Thunderstorm Avoidance on a Converging Track
On a summer afternoon a flight must cross a 200 km leg. Radar shows a line of thunderstorms 40 km wide oriented north–south, with tops above 12,000 m, hail reported, and a gap of about 25 km between two cells. The aircraft cruises at 6,500 ft and has weather radar but no storm scope. The area forecast mentions isolated embedded cumulonimbus and a low-level jet of 40 kt at 3,000 ft. The destination has a temporary forecast of gusts 25 kt and heavy rain showers. Fuel allows 30 minutes of holding. The crew proposes to fly through the gap at cruise level to save time.
1. Assess the hazards of flying through the 25 km gap at 6,500 ft.
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A 25 km gap between cells is not a safe corridor. Thunderstorm hazards extend well beyond the visible precipitation: hail can fall several kilometres downwind, and the anvil and downdraught region can extend 20 km or more. At 6,500 ft the aircraft is below the freezing level in the heavy rain area, so the main threats are severe turbulence, microburst and downburst wind shear, hail, lightning and heavy rain reducing radar effectiveness.
The low-level jet of 40 kt at 3,000 ft increases low-level shear and the likelihood of organised storm outflow. The cells are moving, so the gap may close. Weather radar at 6,500 ft may be attenuated by heavy rain and cannot show the full vertical extent, so the crew cannot confirm the gap is clear above.
Conclusion: the proposed route through the gap at 6,500 ft is unsafe.
2. State a plan, including the use of fuel and the destination forecast, and describe what you would do if the aircraft inadvertently enters a cell.
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Plan: remain clear of the storm line by at least 20 km (more downwind of the cells), route around the northern or southern end of the line rather than through the gap, and if that is impossible, hold clear and wait for the line to move. Use the 30 minutes of holding fuel deliberately, monitoring radar and any amended forecasts. Because the destination has gusty winds and heavy showers, plan for a possible diversion or a second approach, and confirm alternate fuel is intact.
If inadvertent penetration occurs: reduce speed to turbulence penetration speed, maintain a constant attitude and let the aircraft ride the updraught and downdraught rather than fighting to hold altitude, turn to the nearest clear air using radar, and avoid abrupt control inputs. Report the encounter to ATC and request any available information. Do not descend into a downdraught near the ground; climb if performance allows.