Learning hub · Performance · Lesson 3

Runway surface, slope and crosswind limits

The runway under you is part of the performance. This lesson covers surface, slope and wind, how to work out a crosswind component, and why a pilot adds a margin to the figures in the book.

About 12 minutes, plus the quiz. Part of Performance. It builds on Climb, cruise and landing performance.

1. The surface

Flight manual take-off and landing figures usually assume a level, dry, hard (paved) runway. Any other surface changes them.

• Grass, soft ground, mud, snow and standing water slow the acceleration on take-off, because the wheels cannot roll freely. Long or wet grass is worse than short, dry grass.
• Wet or contaminated runways reduce friction, so braking is less effective on landing. With aquaplaning (hydroplaning) the tyres ride on a film of water, and braking and steering can be almost lost.
• Ice is worse again.

Many manuals give no correction at all for these conditions. The pilot must allow for them. Treat any non-standard surface as a reason to add distance.

2. Slope

A runway slope is given as a percentage: a 2 per cent slope rises or falls 2 m in each 100 m. An upslope slows the acceleration on take-off, so the run is longer. A downslope helps it. On landing the effect reverses: an upslope helps to stop the aeroplane, and a downslope makes the landing roll longer. The effect of a small, ordinary slope is relatively small, but it adds to other penalties such as wet grass and a tailwind.

Slope and wind can conflict, for example a downhill runway with a tailwind, so look at the sum, not at each factor alone. Runway slopes are in the AIP aerodrome pages.

3. Headwind and tailwind

Wind changes the groundspeed at lift-off and at touchdown, so it changes the distances. The effect is not symmetrical: a tailwind costs more distance than the same headwind saves. An American handbook gives an example: a tailwind of 10 per cent of the take-off speed lengthens the take-off by about 21 per cent. Take an invented aeroplane that lifts off at 50 kt. A 5 kt tailwind is 10 per cent of that, so a 300 m take-off becomes about 300 × 1.21 = 363 m.

If you must use a tailwind runway for another reason, go through the figures again and consider waiting. Where a manual allows credit for headwind, do not count all of it, and count any tailwind in full or more. How much credit to take is set by the manual and your school or operator; see also the take-off lesson.

4. Crosswind and limits

The wind rarely blows straight down the runway. You split it into two parts: the headwind component along the runway and the crosswind component across it. If the wind is a speed W at an angle a off the runway heading:

crosswind = W × sin a and headwind = W × cos a

A rule of thumb for quick estimates: at 30° off the runway about half the wind is crosswind, at 45° about 70 per cent, and at 60° or more about 90 per cent. Round up, never down. Your chart or flight computer gives better figures.

Example (invented): wind 20 kt, 30° off the runway. Crosswind = 20 × 0.5 = 10 kt, and headwind = 20 × 0.87 = 17 kt. At 60° the same 20 kt wind is 17 kt across.

Many flight manuals give a maximum demonstrated crosswind. This is the crosswind that a test pilot showed the aeroplane could handle in the certification tests. In some flight manuals it is listed as a limitation and in others only as information, so read your own, and in either case it is not a promise that you can handle it. American handbook advice, which is also good practice in Europe, is to set personal limits well below it until you have practised on the type, and to lower them again when you are tired, current less often or landing on a wet or narrow runway. Gusts count: use the gust speed in the sum, not just the mean wind.

5. Why use the tables, and why add a margin

Rules of thumb show how a factor behaves, but each aeroplane is different: the same 10 per cent change in mass or the same wet grass can cost more or less on another type. The manual tables, drawn from tests, are the starting point. The book figures were achieved with a new aeroplane, an engine in good condition and a skilled pilot, so a margin on top is wise. Many operators add a margin, and the figures differ.

For guidance in the UK, the CAA General Aviation Safety Sense Leaflet 7, Aeroplane Performance (as quoted in AAIB accident reports, for example on G-HERB in 2009) recommends applying a factor of 1.33 to the unfactored take-off distance and 1.43 to the unfactored landing distance on all flights, including private ones, although it is guidance and not a legal requirement for a private flight. For landing on wet grass up to 20 cm on firm soil, the same source says the landing distance may rise by 35 per cent, and by up to 60 per cent on very short wet grass. The factors are multiplied together. These are UK guidance figures: your school or national authority may use different ones, so ask.

A sample (invented): the manual gives 350 m from 50 ft. Wet grass: 350 × 1.35 = 473 m. Then the 1.43 factor: 473 × 1.43 = about 676 m. A 600 m LDA is too short for that landing.

Training only. Use the flight manual and the rules of your school or operator for real flights. If the numbers do not fit, delay, reduce the load or choose another runway.

6. Try it yourself

Work each one out on paper first, then open the answer to compare. The numbers are invented.

1. The wind is 25 kt, 40° off the runway heading. Use sin 40° = 0.64 and cos 40° = 0.77. (a) What are the crosswind and headwind components? (b) The manual shows a demonstrated crosswind of 15 kt and your personal limit is 10 kt. Do you land on this runway?
   Show the answer

   1. (a) Crosswind = 25 × 0.64 = 16 kt. Headwind = 25 × 0.77 = 19 kt.
   2. (b) 16 kt is above both the demonstrated 15 kt and your personal 10 kt. No. Choose a runway nearer the wind, divert to another aerodrome, or wait.
   3. If the wind is gusting, the gust figure is larger still.
2. An invented aeroplane needs 350 m from 50 ft on a dry hard runway. You plan to land on wet grass. Apply the 35 per cent wet grass factor and then the 1.43 overall factor, as in the lesson. The LDA is 600 m. Is the landing acceptable?
   Show the answer

   1. Wet grass: 350 × 1.35 = 472.5 m.
   2. Overall factor: 472.5 × 1.43 = about 676 m.
   3. The LDA is 600 m, so the factored distance is 76 m too long. The landing is not acceptable by this method. Use a paved runway, or wait for a drier surface.
3. An invented aeroplane lifts off at 55 kt and needs 300 m in still air. The only runway has a 5.5 kt tailwind. Using the 10 per cent rule from this lesson, what distance should you expect? The runway has 330 m available. What do you decide?
   Show the answer

   1. 5.5 kt is 10 per cent of 55 kt, so the take-off is about 21 per cent longer.
   2. 300 × 1.21 = 363 m.
   3. The runway offers only 330 m, so it is not enough, even before any margin. Do not take off. Wait for a change of wind or use a longer runway.

7. Watch

A short video on what the maximum demonstrated crosswind means and how to set your own limit. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.

Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.

More to watch

A second video, from the Australian regulator, on avoiding runway excursions. It is Australian, so the rules it mentions are not European; use it for the principles.

Open on YouTube if the player does not load. Not made by VectorEFB.

8. Check yourself

Five original practice questions. Answer them, press Check answers and read each explanation.

Next: Fuel mass conversions and a loading sheet. Back to the learning hub.