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A pilot wishes to turn left on to a northerly heading with 10° bank at a latitude of 50° North. Using a direct reading compass, in order to achieve this he must stop the turn on an approximate heading of…

  • A
    030°
  • B
    355°
  • C
    330°
  • D
    015°

Refer to figures.
This is a question from the old syllabus, numerical examples such as this should not appear in the current exams, if they do, appeal!

  • Step 1: Find the direction of the turn – clockwise or anticlockwise?

The aircraft turns to the left onto a northerly heading => anticlockwise.

  • Step 2: Use the acronym ‘UNOS’ to find if you will overshoot or undershoot?
UN – Undershoot = apparent turn to the North.
OS – Overshoot = apparent turn to the South.


By banking in an anticlockwise direction to 000° (northerly heading), you will need to undershoot by a certain amount if you want to stop your turn exactly at 000°.

  • Step 3: Find the angle by which you will need to overshoot/undershoot by using the following formula (Refer to diagram 1):
(Bank angle + Lat) ÷ 2


Remember that the turning error is greatest on a Northerly/Southerly heading. Therefore any time you turn through the North or South the error will be the highest, and any time you turn through West or East the error will be the smallest. Consequently, when your heading is between North and East or South and East, etc. (half-way between two cardinal points) the turning error will be only half as big or twice as big as at these headings. Only in this case, you will use the following formula (Refer to diagram 2) :

(Bank angle + Lat) ÷ 4


In this question, the aircraft turns to the left onto heading 000°, so it’s turning from an easterly heading (no error) onto a northerly heading (biggest error). As we are not turning on a heading that is midway between two cardinal points, there is no need to apply the second formula.

Therefore, you will need to overshoot the heading by:
(Bank angle + Lat) ÷ 2 = (10 + 50) ÷ 2 = 30°

  • Step 4: Add/subtract the angle found in step 3 to find the angle at which you should stop the turn so that the compass indicated the desired heading:

The desired heading is 000°, and in order to stop the turn at 000°, you need to undershoot this heading by 30°:000 + 30 = 030°

Note: We are adding here because undershooting 000° means stopping the turn before passing this value on your compass. Since you turn anticlockwise, the heading values decrease, so you need to add the angle found in step 3 to find the correct undershooting angle.

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This question has appeared on the real examination, you can find the related countries below.

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