Cecco re #1 & #3
”why you have to complete the landing within 60% of the landing distance available.” EU Ops only requires that the landing is planned to stop within 60% of the distance available. You probably appreciate that you don’t have to complete a landing in that distance; although for contaminated operations you might want too – see below.
EU Ops also requires that pilots assess the conditions in-flight to ensure a safe landing; this requires a safety factor to be applied; this is as for pre-flight or similar.
”This factor accounts for the normal operational variability that can be expected in day to day service such that the chances of a landing overrun are remote.” –
Landing performance of large transport aeroplanes, UK AIC 14/06. AFAIR the definition of ‘remote’ in this instance comes from certification criteria and historical aspects as in previous posts.
Max Angle #17
”… although 1.67 (60%) seems like a large margin it is in fact nothing like as large as many people think. It is eroded by …”
Absolutely correct; but re “no more required” might depend on the circumstances. *1
Old Smokey, with respect (OB1), the certification landing tests are not quite as you describe (#15); re landing aim point and thrust management, and the need to use max brake in operations.
The key issues are that the unfactored distance is most unlikely to be achievable, and it is used to define ‘a line in the sand’; but the problem is that the sand is shifting!
Where factored performance data provides a landing distance safety factor – a safety margin, the margin varies with the situation. Thus a 1.92 wet factor might provide a safety margin equivalent to the dry safety margin in ‘good’ wet conditions, the actual safety margin can reduce rapidly with increasing ‘wetness’ to a point where no margin may exist in wet contaminated conditions.
The safety margin can also be eroded much quicker with changes in tyre condition, type of runway surface, and runway maintenance condition; thus even with a ‘legal’ LDR the safety of the operation requires pilot intervention to avoid or mitigate the actual conditions. *2
Furthermore, in contaminated conditions there may be no safety margin (even with a 1.15 factor) due to the way in which performance data is acquired and the assumptions made about the runway conditions - conditions which in general the pilot is poorly informed of. *3.
Thus “
Attempts to land on contaminated runways involve considerable risk and should be avoided whenever possible” (UK AIC 14/06).
Therefore, the pre landing assessment requires knowledge of how far the sand has drifted, and on what basis the line is drawn – drawn more often with a wide brush rather than a fine pencil.
* Refs
(1) “
The current operational dispatch factor of 1.92 for turbojet aircraft landing on wet runways at destination or alternate airports would have to be increased to a value of 2.2 to 2.4 in order to achieve the same level of safety as that which is currently accepted for dry runway operations.”
Transport Canada TP 14273E.
(2) “
…using the braking coefficients obtained during the tests on wet surfaces, indicates that the current operational dispatch factor of 1.92 for turbojet aircraft does not provide an adequate safety margin for landings on wet runways, particularly those with low texture or rubber contamination”.
Transport Canada TP 14627E.
(3) See presentations at
EASA - Workshop Runway Friction and Aircraft Braking.