycock" or BS or whatever, let me set the probable reasons and scenario.
First, lateral control in the bus would normally be done with the fingertips and with small motions of short duration. The normal roll rate depends on deflection from stick centered for both Normal law and Alt 1 Law. Normal law would command a rate of 15 degrees/ sec with full deflection and Alt 1 is "similar to normal law" except you lose spiral stability.
When you get to Alternate 2 law, things are quite different in the lateral axis. Control surface deflection is directly proportional to
control deflection-period. No turn coordination with the rudder. No roll limits, you can roll without the bus fighting you. If the aircraft is in balanced flight at this point, no big deal. Probably just a little bit more twitchy than you are used to. Maximum roll rate is now 20 to 25 degrees per second depending on speed and configuration.
The problem comes when you are not in balanced flight. Suppose the rudder changes its trim point in the switch to Yaw alternate law (this may be a function of rigging of the control system.) The effect of this is that your lateral zero roll position now changes to a point not in the center of the lateral travel, and you have to exert force to hold it there.
How much force?
For the A320 from a 2004 post by 'Max Angle'
Quote:
The figures copied from an Airbus publication and converted from horrid Euro units to pounds are:
Breakout force: 1.1 pounds
Pitch: Fore and aft. +/- 16 degrees 22.5 pounds
Roll: Outboard 20 degrees 5.6 pounds.
Roll: Inboard 20 degrees 7.8 pounds.
Note that the force is different for inboard and outboard roll. Airbus found during development that your arm is stronger moving inboard than outboard and fine tuned the forces to make left and right roll feel the same. Try moving the stick left and right holding it from above next time you are at work, you can clearly feel the force difference.
I have to imagine that the A330 stick is almost identical. They do want an easy transition from one aircraft to the other after all.
What does this mean to lateral control? It suddenly becomes very difficult. Yes 7.8 pounds isn't that much force, you can hold that with one finger-for a while.
But you are flying an aircraft yourself now on the lateral channel. You are fighting transitions across the breakout force point and experiencing reversing stick forces. It is sort of like a cruel instructor trying to make your life very difficult by riding the controls and applying spurious inputs. It is impossible to fly the aircraft smoothly with this type of control interference. The best you can do is get a firm grip on the stick and balance your own muscles against each other so that the interfering forces are small in comparison to your own applied forces.
Problem. How do you keep from making inadvertent pitch inputs in this situation. Answer, you probably can't until you can release your grip on the stick back to more conventional levels.
"Oh that is easy", say you. Just move the rudder trim in the right direction and the plane will straighten up and fly right.
Problem. The PF has a serious roll control problem and a host of other distractions. He doesn't have the free brain cells to realize that his wing problem is correctable by the rudder, a different control surface. That takes a moment of calm insight which was not available. Remember, he's an Airbus guy. He flies with his feet on the floor to keep from accidentally kicking the tail around. His aircraft normally keeps everything in trim for him. He has never experience anything like this, particularly not at night, with a major pin ball show going on in front of him, at 2 in the morning.
Look at the BEA initial data report.
The initial roll was to the
right. (There is your heavy wing.) And the PF made a left nose up control input. I wouldn't be surprised if his thumb was pointed up along the stick He probably was curling his wrist to control the wings. Mama Airbus is keeping the nose more or less where it belongs for now so I'm going to concentrate on getting these wings under control for now. If this thing rolls over, we are going to be doing a split S! Can't let that happen.
I wouldn't be at all surprised that we find that the majority of his control inputs were to the left and that he was accidentally pulling the nose up at the same time.
I'll bet that they find dents in the floor beneath the R seat rudder pedals, in a location indicating that his feet were still on the floor at impact.
Remember-whichever guy was PF, he was in the right seat and was therefore flying with his right hand The limited data made available seems to show that the rudder trim was
never set. Whenever there is a description of the aircraft's roll behavior, it is oscillating.
Of course, once you get into a stall range, wing rock is a common result.
OK, I still have some work to do. Have a chew on this. There is more later if necessary.
Suggested area for discussion is, "What was the PNF doing all this while?"