Tiburon peligroso,
The aircraft will be flying a flatter approach.
There are a few ways to look at this. Imagine a perfectly flat surface with a very long runway.
The first aircraft makes a 5.5 degree approach.
The aiming point is the marks but the trajectory of the aircraft is partly down and partly along the runway. Changing the trajectory at the roundout (rapid increase in drag) uses energy. Therefore assuming perfect technique the aircraft main wheels will impact the ideal touchdown point and the aircraft will at that moment have say X amount of energy pointing along the runway (X has to be absorbed by brakes etc to stop).
Next approach 3 degree slope the aiming point is the same as above. There is less downdard component to the trajectory and more of a horizontal component. Again assuming the pilot uses the perfect technique (less of a roundout - uses up less energy), the aircraft will again impact the ideal point but with more than X energy along the runway. So stopping distance will be longer.
Finally, the aircraft flies just less than horizontally to the runway. Say 0.5 degree approach angle. In this case, there is very little downward energy. Most of the energy is along the runway and with no roundout there is not using up of the energy and so we have the highest amount of along runway energy to be absorbed by the brakes.
The certification requirements are very clear at what type of approach must be used and at what speed for testing the landing performance. This prevents the test pilto from using an idle power steep approach with a big roundout to acheive very short distances.
Next think of the above 3 cases while remembering that the basic Landing Distance is measured from below the point where the aircraft is at 50ft to stop. That would be a very long distance in the last example.
Perhaps the easiest way to understand the principle is to drop a stone vertically onto the ground at an impact speed of say 60Kt. Landing distance = zero.
Now shoot the same stone on an almost horizontal angle so that it again impacts the ground at 60Kt. Measure the distance from where is touches the ground until it skids to a halt. Same stone, same speed, Bigger distance.
As for the PAPI.
If they are all white then you can not tell your approach angle other than it is steeper than the set slope. You could even be in level flight or climbing. If they are all red then the same applies but lower then the set slope.
With 3 white and 1 red, one can check Annex 14 and see what the possible range of angles are which provide that indication. Same goes for 3 red and 1 white.
However, provided the PAPI are not all red or all white and not changing indication then the aircraft is travelling towards the aiming point and the aim point should be a constant angle below the horizon.
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Surely any indication (whether PAPI or ILS GP) that the a/c is low on approach should be acknowledged and corrected for unless specific chart notes or NOTAMs specify otherwise?
Bob,
You know that. I know that. Does the litigation expert who finds "disregard the PAPI indications" on a company instruction after an aircraft hits an obstacle care? They would laugh all the way to the bank with a clear unqualified instruction from the company to disregard a piece of safety equipment!!
Even if it did not have a direct link with the accident they could use it to show that management expected pilots to disregard safety warnings.
