Hi Zeke ...
Thanks for posting the landing performance chart. It's certainly instructive in terms of the level of analysis that TAM has put into their operations to CGH.
Regarding the effect of inoperative ground spoilers shown as a factor of influence on the chart: The scan is a bit hard to read but I think the two conditions stated on the left side of the chart are "1 pair of spoilers inop" and "2 pairs of spoilers inop". These would match permissible dispatch conditions allowed under MEL 27-64-01. These conditions are different than the case with MEL 27-92-02 where the entire Ground Spoiler Control System is declared inoperative and consequently there are no ground spoilers available at all.
The MEL I'm using for reference is an A330 one so there may be some differences with the A320 but for the A330 it shows no landing performance penalty applicable for 1 pair of spoilers inoperative and a penalty of either 5% or 10% applicable for 2 pairs inoperative (depends on the MSN, different PRIM standards?). As the chart you've provided shows, the actual runway specific performance penalty for 2 spoiler pairs inop for all stated wind conditions is less than the 5-10% conservative correction shown in my MEL (and I assume the A320 MEL). It is probable that the same would hold true for the 15% penalty applicable for the Ground Spoiler Control System inoperative case as well.
Having said that, these factors are intended to be applied to the RLD values used for dispatch, not the ALD values which represent the actual landing distance capability. Depending on how the effect of the actual performance decrement is considered when determining the MEL penalty (is it added before or after factoring?) the effect on ALD may be higher. Certainly the QRH factors for inoperative spoilers are higher, but these are probably inherently conservative and must take into account the various uncorrected system failures that may occur during flight as opposed to an aircraft with an inoperative system properly diagnosed on the ground and then dispatched in accordance with the required maintenance actions.
Taking another look at the Airbus data in the TransAsia report suggests that the performance decrement may well be added after calculating the factored distance (though I would have thought the reverse more likely) as the inferred ALD for all spoilers deployed versus retracted rises by approx. 25% in the one engine in reverse one in idle wet runway case. But again this is based on a specific set of factors (speed, weight, flap config, wheel braking effort, and etc.) which don't conform to the assumptions used to calculate an ALD.
ELAC