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11. Aeroplane aerodynamics, structures and systems

A compass has a residual deviation of +1 degree (1 degree east). To fly a magnetic heading of 180 degrees the pilot must steer

  • 181 degrees.
  • 180 degrees.
  • 179 degrees.

Explanation from PART66Online

Deviation is named east or west according to the side of magnetic north on which compass north lies, and easterly deviation is counted as plus. Easterly deviation makes the compass read low, so the compass heading for a magnetic heading of 180 degrees is 180 minus 1, which is 179 degrees. Steering 181 would apply the deviation the wrong way, and steering 180 would ignore it.

G

GEO9816 asked · 2 Aug 2026

Hello , i am thinking that the correct answer in this question is 181 degrees .If the deviation is +1 dergee that means that if the compas heading to 181 degrees the arcraft goes heading 180 degrees ( 181 degrees - (+1) degree = 180 degrees). On the other hand if the pilot steer to 179 degree the compass will show 180 degree but the plane would travel too 179 degrees .
Is this right or am i wrong to somehtink?

thank you for your time , George

Community Comments (1)

L
Lisa_T 2 Aug 2026
Hey George, this one's tricky, you've got the correction running the wrong way round. It's 179, because a plus one deviation means the compass is reading one degree low, so when the aircraft is genuinely pointing at 180 the card in front of you shows 179, and that lower number is what you steer to. Same idea as a watch that runs a minute slow, if you need to be somewhere at 8 o'clock you set off when the watch says 7:59, you work to what the dial shows and not what you wish it showed. The memory line for it is deviation east, compass least, and a compass correction card would read for 180 steer 179, so that's how I'd read it.

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