If we were to plot the take-off distance for any given mass, against the flap setting from zero to maximum (landing flap) we would see something akin to a drag curve. High at each end and lowest somewhere in between. With zero flap the lift-off speed is high, so it takes a long time to accelerate to Vlof. With landing flap Vlof is lowest but there is a lot of drag so the acceleration rate is reduced. This again means that a long distance is required to accelerate to Vlof The minimum distance would be required when using the flap setting which gives the best trade-off between reduced Vlof and increased drag.
If we assume that we normally use this optimum flap setting, then any other setting wil increase take-off distance. So the answer is option A.
This is not just a Cathay Pacific problem, it is typical of the kind of thing that is asked in the JAR ATPL(A) Performance exams. But the questions in these exams also tend to include the effects on climb limited take-off mass and/or obstacle limited take-off mass.
Last edited by Keith.Williams.; 16th Nov 2003 at 02:31.