The Navigation Computer - Multi-drift Winds and Wind Components — Page 182, Lesson 157

Let’s start with the why behind all of this, because it drives everything else. Pilots need to be able to calculate two wind components: the headwind and the crosswind. Let me take headwind first. The threshold speed — that’s the speed at the runway threshold, the start of the landing runway — is given as CAS, or Calibrated Airspeed, which for all practical purposes we treat as IAS, Indicated Airspeed. That CAS corresponds to a TAS, True Airspeed. But here’s the key: stopping on a runway depends on the value of ½mv² — half the mass times the velocity squared. That’s the kinetic energy. The ‘v’ in that formula is ground speed, not airspeed. A headwind will reduce the ground speed at touchdown, which means less brake energy is needed to stop in a given runway length. So for a heavy aircraft on a short runway, there may be a minimum headwind requirement — below that headwind, the aircraft simply cannot stop with an adequate safety margin. Now crosswind — this can be even more critical, and there are several reasons. With a crosswind, the pilot lines up on the approach using the extended centre line of the runway as a reference, then points the nose into wind to lay off for the drift on finals. At the moment the undercarriage is about to touch down, the pilot must apply rudder to re-align the aircraft with the runway. This is usually called “kicking off the drift” — though it should be a controlled application of rudder, not a kick, and it takes skill and judgement. The greater the crosswind, the harder this becomes. Even if the pilot judges it perfectly and the aircraft is precisely lined up with the runway at the moment the tyres make contact, there will still be a sideways velocity. Tyres are mainly stressed to take fore-and-aft loads, not sideways ones — so it is possible to burst tyres on crosswind landings. And finally, once the aircraft is down, the pilot may have trouble holding it on the runway — especially if the aircraft has a high tail fin. As the speed slows, the rudder has less control authority and may not be able to counteract the crosswind component acting on the rest of the fin. The aircraft may run off the runway. For all these reasons, there will be a maximum crosswind component specified for the type of aircraft. Companies may also apply their own rules, quite apart from the aircraft limitation. For instance, an aircraft may be cleared to land in up to 25 knots crosswind, but a company might rule that the first officer may only land with a crosswind of up to 15 knots — above that, it has to be the captain’s landing. Now, one crucial distinction: wind directions — true or magnetic? Any forecast wind is given in true direction. So any written meteorological information — TAF, METAR, and so on — is in true. Forecast winds are used for planning navigation, and navigation tracks are normally initially measured in true. But any information used to help the pilot line the aircraft up on the runway is given in magnetic, because runway directions are always magnetic. So a wind from an ATC controller is magnetic, and the ATIS always quotes magnetic wind direction. So to summarise: forecast winds — true. Runway alignment winds — magnetic. That distinction is the foundation for everything we’re about to do with the navigation computer.