On May 10, 2004, two private pilots flying a Piper Seminole were killed when they descended into mountainous terrain near Julian, California. The pilots had filed an instrument flight plan for the flight from Deer Valley, Arizona, to Carlsbad, California.
The accident airplane, N304PA, was the fourth airplane in a group of five company airplanes flying the same route. Airplane No. 3, N434PA, was directly ahead of N304PA.
N304PA contacted the San Diego North Radar (SDNR) controller, reporting level at 8,000 feet. They were instructed to fly heading 260 after Julian and intercept the Palomar localizer. Three minutes later, SDNR cleared N434PA to descend to 6,000 feet. The pilot of N434PA acknowledged the clearance. Shortly thereafter, the SDNR transmitted, "Seminole four papa alpha, descend and maintain five thousand two hundred." 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 to the clearance.
Two minutes after N304PA started to descend, they informed SDNR that they had ATIS information Zulu. The data block on the controller's screen showed N304PA descending through 6,600 feet, yet 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 SDNR 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 incorrect use of an abbreviated call sign by the sector controller, the issuing of a descent clearance to N434PA, and the sector controller's failure to detect that the pilot of N304PA had read the clearance back with the full call sign. A contributing cause was the N304PA pilot's failure to question a clearance that put them below the published minimum en route altitude. Another contributing cause to the accident was the failure of both the center and tracon controllers to properly respond to the aural and visual MSAW alert.
According to the Aeronautical Information Manual, 4-2-4, Aircraft Call Signs:
a. Precautions in the Use of Call Signs.
Improper use of call signs can result in pilots executing a clearance intended for another aircraft. Call signs should never be abbreviated on an initial contact or at any time when other aircraft call signs have similar numbers/sounds or identical letters/number; e.g., Cessna 6132F, Cessna 1622F, Baron 123F, Cherokee 7732F, etc.
Assume that a controller issues an approach clearance to an aircraft at the bottom of a holding stack and an aircraft with a similar call sign (at the top of the stack) acknowledges the clearance with the last two or three numbers of the aircraft's call sign. If the aircraft at the bottom of the stack did not hear the clearance and intervene, flight safety would be affected, and there would be no reason for either the controller or pilot to suspect that anything is wrong. This kind of "human factors" error can strike swiftly and is extremely difficult to rectify.
Pilots, therefore, must be certain that aircraft identification is complete and clearly identified before taking action on an ATC clearance. ATC specialists will not abbreviate call signs of air carrier or other civil aircraft having authorized call signs. ATC specialists may initiate abbreviated call signs of other aircraft by using the prefix and the last three digits/letters of the aircraft identification after communications are established. The pilot may use the abbreviated call sign in subsequent contacts with the ATC specialist. When aware of similar/identical call signs, ATC specialists will take action to minimize errors by emphasizing certain numbers/letters, by repeating the entire call sign, by repeating the prefix, or by asking pilots to use a different call sign temporarily. Pilots should use the phrase "VERIFY CLEARANCE FOR (your complete call sign)" if doubt exists concerning proper identity.
Civil aircraft pilots should state the aircraft type, model, or manufacturer's name, followed by the digits/letters of the registration number. When the aircraft manufacturer's name or model is stated, the prefix "N" is dropped; e.g., Aztec Two Four Six Four Alpha.
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 the route. Anytime a similar sounding call sign is on the same frequency, use extra vigilance in communicating with ATC.
For more information about how altitudes are depicted on charts and how you can use these altitudes in flight planning, see the AOPA Air Safety Foundation's Terrain Avoidance Plan Safety Brief.
Accident reports can be found in ASF's accident database.
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