BOAC, the devil is indeed in wet landing performance; as above, the basis of and the assumptions in this data are often overlooked. (
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Certificated ‘wet’ landing performance is based dry runway test data (some manufacturers’ spot-test wet landing distances, but only in particular conditions).
The safety distance factors are to account for differences between test conditions and variability in normal operation, i.e. greater air distance than certification, and ‘average’ piloting margins in speed, TCH, flare time, etc. However, the safety factor may not be sufficient for gross deviations, nor for a combination of accepted max deviations – excessively fast and long (see AFM/FCOM).
‘Wet’ distances have an additional factor to account for the runway conditions – the braking action on a wet surface. The wet landing distance safety factor in certification (1.67 – 1.92) assumes a particular value of mu, lower than the dry mu to extrapolate the dry landing distances. Thus the wet distances are only as good as the assumed wet mu, which may be considerably different from the actual (un-measurable mu) in operations – depth of water, basic runway friction, runway material and texture, grooving or not, tyre tread condition, … ; many of these items are unknown (unknowable) to the crew. These differences again use up the safety factor. The baseline wet safety factor may not provide the same level of safety as for a dry runway, and as conditions deteriorate, wet landings involve increasing risk.
The wet certification limit is at 3mm water depth (flooded = contaminated) where different methods of distance calculation (and assumed operation) are used, again with further risk – this is explained in AMC CS 25.1591 (FAR may differ!!!). However, the gap between the assumed wet performance (AFM) and the actual conditions involves many other variables, including flight accuracy, which in near limiting conditions may leave no margin for error, or even be insufficient due to inaccurate reporting – water depth, wind. (
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Hence my question back at #64, 13 May. ‘Wet/good’ relates to the certificated wet data, but ‘medium/fair’ probably does not, and has a different basis for calculation and operation (Boeing ‘actual’ distances), which are more aligned with the risks of operating on a contaminated runway and thus requiring a change in crew’s attitude in the use of this data. Thus the lack of reference to water / wet in the ‘medium/fair’ category might be misleading and not cue an appropriate attitude to risks involved.
It is possible, perhaps more probable with hindsight, that even if the crew in this accident had calculated the distance with factors for the braking action, and landed in the ideal position, with max braking, then the aircraft might not have stopped in the distance available.
Ref
Managing Threats and Errors during Approach and Landing note slides 25 / 26