Landing Performance and Landing Distance Requirements
The landing case
Landing performance requires that the landing distance required (LDR) does not exceed the landing distance available (LDA). LDR is measured from the 50 ft screen to a full stop, factored (typically ×1.67 for a dry runway, ×1.92 for wet/contaminated in older rules; modern CS-25 uses 1.15 on a dry runway with reverse thrust accountability and 1.43 on a wet runway). The candidate must know the mass used is the landing mass - MLM-limited or trip-fuel-limited, whichever is lower, with no allowance for fuel to be burned off.
Approach and reference speeds
VREF is 1.23 VS0 for most types (1.3 VS0 for some), VAPP = VREF + wind correction (usually not more than one-third of headwind, plus gusts). Landing distance increases roughly with the square of speed, so 10% extra speed costs about 21% more distance.
Runway condition
Dry, wet, contaminated (standing water, slush, snow, ice) each have separate factors. A wet runway with a slippery surface can require a braking action check; contaminated runways may demand the use of contaminated runway charts which are often not available and thus prohibit landing.
Worked example
LDR from the chart at 55 t = 1,400 m dry. Wet factor 1.15 gives 1,610 m; if LDA is 1,550 m the landing mass must be reduced until LDR × 1.15 ≤ 1,550, i.e. LDR ≤ 1,348 m.
Traps
Using TOW instead of landing mass; forgetting the 50 ft screen; applying the dry factor to a wet runway; ignoring slope (uphill helps, downhill hurts, typically 5% per 1% slope); assuming reverse thrust is always credit-worthy.