SIGMET and a Blocked VFR Route to Lamu
You are pilot of Cessna 182 5Y-DEF on a VFR flight from Mombasa (HKMO) to Lamu (HKLU) at 9,500 ft. At 30 nm southwest of Malindi, the controller passes: '5Y-DEF, SIGMET valid for embedded thunderstorms, tops FL280, 20 nm northeast of Malindi, moving west at 15 kt. Report your intentions.' You have 2.5 hours fuel, 1 hour to Lamu, and the CB line is visible ahead. Your aircraft has no weather radar, but you have a GPS and a serviceable VHF. Malindi (HKML) is 25 nm south; Lamu is 70 nm northeast. There is no IFR clearance available. You are in VMC now, but the cloud tops are above your level. Mombasa Control is the ATS unit. You have not filed an IFR flight plan.
1. What does the SIGMET tell you about the hazard, and what immediate RT request should you make?
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The SIGMET is warning of embedded thunderstorms, not isolated build-ups. 'Embedded' means the storms are hidden inside other cloud and may not be visible as bright, isolated cells. Tops at FL280 are far above your 9,500 ft, so you cannot overfly them. The line is 20 nm northeast of Malindi and moving west at 15 kt, which means it is moving toward the Malindi area and possibly across your route to Lamu. The hazards include severe turbulence, hail, lightning, microbursts, icing and possible instrument upset. Your immediate request should be specific and should state your inability: 'Mombasa Control, 5Y-DEF, request diversion to Malindi due weather, request weather and landing information.' If you need time to decide, say '5Y-DEF, request 2 minutes, remaining clear of the area.' Do not simply say 'request weather' without stating your intention. ATC needs to know whether you are continuing, diverting or returning so it can protect your flight path.
2. The controller replies '5Y-DEF, maintain VFR, remain clear of the thunderstorm area, advise intentions.' What are your responsibilities, and what would you do if the CB line blocks the route to Lamu?
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The instruction 'maintain VFR' means you remain responsible for seeing and avoiding other traffic and for staying clear of cloud. It does not mean ATC has separated you from the weather or that ATC has checked the route ahead. 'Remain clear of the thunderstorm area' is a safety instruction you must obey. If the CB line blocks the route to Lamu, you should not attempt to pick a way through visually unless you can maintain VFR and remain clear of the storm cells. Divert to Malindi if it is open and suitable, or return to Mombasa. Advise ATC clearly: '5Y-DEF, unable to proceed to Lamu due weather, diverting to Malindi, estimating Malindi 12 minutes.' If you are uncertain about the weather at Malindi, request the current METAR/SPECI and ask for the aerodrome information. If you are caught by deteriorating visibility and cannot maintain VFR, do not press on; declare an urgency condition and request assistance.
3. What is the difference between a SIGMET and an AIREP, and why should you file an AIREP?
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A SIGMET is a meteorological advisory issued for significant en-route weather that may affect the safety of aircraft, such as thunderstorms, severe turbulence, severe icing or volcanic ash. It is a forecast/advisory product and may be based on satellite, radar or model data. An AIREP is a pilot report of actual conditions encountered at a specific position and time. It is direct evidence from the cockpit. You should file an AIREP because it can confirm or correct the forecast, warn other crews, and help ATC and the meteorological office update their picture. A useful AIREP includes aircraft identification, position, time, flight level, weather encountered and the effect on the flight. For example: 'AIREP, 5Y-DEF, 30 nm southwest of Malindi, 1045, 9,500 ft, line of CB northeast, tops above FL280, moderate turbulence, diverting to Malindi.' Keep it brief and do not block the frequency; ATC can ask you to pass it to the meteorological office after landing.