Basil, thanks for the reminder of the need for practicality; a distant past.
However, simple rules-of-thumb often make assumptions which if not understood or considered in context can unknowingly result in operations close the limits of safety. Landing performance is one such area, and continues to irritate me; – I’m biased by experiences in aiding investigations into unnecessary overrun accidents, nearly all involving assumptions.
A continuing (practical) problem for the industry is the interpretation of requirements for landing distance; see several related Pprune threads.
The dispatch requirements are clear, relating to the AFM data – no temp correction. What is far from clear is what a safety factor actually covers. A recent ICAO study could not identify a definitive answer, but there was a history of research undertaken by RAE and FAA in the early 60s – the advent of the jet age, which resulted in the current values.
Also lacking clarity is if the dispatch factor applies to a pre-landing assessment; regulations are ambiguous but tend towards ‘not applicable’.
The UK CAA attempted to get to grips with the factor in AIC 14/2006 (now superseded?) – “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.” N.B. ‘remote’ has a specific meaning to regulators in certification.
The AIC continues “Landing distances achieved operationally are affected by the variation in the values of various parameters from those assumed at certification. These include, threshold speed: height over threshold, time from threshold to touchdown (float): delays in initiating stopping procedures; level of braking applied, runway friction.”
This neither includes Temp correction nor excludes it.
What might have greater importance is how representative the factored AFM distances are in actual operation? 30%+ of the factor could be consumed in achieving a normal ‘good’ touchdown relative to the certificated data, and even more in the assumptions about a wet runway, quality of runway surface, tyres, etc. Thus the actual safety margin between the dispatch data for ‘foggy’ wet conditions, or for monsoon wet (but not contaminated) with poor tyres can be significantly different, and in some cases there is no safety margin. Thus any assumption that Temp is within the factor could leave you short of runway.
The AIC concluded – “However, the existence of these allowances should not be taken as an indication that there will always be adequate protection against an overrun if there if excessive threshold speed onto a limiting runway with questionable braking qualities. In arriving at the factor to cover residual variability such as height at the threshold, 'float' before touchdown, delays in commencing braking procedures, substandard wet runway braking action, the effect on landing distances are combined statistically, not arithmetically. It cannot be assumed that the scheduled landing distances can accommodate a landing in which all relevant parameters are at the limit off the tolerance in the adverse sense.” N.B. the value of the limit of tolerance is not specified.
Many investigations and studies have shown that a pre-landing assessment is useful in helping pilots to identify a safety margin for the landing, particularly if conditions have changed after dispatch. Also, that dispatch and pre-landing data are likely based on differing data and assumptions.
JAR/EU OPS requires a pre landing check of distance, but fails to specify what is ‘safe’.
Again the UK CAA provides some guidance in Safety Note 2011/16 “The operator is required to demonstrate that it can ensure safe operations, and by applying the same additional safety margin to the in-flight performance assessment as to the dispatch assessment demonstrates an equivalent level of safety.”
The manufactures have agreed a common format for reporting, and will use new and more practical Operational Landing Distances (OLD) – see UK safety note. In addition, factored OLD normally 15% (FOLD), may provide similar distances to that in the dispatch data.
Thus even the more complex view of the landing issues as above fails to identify conclusive evidence about Temp correction; this appears to be an OPS issue, not Part 25.
If landing distance is marginal and the assumptions not fully understood (too simple), then the absence of a Temp correction might be significant.
A way forward; ask Dassault, compare with Airbus and Boeing.
Will Dassault be using OLD, if so are they Temp corrected?