On Sunday, June 6, 1971, the McDonnell Douglas DC-9 serving as Flight 706 departed Los Angeles just after 6 p.m. en route to Seattle as a McDonnell Douglas F-4 Phantom II of the United States Marine Corps was approaching Marine Corps Air Station El Toro near Irvine at the end of a flight from Naval Air Station Fallon in Nevada.

The two aircraft collided in midair over the San Gabriel Mountains near Duarte, killing all 49 aboard the DC-9 and the F-4 pilot; the F-4 radar intercept officer ejected and survived.

[2][1] Flight 706 departed from Los Angeles at 6:02 pm PDT, bound for Salt Lake City, Utah, the first of the five intermediate stopovers, followed by Boise and Lewiston in Idaho, and Pasco and Yakima in Washington before ending at Seattle.

[1] '458' was part of a cross-country flight of two aircraft when its radio failed while landing at Mountain Home Air Force Base in southwest Idaho.

[4] As the fighter proceeded to NAS Fallon in Nevada, the oxygen leak deteriorated until the system was disabled completely, and the pilot was instructed to fly at low altitude.

The Phantom II departed NAS Fallon at 5:16 pm following a flight plan routing across the Fresno, Bakersfield, and Los Angeles air corridors.

Under IFR procedures, the pilot guides the aircraft using the cockpit's instrument panel for navigation, in addition to radioed guidance from air traffic controllers and ground radar.

[1] Soon after reaching 15,500 feet (4,700 m), the fighter's DME (radio) showed MCAS El Toro was 50 nautical miles (93 km; 58 mi) away.

[12] The stricken airliner crashed onto Mount Bliss in the San Gabriel Mountains at an elevation approximately around the 3,000 feet (900 m) level,[13] where the bulk of the wreckage landed in a gorge.

[12] The National Transportation Safety Board (NTSB) investigated the incident, assisted by the United States Marine Corps, Federal Aviation Administration (FAA), Hughes Airwest, and the Airline Pilots Association.

[17] Early statements released by the NTSB revealed the F-4B fighter had attempted to swerve away from the DC-9 immediately prior to impact, and that an additional ten feet (3 m) of clearance would have averted the collision.

Other causal factors include a very high closure rate, commingling of IFR and VFR traffic in an area where the limitation of the ATC system precludes effective separation of such traffic, and failure of the crew of BuNo458 to request radar advisory service, particularly considering the fact they had an inoperable transponder.During the course of the accident investigation, the NTSB attempted to recreate the conditions of the accident to determine the visibility of BuNo458 on June 6.

The NTSB, Federal Aviation Administration (FAA), and the Marine Corps flew a series of F-4B fighters along the flight paths described by Schiess, the radar intercept officer, and various witnesses.

While the tests were sufficient to determine the difficulty in locating and identifying the fighter on the radar scope, the many other variables involved in the June 6 incident, including the deteriorated condition of '458', compromised the validity of the study.

These recommendations included: installing recorders for radar displays, installing audio conversation recorders at air traffic control facilities; establishing climb and descent corridors under ATC positive control in the vicinity of air terminals; and establishing more definitive procedures for receiving and handling the emergency transponder code 7700.

Additionally, the NTSB strongly recommended that the FAA and the Department of Defense cooperate to develop a program, in areas where a large intermix of civil and military traffic exists, to ensure that appropriate graphical depictions of airspace utilization and typical flow patterns are prominently displayed at all airports and operational bases for the benefit of all airspace users.

[23] Congressmen Sherman P. Lloyd (R-Utah) and Henry S. Reuss (D-Wis) both decried the actions of the Marine Corps jet fighter,[24] which media at the time indicated had been "stunting" prior to the collision.

[27] Oscar M. Laurel, a member of the National Transportation Safety Board (NTSB) team investigating the crash, was widely quoted saying that now "may be a good time to take another look" at VFR flights near metropolitan areas.

[19][20][28] The validity of the "see and avoid" doctrine as a safe means of aircraft navigation was a point of contention between the NTSB and the Federal Aviation Administration (FAA).

Additionally, near-miss situations involving jetliners occurred on average at least once per day, with the Los Angeles and New York areas noted as being especially high-risk.

Finally, the report noted that the current trend in air casualties indicated that a further 528 people would die in mid-air collisions during the following ten years.

[31][32] On June 21, 1971, 15 days after the collision, the Airline Pilots Association and Professional Air Traffic Controllers Organization issued a joint statement, asking the FAA for a series of safety regulations that included a speed limit of 250 knots (290 mph; 460 km/h) for aircraft operating under VFR.

[35] A week after the crash, the families of various passengers aboard Flight 706 began filing lawsuits and court papers against Hughes Airwest and, later, the government of the United States.

In January 1972, the Gabel family filed a lawsuit against the United States that contained class-action allegations and sought a "declaratory judgment on the issue of liability".