The Earth
Distance
The Solar System & Time
Charts
Using Aeronautical Charts
Distance, Speed & Time
Fuel Planning
Practice Exam

Maximum Drift

The Navigational Computer is needed in the exam and can be used for pre-flight planning, but there is no place for it in the cockpit! Instead, we use Mental Dead Reckoning while airborne to simplify a lot of the calculations.

The Maximum Drift technique is not required for the exam but it is very useful for practical in-flight navigation.

Maximum Drift

Pre-flight we can calculate a number that is the maximum drift the aircraft will experience – i.e. the most it will be ‘blown off course’. This depends on the wind speed & the aircraft’s true airspeed. It can be calculated with the formula:

Maximum Drift Example

For example, if your aircraft’s cruise speed is 90 knots and the wind speed is 20 knots:

So in this case, the most the aircraft will drift due to the wind is 13° (unless the wind speed or aircraft airspeed changes).

Wind Components

The aircraft won’t always drift by the maximum drift amount.

If the wind is directly ahead or directly behind, there will only be headwind or tailwind and no drift
If the wind is a direct crosswind (90° from heading) then the drift will be maximum (13° in this example).

However, when the wind is somewhere between these two, we need to calculate headwind/tailwind and crosswind components – this is covered in the next two lessons.