A History of Hawker Siddeley HS121 Trident Aircraft Accidents

While the Trident’s sales may have failed to attest to the design’s integrity, competition notwithstanding, its accident history may confirm this fact. In the 15-year period between 1964, when it entered service, and 1978, there were only two fatal incidents, neither of which was due to a design flaw. Both, however, entailed what could be considered the “perfect storm” of interrelated circumstances, the elimination of one of which may have prevented their occurrence.

The first did not take place until June 18, 1972. In this, a BEA Trident 1 under the command of Captain Stanley Key, 51, and First Officer Jeremey Keighley, 21, and operated by aircraft G-ARPI bound for Brussels on the continent, initiated its takeoff run from London’s Heathrow International Airport with seven other crew members and 109 passengers aboard, one of whom was an infant.

Rotating into the gray, rain-emitting obscurity at 5:00 p.m. local time, it retracted its tricycle undercarriage and ascended through the onslaught of precipitation. Yet, only minutes into its flight, it lost airspeed and plummeted to the ground near the city of Staines, itself only four miles from the airport.

Losing its tail as it snapped off, the remainder of the fuselage careened 50 yards before coming to rest in front of a line of trees. As Britain’s worst airline crash, it claimed the lives of all 118 on board, including one who perished shortly after being brought to a hospital.

During the ensuing triage, which entailed placing the bodies on a grassy area before they were transported to a temporary, airport-vicinity morgue, speculation as to why a seemingly routine takeoff could so unexpectedly end in disaster. That the aircraft’s undercarriage had been retracted after it had established a positive rate-of-climb, indicated that nothing seemed amiss.

During the subsequent investigation, whose official inquiry began in November of 1972, it was learned that two significant events had led to the crash.

In the first, Captain Key had become embroiled in a heated dispute with another pilot about a pending strike, which he apparently favored. Although he subsequently apologized for his outburst, the exchange proved far more detrimental to him than he was aware of at the time. In the second, which was to prove the accident-causing catalyst, the premature retraction of wing high lift devices at the improper speed occurred.

Although the aircraft’s black box confirmed that both the takeoff and the initial climb had occurred in accordance with operating procedure, the leading edge slats having been retracted at a 162-knot airspeed and at a 1,750-foot altitude, the former far below the recommended 225- to 250-knot one. Both lift and camber, needless to say, were decreased, leaving the tri-jet to attempt to climb at an airspeed that could not sustain its flight. Its resultant stall, especially at a point where insufficient altitude remainder in which to recover, led to its groundward plummet.

The actual leading edge device retraction had been activated by the first officer. While he was relatively young at only 21 years of age, it could not initially be understood why a seasoned captain had not caught the error. But the answer was revealed when the heart condition he unknowingly suffered from and which had been aggravated by his pre-flight argument.

During the post-crash autopsy, it was discovered that the emotional turmoil had served as the catalyst to his atherosclerosis condition and that it had produced sudden, sharp chest pains immediately after the aircraft had taken to the sky. So severe were they, in fact, that they distracted his attention and temporarily impaired his functioning, leaving the first officer to initiate the retraction, most likely in the midst of cockpit chaos.

With the slats now in their stored position, the control column began to automatically shake, warning the crew of imminent stall, in the midst of audible signals and the flashing red and amber warning lights. Two corrective actions that are almost intrinsically instilled in all pilots include the immediate forward push of the control column to bring the nose down and the retraction of the leading edge slats. But the medical condition precluded implementation of either.

At this point during the short, 150-second fight, the automatic stall warning system was activated, moving the control column forward without human intervention. But even this last-ditch action failed to save the aircraft when the system was deactivated by means of an override switch.

Virtually exhausted of all altitude reserve, the Trident 1 continued to stall and dove groundward.

Because the captain’s heart condition was not detectable during routine medical examinations, only the subsequent autopsy revealed it, along with the fact that the heated argument had resulted in his internal hemorrhaging and that that had caused the distraction. Although the premature slat retraction was cited as the direct cause of the accident, the captain’s unknown health condition had precipitated it.

The Court of Inquiry concluded that Captain Key “was, throughout takeoff and initial climb, distracted by some degree of pain and discomfort and that the stage was reached where his powers of reasoning were materially affected.”

Several recommendations resulted from the crash, including more stringent coronary tests for pilots, a mechanism to inhibit premature leading droop retraction at below-minimum airspeeds, and the installation of cockpit voice recorders on all large airliners.

The second of the two fatal Trident accidents occurred four years later, on September 10, 1976, but entailed a mid-air collision between two aircraft.

The first, a British Airways Trident 3 registered G-AWZT and under the command of Captain Dennis Tann, 44, and First Officer Brian Helm, 29, departed London-Heathrow at 9:21 a.m., bound for Istanbul, Turkey, on a three-hour, 24-minute flight. Its flight plan, at 33,000 feet, took it over Dover, Brussels, Munich, Klagenfurt, and Zagreb.

In a seemingly unrelated event, the second aircraft, an Inex Adria DC-9-30 registered YU-AJR, took off from the coastal resort of Split, Yugoslavia, at 10:48 a.m. with five crew members and 108 passengers, operating a charter flight to Cologne, Germany

One other aircraft was presently in the Zagreb-controlled airspace-a Lufthansa Boeing 737-100 at flight level two-nine-zero, flying from Belgrade to Frankfurt.

The paths of the first two were about to cross. While the Trident was following its flight plan at 33,000 feet, the Inex Adria DC-9 was climbing through 33,000 to its assigned, 35,000-foot altitude in accordance with its clearance. But the two radar screen blips in the Zagreb Air Route Traffic Control Center began to merge.

The controller, sparing no time in alerting the Inex Adria crew of the potential collision, blurted out evasive instructions, but in his native Serbo-Croatian. The British Airways crew, needless to say, failed to understand them and the transmission proved no remedy to them. Closing in on each other at a combined, 1,000-mph, the two jetliners, collectively weighing some 250,000 pounds, collided and was reduced to disintegrating metal dust.

The DC-9 smashed into the Trident’s underside, snapping off one of its wings, and proceeded to spiral earthward. The Trident’s starboard wing had, in the meantime, practically disintegrated, leaving what remained of the aircraft without lift and lateral control and sparking its own vertical dive. It was 11:16 a.m.

As both impacted seven miles from one another, the sky thundered from the explosive impact, taking the lives of all but one of the 176 aboard both aircraft with it and marking the worst commercial aviation midair collision up to that time. A girl, clinging to life, miraculously survived the 33,000-foot plunge, but succumbed to injuries an hour later.

Cockpit voice recordings indicated that the British Airways crew could have taken evasive action if it had understood the Zagreb controller’s plea. Eight air route traffic controllers were subsequently charged with “failure to ensure the use of English in conversations with pilots… failure to ensure that subordinates carried out standard regulations… (and) negligence in performing their professional duties.”

One of them, Gradimir Tasic, 28, was initially sentenced to seven years in prison for criminal negligence, marking the first time in commercial aviation history that an air traffic controller had been cited as the personal cause of an aircraft accident.