Navigation – Using Mental Dead Reckoning to Calculate Drift

The Dalton Computer or Whiz Wheel is certainly a very good tool, perhaps better than an electronic calculator because it is visual and allows you to check your answers “by inspection”.  A much easier technique however, is Mental Dead Reckoning.  With practice MDR is easy and quick. In this short blog, I will develop the theory from the last blog “Navigation – intro to MDR” enabling you to use MDR to calculate drift.

Recap – Clock System

I explained in the last blog that using the clock system to calculate the cross wind component is straight forward. Measuring the wind angle from the nose of the aircraft or runway, up to 60 degrees you use the fractions of a clock face.  So 15 degrees would be 1/4, 20 would be 1/3, 30 1/2 etc.  This gives the crosswind component and is reasonably accurate.

Finding Drift

If we divide wind speed by the aircrafts TAS in nm per minute, we get maximum drift.  If we then apply the clock system, based on track, we can learn what fraction of maximum drift we can expect.  Much easier than using a Whiz Wheel! Typically we fly light  aircraft at speeds to facilitate this i.e. 90 knots (1.5 nm per minute) or 120 knots etc.

Examples

We plan to fly at 90 knots TAS (we will look at converting IAS to TAS in a future article, for now presume IAS and TAS are equal). This is 1.5 nm per minute.  The wind vector at our planned height of 2,000′ is 270/15.  Divide 15 by 1.5 = 10, so maximum drift is 10 degree.

If our track is 360, then the required heading would be 350 degrees because the wind angle is 90 degrees (60 or more we use max drift = 10)
If our track is 330, then  the required heading would be 320 degrees because the wind angle is 60 degrees (60 or more we use max drift = 10)
If our track is 300, then  the required heading would be 295 degrees because the wind angle is 30 degrees so we use half the max drift =5
If our track is 290, then  the required heading would be 287 degrees because the wind angle is 20 degrees so we use 1/3 the max drift =3

In the next blog we will develop this further to calculate groundspeed

Happy flying!

Adrian.Willis@BritishAerobaticAcademy.com