What follows is a timeline analysis of the few seconds before and after touchdown, that might explain how a failure to retard ENG2 TL may have occurred, by failure to recognize thrust lever mode (or state) change from a switch function to a throttle function for ENG2 TL (left in the CLB detent), as I've discussed in previous posts. There have been several good theories discussed as to why ENG2 TL was left in the CLB detent, and I thought it prudent to attempt to "apply" the theory I've put forward to the actual CVR and FDR data. I also think theories that there was an electrical or sensor fault in detecting ENG2 TLA cannot be discounted at this point either. Hopefully we'll know the actual causes once the investigation is complete.
I assume from reading the CVR transcript that HOT-1 is PF and HOT-2 is PNF.
18:43:04 - HOT-1 (PF) remember, we have only one reverse.
18:43:06 - HOT-2 (PNF) yes...only the left.
Verbal reminder for both pilots that they only have one reverser discussed about 5 minutes 20 seconds before touchdown. Auto Thrust is activated and remains activated until after touchdown. Everything seems normal until close to touchdown, so I'm picking it up at the RA twenty callout.
18:48:21.0 - FWC twenty.
18:48:21.6 - FWC retard
18:48:22 - FDR - beginning of ENG1 TL movement to idle
18:48:23 – FDR – beginning of ENG2 increase in power
18:48:23.0 - FWC retard
18:48:23 - FDR - L-MLG WOW
18:48:24 - FDR - R-MLG WOW
18:48:24.5 - CAM [sound of thrust lever movement]
18:48:24.9 - CAM [sound of increasing engine noise]
18:48:25.5 - GPWS retard
18:48:26 - FDR - beginning of ENG1 TL movement to reverse
18:48:26.3 - CAM [sound similar to touchdown]
18:48:26 (between 26 and 27) - FDR - NG WOW
18:48:26.7 - HOT-2 (PNF) reverse number one only
18:48:27 - FDR - ENG1 TL in reverse
If I understand the A320 correctly, the retard callout should occur at RA twenty (it's RA thirty for the A340). According to the FDR data the movement of the ENG1 thrust lever begins about 1 second after the retard callout begins, around 18:48:22. Perhaps this movement didn't make a sound loud enough to be picked up by the CAM until 18:48:24. From the FDR data, this movement is the ENG1 TL being moved to idle, which is reached around 18:48:24.
It seems strange to me that the sound of increased engine noise at 18:48:24.9, which corresponded with a rise in engine power of ENG2 from the FDR, does not seem to get the attention of the pilots, as they make no comment about it, nor do they seem to take any action regarding it, because increasing engine power in the flare just prior to touchdown is pretty abnormal, unless your attempting a go-around from the flare. I think the pilots should have both heard and felt the increase in engine power.
The engine noise increase (as picked up by the CAM) occurred about the same time as the ENG1 TL reached the idle position. The FDR data shows the engine power started to increase about a second before the CAM picked it up, during the thrust lever movement to idle. The ENG2 power increase makes sense in light of the fact that the Auto Thrust System was still engaged at this point and a decrease in power on one engine would cause an increase in power of the other engine to maintain airspeed. Some have commented about the spike in power of ENG2 to EPR 1.24 or 1.25 at around 18:48:27 before settling to EPR 1.19 This looks like simple overshoot to me, common in many automatic systems.
Since according to the CVR and FDR, the pilots made no comment and took no action regarding the increasing engine power (which I think they would have heard and felt), I wonder if they were expecting it?
One possibility to explain why they might have been expecting it could be they got "creative" with the thrust levers (outside of Airbus procedures to retard both TLs) by trying to deploy ENG1 TR as soon as possible after touchdown. Both their actions taken and the comment made at 18:48:26.7, strongly suggest that getting ENG1 TR out as soon as possible was the goal of the PNF. Could he (PNF) have thought that the Auto Thrust System was still engaged when he started to move ENG1 TL back to idle (just above touchdown), thus leaving the ENG2 TL in the CLB detent on purpose so the ATS would continue to maintain airspeed (which the ATS did until ATS disconnect after touchdown)? Perhaps he didn't want to unsettle the flare just prior to touchdown, in order to get a good touchdown in the prevailing conditions, while still deploying the TR as soon as possible.
Another possibility is that the sound and vibrations of engine power increasing could have been associated with Reverse Thrust on ENG1 in the pilot's mind, as insidious as this scenario sounds.
18:48:29 (between 29 and 30) – FDR – Auto Thrust System disconnects
18:48:29.5 – HOT-2 (PNF) spoilers nothing
18:48:30.8 – HOT-1 (PF) aaiii [sigh]
The timeline at 18:48:29 and 18:48:30 re-enforces my belief that the pilots missed the state change from the thrust levers changing their mode (or state) from being selector switches to throttle levers. The state change happened at about the same time that the PNF called out “spoilers nothing”. This callout would have been shocking to the pilots, as both of these very experienced pilots would know in an instant what no ground spoilers meant in the prevailing conditions. The “sigh” from the PF suggests immediate recognition of those consequences.
This shocking and immediate turn of events could easily take the mind away from recognizing any state change in the working of the thrust levers, or recognition that the ATS has just disconnected and thus the ENG2 TL HAD to be pulled back to idle to activate the ground spoilers and auto brakes. If the increased engine power noise from ENG2 was associated with the TR from ENG1, how insidious this would have been, as it would sound and feel close to normal for thrust reverse.
I still argue that one habituated reason to leave an A320 thrust lever in the CLB detent is because it “lives” in that detent (as a selector switch) most of the time while an A320 is operating. It is also THE most common position to leave the thrust lever in when the Auto Thrust System is operating. On an all moving throttle lever system, the thrust levers don’t “live” in any particular position in the quadrant, and are thus habitually always thought of as throttles.