Originally Posted by
safetypee
The discussion on PAPI, #142 onwards is intriguing; as is the continuing views of ‘normal’.
Adding to the PAPI issue, is the setting of the angular change between white and red, which adjusts the sensitivity of the aid; e.g. crossing the road 3R 1W - a ‘minor’ (unseen) transgression is a small height difference on a ‘tight’ setting, but a larger height difference on a ‘wide’ setting. Not forgetting that this relates to the height of the cockpit, and not the wheel height judged by video.
If aiming to land at the normal touchdown point is safe, then why advise to aim short of normal on a ‘wet’ runway, if it is ‘less safe’, (increase undershoot risk vs perceived increased risk in landing distance for a wet runway). ‘Wet’ is one view, ‘contaminated’ another, but not the same.
This is another SOP double bind; the procedure makers move responsibility down to the crew - no right answer, except by judged outcome.
Where does the perception that risk is increased on wet runways come from; the published landing distances show adequate margin.
[One view is that wet runways landing distances do not always have equivalent distance margins as for dry - a view which I subscribe to, debatable elsewhere - , but not requiring a change in aiming point.]
If the operation necessitates anything other than normal operations with more restrictive tolerances, then the public safety is being compromised by the workaround. We used to operate a baby B747 into an interesting airport, and there were very specific criteria for completing a touchdown, but the flight path, MEHT/TCH and Vapp were not altered, it was an exactly standard flight profile, but with very specific decision tolerances.
When a decision to alter the procedure due to a perception of a specific threat arises, then the answer is to revisit what it being attempted. It should not be up to the pilot to use an untrained, and unapproved technique in order to satisfy perceptions of risk, there be dragons in that direction, and the liability reverts to the crew, well, they are always the first at the accident site anyway.