Indeed NOD quotes BA's figures as a 24% increase in LDR with no reversers on a slippery runway (but does not specify the level of reverse). That would certainly knacker a BA 320 in CGH on a slippery runway. I see another operator quotes 23%.
BOAC,
You are confusing your terms. NOD did not quote BA figures as saying there was a 24% increase in landing distance on a
slippery runway, what he wrote was:
BA QRH A320 IAE 2 Rev Operative, Max Manual Braking, improves LDR from 50' by 5% (Dry) to 24% Ice over no Rev.
An
Icy runway is a whole different beast from what we are discussing here. By Airbus' definition "
Icy" is "
a condition where the friction coefficient is 0.05 or below" (FCOM 2.04.10 P1). This is a value well below that which would be considered allowable for landing in normal operations by any operator that I know of, and it certainly isn't relevant to the current case.
The numbers you quoted for the B737:
The Boeing 737-700 requires an additional 370m for braking action 'Poor' with 1 u/s and 970m for both, which is far from 'little effect'.
are for braking action "Poor" which is generally understood to be less than or equal to a coefficient of .25. Airbus equates this as being analogous to "
a runway covered with standing water with risk of hydroplaning or wet snow" (FCOM 2.04.10 P11).
A wet runway, which was what was reported at CGH (albeit with the modifier "slippery" which has an unknown value) is described in terms of its effect on performance by Airbus as "
A runway is considered wet when the surface has a shiny appearance due to a thin layer of water. When this layer does not exceed 3mm in depth, there is no substantial risk of hydroplaning." (FCOM 2.04.10 P1). A wet runway is one where the friction coefficient is .40 or greater and is analogous to a braking action report of "Good".
Though we'll have to wait for the completion of the investigation to know for sure, previous discussions regarding the effect of reverse on stopping distance have been predicated on the runway condition being as it was reported by the tower, i.e. wet but not contaminated. If it turns out that this was not the case in reality then the calculations are not relevant, and a prime question would become why the surface was below the stated condition when it had apparently been tested only minutes before the accident. Obviously, if the runway was contaminated with significant standing water then the case for making any landing at all there that night becomes open to question.
Referring to your post it's a bit unclear, but if the figures you quote are coming from the B737 MEL they would likely relate to additives to RLD which is a factored value. Quite possibly the additives are factored as well as, in that they are additives implies that the use of reverse was considered in the original calculation of RLD for this aircraft. Quite possibly the effect of reverse on the published ALD values is less, but not having a B737 manual at hand I don't know.
On the Airbus I think the documentation bias is to the conservative side. The figures published for both RLD and ALD under any runway condition do not take any credit into account for reverse, so the numbers are valid for a stop made with both engines providing forward idle thrust. From the ALD values a decrement is then provided to account for the use of both reversers, should you wish to consider their use. As it's a decrement, the lower the percentage value the more conservative the estimated effect of reverse. For the airplane I fly the range is from -2% to -7% for 2 reversers operative in all cases except the "Icy" condition (coeff. = .05!) where the value is 19%. Possibly the actual effect of reverse thrust on the landing distance may be slightly greater, but if so that’s an unplanned bonus to stopping distance and not the opposite.
Going through Airbus’ performance document “Getting to Grips With Aircraft Performance” I also came across the following statement:
"In other words, the aquaplaning speed is a threshold at which friction forces are severely diminished. Performance calculations on contaminated runways take into account the penalizing effect of hydroplaning." 5.5.2.4 Page 82
The particular sentence was included in a section of the document discussing aquaplaning and its effect on takeoff performance, but I suspect that the statement is equally valid for landing performance calculations, which aren’t generally as limiting in the first place. So, when you look at the factors relating to the effect of reverse thrust on Airbus stopping distances what you are seeing is a value to which it is quite likely a greater degree of conservatism has been applied than may be the case with the B737 numbers you quoted.
Also, while looking into Airbus' recommended braking techniques on another matter I came upon the following in the FCTM which might help explain how the general effect of reverse thrust is considered from a performance perspective and why in a previous post I referred to the effect of one reverser below 100 kts as being negligible:
THRUST REVERSERS
Thrust reverser efficiency is proportional to the square of the speed. So, it is recommended to use reverse thrust at high speeds.
Pull to reverse IDLE at main landing gear touchdown (not before). When REV is indicated in green on ECAM, MAX reverse may be applied.
The maximum reverse thrust is obtained at N1 between 70% and 85% and is controlled by the FADEC. Below 70 kts, reversers efficiency decreases rapidly, and below 60 kts with maximum reverse selected, there is a risk of engine stall. Therefore, it is recommended to smoothly reduce the reverse thrust to idle at 70 kts. However, the use of maximum reverse is allowed down to aircraft stop in case of emergency.
If airport regulations restrict the use of reverse, select and maintain reverse idle until taxi speed is reached.
Stow the reversers before leaving the runway to avoid foreign object ingestion. (FCTM 02.160 P7)
ELAC