Compass Conversion with Changing Deviation on a Long Leg
An aircraft departs on a magnetic heading of 095°(M) with the compass reading 092°. The deviation card shows 3°E on heading 090°. En route the pilot plans to turn onto a new track of 210°(T). The local magnetic variation at the turning point is 7°W. The aircraft's compass swing was carried out with the radios on, and the engineer notes that deviation on southerly headings is 4°W. The pilot also carries a standby compass that reads 6° different from the panel compass on the same heading. Fuel is sufficient for 4 hours and the leg is 2 hours 40 minutes. The pilot must decide which compass to use and how to convert between true, magnetic and compass headings for the new track.
1. Convert the new true track of 210°(T) to the required compass heading, showing each step.
Read the worked answer
Work from true to magnetic to compass.
True to magnetic: variation is 7°W. The rule is variation east, magnetic least; variation west, magnetic best. With westerly variation the magnetic heading is larger than the true heading, so magnetic = 210° + 7° = 217°(M).
Magnetic to compass: deviation on southerly headings is 4°W. Deviation west means the compass reads less than magnetic, so compass = magnetic + 4° = 221°(C).
Check the direction of the correction: with westerly variation and westerly deviation both corrections add, which is consistent with the rule that westerly errors require a larger compass number. The pilot should turn to 221°(C) to fly 210°(T).
2. The pilot notices the panel compass and standby compass disagree by 6°. Explain how to resolve this and what it implies about the deviation card.
Read the worked answer
A 6° disagreement between two compasses on the same heading cannot be resolved by averaging, because one of them is wrong or is being disturbed.
First, check the conditions. The deviation card was produced with radios on, so the panel compass deviation includes radio interference. The standby compass is usually mounted away from the panel and is less affected by electrical equipment, but it may be affected by magnetic material carried nearby, such as a torch or headphones.
Second, check whether the aircraft is on a known heading, for example lined up on a runway or a surveyed taxi line, and compare both readings with the known magnetic heading. Whichever agrees with the known heading plus variation is the more trustworthy.
Third, if the panel compass is the one in error, the deviation card is suspect and the 4°W figure cannot be relied on. The pilot should use the standby compass for the leg, apply its own deviation if known, and have the panel compass swung and the card reissued. If neither compass can be validated, the pilot should navigate by GNSS track and treat compass headings as approximate.
3. Assess how a 6° heading error would affect the flight over the 2 h 40 min leg.
Read the worked answer
A constant 6° heading error produces a lateral drift that grows with distance flown. The cross-track error is approximately distance flown times the sine of the error angle. For small angles, sin 6° is about 0.105, so after 100 NM the aircraft is about 10.5 NM off track.
At a groundspeed of, say, 110 KT, the 2 h 40 min leg covers about 293 NM. If the error persisted for the whole leg, the cross-track error would reach about 293 × 0.105 ≈ 31 NM. That is well beyond the accuracy needed for a VFR arrival at an unfamiliar airfield.
The error also changes the along-track distance slightly, but the lateral error dominates. The pilot should not fly a single heading for the whole leg on the basis of an unverified compass. Instead, use GNSS track to monitor actual track made good, compare it with the planned true track, and adjust heading by the observed difference. A position fix every 30 minutes will catch a 6° error before it becomes significant.