Landing
Landing performance questions test the legal landing distance, the 50 ft screen, runway condition and the landing climb limits. The exam expects you to apply the certification factors correctly and to recognise when a runway is legally unusable at the planned mass.
Landing distance
For a Class A aeroplane under CS-25/FAR 25, the landing distance required (LDR) on a dry runway is 1.67 × the demonstrated landing distance from 50 ft at Vref. For a wet runway, the LDR is a further 1.15 × the dry LDR, equivalent to 1.92 × the demonstrated distance. Contaminated runways use approved contaminated data. LDR must not exceed the landing distance available (LDA).
Vref is the landing reference speed at the 50 ft screen, normally 1.3 Vs in the landing configuration for ATPL purposes. It is not a threshold speed to be held after touchdown, and reverse thrust is not credited in the dry certification distance unless the AFM specifically allows it.
Landing climb
Landing mass is also limited by climb. All engines operating, the landing climb gradient must be at least 3.2%. With one engine inoperative in the balked landing case, the minimum gross gradient is 2.1% for a twin, 2.4% for a three and 2.7% for a four. These limits can be more restrictive than the runway length at high altitude or high temperature.
Worked example
LDA is 2,200 m. The demonstrated dry landing distance is 1,300 m. Dry LDR = 1,300 × 1.67 = 2,171 m, which is within 2,200 m, so the landing is legal on a dry runway. On a wet runway, LDR = 2,171 × 1.15 = 2,497 m, which exceeds 2,200 m. To use the wet runway, reduce mass until the demonstrated wet distance is no more than 2,200 / 1.92 = 1,146 m, or use approved wet data.
Traps that cost marks
Forgetting the 1.67 factor; applying 1.15 to the demonstrated distance instead of to the dry LDR; using TORA or TODA instead of LDA; assuming reverse thrust is always credited; ignoring the 50 ft screen; and checking only runway length while missing the landing climb limit. Runway slope and braking action also change the demonstrated distance, so the chart basis must match the actual condition.