Background
The accident involved N304PA, a twin-engine airplane operating under Instrument Flight Rules (IFR) at night. The flight was one of five company aircraft traveling the same route, with separation of about 5 to 10 minutes between each. The preceding aircraft was N434PA.
Flight Progress
After being handed off from Los Angeles Center, the crew of N304PA contacted the TRACON sector controller at 2043:48 Pacific Daylight Time, reporting level at 8,000 feet. The controller instructed the pilot to fly heading 260 after the Julian VOR and intercept the localizer.
At 2045:47, the controller cleared N434PA to descend to 6,000 feet, which was acknowledged. At 2047:55, the controller 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." The controller stated that this clearance was intended for N434PA, but did not recognize that N304PA had acknowledged it instead.
At 2049:55, the pilot of N304PA reported having ATIS Information Zulu. The controller responded, "Seminole three zero four papa alpha thank you very much." At that time, the data block on the TRACON controller's display showed N304PA descending through about 6,600 feet. The Minimum Enroute Altitude for the airway segment was 7,700 feet.
At 2050:27, the controller again cleared N434PA to descend to 5,200 feet.
Air Traffic Control Warnings
A Minimum Safe Altitude Warning (MSAW) alert on N304PA was presented to the Center controller at 2049:03. The Center controller did not notify the TRACON sector controller of the MSAW alert as required by FAA directives. The aircraft's data block remained on the Center controller's display until it was forced off at 2050:07.
The TRACON's MSAW system generated two predicted altitude alerts on N304PA at 2050:46 and 2050:51. According to FAA documentation, two consecutive predicted alerts would initiate a 5-second aural alert starting at 2050:51 and a flashing red "LA" in the data block from 2050:51 to about 2051:06. The aircraft then descended below radar coverage, its track entered coast status, and the alert terminated.
Collision
The wreckage of N304PA was located on a ridgeline 200 yards south of the Julian VOR at 5,537 feet mean sea level. No preimpact mechanical anomalies were identified.