Squall line at departure with the weather radar deferred

What the deferral means

A minimum equipment list deferral is a permission with conditions attached, not a licence to ignore the failure. Here the condition is explicit: no operation into areas of forecast or reported convective activity. The radar is the equipment that lets you see and avoid cells; without it, the flight can only be conducted where convection is not expected.

Applying it to this departure

• Kano is reporting an active thunderstorm with cumulonimbus at 1,000 ft, a 40 kt gust and heavy rain - the very condition the deferral excludes.
• The SIGMET puts embedded cumulonimbus on the route with tops at FL400, so the cruise phase is also excluded.
• Both the departure and the en-route environment therefore breach the conditions of the deferral.

Decision

Do not depart. The correct action is to delay until the squall line has passed and the SIGMET is cancelled, and then re-check the radar status and the forecast along the whole route. A squall line moving at 20 kt typically clears an aerodrome in under an hour, but the associated gust front and standing water may outlast the rain. If the line is slow-moving, the flight should be cancelled or a serviceable aircraft substituted.

Pressing factor to resist

Passenger or commercial pressure does not alter the conditions of the deferral, and the pilot in command remains responsible for the decision.

Recognise the position

At FL180 you are inside the vertical extent of the convective build-up (tops FL400) and cannot climb over it. Without radar you have no way of seeing where the gaps are, so threading through the line is not an option - the gaps you think you see are an illusion.

Actions, in order

1. Slow to turbulence penetration speed only if penetration is unavoidable; the aim is to stay out.
2. Ask ATC for a deviation, stating the reason and the distance you need - a 40 NM offset left or right is a reasonable opening request, and ask for pilot reports from traffic ahead.
3. If ATC cannot offer a safe routing, hold well clear of the line (at least 20 NM) and reassess the movement and the fuel.
4. Check fuel: with 4 hours 20 minutes total and about 2 hours 10 minutes of trip fuel, you have roughly 90 minutes of decision time, but a long deviation eats it quickly. Brief a diversion aerodrome clear of the line before you need it.
5. Report the conditions to ATC and to following traffic.

Do not attempt to pick a path through the line at night without radar, and do not accept a clearance that takes you into the SIGMET area on the assumption that the cells are isolated.

Icing

The freezing level is 15,000 ft, so at FL180 you would be flying in air below freezing whenever you are in cloud - and cumulonimbus cloud contains large amounts of supercooled water. Icing in convective cloud can be severe, and a light twin's de-icing equipment is not certified to cope with it.

Performance

• Ice accretion increases drag and stall speed and reduces climb performance, so the aircraft's ability to climb out of trouble at FL180 may be far less than the book figures suggest.
• Turbulence in and near the cell adds gust loads and makes altitude holding difficult.
• High temperature and humidity at Kano reduce take-off and climb performance; a departure in a 40 kt gust with a wet runway also reduces the margins.

Altitude choice

• Below 15,000 ft you are below the freezing level and the icing risk in cloud is much lower (rain rather than ice), although turbulence and hail remain.
• Above the tops (FL400) is not achievable.
• A lower level does not solve the avoidance problem: the squall line extends to the ground, so the correct choice is lateral avoidance, not a change of level.

If icing is encountered anyway: change level, report it, and if the ice cannot be shed, divert and land. Do not accept visible ice accumulation on a flight that is already constrained by a deferred radar.