There are several contributing factors to this accident. While the discussion of braking method is very interesting, I think it's almost academic given that the ground spoilers failed to deploy in this accident, as several previous accidents demonstrate how ineffective braking can be on wet runways without ground spoilers.
My own focus has been on the fact that the ENG2 TL was left in CLB detent, which prevented the GS from deploying (thus minimizing the braking action), and adding forward thrust to a decelerating aircraft. My particular focus has been on why the pilots failed to retard the ENG2 TL, which to me seems central to this accident.
Regarding PBL's comments about Raskin, the A320 is a heavily computerized aircraft, the first of its kind (FBW) in airline operations. The rest of the A320 family, the A330, A340, A380, 777, and 787 have since followed it into airline service. The very fact that all primary control inputs are feed into and processed by a computer makes Jef Raskin's Human Interface Design Rules for computers extremely relevant in my view. If a joystick for primary flight control input isn't a computer interface subject to HMI design rules, I don't know what is. On the A320, thrust levers also provide primary control input to a computer and are processed. While "modes" can be made to exist in mechanical controls, they are primarily the creation of computer programs, where the programmer determines what the control inputs mean, and how they are processed. This is the main reason why some set of Human Machine Interface Design Rules are needed when writing control interface programs, because without the disciplined guidance of design rules, a programmer could write anything (from good to terrible) as a control interface.
Regarding how the pilots interacted with the A320 throttle system in this accident, and given that the failure to retard ENG2 TL to idle caused everything to go wrong during this landing rollout, I'm going to try to put myself in the cockpit (as uncertain as this may be) during the landing.
As we know from the CVR transcript, both pilots were aware of the runway length and conditions (wet and slippery). Both were aware that ENG2 TR was inop. They briefed each other on these things, though some have correctly posted that the briefing was not extensive and did not include a concise plan of action.
My own opinion is that fear (and it relatives like anxiety, etc), cause the narrowing or tunneling of mental processing that's been discussed often in this thread. Fear seems to create a narrow focus on the circumstances causing the fear. But this narrowing focus seems to also include a kind of mental "load shedding", where the mind seems to "select out" anything in the environment that doesn't seem relevant. A fear response also seems to move more towards a "fight or flight" physical action based response, and away from higher levels of mental reasoning as a response. As others have pointed out, hearing seems to be one of the first things "selected out".
As these pilots neared touchdown, a couple of interesting things happened. First, the "retard" callout began about 5 seconds prior to touchdown. Then, 3 seconds after the retard callout started, one of the pilots pulled the ENG1 TL back to idle, but left the ENG2 TL in the CLB detent. Why was the "retard" callout ignored and why leave the ENG2 TL in the CLB detent? Or was the "retard" callout actually responded to by a pilot who simply pulling the ENG1 TL only back to idle? It's possible the pilots actually responded to the "retard" callout by pulling the ENG1 TL back to idle. However I think the reason for selecting ENG1 TL only to retard, was because the pilot who did this wanted to get the ENG1 TR deployed as soon as possible after touchdown, which in fact he did.
However these actions may already show some cognitive narrowing due to anxiety over landing in these conditions. Instead of pulling ENG1 TL only back to idle, they should have pulled both throttles back. They also should have delayed the deployment of reversers (or the one good reverser) until confirmation that the spoilers were deployed, otherwise brake performance would be abysmal. What the pilot did was a more action oriented response (get that reverser out ASAP) rather than a more reason oriented response (pull both TLs back, wait for touchdown, wait for spoilers, deploy TR).
Three seconds after touchdown, the callout was made that there were no spoilers. If the anxiety level was pucker factor 3-4 on approach to landing, the realization and callout 3 seconds later that there were no spoilers, must have created immediate pucker factor 10 for both of these very experienced pilots, who knew exactly what no spoilers meant in those conditions. At pucker factor 10 the cognitive reasoning would have been narrowed greatly, as we know they never figured out in the 16 seconds from that callout until impact, that they needed to retard the ENG2 TL to have a chance at stopping (or a much lower speed overrun). In those 16 seconds, the responses all seem to be action based, "decelerate", "it can't", "oh my god", "turn, turn", rather than reason based, "why are there no spoilers", etc. Nothing against these pilots for what happened, as I think this is human nature that all of use are subject to.
Now to conclude my argument that the multimode throttle system in the A320 may have played a role in this accident, I would argue that since auto throttles is used a lot during the gate to gate time of an A320, the mode (or state) of "thrust lever angle DOES NOT equate to engine power" is probably the predominate mode (or state) of the throttle system while pilots are operating the A320. This however in not the mode (or state) used when the aircraft is on the ground, where trust lever angle DOES equate to engine power, and where the ground spoiler system thinks it does as well. The whole idea behind a single mode primary control is that the action and responses of that control are so consistent and predictable, that using the control becomes a habit so that one no longer has to think about or mentally process how that control functions. This is what Jef Raskin meant when he said you can "habituate" the control, if it’s modeless.
However it's not possible to fully "habituate" the function of the throttle system of the A320, because it has multiple modes. This means you have to think about how it functions at times. In the circumstances these pilots were in after touchdown, they apparently were not in a condition to notice (however I still wonder what “look this” was referring to in the transcript) or think through the meaning of the ENG2 TL position. My argument is that if the throttle system had been single mode in the sense that throttle lever position ALWAYS equated to engine power, then you wouldn't have to think about how it worked when you're in a situation where it's hard to think. It's very possible the pilots could have noticed and acted instinctively (meaning to act without thinking) to pull the ENG2 TL back. At pucker factor 10, performing actions is about all you can do.
Would a single mode throttle system have saved the day in this accident? I don't really know, but I do think it would have increased their chances.
Last edited by Flight Safety; 28th Aug 2007 at 09:26.