It may be just one of those days, (multitasking or I’m just below average), but I don’t follow all of the reasoning in accepting a 3120m actual landing distance on a 3300m runway without explanation of the assumption. As discussed extensively above, the industry accepts a 1.97 ‘factor’ (1.67x1.15) on top of the actual dry landing distance. ‘Accepts’ is used in the sense that this does not necessarily provide the same margin of safety when landing on a dry runway with a factor of 1.67, but it is equivalent (“equal, all things considered”) i.e. satisfactory for a range of normal operations provided the relevant ‘things’ are considered.
So why should we accept a significant reduction in safety margin in the use of 3120m as opposed to the fully factored distance? Is this because whenever there is a failure the normal level of safety is reduced?
I calculate that for the Hyd failure case the fully factored distance is approx 5590m, a significant increase compared to 3300m available - I am not suggest that 5590 is a ‘must have’ distance, it’s for comparison. So what are the justifications for accepting a low ‘safety factor’ of approx 1.06 (3300/3120) vs the standard factor of 1.97 (wet factored failure case 5590/3120).
In exceptional circumstances the Captain has the authority to accept additional risk, i.e. lower the level of safety. Is the hydraulic failure a qualifying event? This depends on many other aspects of the situation, particularly the availability of a suitable runway. Is this what Airbus state, imply, or leave open to the operators interpretation in calculating risk (and bearing the responsibility). In the failure case, the question to be considered is ‘should we be making this landing’, not ‘can we make this landing’.
However, confining the discussion to 3300m available and using the actual landing distance as the basis (Decision made - yes we should be making this landing, even after considering the distances involved, because there is no other alternative runway), then the calculations above should be used to quantify the change in risk – the level of safety of the planned operation. Thus equally important are the changes to procedures, operating conditions, specific actions, etc, based on the additional risk and any particular characteristics of the runway/airport in this situation, i.e. this is not just a landing distance question it is one of risk management.
I base my comments on the UK CAA AIC 14/2006 ‘Landing performance of large transport aeroplanes.’ I quote -
“2.3 This gross distance (actual) should be regarded as a theoretical minimum, consistent achievement of which requires a high level of pilot skill under favourable conditions, and concluding in a level of deceleration that would normally be considered excessive from the passenger comfort point of view. Consequently, in order to provide an operationally realistic level of performance, this gross distance is multiplied by a field length factor to obtain the Landing Distance Required.
2.4 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. It should be appreciated that the value of the factor is not the same for all aeroplane types. For example, propeller aeroplanes are not as sensitive to excessive approach speeds as are jet aeroplanes, consequently they are subject to a smaller factor.”
“6.2 Where a failure occurs in flight to other systems (eg restricted use of high lift devices) crews will normally be provided with advice on their effect on landing distance. Allowing for the low probability of such an occurrence, lower safety factors, if any, may have been applied compared to those used for the normal conditions. Therefore, even when such performance corrections are taken into account there may be a reduction in safety margins available to account for variations in, for example, touchdown accuracy or stopping technique. Other conditions, such as reduced wind limits, may also apply in such cases.”
So for a 3300m runway using a relative safety factor of 1.06 / 1.97 as a guide to the risk involved, and also against the need for “a high level of pilot skill under favourable conditions”; then do you land on 3300m?
If yes, what additional restrictions and guidance would be given? If operational managers cannot be assured that every pilot will be able to provide the high level of skill required on that day or in those particular circumstances (wee-one, you cannot determine “P1 non-factored landing distance’), then are they prepared to mitigate the consequences of landing (or overrun), because they, like the crew may have no other options when deciding to land.
Avoid wet runways?