Cause Unknown- page 2 | History | Air & Space Magazine
Reconstruction of a South African Airways Boeing 747 has failed to reveal what started an onboard fire, which led to the loss of 19 crew members and 140 passengers. (Courtesy BEELD)

Cause Unknown

What brought down these five airplanes?

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(Continued from page 1)

Seven and a half months before the accident, another Northwest Airlines DC-7C had ditched, this time in the water off Sitka, Alaska. All 102 on board survived. It was the same flight number, with the same origin and destination, as the aircraft that later crashed in June. The earlier aircraft had lost power in one engine, followed by uncontrollable propeller overspeeding.

Aer Lingus Flight 712

Aer Lingus Flight 712 was to be a short trip, covering the 361 miles from Cork, Ireland, to London’s Heathrow Airport. The Vickers Viscount, named St Phelim, took off the morning of March 24, 1968. Less than 45 minutes later, it went down in the Irish Sea. All 57 passengers and a crew of four were lost.

The four-engine turboprop had reached 17,000 feet in a clear sky, yet soon after, as the pilots reported in their final radio contact, it was at “12,000 feet, descending, spinning rapidly.” The crew managed to regain control and fly for about 10 minutes before the dive into the sea.

Only 14 bodies were recovered. The St Phelim carried no recorders, and even though the main debris field was only six miles from the Irish coast, much of the wreckage was either unrecovered or damaged further during salvage. Still, analysis of that wreckage enabled Irish government investigators to rule out engine failure or some type of explosion. Something had happened to impair the Viscount’s pitch control, but investigators could not determine what it was.

But that was hardly the end of the case. Among numerous accident scenarios, the investigation had considered the possibility that something manmade flew close to the St Phelim or even struck its tail. The report called this nothing more than a “remote possibility,” but indicated that it did constitute a “coherent” hypothesis for explaining all the evidence, including eyewitness statements. Decades of speculation and conspiracy theories followed, with chief suspicion focused on the British military. In one account, the airliner was downed by an errant missile, fired from a base on the west coast of Wales or from a warship. Another explanation blamed a drone aircraft.

Following the 30th anniversary of the accident, families of the victims organized and called for a fresh inquiry. The Irish government agreed, and established a new review team, including investigators from France and Australia. Their 2002 report dismissed suggestions of a missile strike or other such encounter and pointed instead to a failure in the left horizontal stabilizer and elevator. The report cited metal fatigue, corrosion, control surface vibration, and bird strike as possible causes.

Southern Airways Flight 932

On the campus of Marshall University in Huntington, West Virginia, a prominent building is the Memorial Student Center. It commemorates the evening of November 14, 1970, when the school lost most of its football team in an airplane crash at nearby Tri-State Airport. With two pilots and two flight attendants, Southern Airways Flight 932 was a charter for the school, returning 71 team members, coaches, university staff, and officials from a game in Greenville, North Carolina, where Marshall had lost to East Carolina University, 17 to 14. Mist and light rain restricted visibility in the Huntington area as the pilots were attempting to land. One mile short of the runway, the McDonnell Douglas DC-9 struck trees, then crashed and burned, killing all on board. (The accident and its aftermath are the subject of the 2006 film We Are Marshall.)

Some airport instrument landing systems are equipped with both a localizer antenna array, which provides inbound aircraft with lateral guidance, and a glide slope for vertical guidance. Pilots flying a precision approach follow the glide slope to the touchdown point. But like many airports, Tri-State had only the localizer (following the accident, a glide slope was installed at the airport). Without a glide slope, pilots fly a non-precision approach, which requires them to stay above a designated minimum descent altitude until the runway is in sight. An individual MDA for approaching each runway, based on terrain and other considerations, is established and published. In its investigation, the NTSB determined that the DC-9 was flying below the MDA, but the board could not establish why.

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