Originally Posted by Rob21
learned here that there are two conditons when "Retard" warning will cease:
1. Both TLs at idle
2. At least one TL at reverse.
If this is correct, why "Retard" warning ceases before condition 2 is met?
(per CVR @ 18:48:25.5)
Thanks for the question, Rob - cost me a bit of researching time, but it's actually uncovered something of a new angle. Before I explain, apologies all round; I usually make an effort to keep my posts short, but this one has to be a bit longer than usual.
At first I thought that the Philippines investigators might have got it wrong; or that the system logic might have been changed since Bacolod in 1998 to 'at least one lever to idle' only.
But then I consulted Captain Chen's excellent report on Taipei, and discovered that not only did he pick up and discuss the point; he specifically questioned the Airbus reps. to establish the position.
This is from the body of his report:-
"(Page 95) When auto throttles are used under normal conditions, if any of thrust control levers is not pulled to idle position, no matter using the automatic landing or the manual landing, at 10 ft or 20 ft RA the FWC will deliver “RETARD” aural alert and then automatically stop when both thrust control levers are pulled back to idle position. From 1959:23(at 23 ft RA), the FWC began to deliver “RETARD” aural alert. After four times alert, it automatically stopped but not caused by the both thrust control levers at idle position. According to the documents provided by the aircraft manufacturer, the reason of “RETARD” stop was caused that the thrust control lever no.1 was at reverse position. When the FWC detected an internal signal of TLA inhibition which includes either thrust control lever at REV position and then FWC stopped the aural “RETARD” alert.
"During landing, the FWC has delivered four aural “RETARD” alerts and then after touchdown two seconds it came to stop. But at this moment the thrust control lever no.2 was not pulled back to idle or reverser position yet. In this situation, one of thrust control lever is not in proper position but the aural “RETARD” alert already stopped. The aural “RETARD” alert should continue or there should be other ways to remind the pilots of pulling back thrust control lever to reduce the probability of an accident caused by human error."
He asked Airbus a direct question about this, and received a response:-
"(Page 146) Investigator 2/ Why does the message "Retard" stop 2 s after the nose gear touched down? Is it as per design?
"Airbus - As soon as one TLA is set to REVERSE (whatever the other TLA position), the internal FWC signal "TLA inhibition" becomes true. If "TLA inhibition" is true, RETARD is inhibited."
So it appears that the logic applying at Bacolod in 1998 still applied at Taipei in 2004; namely that the 'Retard' call is designed to continue until both TLs are retarded to idle, OR at least one TL is put into reverse, OR speed is reduced below 80 knots.
But in both cases (Taipei AND Congonhas) the call SEEMS seems to have ceased before the reverser was deployed?
I think the CVR transcript may provide a clue as to what happened:-
"18:48:14.9 HOT-2 one dot now. okay.
18:48:16.8 HOT-1 okay.
18:48:21.0 FWC twenty.
18:48:21.6 FWC retard.
18:48:23.0 FWC retard.
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
18:48:26.3 CAM [sound similar to touchdown]
18:48:26.7 HOT-2 reverse number one only.
18:48:29.5 HOT-2 spoilers nothing.
18:48:30.8 HOT-1 aaiii. [sigh]"
Note that the 'retard' call occurs at intervals of about 1.5 seconds - and that the last one recorded occurs virtually at the moment of touchdown.
I occurs to me that the pilot may have been so conscious of the need to deploy the single reverser ASAP that he kept his hand on the No. 1 lever - and slammed it into reverse the first moment he could, as soon as the main wheels were down. The comment on HOT-2,
'reverse number one only,' might just have been a reminder from the PNF, not a callout indicating the time of deployment. In any case, the computers would have been working on the time the lever began moving through the gate, not the time the actual deployment took place.
Having found all that I was still left wondering whether it was possible to move the TL into reverse in the matter of a second or two of touchdown. But then I looked again at Captain Chen's Taipei report; and found that he had provided at least part of the answer:-
"(Page 87) The analysis of FDR recorded data shows that thrust lever 1 was at the positions of 0, -19.7 and -22.5 degrees in sequence within 1 to 3 seconds after the main landing gear touched down. 1 second later, i.e. at 1159:32 hr, thrust lever 2 was still remained at the position of 22.5 degrees."
Not sure what to make of it all - but I hope it's interesting information.
All rather confirms the 'logic' problem in the system, though. It's one thing to deny the pilot ANY stopping devices if he fails to retard both levers properly; quite ANOTHER to give him reverse thrust on the strength of just one lever having been moved, and use that event to cancel the warning callout while STILL denying him spoilers and autobrakes..........