Haze, a Precessed Heading Indicator, and No Fix
You depart Kaduna (KAD) for Abuja (ABV), 110 nm, planned track 160°T, TAS 100 kt, groundspeed 95 kt, variation 3°W. After 35 minutes, harmattan haze reduces visibility to about 6 km. The GNSS shows “no fix” and the only chart feature ahead is a road junction you cannot positively identify. The heading indicator reads 160°, but the magnetic compass reads 173°. You have been steering by the heading indicator since departure. Fuel remaining is 28 US gal and burn is 9 gal/h. Abuja is still reporting VFR, but you are no longer sure of your position. You must decide what to do first, how much to trust the instruments, and when to ask for help. There is no radar service, but Abuja Approach is available on the radio.
1. Estimate your lateral position error relative to the intended track using the instrument discrepancy.
Read the worked answer
The heading indicator reads 160° but the magnetic compass reads 173°, so the heading indicator has precessed by about 13°. If the compass is correct, you have been flying a magnetic heading of 173°, not 160°. Planned magnetic heading is 160° + 3° = 163°M. The error is therefore about 10° right of the planned heading. In 35 minutes at 95 kt you have covered about 55 nm. Lateral offset = 55 × sin 10° ≈ 9.5 nm to the right of track. That is an estimate, not a fix: it assumes the compass is accurate and the precession has been constant. It is enough to tell me I am not where the heading indicator suggests, and that I should not simply continue on the indicated heading.
2. State your immediate actions in order, including what you would do with the heading indicator and compass.
Read the worked answer
First, maintain control and fly a steady heading using the magnetic compass, not the precessed heading indicator. Second, note the time and start a log of heading, altitude, and fuel. Third, attempt to identify a feature: a road junction, a town, a river, or the sun’s bearing at a known time. Fourth, compare the compass with the heading indicator and reset the heading indicator only if I am sure of the compass; in haze, a magnetic compass turn error can make that uncertain. Fifth, call Abuja Approach, state that I am uncertain of position, give my last confirmed fix, heading, altitude, and fuel, and request assistance. I would not wait until fuel is low or visibility closes in further.
3. If you decide to proceed to Abuja, what heading would you steer from the estimated position, and what reserve would you protect?
Read the worked answer
From an estimated position 9.5 nm right of track and about 55 nm from Abuja along track, the angle back to Abuja is arctan(9.5/55) ≈ 10° left of the planned track. The track from the present position is therefore about 150°T. With 3°W variation, the magnetic heading is about 153°M, assuming no wind correction; the forecast wind is light, but I would check drift visually or with a feature. Time at 95 kt for 55 nm is about 35 minutes, using about 5.3 gal and leaving roughly 22.7 gal. I would protect a 45-minute reserve and a 30-minute alternate. If I cannot identify Abuja by the revised ETA plus 10 minutes, I would divert to a known VFR airport or declare an emergency rather than continue into haze.