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

I want to walk you through the start of the Navigation Computer chapter on multi-drift winds and wind components. This is a critical operational skill for every pilot, and we're going to cover why we need these calculations, what the risks are, and how wind direction is referenced. Let's begin with the requirement for wind component calculations. As a pilot, you need to be able to calculate two things: the headwind component and the crosswind component. Let's take headwind first. Your threshold speed — that's the speed you cross the runway threshold at — is given as CAS, or Calibrated Airspeed. For all practical purposes in this context, that's the same as IAS, Indicated Airspeed. That CAS corresponds to a TAS, or True Airspeed. But here's the key point: stopping on a runway depends on the value of ½mv² — that's half times mass times velocity squared. The 'v' in that equation is ground speed, not airspeed. A headwind will reduce your ground speed at touchdown. Because you're touching down with less ground speed, you need less brake energy to stop within a given runway length. For a heavy aircraft on a short runway, there may even be a minimum headwind requirement — below that headwind value, it simply won't be possible to stop with an adequate safety margin. Now, crosswind can be even more critical. Let me explain why, step by step. First, with a crosswind, the pilot needs to line up on the approach using the extended centre line of the runway as a reference. You then point the nose into wind to lay off for the drift on finals — that means you crab the aircraft into the wind so your actual track stays aligned with the runway centre line. Second, at the moment the undercarriage is about to touch down, you need to apply rudder to re-align the aircraft with the runway. This manoeuvre is usually called "kicking off the drift" — but I want to be clear: it should be a controlled application of rudder, not an actual kick. Applying the correct amount requires a certain amount of skill and judgement. The greater the crosswind, the more difficult this problem becomes. Third, even if you judge this 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 at the moment of touchdown. Tyres are mainly stressed to take fore-and-aft loads — loads along the direction of travel — not sideways ones. It is possible to burst tyres on crosswind landings because of this sideways force. Fourth, once you've got the aircraft down, you may have trouble holding it on the runway, especially if the aircraft has a high tail fin. As the speed slows down, the rudder has less control authority — it becomes less effective — and may not be able to counteract the crosswind component acting on the rest of the fin. The aircraft may run off the runway. Because of all these risks, there will be a maximum crosswind component specified for the type of aircraft you're flying. That's an aircraft limitation. But 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 have rules 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, let's talk about a fundamental question: wind directions — true or magnetic? Any forecast wind is given in true direction. So any written meteorological information — that includes TAF (Terminal Aerodrome Forecast) and METAR (Meteorological Aerodrome Report) — will be in true. Forecast winds are used for planning navigation as well as landing, and navigation tracks are normally initially measured in true. However, any information used to help the pilot line the aircraft up on the runway will be given in magnetic, because runway directions are always magnetic. So a wind from an ATC controller will be magnetic, and the ATIS — Automatic Terminal Information Service — always quotes magnetic wind direction. Let me summarise that clearly: forecast winds from written sources are true; winds from ATC and ATIS for lining up on the runway are magnetic. That distinction is essential for your calculations.