Widerøe Flight 933, also known as the Mehamn Accident (), was the crash of a de Havilland Canada DHC-6 Twin Otter operated by Norwegian airline Widerøe. The Twin Otter crashed into the Barents Sea off Gamvik, Norway on 11 March 1982 at 13:27, killing all 15 people on board. The results of the four official investigations were that the accident was caused by structural failure of the vertical stabilizerAlso called the vertical fin, the vertical stabilizer is part of the empennage or tail. The aircraft's rudder is mounted on the rear of the vertical stabilizer. during clear-air turbulence. A mechanical fault in the elevator control system caused the pilots to lose control of pitch; and either a series of stalls or a high-speed gust of wind caused the aircraft to lose altitude without the ability of the crew to counteract, resulting in the failure of the vertical stabilizer.
The accident occurred during a NATO military exercise, within a self-declared no-fly zone for allied military aircraft. An extensive search and rescue operation was carried out and the submerged wreck was found on 13 March. The aircraft and all but one of the deceased were retrieved. An official investigation was concluded on 20 July 1984.
A conspiracy theory later emerged after the accident investigation was concluded, claiming that the accident was caused by a mid-air collision with a Harrier jump jet of the British Royal Air Force. The theory is based on reports which emerged years or decades after the accident. The claims and renewed press interest resulted in three additional investigations, established in 1987, 1997, and 2002. All four investigations came to the same general conclusions and rejected a collision.
Aircraft
The accident aircraft was a 19-passenger de Havilland Canada DHC-6 Twin Otter, registration LN-BNK.Parliament: 30 It was built by de Havilland Canada in 1977, delivered new to Widerøe and registered in Norway on 9 February 1978. The aircraft had been damaged by jet blast from a Douglas DC-9 at Tromsø Airport in March 1980, after which the rudder was replaced. Beyond this, the Twin Otter had not been subject to any other extraordinary incidents. It had met all requirements regarding maintenance and certification and had operated for about 10,000 hours at the time of the accident.Parliament: 36
Flight Widerøe Flight 933 was a scheduled service from Kirkenes Airport, Høybuktmoen to Alta Airport, with intermediate stops at Vadsø Airport, Berlevåg Airport, Mehamn Airport and Honningsvåg Airport, Valan.Jaklin: 287 Captain Peer Christian Hovring was aged 38 and First Officer Hans Breines was 26 years old.Parliament: 29 The aircraft had thirteen passengers on board, including two children, when departing Berlevåg. The weather was clear, but with a strong wind from the south. The captain chose to fly with visual flight rules. Other aircraft that passed through the accident area after Widerøe Flight 933 experienced strong turbulence between and Aircraft use feet as the unit of measure for altitude instead of meters in altitude.Parliament: 22
On 11 March 1982 the Twin Otter left Berlevåg at 13:19 – 11 minutes early, thus causing one passenger to miss the flight. The first officer reported to Mehamn Aerodrome Flight Information Service (AFIS) at 13:22 that the aircraft was at altitude over the Tanafjord and had an estimated time of arrival of 13:33. Radio contact ended at 13:22:53. The flight path was monitored by an officer at the Royal Norwegian Air Force (RNoAF) control and reporting center in Honningsvåg between 13:23:20 and 13:25:25, after which the aircraft no longer appeared on radar. The officer at the RNoAF center presumed that the aircraft had fallen below the radar horizon of in altitude.
Mehamn AFIS radioed Flight 933 at 13:35:52 but received no answer. After several attempts, Mehamn AFIS contacted Berlevåg AFIS and Kirkenes Airport, which also failed to make radio contact. A Widerøe aircraft en route from Honningsvåg to Mehamn also attempted to make contact. The Joint Rescue Coordination Centre of Northern Norway was informed of the situation at 13:41 and immediately coordinated a search and rescue operation. Three groups of the Norwegian Red Cross Search and Rescue Corps were dispatched and ten ships in the area volunteered to assist in the search. They were supplemented by two search and rescue vessels from the Norwegian Society for Sea Rescue (NSSR); the Royal Norwegian Navy diving vessel Draug; and the Norwegian Coast Guard vessel Horten.Parliament: 31
Two RNoAF CF-104 Starfighters and a Westland Sea King at Banak Air Station participated in the search along with two other military helicopters, as did a Twin Otter of Widerøe and an aircraft of Norwegian airline Norving. Parts of the crashed aircraft were found at about 18:00 on the day of the crash. The breakthrough came at 17:39 on 13 March, when an NSSR vessel found the wreck at depth, north of Teistbergan. The following day two police divers explored the site and confirmed it was the missing aircraft. One of the divers suffered from decompression sickness, which caused a lifelong brain injury and subsequent disability.
A troop of divers arrived from Ramsund Naval Base that evening and started bringing up the bodies. The last body was retrieved on 20 March.Parliament: 32 Everyone except the captain was found, although one body was located from the wreck. As the passengers would have been exposed to a force of 50 to 100 g at the time of impact, they would all have died instantly. Widerøe paid a maximum compensation of 330,000 Norwegian krone (NOK) per casualty, costing the airline between NOK 4 and 5 million.
