Harmattan dust, ITCZ position and a Lagos TAF with thunderstorms
January, 1200 UTC. You plan Kano (DNKN) to Lagos (DNMM), ETA 1600, with Abuja (DNAA) as the only realistic alternate. Kano METAR: DNKN 121200Z 06008KT 2500 HZ SKC 28/05 Q1014. Kano TAF: DNKN 121100Z 1212/1312 06008KT 3000 HZ SKC TEMPO 1212/1218 1500 HZ. Lagos TAF: DNMM 121100Z 1212/1318 22010KT 9999 SCT020 TEMPO 1214/1218 3000 TSRA BKN012 FEW025CB BECMG 1218/1220 24005KT. Abuja METAR: DNAA 121200Z 20006KT 8000 SCT018 32/23 Q1010. The company alternate minima are 800 ft ceiling and 3,000 m visibility, and destination minima are 500 ft and 1,500 m. The aircraft has fuel for 45 minutes holding plus alternate. The ITCZ is analysed near 5°N. The captain asks whether Lagos is acceptable at ETA, whether Kano can be used as a return alternate, and what the harmattan is doing to the operation.
1. Decode the Kano and Lagos TAFs and identify the hazard windows relevant to an ETA of 1600.
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- Kano TAF: issued 121100Z, valid 1212 to 1312. Prevailing wind 060/8 kt, visibility 3,000 m in dust haze, sky clear. TEMPO 1212/1218: visibility 1,500 m in dust haze.
- Kano METAR at 1200Z already shows 2,500 m in HZ, which is below the TAF prevailing 3,000 m but above the TEMPO 1,500 m. The trend is toward the temporary lower visibility during the afternoon.
- Lagos TAF: issued 121100Z, valid 1212 to 1318. Prevailing wind 220/10 kt, visibility 10 km or more, scattered cloud at 2,000 ft. TEMPO 1214/1218: visibility 3,000 m in thunderstorm rain, broken cloud at 1,200 ft, cumulonimbus at 2,500 ft. BECMG 1218/1220: wind 240/5 kt.
- Hazard window for Lagos is 1400-1800, which contains the 1600 ETA. The temporary conditions are above destination minima but include TSRA and CB, so the approach may be interrupted by thunderstorm activity, wind shear, or a sudden visibility reduction.
- Abuja METAR is 200/6 kt, 8,000 m, scattered at 1,800 ft: above alternate minima.
2. At ETA 1600, is Lagos acceptable as destination? Can Kano be the return alternate? What alternate do you nominate?
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- Lagos destination minima are 500 ft ceiling and 1,500 m visibility. The TEMPO forecasts 1,200 ft and 3,000 m, so the forecast values are above minima. On a strict forecast basis Lagos is acceptable at 1600.
- Operationally it is marginal: TSRA, a CB at 2,500 ft, and a 1,200 ft broken ceiling mean a real risk of go-around, wind shear, and rapid changes. The prevailing forecast is much better, so the TEMPO may not be continuous, but the crew must carry alternate fuel and brief a diversion.
- Kano as return alternate: at a return ETA of about 1700, Kano is still inside TEMPO 1212/1218 with 1,500 m in HZ. Alternate minima are 3,000 m and 800 ft. Kano would not meet the visibility requirement, so it is not a suitable return alternate at that time.
- Abuja is the correct nomination: 8,000 m visibility and scattered cloud at 1,800 ft exceed the 3,000 m and 800 ft alternate minima. If Abuja were unavailable, the flight would need additional fuel or a delayed departure.
- The ITCZ near 5°N places Lagos in moist monsoon air and Kano in dry harmattan air. That is why Kano has dust haze and no cloud while Lagos has convective cloud.
3. What operational precautions would you take for the harmattan leg and for the Lagos arrival?
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- Kano departure: visibility 2,500 m in dust haze, with a forecast temporary 1,500 m. There is no cloud, so the threat is visibility and possible engine intake dust, not icing or convective turbulence. Use instrument departure procedures, confirm takeoff alternate and departure minima, and brief the possibility of a return to Kano only if it meets alternate minima at the relevant time.
- En route: the ITCZ near 5°N marks the transition from dry harmattan air to moist monsoon air. Expect a visibility improvement southbound, but also the build-up of convective cloud near Lagos.
- Lagos arrival: the 1400-1800 TEMPO includes TSRA and CB. Use weather radar, avoid the CB cells, expect a go-around, and brief wind shear. Do not descend into a cell to maintain the schedule.
- Fuel: carry the 45-minute hold plus Abuja alternate. If Lagos is active with thunderstorms at ETA, divert early rather than burn the alternate fuel holding.