What follows is an academic discussion of the effect of ground spoilers on landing distance. It probably won't be of interest to many, but is worth considering in light of the questions this incident has us asking. All below is only my opinion, and not published fact except as indicated.
According to A320 FCOM it takes about 40% - 50% longer to stop without ground spoilers available.
TripleBravo ...
I've been giving thought to the differences between the MEL 15% factor for ground spoilers and the QRH/FCOM 50% coefficient for a while now, trying to fully explain why the two are different and which one expresses actual performance as they obviously aren't both correct for the single case where the spoilers are not extended.
To start, my thinking is that a case where the Ground Spoilers don't extend simply due to the lack of the activation requirements being met is more analogous to a case where the aircraft has been dispatched with the system failed and deactivated on the ground as opposed to failed in flight after departure. The governing regulations (JARs/FARs/CARs) and Airbus requirements for the consideration of a system failure known prior to dispatch are as described in Airbus' Flight Operations Support document "Getting to Grips With Aircraft Performance", page 121, Dispatch Requirements:
In the event of an aircraft system failure, known prior to dispatch and affecting the landing distance, the available runway length must at least be equal to the required landing distance with failure. This distance is equal to the required landing distance without failure multiplied by the coefficient given in the MMEL, or to the performance with failure given by the Flight Manual.
My Note:
Flight Manual refers to the Aircraft (sometimes referred to as Approved) Flight Manual (AFM), not the FCOM.
These are the dispatch requirements, and as we all know the performance values used to generate dispatch limiting conditions are always the very best ones possible for the conditions as allowed by the regulations. If it were possible to prove that a lower factor was possible it would be used, and so conversely the factor provided will not be lower than the actual decrement to performance that the failure will cause to occur.
In the Taipei report the MEL page relevant to ground spoilers inoperative for that aircraft was reproduced showing that for the Ground Spoiler Control System Inoperative the requirement was to multiply the RLD*1.15. Presumably, but not necessarily the factor is the same for the TAM aircraft. In my manuals for one carrier's A330 fleet I found three seperate factors applying to aircraft of different mod status for the same failure so unless we see TAM's MEL page we won't know for sure.
In practical terms, on a dry runway if the MEL factor is 1.15 this indicates that the actual effect of the spoiler system being inoperative would be to increase the actual landing distance by a maximum of 25%. By Demonstration:
If: ALDdry = 1200m
Then: RLDdry = 2000m (1.66*ALDdry)
And: RLDns = 2300m (RLD Dry*1.15)
Thus: Effect of Spoilers = (RLDns-RLDdry)/ALDdry
= (2300-2000)/1200
= 300/1200 = 25%
In the event the runway is wet the percentage contribution of the ground spoilers to the landing distance actually decreases slightly even though the absolute value increases. By example, following from the above:
ALDdry = 1200m
ALDwet = ALDdry +25% (the 15% Wet factor on RLD results in a 25% greater ALD the by the same calculation as above)
ALDwet = 1500m
RLDdry = 2000m
RLDwet = 2300m (RLDdry*1.15)
RLDwns = 2645m (RLDwet*1.15)
Effect of Spoilers = (RLDwns - RLDwet)/ALDwet
= (2645-2300)/1500
= 345/1500 = 23%
Now, one thing to consider with these numbers is that the percentages relate to ALD, actual landing distance and not the ground stopping distance from touchdown. It's self evident that for the distance covered from 50' at the runway's edge to the touchdown point the contribution of the ground spoilers is zero. All of their effect occurs in the distance between touchdown and bringing the aircraft to a full stop. Allowing for the fact that the airborne phase to touchdown point for an ALD certification test landing will be approx. 300m with a minimal flare (!), the percentage effect of the Ground Spoilers on Ground Stopping Distance, not Actual Landing Distance, moves back upwards to higher values. In the sample cases here:
Stopping Distance Dry = 1200-300 = 900m
Effect of Spoilers Dry = (2300-2000)/900
= 300/900 = 33%
Stopping Distance Wet = 1500-300 = 1200m
Effect of Spoilers Wet = (2645-2300)/1200
= 345/1200 = 29%
If you wanted to you could also make a baseline comparison by using the Effect of Spoilers Wet versus Dry Stopping Distance in which case you could say that the loss of Ground Spoilers on a wet runway will increase ground stopping distance by 38% (345/900) versus the normal stopping distance on a dry runway.
Having said all this, the numbers we are used to using are the ALD numbers from the FCOM's and the QRH, and in this wet runway case the relevant factor would be an approx. 23% increase in ALD. However, when we refer to your FCOM page 3.02.80 it shows that for "ALL SPOILERS FAULT" a coefficient of 1.50 or 50% that has to be added to the Landing Distance (Conf Full). Why the difference between the MEL factor which adds up to 23%-25% and the FCOM/QRH which says 50% when there can only be one correct factor for a specific condition?
"Getting to Grips With Aircraft Performance" is very useful in clearing up the confusion. From page 124, Inflight Failure:
In the event of an aircraft system failure occurring in flight, and affecting landing performance, the runway length to be considered for landing is the actual landing distance without failure multiplied by the landing distance coefficient associated to the failure.
These coefficients, as well as the ALDs for each runway state, are published in Airbus’ operational documentation (Flight Crew Operating Manual and Quick Reference Handbook).
My Note:
The reference documents for inflight failures changes from the MMEL & AFM to the FCOM & QRH
So, the starting point for the difference is that the coefficients published in the FCOM and QRH only come into play when the failure occurs during the flight, not when it was known prior to dispatch. The documentation is silent on the reasons why the coefficients would be different, but I can think of at least three reasons that might apply:
- The dispatch case starts from a known secured condition (i.e. affected system deactivated) as opposed a case where the system has failed in flight. In the failure case there may be other subsidiary effects depending on where the failure starts or ends that have to be considered.
- The dispatch factor is intended to be no more limiting than it actually has to be to conform to regulations. Hence the factor represents what must be added assuming the maximum efforts of the crew and aircraft to minimize the effect. Actual pilot landing and braking techniques in the real world are likely to be less aggressive resulting in longer stopping distances than required by dispatch regulation.
- Once airborne, the dispatch regulations requiring a factor to be applied to RLD no longer apply. Consequently, the manufacturer can provide more conservative stopping distance guidance without it having a negative effect on dispatch capability. I would expect that most pilots would want to err to the cautious side of whatever values were produced using the inflight co-efficients anyway, but allowing a little more give costs the manufacturer nothing and allows both the pilot and the designer a little more breathing space for the unknown. There’s sense in this explanation in that the A330’s with different MEL dispatch factors all had the same inflight co-efficient.
From here I'd like to move on to a speculative discussion about what the effect of the ground spoilers not extending might have been in this specific case and whether a stop within the runway length might have been possible.
ELAC