Aviation safety requires avoidance of unnecessary risk; risk depends on the hazard, frequency of encounter (likelihood), and the severity of the consequences. The process (risk assessment) requires judgment, which in turn requires dependable knowledge and appropriate skills of thought.
A safety factor provides a buffer between the hazard and the planned operation. It is a margin for ‘error’ or deviation from the ideal to minimize the risk(s) and/or alleviate consequences of the hazard.
Haroon discusses
’ the ‘ safety factor (#34), but the issue is
’ a ‘ safety factor, relating to a specific situation and the assumptions made about human behavior/capability (in that situation). Hence, in an emergency (change of situation), the landing-distance safety-factor can be reduced. Note that although a wet landing uses a landing–distance factor of 1.92 it is not necessarily the same safety factor as for a dry runway as the increasing number of the accidents indicates.
Thus, the debate is firstly whether the event is an emergency or not – it’s not; and secondly if an over weight landing were to be attempted what distance is required to provide a margin of safety
equivalent to a max wt landing on a dry runway.
EU-OPS 1.475 requires a 1.67 factor (normal operation – in flight replanning).
The QRH actual landing distance (5700ft) is probably shorter than the certificated actual landing distance because reverse detent is used (I assume that this is idle, vice ‘max reverse’ quoted elsewhere in the QRH).
Thus by adding 220ft (QRH adjustment for no reverse), the certificated actual (overweight) distance is ~ 6000ft.
A 1.67 factor would require ~ 10,000ft runway.
However, this calculation does not consider other / new risks such as brake energy limit, brake fade, landing gear limits (vertical rate and side loads).
As all risk assessments should be specific to the situation, the calculations for the overweight landing above does not use an equivalent basis as a for max wt landing, and it is these differences which might sway the argument for not landing overweight without an emergency as the aircraft is not certificated for this on a regular basis.
Re
The flight safety foundation have a good briefing. (
F E Hoppy), is this the one?
http://www.skybrary.aero/bookshelf/books/874.ppt
A recent FSF report outlines the nature of the risks, but provides few practical remedies -
http://www.skybrary.aero/bookshelf/books/900.pdf
A similar but more practical document is
‘Runway excursions’ by ASTB.
For some ‘How To’ aspects – Judgement etc see
Aviation.org, - ‘library’ section, presentations on Critical Thinking, Situation Awareness, and Decision Making (free registration required).