July 15, 2011
Looking for the airport from several miles out on a hot, humid afternoon, it’s not easy to judge how close to the field those dark cumulus clouds up ahead really are. The airport’s automated weather report may mention high ceilings and moderate winds, but how long will those manageable conditions last? It’s not yet a question of making an unwise dash to beat incoming weather, but it is time to be cautious.
You may remember from studying weather in ground school that thunderstorms are not just a direct hazard to aircraft. Their effects can be felt at a considerable distance from active cells. Hail can be ejected from a thunderstorm as far as 20 miles from the cloud, as described in the July 17, 2009, “Training Tip: Hail.”
Not all thunderstorms produce hail, but you can count on turbulence from a gust front. Nor does it require special study of aviation weather to visualize a gust front; any pilot who has ever stood outside and noticed changes with a storm’s approach has had a first-hand look.
“If you've been on the ground when thunderstorms are in the neighborhood, you've probably felt a cool breeze, maybe a breeze that changes the wind's direction,” wrote meteorologist Jack Williams in the December 2007 Flight Training. “This is the gust front created by a downburst and when you're on the ground on a muggy day, you appreciate the cooling effect. If you're on final approach or just taking off when a gust front hits, you might wish you were on the ground, at least for a few moments.”
The conditions described are a form of wind shear, which comes in three varieties—increased headwinds, decreased headwinds, and downdrafts that could occur without warning. “Do not take off or land if a shaft of rain covers an airport or its departure or approach corridors,” advises the discussion of turbulence in AOPA’s Handbook for Pilots.
The Air Safety Institute’s interactive safety course WeatherWise: Thunderstorms and ATC shows how air traffic control may be able to help you make the right call. If you are out of contact when pondering a grim view ahead, consider diverting, or turning around. Don’t risk a nasty surprise that can arrive on a gust front’s breeze.
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Question: Do the federal aviation regulations or Aeronautical Information Manual (AIM) state the proper altitude to begin your turn from the departure leg to crosswind while staying in the traffic pattern? My flight instructor always told me it was the traffic pattern altitude (TPA) for the particular airport minus 300 feet. Was that just sound advice from my CFI, or can I find it anywhere in writing?
Answer: This has long been a topic of hangar conversations; however, the recommendation is clearly stated in Chapter 4 Section 3 of the AIM. It explains, “If remaining in the traffic pattern, commence turn to crosswind leg beyond the departure end of the runway within 300 feet of pattern altitude.” The AIM also defines the departure leg as “the flight path which begins after takeoff and continues straight ahead along the extended runway centerline. The departure climb continues until reaching a point at least one-half mile beyond the departure end of the runway and within 300 feet of the traffic pattern altitude.” So the AIM recommendation agrees with your flight instructor’s good advice to turn onto the crosswind leg once the departure leg is completed, which is 300 feet below pattern altitude.
Got a question for our technical services staff? E-mail email@example.com or call the Pilot Information Center, 800/872-2672. Don’t forget the online archive of “Final Exam” questions and answers, searchable by keyword or topic.
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