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PPL Air Law
by Jon Williams - Thursday, 20 February 2014, 5:14 PM

Users of our products have brought to our attention that questions in the Air Law examination on flight levels flown in accordance with the Quadrantal Rule sometimes contain elements which are also a stiff test of a candidate's knowledge of altimetry.

Below is a question adapted from one sent in by a customer, together with our reasoning to arrive at the answer.

A pilot, flying VFR, is making good a true track of 084deg while correcting for starboard drift of 8°.

The magnetic variation is 6°W and the aircraft's compass on this heading displays a deviation of 2°E.

If the safety altitude is 3400 ft and the QNH is 1003mb, what is the lowest available flight level that the pilot can elect to fly at, if he does not wish to descend below the Safety Altitude.

Answer options

a) FL30
b) FL35
c) FL55
d) FL40

As you can see, in order to answer this question, a student requires a good knowledge of altimetry, and only an elementary knowledge of Air Law. There are also two distractors in this question. Here is our solution.

The straight Air Law consideration is simply that, above the Transition Altitude, it is recommended (it is obligatory for IFR) that a pilot flying in accordance with the Visual Flight rules fly at Flight Levels stipulated by the Quadrantal Rule (QR) which is based on his magnetic track (MT). The QR can be read off the Air law CD-ROM and states that for MTs from 000 -089° you fly at odd flight levels, from 090 -179° you fly at odd flight levels + 500, from 180 -269° you fly at even flight levels, and from 270 -359° you fly at even flight levels + 500.

Your question says that your aircraft "has a True Track" of 084° so we must assume that you are flying whatever compass heading is required to achieve that true track. With a variation of 6°W, your track would be 090° Magnetic. You should, therefore, above the Transition Altitude, fly at odd flight levels plus 500. So the answer has to be either FL 035 or FL 055 . That's really the Air Law bit.

But, of course, you now need to bring in your altimetry knowledge (a Met subject or Navigation subject).

You are given a Safety Altitude of 3400 ft. Safety Altitudes have nothing to do with VFR flight when you have to be clear of cloud and in sight of the surface. All you have to do is remain VFR and stay above the appropriate minimum height depending whether you are flying over a built up area, open air assembly or open countryside. But it is good airmanship, as a VFR pilot, to know what the Safety Altitude for your route is; so we'll assume ( a wrong assumption in real life, but required by your question) that you have to remain above 3 400 ft above sea level. You know that the QNH is 1003 mb , so we must presume that you must stay above 3400 feet on 1003mb. From that altitude you must now select an altimeter sub-scale setting of 1013mb in order to fly at flight levels. So, if you are at 3 400 ft and wind your sub-scale setting from 1003 to 1013 you will have an altimeter reading of 300 ft higher (1mb = 300 ft), putting you at an indicated altitude of 3700 feet. So with a QNH of 1003mb, the Transition Level would be FL35. But you aren't supposed (according to the questioner) to fly below your present indicated altitude; so as you have to fly at a flight level required by the Quadrantal Rule – i.e. odd flight levels + 500 - you should fly at FL 55.