I've come very late to the discussion of this accident, as I've had a busy schedule. I tried to read the whole thread, but simply ran out of time. Regarding the mistake the pilots made in leaving ENG2 throttle in the Climb power position upon landing, I've come to think there's a design flaw in the AB throttle system. This accident and the nearly identical 1998 Philippines and 2004 Taipei accidents seem to illustrate this.
The following discussion is about modeless primary control design, which I’m applying to the AB throttle system).
The following Human Machine Interface (HMI) design rules are from a Dutch design engineer. These design rules apply to primary machine controls, and in my view especially apply where safe machine operation is ia concern. In the following descriptions, the word “gesture” means “control action”. Rule 1a in particular, is the most relevant design rule to this accident.
Rule 1. An interface should be habituating.
If the interface can be operated habitually then, after you have used it for a while, its use becomes automatic and you can release all your attention to the task you are trying to achieve. Any interface will have elements that are habituating, but the principle here is to make the entire interface habituating.
Rule 1a. To make an interface habituating, it must be modeless.
Modes exist where the same gesture (control action) yields different results depending on system state at a time when your attention is not on system state. In the presence of modes, you will sometimes make mode errors, where you make a gesture intending to have one result but get a different and unexpected result, distracting you from your task.
Rule 1b. To make an interface habituating, it must be monotonous.
"Monotony" here is a technical term meaning that you do not have to choose among multiple gestures to achieve a particular sub-task. Crudely, there should be only one way to achieve a single-gesture subtask.
In the application of these HMI rules to primary controls on automobiles, we have the following primary control behaviors, which all of us who drive are familiar with:
Primary Auto Control – Steering Wheel (primary directional control)
This control is modeless and habituating on all autos I’m aware of, as the operating mode of the steering wheel never changes. The basic relationship of the wheel’s position to directional control does not change. To go straight, hold the wheel straight. To turn the auto left or right, turn the wheel left or right. To increase the rate of turn, turn the wheel more. All control actions are habituating, and comply with the above design rules.
Primary Auto Control – Gas Pedal (primary speed control)
This control is modeless and habituating on all autos I’m aware of, as the operation mode of the Gas Pedal never changes. Even the presence of a cruise control system (auto throttle) does not change the basic operating mode of the Gas Pedal. The basic relationship of the pedal’s position to engine power (speed) does not change. To hold speed (or power output) hold the pedal in the same position. The increase speed (power), press farther on the pedal. To reduce speed (power), lift pressure from the pedal. All control actions are habituating, and comply with the above design rules.
Primary Auto Control – Brake Pedal (other primary speed control)
This control is modeless and habituating on all autos I’m aware of, as the operation mode of the Brake Pedal never changes. The basic relationship of the pedal’s position to speed reduction does not change. To hold speed, do not press on the pedal. To decrease speed gradually, press lightly on the pedal. To decrease speed more strongly, press more firmly on the pedal. All control actions are habituating, and comply with the above design rules.
One can argue that the sudden loss of power assist of the steering wheel or brake pedal changes the forces involved on operating these primary controls, but the basic mode of operations of these controls does not change.
Now to airplanes, we have the throttle system, a primary speed control on an airplane.
Primary Airplane Control – Engine Throttle (primary power and speed control)
On nearly all models of airplanes, this control is modeless and habituating, as the operating mode of the throttle(s) does not change. Even the present of an auto throttle systems does not change the basic operating mode of the throttle. The basic relationship of the throttle position to engine power (speed) does not change. To hold engine power (speed), hold the position of the throttle. To increase engine power (speed) move the throttle forward. To decrease engine power (speed) move the throttle aft. On most airplanes, all control actions are habituating, and comply with the above design rules. On some airplanes, the throttle movement is the reverse of that described, but even for these airplanes, the control actions do not change.
On the A320, we have something different. We have a throttle system with more than one mode, which violates the basic HMI design rules listed above. The throttle system on an A320 is still a safety related Primary Control for this airplane. The main problem as far as I can see, is a mode change occurs at touchdown, when auto throttles are used on approach. There’s a mode where throttle position must be in climb detent on approach (where throttle position does NOT relate to engine power), then when auto throttles disconnect at touchdown, a sudden mode change occurs where now throttle position DOES equate to engine power”. The very failure possibility mentioned in rule 1a is at work here.
The mental mistakes introduced by this design have happened before, and will happen again on the A320, as long as this multimode throttle system exists. A Rube Goldberg machine if I ever saw one.