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The following stories from the July 24, 2009, edition of AOPA ePilot were provided to AOPA members who expressed an interest in the particular subject areas. Any AOPA member can receive information tailored to their areas of interest by updating their preferences online.


Convective sigmets

The weather looked great when you launched on today’s solo cross country. There was a cold front headed into the area, your weather briefer said, but frontal passage and any associated weather activity was not expected until tomorrow. But as the day wore on, those typical fair-weather cumulus clouds seemed to take on a more ominous appearance, growing higher and darker. Acting on your gut feeling that weather changes might be afoot, you dialed in a HIWAS frequency or contacted Flight Service by radio for an update. HIWAS was discussed in the Aug. 22, 2003, “Training Tip: Keeping Tabs on the Weather.


Two words that you heard over the radio got your immediate attention: Convective sigmet 45E has been issued near your route “for a line of severe thunderstorms 15 nautical miles wide, moving from 250 degrees at 15 knots, tops above Flight Level 450. Hail to one inch, gusts to 50 knots possible.” Hail greater than three-quarters of an inch in diameter is always included in a convective sigmet or an urgent pilot report; see the July 17, 2009, Training Tip.


What exactly is a sigmet? It is a National Weather Service contraction that stands for significant meteorological information. A sigmet is an in-flight weather advisory “considered significant to all aircraft,” as explained in Chapter 11 of the Pilot’s Handbook of Aeronautical Knowledge . When such an advisory is concerned with convective weather, the sigmet is named accordingly.


Updating your weather information from the air is not the only way you may learn that a sigmet is in effect. Aviation weather forecasts refer to these weather advisories as well. The AOPA Air Safety Foundation publication Weather Strategies contains a transcript of a telephone weather briefing in which the briefer reviews a convective sigmet such as the one described above with a pilot.


Take some time to familiarize yourself with the information a sigmet contains. Then try your hand at the two questions about sigmets in the August 2008 AOPA Flight Training ’s Final Exam.


Convective or not, the issuance of a sigmet is need-to-know information for any pilot. So stay informed about weather, and be ready with a plan of quick action.


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Question: I am planning a flight into a small, nontowered field. Looking in the Airport/Facility Directory, I noticed that there is both a unicom frequency and a CTAF. Can you tell me what the difference between these two frequencies is and which one I should use to announce my position?


Answer: CTAF is the common traffic advisory frequency. This is the primary and preferred frequency for providing position reports while flying at or near an airport. The unicom frequency is shortened from "universal communications." This is the frequency you would use to communicate with the FBO or airport itself. You can use the unicom frequency to inquire about the preferred runway, wind direction, and other pertinent information. For more information on radio communications, check out the AOPA Air Safety foundation’s newest interactive course, Say It Right: Mastering Radio Communication.

Got a question for our technical services staff? E-mail to [email protected] 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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