Hughes Airwest Flight 706 was a regularly scheduled domestic flight operated by Hughes Airwest from Los Angeles, California, to Seattle, Washington, with several intermediate stops. On Sunday, June 6, 1971, the aircraft involved in the incident, a two-year-old McDonnell Douglas DC-9-31 with the registration N9345, had taken off from Los Angeles International Airport (LAX) and was en route to its first stop at Salt Lake City International Airport (SLC).

The second aircraft involved in the midair collision was a United States Marine Corps F-4B Phantom on an overnight cross-country flight to McChord Air Force Base (AFB), located just south of Tacoma, Washington. During a refueling stop at Mountain Home AFB, Idaho, the aircraft was grounded for repairs to have mechanical difficulties sorted out. The plane's radar transponder was not working, and there was a leak in the oxygen system. Despite being unable to fix the transponder issue at a second stop at Naval Auxiliary Air Station (NAAS) Fallon, Nevada, the plane was instructed to fly at a low altitude to Marine Corps Air Station El Toro near Irvine, California.

The planes collided over the San Gabriel Mountains

Hughes Airwest Flight 706 departed LAX at 18:02 for the first of five stops en route to Seattle–Tacoma International Airport (SEA). Four minutes after takeoff, while passing through 12,000 feet, the aircraft was instructed to head 040 (magnetic) until receiving the Daggett VOR, then direct. Flight 706 acknowledged the request, which was the last communication from the plane.

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McDonnell-Douglas F-4 Phantom. Photo: Bengutterly via Wikimedia Commons.

The Marine Phantom, operating as flight "458," decided to deviate from its flight plan to avoid air traffic in and around Los Angeles. Climbing to 15,000 feet to avoid deteriorating weather conditions, the plane performed a 360° aileron roll to allow the pilot to observe any air traffic above or below him. Meanwhile, the aircraft radar intercept officer could not detect any aircraft until he suddenly glanced up and saw the DC-9. He immediately shouted a warning to the pilot, who attempted an evasive maneuver but was unable to avoid the oncoming airliner.

Witnesses in the nearby town of Duarte, California, reported hearing a loud explosion and two aircraft in flames crashing into the San Gabriel Mountains. The radar officer had managed to eject and parachute to safety while the pilot of the Phantom went down with the plane. All 49 passengers and crew aboard the DC-9 died in the crash.

The NTSB investigation

Assisted by the United States Marine Corps, the National Transportation Safety Board (NTSB) immediately opened an investigation into what had caused the crash. Early statements released by the NTSB suggested that the military jet had attempted to swerve away from the airliner and that an additional ten feet of clearance could have avoided the collision.

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Wreckage of the F-4 Phantom. Photo: NTSB via Wikimedia Commons.

The NTSB also noted that the F-4 Phantom's right side wing had slashed through the front of the passenger cabin and cockpit. Despite having recovered the DC-9s flight data recorder, the cockpit voice recorder tapes were destroyed by fire.

The probable cause of the collision and recommendations

In its final report on the accident, released on August 30, 1972, the NTSB concluded that the cause of the accident was the failure of both aircraft to see and avoid each other. It also recognized that both planes had a marginal capability to detect, assess, and avoid the collision. Other factors contributing to the collision included:

  • A very high closure rate
  • Limited Air Traffic Control (ATC) in the area
  • A failure of the F-4 crew to request a radar advisory because they had an inoperable transponder.

In its final recommendations, the NTSB recommended that the Department of Defense restrict high-speed, low-altitude flights in civilian aircraft corridors. Consider installing collision avoidance technologies on military aircraft, and make pilots aware of the FAA's radar advisory service.