full text of the classic FAA guide
DO'S AND DON'TS OF THUNDERSTORM FLYING
Above all, remember this: never regard any thunderstorm as “light” even when radar observers report the echoes are of light intensity. Avoiding thunderstorms is the best policy. Following are some Do's and Don'ts of thunderstorm avoidance:
Don't land or take off in the face of an approaching thunderstorm. A sudden wind shift or low level turbulence could cause loss of control.
Don't attempt to fly under a thunderstorm even if you can see through to the other side. Turbulence under the storm could be disastrous.
Don't try to circumnavigate thunderstorms covering 6/10 of an area or more either visually or by airborne radar.
Don't fly without airborne radar into a cloud mass containing scattered embedded thunderstorms. Scattered thunderstorms not embedded usually can be visually circumnavigated.
Do avoid by at least 20 miles any thunderstorm identified as severe or giving an intense radar echo. This is especially true under the anvil of a large cumulonimbus.
Do clear the top of a known or suspected severe thunderstorm by at least 1,000 feet altitude for each 10 knots of wind speed at the cloud top. This would exceed the altitude capability of most aircraft.
Do remember that vivid and frequent lightning indicates a severe thunderstorm.
Do regard as severe any thunderstorm with tops 35,000 feet or higher whether the top is visually sighted or determined by radar.
If you cannot avoid penetrating a thunderstorm, following are some Do's Before entering the storm:
Tighten your safety belt, put on your shoulder harness if you have one, and secure all loose objects.
Plan your course to take you through the storm in a minimum time and hold it.
To avoid the most critical icing, establish a penetration altitude below the freezing level or above the level of —15° C.
Turn on pitot heat and carburetor or jet inlet heat. Icing can be rapid at any altitude and cause almost instantaneous power failure or loss of airspeed indication.
Establish power settings for reduced turbulence penetration airspeed recommended in your aircraft manual. Reduced airspeed lessens the structural stresses on the aircraft.
Turn up cockpit lights to highest intensity to lessen danger of temporary blindness from lightning.
If using automatic pilot, disengage altitude hold mode and speed hold mode. The automatic altitude and speed controls will increase maneuvers of the aircraft thus increasing structural stresses.
If using airborne radar, tilt your antenna up and down occasionally. Tilting it up may detect a hail shaft that will reach a point on your course by the time you do. Tilting it down may detect a growing thunderstorm cell that may reach your altitude.
Following are some Do's and Don'ts During thunderstorm penetration:
Do keep your eyes on your instruments. Looking outside the cockpit can increase danger of temporary blindness from lightning.
Don't change power settings; maintain settings for reduced airspeed.
Do maintain a constant attitude; let the aircraft “ride the waves.” Maneuvers in trying to maintain constant altitude increase stresses on the aircraft.
Don't turn back once you are in the thunderstorm. A straight course through the storm most likely will get you out of the hazards most quickly. In addition, turning maneuvers increase stresses on the aircraft.