Several inter-related aspects in recent posts.
Runway length per se relates to the landing distance required, which is referenced to "15m (50 ft) above the landing surface", taken to be the nominal 'threshold'. AMC CS 25-1592
Certification accepts (appears to assume) that there will be some variation in TCH re 'use of procedures' and the nebulous 'pilots of average skill'; as indicated by the distribution of wheel touchdown positions.
PAPI is an angular guidance system indicating the required flight path angle.
An approach flown with constant 3Red or 3White will have an angular offset, but still guiding to the same position on the runaway. The TCH for approaches with one light deviation will have different TCH, but not significant wrt to the range of crossing altitudes in normal operations i.e. normal flight path variations (beware combinations of extremes - lower designed TCH, low approach + wind-shear).
An non-standard approach 3W becoming 4W, flown parallel to the approach path (pseudo long body) requires judgement of the longitudinal distance relative to PAPI as to where the change in indications occur (2W - 3W - 4W).
Without real-time computation the height displacement from the PAPI flight path is unknown - a judgement, which if excessive can result in a touchdown further down the runway, invalidating landing distance calculations (risk of overrun). A mis judged deliberate offset vs normal variability.
Increasing the distance between PAPI and the threshold (higher TCH):
If the PAPI was repositioned SW on the existing runway 22 then the TCH would be higher. This would not reduce landing distance available (same threshold); but the hazard of the lip between runway / grass still remains - possibility of wheel brake damage as discussed.
Retaining the current PAPI location but extending the runway by moving the threshold of 22 NE would again increase TCH, and ironically increase the LDA - new threshold, but again the hazard remains.
However, retaining the existing threshold and adding hardstanding to the NE should reduce the hazard - less likely to occur, compare with 27 wheel marks, but this may not reduce the perception of 'safety risk' from an early touchdown 27 vs 22 - already discussed.
Thus if 'it' can happen, 'it' will happen, and if 'it' happens on 22, then the higher safety risk from the hazard.
This is an issue for operational awareness, but not within pilots capability to reduce the risk; that's the 'hard' task for the airport - remove / reduce the hazard.
Bloggs #50
swh, thanks for the info; don't forget that with the correct TCH eye ht for the flt deck, the wheels are behind and lower, and lower still when they cross the threshold.
BraceBrace "
The PAPI's are perfectly safe to be used by any 777 "2 reds, 2 whites", whatever the body length, " This is debatable; requires the equivalent data for 777 as in #50, and context re PAPI - threshold distance i.e. rwy 22 and grass.