OK, I have to admit to a little subterfuge. I wanted to ask a question which I thought might be a little embarrassing and so, rather than ask the tricky question straight out, I hid it inside a quiz. For all those who took part in our recent runway marking quiz, I thank you for helping out and apologize for the less than straightforward approach.
Some while ago I wrote a blog about adjusting the track position of an aircraft which does not have accurately recorded latitude and longitude. When the position is inaccurate, we try to refine the position by placing the landing aircraft on the runway centreline. (For this discussion, we will assume that the aircraft is landing at a long runway, but without using an ILS). Knowing the ends of the runway allows us to correct for lateral position quite well, but that still leaves us not knowing the position of the aircraft along the runway.
When I originally wrote the algorithm for this I took the vertical profile of the approach from 500ft to 200ft and assumed that the pilot would be flying on a line that took him to the touchdown point, 1000ft from the start of the runway. This went into use for some years and gave fair results.
Over time it became apparent that, quite often, the ground track showed the aircraft turning off the runway past the appropriate taxiway. We tried to find an error in the arithmetic, but could not trace the difference to integration, scaling or any other type of error. We then found that the ground track for one operator was consistently about 500ft, and the suggestion was that the pilots were aiming at a point 500ft from the runway start, not 1000ft.
I needed to find out where pilots aim to land when approaching a runway. Simply asking this outright would be too obvious, so we “hid” the question in a set of other questions, hence the quiz. Again, I would like to thank the 40 people who replied, and the six pilots who visited our office recently and were given the verbal version of the quiz!
While pilots all know what the stopway and runway designation look like, and almost all know the runway threshold marking, almost half get the first touchdown zone marking muddled with the aiming point marking. Evidence from the data implies that the actual rate may be higher (or it may be that the pilots who fly visual approaches are more likely to aim at the first touchdown zone marking).
For this reason, our analysis algorithms have been altered to set the visual approach aiming point 500ft from the start of the runway, and we will see how this works out in practice.