032 Performance topic guide
Landing Distance and Dispatch Factors
The actual landing distance, ALD, is what the aeroplane demonstrates in a landing, from the screen height on approach to a full stop, flown to a defined technique with no unusual margin built in. Regulations do not let dispatch planning use that raw number directly: a factor is applied so that ALD is never allowed to reach a set proportion of the landing distance available.
For a turbojet, ALD must not exceed 60 per cent of LDA, which is the same as saying the required LDA is ALD divided by 0.6. For a turboprop, the figure is 70 per cent, so required LDA is ALD divided by 0.7. A wet runway adds a further 15 per cent on top of the dry requirement, because the same factors that stretch a landing roll, reduced braking friction chief among them, are exactly what a wet surface makes worse.
Actual versus required landing distance
ALD comes from flight test data for the specific aeroplane, in a defined configuration and with a standard technique, so it already reflects the aircraft's aerodynamics and braking hardware. It is not, by itself, a safe planning figure.
The dispatch factor exists to absorb the difference between a demonstrated distance flown by test pilots to a fixed procedure and the more variable distances flown in normal line operations, so the aeroplane is never planned to use the runway down to the last metre.
The dispatch factors
The two aircraft-type factors are not interchangeable, and using the wrong one for the wrong type is the single most common slip in this topic: turbojet, dry, required LDA equals ALD divided by 0.6; turboprop, dry, required LDA equals ALD divided by 0.7.
For turbojets, the wet runway penalty is a further 15 per cent added to the already-factored dry requirement: required LDA equals the dry requirement multiplied by 1.15. The wet addition is applied on top of the dry factor, never to the raw ALD, because the dry factor and the wet penalty protect against two different things, normal line-operation variability against the specific loss of braking effectiveness on a wet surface.
What changes the raw ALD before any factor is applied
Before any dispatch factor comes into play, the actual landing distance itself changes with the conditions of the day: a tailwind component lengthens it and a headwind component shortens it, an uphill slope shortens it and a downhill slope lengthens it, an increase in pressure altitude lengthens it because true airspeed for a given indicated speed rises, and an increase in mass lengthens it because more kinetic energy has to be dissipated over the same distance.
Every one of these adjustments is built into the ALD before the 60 or 70 per cent rule, or the wet addition, is ever applied on top, which is why the order of operations always runs from raw distance, to aircraft-type factor, to wet or contaminated addition.
Worked example
Worked example: a wet turbojet dispatch distance
Not quite. The correct answer is C.
Correct answer: C
- A. This is the dry requirement, 1200 divided by 0.6, with no allowance made for the wet runway.
- B. This applies the 15 per cent wet addition to the raw 1200 m ALD instead of to the already-factored dry requirement.
- C. Correct. Dry requirement first: 1200 / 0.6 = 2000 m. Then the wet addition: 2000 x 1.15 = 2300 m.
- D. This is the raw ALD, with neither the dispatch factor nor the wet addition applied. A demonstrated distance is never used as the dispatch minimum on its own.
Step by step
- Apply the turbojet dry factor first: required dry LDA = ALD / 0.6 = 1200 / 0.6 = 2000 m.
- Apply the wet penalty to that factored figure, not to the raw ALD: 2000 x 1.15 = 2300 m.
- State the answer: the runway must offer at least 2300 m of landing distance available.
- Sanity check: 2300 m is larger than both the raw 1200 m ALD and the 2000 m dry requirement, since both steps only ever add margin, never remove it.
Common mistakes
Using the wrong aircraft-type factor
60 per cent for turbojets and 70 per cent for turboprops give different required distances from the same ALD, and swapping them changes the answer by a wide margin, not a rounding error.
Applying the wet addition to the raw ALD instead of the factored dry distance
The two adjustments protect against different things and stack in a fixed order: aircraft-type factor first, then the wet or contaminated addition on top.
Treating the demonstrated ALD as a usable planning figure
ALD is a flight-test result, not a dispatch minimum. Any answer that stops at the raw ALD has skipped the entire factor structure the topic is testing.
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Last reviewed July 2026