Bernd, thank you for the clarification. Indeed responsibility for clarification appears to be in the first instance with Airbus, and perhaps second, with the regulators; - my poor choice of words in not leaving these options open. Re ‘qualified’ – in the sense of not being explained in sufficient detail or in context – bounded or limited.
IMHO a manufacturer’s recommendation provides guidance and is not a limit, but I recognise that many interpretations (including some regulators) mandate recommendations.
I still don’t follow the description of the ‘immediate’ activation of brakes with MAX, nor de-rotation (what about low torque with antiskid on a low friction runway?), but this not for discussion here.
The Airbus advice ‘to use MED braking’ (MAX not recommended) could be interpreted in a way which might reduce the safety margin during landing. Consider the following:-
Landing distance is based on the unfactored distance plus a safety margin.
The unfactored distance is a theoretical minimum, using maximum braking etc, etc.
The safety margin … “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.” (
UK AIC 14/2006)
Some of the safety margin may be used in ‘normal braking’ … “Passenger comfort and brake wear considerations generally induce a reluctance to utilise the aeroplane's full braking potential. This is acceptable where other factors affecting stopping performance are favourable, for example when the braking action is good, and when the landing distance available is clearly not limiting. However, reduced braking will result in the erosion of the factors built into the scheduled landing distances and is not appropriate where the margin has been eroded for other reasons. ”
The second quote covers the key issues of limiting distance and eroded safety margin, e.g. tailwind limited distance / poor braking action.
If auto brake MED is equated to ‘reduced braking’, then the Airbus recommendation appears to lack any advice about other situations such as operating with limiting landing distances. In this instance a clear instruction to use manual brake in unfavourable conditions might be appropriate to alert crews to evaluate the need to change procedures. Perhaps this is documented elsewhere, or considered to be knowledge within ‘airmanship’ – an assumption (mine, the manufacturer, the industry?).
IMHO the AIC text provides a clear rationale of when not to use ‘reduced braking’, i.e. when not to use auto brake settings other than MAX. If pilots, do not interpret the Airbus advice in this way (nothing says that they should), then the continued use of MED auto brake in unfavourable conditions might reduce the overall margin of safety during landing, and furthermore mask or delay the identification of a deteriorating condition as discussed previously.
This view identifies with the operating differences between Airbus and Boeing (or other auto-brake aircraft), where MAX can or cannot be used ‘routinely’ (a consideration in decision making).
Perhaps the origins of this problem are beyond the immediate FCOM documentation, and come from training or guidance in SOPs; either of these can lead to bias in decision making or an inappropriate mind set based on the habits of normal operations.
A solution, in addition to clarifying procedures, might be to consider how to transfer knowledge, particularly the ‘know how’ which forms experience as related by – NVpilot (#381), FlexibleResponse (#392)
Such are the problems of explanatory materials, recommendations, assumptions, and the choice of words.