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Air Safety Institute Safety Spotlight

Listen closely

On May 10, 2004, two private pilots flying in a Piper Seminole were killed when they descended into mountainous terrain near Julian, California. The pilots had filed an instrument flight plan from Deer Valley, Arizona, to Carlsbad, California.

The accident airplane, N304PA, was number four in a group of five company airplanes flying the same route. Airplane number three, N434PA, was directly ahead of N304PA.

N304PA contacted the San Diego North radar controller, reporting level at 8,000 feet. The pilots were instructed to fly heading 260 after Julian and intercept the Palomar localizer. Three minutes later, the controller cleared N434PA to descend to 6,000 feet. The pilot of N434PA acknowledged the clearance. Shortly thereafter, the controller transmitted, "Seminole Four Papa Alpha, descend and maintain 5,200." The pilot of N304PA responded, "Down to five thousand two hundred for Three-Zero-Four-Papa-Alpha." This clearance was intended for 434PA, but the controller did not recognize that the incorrect airplane had responded.

Two minutes after N304PA started to descend, its crew informed the controller that they had ATIS information Zulu. The data block on the controller's screen showed N304PA descending through 6,600 feet, although the minimum en route altitude for the airway segment is 7,700 feet. Shortly thereafter, the controller again cleared N434PA to descend and maintain 5,200 feet.

After another minute, the controller was presented with a minimum safe altitude warning (MSAW) alert on N304PA. The controller did not notify the tracon sector controller that he was receiving an MSAW alert, as required by the FAA. Two additional alerts were received by the tracon controller, but not acted upon. The aircraft then descended below radar coverage and disappeared from the controller's screen. The wreckage of N304PA was found on a ridgeline 200 yards south of the Julian VOR at 5,537 feet.

The NTSB determined the cause of this accident to be the sector controller's incorrect use of an abbreviated call sign to issue a descent clearance to N434PA and the controller's failure to detect that the pilot of N304PA had read the clearance back with the full call sign. A contributing factor was that the pilots of N304PA failed to question a clearance that descended them below the published minimum en route altitude. The failure of both the center and tracon controllers to properly respond to the MSAW alerts was another contributing factor.

Section 4-2-4 of the Aeronautical Information Manual cautions that improper use of call signs can result in pilots executing a clearance intended for another aircraft. "Call signs should never been abbreviated on an initial contact or at any time when other call signs have similar numbers/sounds or identical letters/numbers, e.g., Cessna 6132F, Cessna 1622F, Baron 123F, Cherokee 7732F, etc."

This accident could have been prevented with the use of full call signs as well as the accident pilots' prior knowledge of the terrain along their route. Any time a call sign similar to yours is on the same frequency, use extra vigilance when communicating with ATC. For more information about how altitudes are depicted on charts and how you can use these altitudes in flight planning, download the AOPA Air Safety Foundation's Terrain Avoidance Plan Safety Brief from the AOPA Online Safety Center.

Kristen Hummel manages the GA accident database for the AOPA Air Safety Foundation. She holds a commercial pilot certificate with multiengine and instrument ratings.

By Kristen Hummel

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