Legazpi Dispatch with Mayon Ash and a Marginal Alternate
At 0400 UTC a turboprop is planned RPLL–RPLP (Legazpi) with ETD 0500 and ETA 0625. RPLP TAF: 04012G22KT 5000 -RA SCT015 BKN025; TEMPO 0400/0600 2500 +TSRA BKN010; BECMG 0600/0700 9999 SCT020; PROB30 0600/0700 3000 +TSRA BKN012. RPUN (Naga) TAF: 9999 SCT020. RPLP's available approach has company minima of 900 ft ceiling and 3,200 m visibility; RPUN's circling approach has minima of 1,500 ft and 5,000 m. A SIGMET reports Mayon volcanic ash at FL080–FL120 drifting WSW; the planned track passes 12 nm north of the summit. Fuel plan includes 30 minutes holding and 45 minutes final reserve; RPLL is 55 minutes from RPLP, RPUN is 20 minutes. The captain asks whether to nominate RPLL as alternate and to continue visually around the ash.
1. Evaluate the destination and alternate forecasts at ETA 0625 against operating minima. Is the flight legally dispatchable?
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At 0625 the TEMPO period 0400/0600 has ended. The BECMG 0600/0700 group means improvement to 9999 SCT020 is expected by 0700 but may still be in transition at 0625. The prevailing forecast at 0625 can therefore be taken as at least 5000 m visibility, -RA, SCT015 BKN025: ceiling 2,500 ft and visibility 5,000 m. That meets RPLP's company minima of 900 ft and 3,200 m. The PROB30 group is not a prevailing condition, but it signals a 30% chance of 3,000 m visibility and +TSRA with BKN012 during 0600–0700, so the alternate must be solid.
RPUN's TAF 9999 SCT020 gives cloud at 2,000 ft and 10 km visibility, which meets its circling minima of 1,500 ft and 5,000 m. The flight is therefore dispatchable with RPUN as alternate, provided Naga is open, the crew is circling-qualified, and the ash route is resolved. If company policy treats PROB30 as an operating condition requiring a firmer alternate, the dispatcher should delay until the BECMG improvement is established or choose another alternate.
2. Can the planned track be used with the volcanic ash SIGMET? What specific action is required?
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No. The SIGMET places ash at FL080–FL120 near the route, and the planned track passes only 12 nm north of the summit. Visual avoidance is not an acceptable mitigation: ash can be invisible in haze or cloud, and engine ingestion can cause damage before the crew sees it. The aircraft must remain clear of the SIGMET area and the ash cloud.
Reroute south of the volcano by a margin that keeps the aircraft well outside the SIGMET, for example at least 30–50 nm from the reported ash boundary, and obtain ATS concurrence. If the reroute adds more fuel than the 30 minutes holding, replan fuel and possibly payload. If no safe route is available, delay or cancel. The dispatcher should also obtain the latest ash advisory and pass it to the crew; the decision must be based on the SIGMET and not on a visual assessment.
3. Is RPLL a suitable alternate with the current fuel plan? How should the dispatcher answer the captain?
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No. RPLL is 55 minutes from RPLP, while the fuel plan only carries 20 minutes to RPUN plus 30 minutes holding and 45 minutes final reserve. Nominating RPLL would require about 35 minutes more alternate fuel than planned. Using final reserve to reach RPLL would be illegal and unsafe. RPLL may be VMC with an ILS, but it is not a legal alternate unless the fuel plan is rebuilt.
The dispatcher should tell the captain that RPUN is the planned alternate and meets its minima, while RPLL would require additional fuel and a new load sheet. If the captain wants RPLL for operational reasons, the dispatcher can offer a replanned fuel load or a delay, but must not authorize dispatch on the existing fuel figure. The reply should be factual and include the fuel/time comparison.