View Full Version : Radial intercept rules of thumb?

Banana Joe

22nd May 2018, 11:36

I was wondering if there's a simple rule of thumb to use on the go to 'eye-ball' by how many degrees of CDI deviation to anticipate the turn to intercept the required radial or QDM/QDR based on ground speed, distance from the station, and intercept angle.

Simple stuff, not mathematicaly exact but good enough to fly by reference to instruments.

what next

22nd May 2018, 17:30

Google for "angle of lead" (many of the search results will point towards this very forum...). Seems to work but I would recommend to do it often enough so as to develop an intuitive feeling for getting it right without applying any rules. Same for descent planning.

Tinstaafl

23rd May 2018, 00:52

If you're using a HSI then there's an easy way: Keep the HDG lubber line touching the top of the CDI. The closer the CDI moves towards the centre, the intercept angle will reduce. Eventually the intercept angle will equal zero when the CDI reaches the centre.

There are a couple of caveats though:

1. If there is a wind from the other side of the radial ie a headwind component during the intercept, then eventually the radial closure will stop when the intercept angle equals the wind correction angle. Not a big deal - you'll just have to recognise it and then increase the intercept angle a bit to get the CDI centred. But then the advantage is that once it's centred you'll already know what WCA to lay off to anticipate the drift.

2. If a tailwind component then the CDI could move quite rapidly, needing a fair amount of turn rate from you, and you will also have to anticipate the required WCA by turning a bit more as the CDI centres.

Ascend Charlie

26th May 2018, 10:41

You can use the 1:60 rule. If your turn radius is 1nm, then at 10 nm from the station, it would be 6 at 60nm, so call it 6 degrees.

For 2nm turn radius, allow twice as much, Einstein. And that is presuming you are tracking the arc, at 90 degrees to the oncoming radial. If you are at a 30 degree intercept, use half the lead angle.

Any other angle, pluck a number out of your fundamental orifice and have a go.

But try to keep your heading line between the fixed part of the CDI line and the moving part of the CDI.