I don't know if someone has already posted this, but... Here's the FDR Graphs just released...
Good. So the first thing to point out is about the "delay" concerning manual braking, that was in the Airbus Telex.
Spoilers partially extend with WoW on one main, and reverse one one engine (provided the other engine is at idle, or also in reverse). Full extension is with WoW on both mains (FCOM 1.27.10).
Please note that WoW, brake pedal deflection, TLA and so on are depicted in the transcript as being sampled at one-second intervals.
Wow on L was registered at 18:48:25, WoW on both (with WoW on R) at 18:48:25 (FDR p2). "No spoilers" was called at 18:48:29+, five seconds after theoretical begin of spoiler deployment. Manual braking from nothing to max (80 degrees deflection) took place over the two seconds from 18:48:34-36.
There are less than 5 seconds from "no spoilers" call to activation of manual braking. Given 1 second (minimum!) reaction time, that is at most a 3-4 second "delay". Similarly, one could argue that it would be unreasonable to expect a "no spoilers" call to happen earlier than 18:48:27. That would allow 2 seconds after when they should be fully activated for the two actions: judge they are not, and call it.
So one could argue that the "no spoilers" call took 2 seconds longer than optimal, and the manual braking 3-4 seconds longer than optimal. That is 5-6 seconds longer than optimal, and paints a different picture from saying "manual braking was delayed 11 seconds".
That is my first point. Here is my second.
Let us compare optimal autobrake-with-no-braking against manual braking.
I have argued that optimal in this situation ("optimal" here means something like: perfect behavior by perfectly programmed robots) would have been a "no spoilers" call at 18:47:27, and, as reaction to this, manual braking to full starting at 18:47:28.
Had one been landing with manual braking, then manual brakes would have been activated as a reaction to WoW on both mains, which was at 18:48:25. Allowing a reaction time of 1 second, this means manual braking would have been activated over the 2-second time frame 18:48:26-28.
But we are not perfectly-programmed perfect robots. A human-factors issue arises. One does not expect "no spoilers", so the PM waits an extra second to allow him to believe his eyes that the spoilers haven't deployed. So a call at 18:48:28 (now the call at 18:48:29 is not looking so slow). And the PF reacting to this over an unexpected-event reaction time, which is understood by many engineering psychologists to be between 1 and 2 seconds, and we have application of manual braking at 18:48:30-32.
That is four seconds difference between the situation in which one is landing with manual braking, and the situation in which one is landing with autobrake and one must apply manual braking. 4 seconds at 70 m/s is 280 m.
Hydroplaning or no, thrust levers to idle or no, perfect reactions or no, this 280 m represents what they lost simply through the choice of autobrake over manual braking, which they made in the air when everything was OK.
I conclude that 300 m is the price you nominally pay in an abnormal braking situation for having chosen autobrake over manual braking. Food for thought.
PBL