Cause Unknown

Like the Royal Canadian Mounted Police, who always get their man, aircraft accident investigators are expected to solve every case. But Mounties don’t nab every crook, and occasionally, an airplane crash can’t be explained. Mystery compounds an accident’s tragedy, leaving families to wonder about the fate of those they lost. And from the perspective of those in the airline industry, understanding the cause of an accident is the only way sufficient measures can be taken to prevent a recurrence.

The tools and techniques of accident investigation continue to advance. In the past, flight data recorders could capture only five aspects of aircraft performance; some current digital devices can record more than 1,000. With new software, investigators can also make better use of that data in documenting an aircraft’s final moments. Not only has accident investigation grown more sophisticated, air travel has become safer. With these strides, unsolved—or partially unsolved—cases have become rarer. Still, one such rarity may be developing in the South Atlantic. As this article goes to press, the inquiry into the 2009 crash of Air France Flight 447 continues.

Investigators try to learn not only what happened in an accident but why. The “why” is usually the tougher question. In 1972, for example, as a British European Airways Hawker Siddeley Trident departed London’s Heathrow Airport, the leading-edge slats were prematurely retracted, and the airliner fell to the ground. Did a medical emergency incapacitate the captain? There is evidence suggesting this, but no proof.

Sometimes an accident stumps investigators until similar subsequent disasters establish a pattern and point the way to solution and prevention. That was the case with the infamous 1954 de Havilland Comet crashes, which stemmed from a flaw in the design of the airliner’s windows. In other cases, solutions are found but disputed. National Transportation Safety Board (NTSB) investigators concluded that the 1999 loss of an EgyptAir Boeing 767 and the 1997 loss of a 737 from Indonesia’s SilkAir were both caused by the pilots intentionally crashing the aircraft, but in each case, the airline’s government does not accept the verdict.

In some cases, investigators may need decades to close the books. In 1947, a British Lancastrian airliner crashed high in the Andes Mountains. It took 53 years for a glacier that had encased part of the wreckage to melt sufficiently for investigators to find the remains and examine them for clues. A team organized by novelist Clive Cussler has been searching the bottom of Lake Michigan for a Northwest Airlines Douglas DC-4 lost in 1950.

Here are five of the most stubbornly unyielding mysteries in aircraft accident investigation.

Northwest Airlines Flight 293

In the history of unsolved aviation accidents, the shroud of mystery has most often been the water that covers nearly three-quarters of Earth’s surface. That was the case for a Northwest Airlines DC-7C carrying six crew members and 95 passengers on June 3, 1963. Flight 293 was a charter, transporting members of the military and their families, as well as Department of Defense employees, from McChord Air Force Base, near Tacoma, Washington, to Elmendorf Air Force Base, near Anchorage, Alaska.

During the first half of the trip, radio communications indicated an uneventful flight. About two and a half hours after departure, though, the pilots requested clearance to climb from 14,000 to 18,000 feet. Controllers told them there was traffic at the requested altitude. No one replied. In that interval, something catastrophic occurred, but what? The answer lies under more than 8,000 feet of water in the Gulf of Alaska.

The report on the crash of Flight 293 is a slim document. The investigation chronicled the seemingly innocuous prelude to the flight: Aircraft mechanical condition, crew qualifications, and the like all seemed to be in order. In the aftermath of the accident, a recovery operation yielded only about 1,500 pounds of wreckage. Investigators conducted as much analysis as they could—determining, for example, that there was no indication of an inflight fire or explosion. The degree of fragmentation suggested that the aircraft hit the water at high speed. And the deformed shape of the seat backs indicated that the fuselage came down nearly inverted. The pattern of floating wreckage showed that the airframe probably remained intact until impact. The report considered the possible reasons the pilots requested altitude change, such as to avoid icing or turbulence. In the end, the Civil Aeronautics Board concluded its inquiry without a finding of probable cause.

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.

Commercial aircraft are customarily equipped with several altimeters, both barometric and radio. According to the NTSB, the barometric altimeter might have been malfunctioning or the pilots might have been relying too much on the radio altimeter in an area of uneven terrain. The crash would likely have been averted if the cockpit had been equipped with a Ground Proximity Warning System, which is now required in all airliners in the United States.

South African Airways Flight 295

South African Airways’ Helderberg was a Boeing 747 Combi, an airliner whose main deck could be partitioned to carry both passengers and cargo. On November 28, 1987, the Helderberg was carrying 140 passengers, 19 crew members, and six pallets of cargo on a flight from Taipei, Taiwan, to Mauritius and then on to Johannesburg, South Africa. Less than an hour before the estimated arrival in Mauritius, Flight 295 reported smoke in the aircraft. Controllers could hear the pilots struggling with the emergency before communications went out. The Helderberg crashed in the Indian Ocean. No one survived.

Search aircraft and vessels retrieved a small amount of floating wreckage and human remains, but most of the debris settled at the bottom of the ocean. More than a year after the accident, a deep-sea salvage operation retrieved the cockpit voice recorder and a substantial quantity of wreckage. The recorder tape provided some information, primarily showing how rapidly the aircraft’s systems were compromised, but it was not as revealing as hoped. South African investigators concluded that fire had broken out in the Helderberg’s right forward cargo pallet and been fed by plastic and cardboard packing materials, but the ignition source remained unknown.

The Helderberg disaster was controversial because of the apartheid policy of the government that owned the airline. Initial supposition was that Flight 295 was the victim of anti-apartheid sabotage. But when the government’s own investigation found no evidence of explosion, suspicions shifted. Conspiracy theories to this day hold that the Helderberg carried dangerous cargo not listed on the manifest, and that the government of the time was using commercial aircraft such as the 747 to circumvent the international arms embargo against the white-controlled regime.

Payne Stewart Learjet 35

All aircraft accidents are terrifying, but the story of the last flight of professional golf champion Payne Stewart has an added element of strangeness. It was an incident that unfolded over nearly four hours and 1,500 miles. Stewart, known for competing in the colorful golfer attire of an earlier era, had won the U.S. Open for the second time four months before he boarded a Learjet 35 on October 25, 1999. He, two agents who represented him, and a golf course designer were flying from Orlando, Florida, to Dallas, Texas.

The flight had a two-pilot crew, and for eight minutes after takeoff, their radio messages were routine. But when communications suddenly ceased and the aircraft began to veer off course, military aircraft were dispatched to intercept. F-16 pilots found the jet with opaque cockpit windows, dark interior, and unmoving flight controls. With the autopilot apparently following an unchanging course, the ghost ship cruised above 46,000 feet until it exhausted its fuel and spiralled into a South Dakota field. The jet hit the ground at a steep angle and near-supersonic speed, shattering at impact.

From the start, the Learjet’s performance, the frosted windows, and the crew’s unresponsiveness made clear what led to the crash: crew incapacitation from hypoxia, stemming from cabin pressure loss. What could not be determined is why the cabin lost pressure. Also unknown is whether the pilots were using the aircraft’s supplemental oxygen, and if so, why it failed to keep them conscious long enough to act. Unfortunately, the aircraft carried no flight data recorder, and its cockpit voice recorder could capture only the last 30 minutes of the flight.

Lester A. Reingold worked for seven years at the National Transportation Safety Board. He thanks airline historian Robin MacRae Dunn for his contributions to this article.