Why did the pilot leave ENG2 TL in the climb detent after pulling ENG1 TL back to idle 2 seconds after the "retard" call in the flare, just prior to touchdown? EMIT may have a point regarding engine spool up time creating part of the anticipation, but what does seem clear to me from the CVR and FDR data, is that the pilot wanted to get the one working TR (on ENG1) deployed as soon as possible after touchdown.
I'm going to argue the same points (single mode vs multimode primary flight control design) as I have previously, but approach it in a different way. I know what follows may seem basic at first, but please bear with me.
What exactly is a "throttle"? A little research shows the word originated in the 1300s, from ME (middle English) “throtelen” (throttle), which probably derived from ME “throte” (throat). In the 1300s the word meant to strangle or choke. The first recorded use of the word “throttle” for mechanical devices was in the early 1800s and referred to a control operating a steam engine. A valve was used for the “choking” function that had a mechanical linkage to a lever used by an operator. Later the word “throttle” was also used for internal combustion engines and turbine engines. For steam engines, the power output of the engine is controlled by “choking’ the flow of steam into the engine. For internal combustion engines, the flow of air into the engine to burn fuel is “choked” by a valve (usually a butterfly valve). For turbine engines the flow of fuel into the engine is also “choked” by a fuel valve. In all 3 engines types, the choking valve operated by a mechanical or electronic connection to a human (via a lever or sometimes a wheel), controls the power output of the engine. Historically, the “choking” valve provides an “infinite” number of power settings providing the operator with precise control of the engine’s power output.
In aircraft, the basic function of an engine throttle is no different from the classic historical uses. When the A320 is on the ground, the throttle levers work in the classic way for controlling engine power. But from takeoff to landing, these levers no longer function as throttles in any classic sense when the auto thrust system is engaged. When auto thrust is engaged, the thrust levers become the big switch arms of multi-position selector switches, instead of throttles.
Switches and throttles work in very different ways. While throttles are described above, switches work by turning things on and off. A multi-position selector switch normally works by turning on one of the possible selections, while turning off the other selections. When auto thrust is engaged on the A320, switch arm positions CLB, FLX, and TOGA are available as possible selector switch positions.
Here we have 2 primary modes of operation for the A320 thrust levers, throttle mode and selector switch mode. In throttle mode the lever position equates directly to engine power, directly controlling the power output of the engine in the classic throttle sense. In selector switch mode however, we have the thrust levers being used as big switch arms to select power modes for the auto thrust system. In selector switch mode, lever position no longer equates to engine power thus the levers have no direct control over engine power, because they no longer function as throttles in any classic sense. It was possible to design the auto thrust system so that a selector switch on the panel performed the same power mode selection function as the thrust levers on the A320, but that design choice was not made. Instead it was decided to add a second operating mode to the thrust levers, because these levers only provide inputs to a computer, where the software determines what those inputs mean. Manufacturer B decided the levers in their cockpit would always be throttles, and would be presented to the pilots only as throttles. Thus B’s thrust levers are single mode in function because they are only throttles, not dual mode s in the AB system, where the levers are used both as throttles and selector switches, but only as one or the other at a time.
Now back to the TAM accident. I think it’s interesting that the primary function (or operating mode) of the thrust levers in the A320 is that of the auto thrust selector switch, because this mode is used the majority of the time the A320 is operating.
In the A320, the TL mode changes from the auto thrust selector switch mode, to the classic throttle mode when the aircraft touches down (or slightly before touchdown in the flare when the pilots pull the TLs back to idle in response to the “retard” callout). I find it interesting that “retard” in not a selector switch mode related callout, but it’s a throttle lever mode callout instead, because “retard” has no meaning for a switch but does have meaning for a throttle.
What’s interesting is that the pilot waited for the “retard” callout in the flare before he pulled the thrust lever back for ENG1 to idle 2 seconds after the “retard” callout started. So I believe he responded to this callout. But why leave the ENG2 TL in the CLB detent? There are several possibilities but I’ll argue for this one. When the thrust levers are in selector switch mode, CLB detent is the correct setting for the thrust levers on approach (with auto thrust engaged). Keep in mind that selector switch mode is THE predominate operating mode for the thrust levers on the A320, because auto thrust is used the majority of the time when an A320 is operating. In the mind of the pilot, he may have been thinking about “selecting reverse” in 2 steps (idle position then reverse position) on the ENG1 TL (thus selector switch mode thinking) instead of pulling engine power back to idle in preparation for reverse (throttle mode thinking)? Again we know that ENG2 TL was left in the CLB detent (the correct setting up to that moment), which suggests that he may still have been in selector switch mode thinking instead of throttle mode thinking. Then to leave ENG2 TL in the CLB detent during the rollout, further suggests to me that the pilots never transitioned from selector switch mode thinking to throttle mode thinking once on the ground. Thus we have what I believe is a classic manifestation of Jef Raskin’s Rule 1a.
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 (control action) intending to have one result but get a different and unexpected result, distracting you from your task.
If the A320 thrust levers are selector switch arms most of the time and only sometimes throttle levers, it’s possible to get the 2 modes confused right at the point of transition from one operating mode to the other (during the flare and touchdown). I also wonder that since the thrust levers spend most of their time in selector switch mode, if this habitually takes the pilots thinking away from thinking of the thrust levers as throttles (direct engine power controls)
When considering the logic of the Ground Spoilers and the Auto Brakes (which has been eloquently discussed), one has to think about which of these 2 operating modes of the thrust levers that logic applies to. It seems clear to me that the logic was built assuming that the thrust levers would be operating in the classic throttle mode, while the aircraft was on the ground rolling out after touchdown.