Following from the academic discussion above, below is an entirely speculative attempt to calculate the effect of the various conditions that seem to have been factors in this accident. None of it amounts to anything more than guesswork, but it does provide the basis for some broad conclusions about the likely importance of the different variables. Again, all below is strictly my own opinion except where referring to particular published documents.
without spoillers and just one reverse, is it possible to stop on a short runway only applying brakes?
To give sdruvss an answer (at least as it applies to this situation) requires the use of some speculative mathematics which I have engaged in purely out of personal curiosity resulting from what is a very reasonable question.
Referring to Zeke’s post #1224 which show’s TAM’s Wet RTOLW chart for landings on 35L at CGH, given a 3 kt headwind component (the calculation basis assumes Vls for approach, not Vapp, so 5 kts has to be deducted from the headwind component), the RLD value wet for the Max. Landing Weight of 64.5 tons is interpolated as 1751m. Using this information and the calculations as in my post above we can work our way backwards to determine what the likely maximum effect of the ground spoilers would have been had the brakes been fully applied and maximum reverse was used on 1 engine only. No credit is allowed in regulatory calculations for the effect of reverse on ALD but it does exist in reality with Airbus showing 6% credit for two on a wet runway. A 3% credit for the single reverser is probably reasonable. We must also assume that the dispatch factor for ground spoilers inoperative is 1.15*RLD, the same as was the case for the A320 at Taipei. Thus at a maximum:
Max RLDwet = 1751m
Max RLDwns = Max RLDwet*Dispatch Factor = 1751*1.15 = 2014m
Effect of Spoilers Inop = Max RLDwns - Max RLD Wet = 2014-1751 = 263m
Max RLDdry = Max RLDwet/1.15
= 1751/1.15 = 1523m
Effect of Wet = Max RLDwet – Max RLDdry
= 1751-1523 = 228m
Max ALDdry = Max RLDdry/1.66
= 1523/1.66 = 917m
Max ALDwet = Max ALDdry + Effect of Wet
= 917+228 = 1145m
Effect of Reverse = Max ALDwet*.03
= 34m
Max ALDwns = Max ALDdry+Effect of Wet + Effect of Spoilers Inop – Effect of Reverse
= 917+228+263-34
Max ALDwns = 1374m
So, at the maximum landing weight of 64.5 tons the loss of the ground spoilers would probably have added a maximum of 263m (about 855') to the landing distance, making it approximately 1374m (about 4465')
if all else was normal and a maximum braking technique was used.
However, in this accident we know this wasn't the case. The additional factor of the #2 engine producing 1.20 EPR means that the ALD would be significantly higher. Referring to the Taipei report once again, the effect of the additional 1.08 EPR there was to add 30% to the ground stopping distance with spoilers retracted on a wet surface. For this case it might be more like 40%-50%. Taking a median value of 45% of the ground stopping distance (which is 50% greater than the Taipei value) the ALD for the condition will increase as follows:
Ground Stopping Distance 1.2 EPR = (ALDwns - Airborne Phase)*1.45
GSD1.2EPR = (1374-300)*1.45 = 1557m
ALD1.2EPR = GSD1.2 + Airborne Phase
= 1557+300 = 1857m
The actual landing weight was about 3% below the max landing weight which would reduce the above values perhaps by 2% (37m), suggesting that the actual landing distance for this configuration would be 1820m if maximum braking was applied and the runway friction coefficient was equivalent to the wet value. There is also 60 meter displaced threshold which might have facilitated the pilot’s intention (18:48:14.9 “
One dot now. Okay.”) to have an early touchdown point on the runway. All of this taken together suggests that they might, just might, have been able to stop on the runway even allowing for the effect of the thrust on the #2 engine if the runway condition was no worse than wet, the touchdown point was less than 420 meters from the start of the runway (360m from the displaced threshold) and maximum braking was applied. That's a lot of if's I grant you, and most pilots would not accomplish it given normal reactions to the evolving situation, but perhaps it was not impossible.
What this little exercise, which I admit is entirely speculative, suggests to me is three things:
- It’s as likely having now done the math as it appeared intuitively at the start that it was the combined effect of the thrust on #2 remaining at 1.20 EPR and the 11 second delay in braking which took what might have been a low speed over-run, possibly contained within the airfield, and turned it into a high speed departure that proved unsurvivable. If either factor is not present the dynamics of the situation change dramatically. To get into the situation required at least 3 substantial deviations from normal procedure. Most of the questions related to why these deviations occurred have already been asked. Hopefully investigation will discover the answers.
- The actual runway braking co-efficient versus reported is of relatively low relevance because of the 11 second delay in applying the brakes. The FDR data reported tells us that no effective deceleration occurred until braking began. From a calculation perspective this is the same as moving the touchdown point 800m (2600’) further down the runway. From there the high-speed over-run was inevitable. However, had maximum braking been applied without delay the braking co-efficient would have played a critical role in determining whether the aircraft would have stopped on the runway. If the, so far as I know, undefined value implied by the modifier “slippery” reduced the braking coefficient below wet values then the chances of stopping on the runway went from slightly possible to not possible. One has to ask what guidance the crew had in terms of evaluating the impact of the “slippery” condition on the safety of the intended landing?
- The answer to sdruvss's original question “without spoillers and just one reverse, is it possible to stop on a short runway only applying brakes?” is: Yes.
It looks to me like the aircraft could have stopped on this runway without the ground spoilers so long as maximum brakes were used and/or the other engine's thrust lever had been retarded to idle. The contribution of the loss of ground spoilers towards increasing the stopping distance is significant, but substantially less than that of the thrust from engine #2 or from the lack of timely application of maximum braking. Take either of these two factors away from the event and a stop on the runway was at least theoretically possible and an over-run if there was one would have been at low speed as opposed to high resulting in a much greater potential for survival.
ELAC