En Route to Singapore with a Forecast of Embedded CB and a Fuel Decision
You are planning a flight from Brunei to Singapore at FL180 in a jet. The route crosses the intertropical convergence zone. The area forecast mentions embedded cumulonimbus with tops to FL450, and a SIGMET for severe turbulence is current. The aircraft has weather radar and a lightning detector. The flight plan fuel allows a 30-minute reserve plus a 15-minute holding allowance. The alternate is 120 NM away. The departure is at 0200 local time, when the ITCZ is most active.
1. Explain how the ITCZ and the time of day affect the likelihood of embedded CB and what that means for your fuel planning.
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The ITCZ is a zone of convergence, low pressure, and deep convection near the equator. It is most active in the early morning hours when the nocturnal cooling of cloud tops and the convergence maximum combine to produce widespread embedded cumulonimbus. Embedded CB are hidden within layered cloud and may not show clearly on radar, so the aircraft may encounter turbulence and icing without warning. The SIGMET confirms severe turbulence. The 30-minute reserve plus 15-minute holding is a legal minimum, but with embedded CB the aircraft may need to deviate significantly. Add fuel for the deviation, or plan a route that avoids the ITCZ, or delay the departure to the afternoon when convection is more isolated and visible. If the deviation fuel is not available, the flight should not depart.
2. Describe how you would use the weather radar and lightning detector to navigate through the ITCZ, and what would make you divert.
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Use the radar in the mapping mode to identify the strongest echoes and the lightning detector to confirm active cells. Maintain at least 20 NM from any echo with tops above FL300, and avoid the anvil, which can extend 20-30 NM downwind. If the radar shows a continuous line of echoes with no gap wider than 20 NM, divert around the line or climb above the tops if performance permits. Divert if the aircraft encounters severe turbulence, if the lightning detector shows frequent strikes along the route, or if the fuel margin falls below the reserve. Report the conditions to ATC and request a deviation.