Alph2z, Re #40, the problem with assumptions is that they are often wrong.
JAR-OPS 1.550 requires that the landing
mass is sufficient for a landing within 60% of LDA (and for a wet runway the distance is 115% of the dry distance). Thus, provided the crew compares landing weight with the max allowable landing weight (wet) for a given runway, an operator meets the regulatory requirement; distance is a by-product of the calculation. (FAR 121.195 is similar ‘…
weight of the airplane on arrival…’).
There is a further requirement in JAR for an in-flight check, which is probably more relevant to the circumstances of this accident, vis runway condition, safe landing, and missed approach.
JAR-OPS 1.400 “Before commencing an approach to land, the commander must satisfy himself that, according to the information available to him, the weather at the aerodrome and the condition of the runway intended to be used should not prevent a safe approach, landing or missed approach, having regard to the performance information contained in the Operations Manual.”
The problem which I think that you allude to is that most crews do not understand or cannot determine the margin of safety that is available during the landing which enable the safety requirement of JAR to be met (safety as such is not defined). However, considering safety as the need to minimize risk requires the Commander to have all pertinent information to evaluate the risks in the planned approach and landing, and thus confirm the safety of the operation.
The accident report refers to this as a margin for error; I prefer ‘margin of safety’, a margin in which risk is as far as possible quantified and controlled.
UK AIC 14/2006 refers to this margin as ‘field length factor’ which 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. Thus, any additional factor or error could reduce the margin of safety below that which we are familiar with in normal operations. The AIC concludes “It cannot be assumed that the scheduled landing distances can accommodate a landing in which all relevant parameters are at the limit of their tolerance in the adverse sense”; which again excludes additional error or adverse factors beyond the crews’ control (i.e. threats to a safe operation).
One of the fundamental problems which crews face is that they do not always have information enabling them to determine the runway condition, wind speed/direction, the precise weather (visibility/rainfall), etc, all of which affect the margin of safety.
Thus in this accident it was possible that from a crew perspective all of the parameters under their control were within normal operational variability (I am not saying that they were or were not, nor I as I interpret, did the report). However, the significant issues involved the late and rapid weather changes which invalidated the crew’s perception and plans, which reduced the margin of safety for operational tolerances to zero (220ft).
It might be debatable that a crew should have identified the potential for a wet contaminated runway at an earlier stage and reconsidered the landing distance (i.e. mass – too heavy for a short wet contaminated runway).
It may be appropriate for operators to require crews to adjust the planned margin of safety before landing in adverse conditions, e.g. tighten the stabilized approach criteria thus forcing an earlier go around, or reduce the landing weight. Unfortunately as illustrated in this accident, these changes still might not accommodate ‘normal’ human behavior (error prone) where the failure to perceive changing weather, runway state, visibility, or a long landing, may remove all of the margin of safety. In these rare events, the industry requires addition safety margins, preferably a good overrun area or increased landing distance requirements.
‘Landing Performance of Large Transport Aeroplanes’ UK AIC, free but registration required; then select PUBS / AIC / Pink.
Also, see AIC 86/2007 ‘Risks and factors associated with operations on runways affected by snow, slush or water’.
Safety aspects of tailwind operations.