BOAC,
I appreciate your point but I think we are talking in circles here.
I did see NOD's reply when I was half way through with my own, but I didn't think he caught the essential distinction that in your post you took a value that applies to a condition (icy - .05) well below acceptable normal limits and applied it to the undefined subjective condition of "slippery", which it seems was deemed within normal limits by the operators using the runway that night. The comparison is invalid and so then is the use of the figure of 24% in any discussion regarding what the effect of reverse might be in this case. If slippery were in fact anything approaching a coefficient of .05 every airplane that landed would have gone off the end, not just this one.
In respect of the effect of reverse on landing distance I can't quote you exact figures for the A320, but figures for the A330 which are in a roughly similar range are as follows:
Runway Condition vs.
Effect of Reverse
Dry -1%
Wet -4%
1/4" Water -7%
1/2" Water -7%
1/4" Slush -7%
1/2" Slush -6%
Compacted Snow -6%
Ice -19%
As you can see, the effect of reverse on all contaminated surfaces other than ice is fairly much the same and is in a consistent range of 6%-7%. For the 320 the range is probably a bit higher, say 10%, but that would cover all contaminants with a friction coefficient of .20 or higher and very likely includes an allowance for hydroplaning.
As far as the actual runway condition being "poor" that's definitely open for investigation, but there was a very recent runway test and there will be several previous flights data to be examined. However, I don't think that the second previous landing's (the one at CGH) FDR data provides any indication of less than normal braking earlier in the day. The traces show a consistent 2.32 m/sec deceleration in response to 40º (about 50%) manual brake pedal application. Held all the way to 0 kts. that flight would have required about 1050m of ground roll to stop which I suspect would be within the normal distance for the force applied. As to the ATR landing the previous day, the number of factors differentiating that from this are too great to make meaningful correlation, but I'd wager my months pay packet that it takes more than braking action poor for an ATR to use 1800m+ on landing to stop.
Regarding operations with one reverse inoperative, this is not PROHIBITED in the Airbus MELs that I have seen. The MEL guidance says:
"One or both may be inoperative ... Provided that no operation or procedure is predicated on their use"
Followed by Operational Procedure guidance:
"Refer to Operational Regulations for performance on wet, contaminated and slippery runway."
For the Airbus at least there is no prohibition, unless it is otherwise required by the regulator or the operator. This is probably a reflection of the fact that reverse wasn't used in calculating the basic performance in the first place.
In practice an operator would require a specific takeoff performance calculation to be done if they proposed a takeoff as the standard allowances consider reverse to be operative, but there is no similar limitation in terms of considering a landing, which is what we are talking about here.
I do understand the point that you are trying to make regarding the use of reverse. The problem is that you are basing that point on procedures and figures that don't apply to this aircraft and relating it to conditions that it does not yet appear applied to this landing. From my perspective the use of reverse, particularly as it was on just the one engine, is likely to be entirely irrelevant to the outcome here, save for the investigation into the presumed confusion about how the PF handled the thrust levers.
ELAC
PS - Not having used these engines I don't have any info as to what the EPR would be for max. reverse, sorry.