Cause Investigation of the wreckage showed that, prior to the accident, there were cracks in the torque tube connecting the port elevator to the elevator control system. Widerøe, de Havilland Canada, and the initial investigation commission were of an opinion that this was irrelevant to the accident, while Transport Canada and the Swedish National Defence Research Institute were of the opinion that this caused a weakening in the structure. The accident area was suffering from strong clear-weather turbulence up to altitude. The wind speed was comparable to the levels used to certify the aircraft. Twin Otter pilots from both Widerøe and the RNoAF confirmed that control of the aircraft's altitude could be difficult in conditions of strong turbulence.Parliament: 302
It is possible that the turbulence caused the torque tube driving the port elevator to break; this would allow the port elevator to move freely, but the pilots would still retain approximately half of their attitude control. The aircraft also had a defective push–pull rod and worn steering cables, although it is not certain if this was part of the cause.
The direct cause of the accident is presumed to be the collapse of the vertical stabilizer. There are two alternative explanations for how the crash may have happened. The first explanation is the lack of vertical control caused by the broken elevator torque tube stalled the aircraft. Because of the lack of pitch control, it would have been easy for the aircraft to fall back into a stall repeatedly, each time losing altitude. The aircraft would thus eventually have fallen into the sea. The vertical fin and rudder would have broken off on impact. This theory does not explain why the vertical stabilizer was found at such a distance from the aircraft.Parliament: 303
The second explanation is based on the aircraft reaching a speed of at least ,Aircraft use knots (nautical miles per hour) as the unit of measure of airspeed compared to the aircraft's ordinary cruise speed of . This is possible under extreme wind conditions, given that the pilots had lost the use of the elevator. Simulations show that each gust of wind had a 0.5 percent possibility of the rudder breaking, and the last commission of inquiry found that it was likely that the aircraft could have been hit by at least ten such gusts. This alternative explains why the vertical stabilizer was found at such a distance from the aircraft.
Military activity NATO was carrying out the military exercise "Alloy Express" in Northern Norway from 24 February to 24 March 1982.Jaklin: 298 The exercise was practicing the deployment of NATO forces into Northern Norway in response to a Soviet invasion. Among the aircraft participating were RNoAF F-5 Freedom Fighters, F-104 Starfighters and F-16 Fighting Falcons, as well as United States Air Force F-4 Phantom IIs and F-15 Eagles. The Royal Navy's aircraft carrier HMS Invincible was anchored in Vestfjorden and her Sea Harriers were participating, as were a detachment of Hawker Siddeley Harriers of No. 1 Squadron RAF flying out of Tromsø Airport. Norwegian policy prohibited any NATO aircraft from operating east of the 24th meridian east, unless they had explicit permission from the government; no such permission was granted for "Alloy Express".Jaklin: 291
On the day of the accident, two Harriers flew on missions from Tromsø, named Red 1 and Red 2. The first was the two-seat Harrier serial number XW925 with Wing Commander Peter Squire as a pilot and Norwegian Major Bjørnar Vollstad.Jaklin: 295 The aircraft left Tromsø at 14:27, nearly an hour after the crash.Jaklin: 296 They flew to a gunnery range near Setermoen, followed by Red 2, a Harrier GR.3. The shooting range was enveloped in a blizzard, but Squire chose to carry out the practice. Red 1 was subject to an air burst; Squire did not register any damage, but chose to return to Tromsø and asked for a priority landing to be on the safe side. The pilot of Red 2 believed his weapons were not secured and chose to carry out a priority landing at Bardufoss Air Station, located nearby.
Breaching of the 24th meridian policy by military aircraft during exercises would be discovered by the control and reporting centers, which would order the aircraft to turn back. Any incidents would be logged at Control and Reporting Centre Sørreisa (CRC) and at the radar station tracking the aircraft. During an exercise three years later on 13 March 1985, two Sea Harriers from HMS Invincible, then anchored off Andøya, were recorded as being inside the no-fly zone, comparatively close to the 1982 site of Invincible.Jaklin: 294 This was the only incident that the parliamentary commission was able to verify. The commission's members carried out interviews with dozens of civilian and military air traffic controllers, pilots and boat skippers; none had ever observed foreign aircraft in the no-fly zone, although all had heard rumors of such activity. The commission concluded that alleged frequent breaches of the zone were the result of an urban legend.Parliament: 332
Investigations
First investigation The first investigation was carried out by a military commission, led by Lieutenant General Wilhelm Mohr and consisting of Captain Stein I. Eriksen, Police Inspector Liv Daae Gabrielsen, and pilot Hallvard Vikholt. The wreckage was salvaged by MS Hugo Trygvasson starting on 16 March. The pieces of wreckage were hoisted on board and stored before being freighted to a hangar at Bodø Airport, where the investigation was carried out. Lack of space on board the ship meant that the wreckage was stored in a heap and could have suffered damage during loading and transport. Some of the technical analyses of the aircraft's components were carried out by de Havilland Canada and by Transport Canada.Parliament: 312