Thunderstorm avoidance distances

Avoiding beats penetrating; decide before getting close. Never treat a thunderstorm as light, even with a light radar echo (TC AIM AIR 2.7.2).

Lateral clearance:

• Hail may fall miles outside the cloud (PHAK ch.12).

Vertical clearance (AIR 2.7.2(f)): clear top of known or suspected severe thunderstorm by 1,000 ft per 10 kt of wind at cloud top.

• Example: top 30 000 ft ASL, 40 kt: 4 x 1,000 = 4,000 ft, so fly at 34 000 ft ASL or higher.

Embedded cells: no flight into cloud mass with embedded thunderstorms without airborne radar (AIR 2.7.2(d)).

Do not fly under:

• Any thunderstorm, even with far side visible (AIR 2.7.1, 2.7.2(b)).
  • Turbulence under the storm can be disastrous.
  • Hazard may be invisible until already in strong out-flowing winds and severe turbulence, especially in low humidity.
• Anvil: severe to extreme clear-air turbulence risk (FAA AWH 22.8.2).

On the ground:

• No landing or takeoff with a thunderstorm approaching: gust front wind shift or low-level turbulence can cause loss of control (AIR 2.7.2(a)).
• If flying around not possible, stay on ground until it passes (PHAK ch.12).

Isolated storm at destination: hold over a known point clear of it until well past the aerodrome. Do not land into it (PSTAR 11.07 key 2).

Trap: judging a storm "light" from a light radar echo, then closing in. Distance and altitude rules still apply.

Practice