Convective Line, Severe Turbulence PIREP, and a Deferred Radar
Flight 88, a twin-engine jet at FL340, is planned KDFW-KATL, 2:00 enroute, ETA 2145Z. A squall line 40 nm wide lies across the route, tops to FL450, moving east at 30 kt, covered by a Convective SIGMET. A PIREP from an aircraft 20 minutes ahead reports severe turbulence and a 6,000-foot altitude loss at FL340 inside the line. The aircraft's weather radar was deferred this morning under an MEL item whose operational procedure reads: 'May be inoperative provided the airplane is not operated in known or forecast thunderstorm activity.' KATL forecast: TSRA, BKN015, improving to 4 SM after 2200Z. KCLT, the alternate, is 55 minutes and 5,000 lb of fuel away. The captain wants to file direct and 'thread the gap' using a 90-minute-old radar image passed along by another crew.
1. Can the flight be dispatched on the planned route with the radar deferred?
Read the worked answer
No. The MEL operational procedure is a limitation on the release, not a suggestion. Thunderstorm activity is both known (active Convective SIGMET, PIREP) and forecast (TSRA at destination, line across the route) along the planned track, so the condition under which the airplane may fly with the radar deferred is not met. Releasing into that environment violates the MEL and, because the MEL is part of the operator's approved document, makes the flight non-airworthy for this routing.
A 90-minute-old image relayed by another crew is not an acceptable substitute. It is not a current observation of the gap, it was painted by a different radar at a different altitude and tilt, and the line is moving 30 kt - in 90 minutes it has moved roughly 45 nm. The gap may not exist where the image shows it.
Two legal paths: (a) reroute to remain clear of known and forecast thunderstorm activity - a lateral deviation clear of the line with a defined return point, verified against current data; or (b) delay until the line has crossed the route and the forecast no longer contains TS. Swapping to a radar-equipped airframe is the third and usually the cheapest option.
2. Evaluate the severe turbulence PIREP and describe how it should change the plan.
Read the worked answer
A severe turbulence report is the strongest in-flight evidence available and is treated as a hard constraint on routing, not a data point to be averaged with the forecast. A 6,000-foot altitude loss at FL340 indicates the crew ahead encountered something the smooth-ride forecast did not capture - typically the anvil or the turbulent zone downwind of the core.
Practical consequences:
- Do not plan a penetration at FL340 through or near the reported location; a deviation that clears the line laterally is required regardless of the radar status.
- The PIREP should be relayed to ATC and the company so other dispatchers and crews get it; the dispatcher is often the only node that can disseminate it to the rest of the fleet.
- Expect that the 'gap' may be displaced or closed; build the fuel plan around a full deviation, not the optimistic one.
- If the reroute still requires flight near the line, the dispatcher should plan ride-quality alternatives (higher or lower altitude in clear air) rather than relying on seat belts and a PA.
3. Assuming a 90-nm north deviation adding 15 minutes, what does the fuel and alternate picture look like, and what goes on the release?
Read the worked answer
At 3,600 lb/hr the deviation costs about 900 lb (15 minutes x 60 lb/min). That has to be additive to the release, not carved out of the alternate or reserve:
- Taxi + trip + deviation + alternate (KCLT, 5,000 lb) + 45-minute reserve, with the reserve computed at the end of the alternate leg.
- If the crew is expected to hold for the KATL TSRA to clear after 2200Z, add holding fuel at the planned hold altitude.
The release remarks should carry:
- The Convective SIGMET identifier and the PIREP, with the instruction to remain clear of the line.
- The MEL item reference and the explicit statement that the airplane is dispatched on a route clear of known and forecast thunderstorm activity, per the operational procedure.
- The deviation routing and the planned rejoin point.
If the numbers do not close with the deviation plus a legal alternate plus reserve, the correct answer is a delay past the line passage, not a thinner reserve.