AIM ¶ 7-1-27 — Thunderstorm Flying

7-1-27. 7-1-27. Thunderstorm Flying Thunderstorm Avoidance. Never regard any thunderstorm lightly, even when radar 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 takeoff in the face of an approaching thunderstorm. A sudden gust front of 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 and wind shear under the storm could be hazardous. Don't attempt to fly under the anvil of a thunderstorm. There is a potential for severe and extreme clear air turbulence. Don't fly without airborne radar into a cloud mass containing scattered embedded thunderstorms. Scattered thunderstorms not embedded usually can be visually circumnavigated. Don't trust the visual appearance to be a reliable indicator of the turbulence inside a thunderstorm. Don't assume that ATC will offer radar navigation guidance or deviations around thunderstorms. Don't use data-linked weather next generation weather radar (NEXRAD) mosaic imagery as the sole means for negotiating a path through a thunderstorm area (tactical maneuvering). Do remember that the data-linked NEXRAD mosaic imagery shows where the weather was, not where the weather is. The weather conditions depicted may be 15 to 20 minutes older than indicated on the display. Do listen to chatter on the ATC frequency for Pilot Weather Reports (PIREP) and other aircraft requesting to deviate or divert. Do ask ATC for radar navigation guidance or to approve deviations around thunderstorms, if needed. Do use data-linked weather NEXRAD mosaic imagery (for example, Flight Information Service-Broadcast (FIS-B)) for route selection to avoid thunderstorms entirely (strategic maneuvering). Do advise ATC, when switched to another controller, that you are deviating for thunderstorms before accepting to rejoin the original route. Do ensure that after an authorized weather deviation, before accepting to rejoin the original route, that the route of flight is clear of thunderstorms. 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 circumnavigate the entire area if the area has 6/10 thunderstorm coverage. Do remember that vivid and frequent lightning indicates the probability of a severe thunderstorm. Do regard as extremely hazardous any thunderstorm with tops 35,000 feet or higher whether the top is visually sighted or determined by radar. Do give a PIREP for the flight conditions. Do divert and wait out the thunderstorms on the ground if unable to navigate around an area of thunderstorms. Do contact Flight Service for assistance in avoiding thunderstorms. Flight Service specialists have NEXRAD mosaic radar imagery and NEXRAD single site radar with unique features such as base and composite reflectivity, echo tops, and VAD wind profiles. 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 installed), if and secure all loose objects. Plan and hold the course to take the aircraft through the storm in a minimum time. To avoid the most critical icing, establish a penetration altitude below the freezing level or above the level of -15ºC. Verify that pitot heat is on and turn on carburetor heat or jet engine anti-ice. Icing can be rapid at any altitude and cause almost instantaneous power failure and/or loss of airspeed indication. Establish power settings for turbulence penetration airspeed recommended in the aircraft manual. Turn up cockpit lights to highest intensity to lessen 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 stress. If using airborne radar, tilt the antenna up and down occasionally. This will permit the detection of other thunderstorm activity at altitudes other than the one being flown. Following are some Do's and Don'ts duringthe 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 the recommended turbulence penetration airspeed. Do maintain constant attitude. Allow the altitude and airspeed to fluctuate. Don't turn back once you are in the thunderstorm. A straight course through the storm most likely will get the aircraft out of the hazards most quickly. In addition, turning maneuvers increase stress on the aircraft.