MFS Re “
… the 60%/67% factor applies only to planned operations. Once you are en route, changes to circumstances (weather, aircraft failures, runway availability) may mean you don't have the margin available any more, which is acceptable - indeed, it's the whole point of the margin in planning, to account for these circumstances.”
This doesn’t appear to be the interpretation of the UK CAA –
Large Transport Aeroplanes - Landing Performance paras 2.3, 2.4, & 7.1, 7.2.
The inference in the AIC (assumptions) is that in normal circumstances, an in-flight reassessment should aim to keep the same margin of safety as calculated before take off.
The 60 – 67% factor is not a margin of safety as implied by some; it is a ‘field length factor’ for determining the landing distance required(LDA), which provides an excess distance over the theoretical minimum; this margin provides an acceptable level of safety in normal operations.
In ideal conditions the margin (40% excess) is unlikely to be achieved even when considering the variables which the pilot can control (speed, touchdown position). The unknown factors and error in reporting (runway texture, rubber deposits, wind strength), further reduce the margin of safety.
When landing on a wet runway, the margin, probably already a smaller % distance than dry, is also reduced by ‘unknowns’, e.g. depth of water, surface composition, or other contaminants (rubber, dirt, paint). These reductions are judged as acceptable up to the boundary of ‘flooded’ or ‘contaminated’ depth of water (3 mm, 1/4 in), i.e. the ‘normal’ margin of safety is reduced, but is judged (by the regulations) acceptable although in practice it might be a very small distance. However, the hidden assumptions in deteriorating wet conditions might include mitigation of the risks with close attention to touchdown speed (and wind), touchdown position, and braking level.
The FAA view in
AC 91-79 is similar to the UK AIC, but with some ambiguities.
A particular point of interest is that the FAA ‘15%’ additive “
is intended only to account for slight variations in achieved performance” (para 6) and thus crews might have to consider other risk-increasing variables – runway surface type, texture, depth of water, tyre tread, contaminants (dirt, dust, rubber,) in their calculation of LDR. Note the preceding text in Para 6 d (2).
The example in Table 3 shows how a nominal 3000 ft can become 5660 ft dry before adding a wet factor (I don’t think that this 15% is necessarily the same 15% as elsewhere in para 6).
Similarly, it is not clear how the manufacturers’ recommended approach and landing speeds above Vref should be accounted for. Some may ‘hide’ the excess (500ft/10kts) in the safety margin, thus further reducing the 15% ‘margin’, whilst others adjust the distance to retain some safety margin.
For in-flight assessment, some manufacturers publish advisory data which includes the use of reverse thrust; this from the experience of normal operations might give a false sense of security – certification may not give any credit for reverse due to it’s ‘availability’.
With in-flight failures, it may be possible to reduce the safety margin. This suggests that first you know what the normal margin is, what the changes could be, and hence how much it can be changed. Second, the crew have to ensure that any mitigation of the increased risk (accurate Vref, ‘short’ touchdown, max reverse) will be achieved – if not, go around: are you always able to GA with the failure?
Landing is a ‘risky’ operation. Justification of the decision should be supported by the best possible knowledge of landing operations, e.g. how the certificated performance is obtained, what the dispatch requirements mean, what assumptions are associated with published performance, and what the safety factors may/may not consider.
There are many other practical aspects relating to a particular situation, requiring good situation assessment.
Also, have some scepticism about what is reported about the runway condition and the braking action; beware of PIREPS.
IMHO, aiming to have the factors required for dispatch would provide a reasonable safety margin. The better option is to ‘know before you go’, and thus for unforeseen in-flight circumstances pilots might wish to prepare a quick reference guide to landing performance for a range of general conditions.
Managing Threats and Errors During Approach and Landing.
Flight Safety Foundation ALAR Additional Resources chapters 7 & 8.
Running out of runway.