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In order to avoid the associated hazards when circumnavigating a severe thunderstorm using airborne weather radar, what is the minimum distance from echoes that the aircraft should fly?
  • A
    10 NM
  • B
    50 NM
  • C
    5 NM
  • D
    20 NM

Refer to figure.


Avoid thunderstorms by at least 20 NM => this general guideline applies even to jet fighters and large airliners.
Hail and severe turbulence can exist well outside of the storm cloud.

  • If you plan to deviate around a thunderstorm => fly on the upwind side so that your path does not converge with the path of the storm.
  • Do not be tempted to try to fly under a thunderstorm, even if you can see through the rain on the other side => besides rain, hail and lighting, the area under a thunderstorm typically contains severe turbulence (including microbursts, windshear and downdrafts
  • Trying to fly over a developing storm is also dangerous => the rising clouds can usually outclimb your aircraft.

If your aircraft is equipped with a weather avoidance system, such as a weather radar, you can use it to avoid thunderstorms. You should avoid intense thunderstorms echoes by at least 20 NM.
Airborne weather radar is designed for avoiding severe weather, not for penetrating it. It detects drops of precipitation. Therefore, do not try to use it to avoid instrument weather associated with clouds and fog – it does not provide any assurance of avoiding IFR weather conditions.

Furthermore, as discussed above, Airborne Weather radar detects large droplet precipitation and hail (present in CBs), it does not detect stratus/stratocumulus or very small water droplets => making it relatively easy to identify embedded cumulonimbus.

According to ICAO AC 120-88A Appendix 1:
AVOID any convective activity (CBs) en route by at least 20 nautical miles.

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