Belgique thanks for the technical details. More issues; is the Rad Alt susceptible to errors when over strong reflective surfaces such as water / ice?
Further thoughts, what if one or both the Rad Alts is above 10ft due to error / failure? Is the reverser / spoiler logic then solely dependent on leg compression, or vice versa?
Can the reverser ‘baulk’ be trapped in the engaged state if the thrust levers are forced rearwards?
tribo thanks for the Boeing link, most enlightening. This restates the problems / unreliability of Mu measuremnt:
“If runway is reported to have slush/standing water covering, the flight crew should be suspicious of braking action reports and measured friction.”
“Ground friction measuring vehicles are unreliable when the runway is covered with a depth of contaminant that exceeds: Water - 1 mm. Slush/wet snow - 3 mm. Snow - 2.5 cm”
The presentation runs into the problems of reported braking action: numerical values of Mu are associated with the definitions “good, fair, medium, poor, nil” when calculating the landing distance, but Boeing refers pilots to PIREP’s (pilot reports). I cannot see any way of a pilot being able to give a quantifiably accurate report of braking action that relates to a performance figure, especially where different types of aircraft are involved. The presentation shows the wide variation in unfactored stopping distance due to technique or systems used on just one type.
Do the issues above mean that the industry operates on contaminated runways with a high probability that the reported braking conditions do not reflect the actual conditions? It appears that the best information would come from a report of what is on the runway - snow, wet, slush, standing water, etc and the crew then evaluate the situation (as recommended by Boeing). However, this gives opportunity for erroneous judgment, with potential for further error in the presentation of landing distance (factored / unfactored); thus, the risk of landing on a contaminated runway is much higher than we appear to realize.
It is interesting to note that manual braking gives the best unfactored stopping distance, but Boeing recommend the use of A/B Max on a slippery runway (another definition of surface condition?).
The 1000 ft air distance (737) appears somewhat short in comparison to JAA assumptions:- a 7 sec flare time with a 7% speed loss. Even if the average speed was 100 kts then the resultant 1125 ft exceeds the Boeing allowance.
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