The crash of Air Moorea flight 1121: Analysis

The tailfin of Air Moorea flight 1121 after its recovery from the sea. Image source: the BEA

On the 9th of August 2007, one of the shortest flights in the world ended in disaster when an Air Moorea de Havilland Canada DHC-6 Twin Otter suddenly plunged into the Pacific Ocean, killing all 20 people on board. The crash on French Polynesia’s most popular route sparked a years-long investigation that ultimately uncovered multiple threats affecting not just the Twin Otter, but every small plane operating out of a major airport. This article delves into the obscure sequence of events that brought down flight 1121, and the safety recommendations that followed it.

An Air Moorea Twin Otter, and the route of flight 1121. Image sources: Paul Spijkers and Google

Air Moorea was a small air carrier based on the island of Moorea in French Polynesia. It specialized in short commuter flights between the scattered islands of the archipelago using its fleet of four de Havilland Canada DHC-6 Twin Otter propeller planes, which could carry 19 passengers and one pilot. Flight 1121 was Air Moorea’s busiest route, from Moorea to Faa’a on the neighbouring island of Tahiti. This flight lasted just seven minutes and Air Moorea ran it more than 40 times per day.

The Twin Otter has fully manual flight controls that are directly connected to the pilot’s yoke via steel cables. The Twin Otter operating this flight had been acquired separately from Air Moorea’s other three aircraft, and there was a small, seemingly insignificant difference between them: whereas the other Twin Otters had carbon steel control cables, this Twin Otter had stainless steel control cables. According to the manufacturer, both types were to be treated identically, and the only indication that this aircraft had stainless steel cables was a single reference number in the mountains of documentation that came along with it. As a result, Air Moorea had no idea that this plane was any different.

However, there actually was a difference between the two types of cables. The original reason for using stainless steel was that it suffered much less corrosion than carbon steel. But there was a trade-off: the stainless steel cables suffered more frictional wear than the carbon steel ones. Every time a pilot moves the control surfaces, the cables rub against various pulleys and guide holes, causing them to wear down over time.

Comparison of wear on stainless steel and carbon steel control cables. Image source: the BEA

The manufacturer appeared not to know anything about this tendency, nor had it been required to test for it. As a result, airlines independently discovered during inspections that the stainless steel cables on their Twin Otters wore down surprisingly fast, and replaced them early without informing the manufacturer. However, because the manufacturer offered no guidance on the difference between carbon and stainless steel cables, and because Air Moorea didn’t know it had both types in its fleet, it replaced all its control cables on the interval specified for carbon steel cables — about once a year.

The particular cables of interest in this incident are the elevator cables. The Twin Otter elevator control system consists of a “pitch up” cable and a “pitch down” cable which form a closed loop, allowing the elevators to move up or down when the appropriate cable is in tension. On the Air Moorea Twin Otter with stainless steel cables, the elevator pitch up cable started wearing against a guide hole, a point where the cable passes through the airplane’s internal structure. The cable is made up of seven interwoven strands, each of which is composed of 19 individual wires. By August 2007, 72 of the 133 total wires had worn through. Nevertheless, sufficient strength remained for the cable to continue bearing all the normal loads associated with flight. That is, until an unfortunate coincidence pushed it to the breaking point.

The night before flight 1121, the Twin Otter was parked overnight at Papeete-Faa’a International Airport, the main international gateway to French Polynesia. The outermost berth in Parking Area G, where Air Moorea stored its Twin Otters, was located close to a gate used by Air France’s massive wide body Airbus A340s. When jet engines spool up, they hammer everything behind them with a powerful burst of wind called a jet blast. As it turned out, if an A340 pushed back slightly too far from this gate, the plane parked in the outermost berth in Parking Area G could be struck by its jet blast, subjecting it to winds of up to 162kph.

Diagram of the possible positions of the accident aircraft and an Airbus A340. Image source: the BEA

It is highly likely that the aforementioned Twin Otter with the badly worn elevator cable was struck by just such a blast that night. The jet blast put enormous strain on the elevator, which transferred the stress to the cable. The cable could not move to alleviate the stress, however, because it was held in place by the gust lock, a device that prevents wind from moving the elevators while the plane is parked. A normal cable would not be seriously damaged by such a blast, but in this case the severely worn cable had a reduced ability to withstand the strain, and several strands snapped in the worn area. The elevator pitch up cable was left with just one of its seven original strands intact.

That last strand was enough for the elevators to keep functioning until shortly before noon the next day, when the Twin Otter took on 19 passengers for flight 1121 from Moorea back to Tahiti. At the controls was pilot Michel Santeurenne, who had just moved with his family to French Polynesia three months earlier, where he started his dream job flying for Air Moorea.

Before takeoff, Santeurenne performed the standard elevator checks, and the elevators functioned normally. Flight 1121 was soon cleared for takeoff and took to the sky shortly after 12:00, climbing out over the Pacific Ocean past popular beaches and tourist resorts. About half way to the flight’s cruising altitude of 600 feet, Santeurenne retracted the flaps, which increase lift on takeoff and landing but must be stowed at higher speeds. The tendency of the Twin Otter with the flaps retracted at that stage of flight was to pitch down, so when he retracted the flaps, Santeurenne naturally pulled up with the elevators to continue climbing. This was the largest force applied to the critically damaged elevator pitch up cable that day, and it proved incapable of handling the stress. The pitch up cable snapped, causing the airplane to succumb to its natural desire to pitch down.

Simulation of the moment of the cable failure. Video source: Mayday (Cineflix)

Santeurenne pulled up hard, but there was no response from the elevators. He uttered an expletive, the only word recorded on the cockpit voice recorder, while the plane entered an increasingly steep dive toward the water. Within moments, Santeurenne ran out of options. Just eleven seconds after the cable broke, Air Moorea flight 1121 plunged nose-first into the channel between Moorea and Tahiti, destroying the aircraft and instantly killing all 20 people on board.

Simulation of the crash. Video source: Mayday (Cineflix)

The crash occurred in full view of numerous witnesses on shore, and rescuers immediately rushed out to the crash site to look for survivors. Instead, they found only floating bodies and light debris; the main wreckage had already sunk to the bottom of the sea, taking several of its passengers with it. The tragedy hit the local community hard, and left French Polynesians asking whether something might be wrong with one of the island nation’s most popular planes.

The investigation into the crash by France’s investigative authority faced large hurdles early in the process. The wreckage came to rest on a steep underwater slope 700 meters below the surface, and a specialized search vessel had to sail more than 4,000 kilometers from New Caledonia to recover the plane. It was not until several weeks after the crash that investigators finally saw the elevator cables and noticed the damage. Even then, the full story was far from obvious. Tests showed that the wear on the cable was by itself insufficient to cause its failure. Without the coincidental jet blast encounter, the cable would likely have lasted until the next inspection, at which point it would have been replaced.

Photo of the actual cable that broke. Image source: the BEA

The investigators also found numerous points at which the accident could have been prevented. In fact, the parking area at Faa’a used to have a fence meant to protect parked planes from the jet blast effect, but it had been taken down in 2004 to make way for a new taxiway. And most importantly, the lack of separate guidance from the manufacturer regarding stainless steel control cables represented a blatant safety deficiency. The original manufacturer, de Havilland Canada, had long since given up the aircraft’s production rights to Canadian aircraft producer Viking Air, and Viking Air had not conducted any wear rate tests on stainless steel cables, apparently assuming that the existing replacement interval would be sufficient. Airlines operating the Twin Otter in the field discovered significant wear during inspections but did not pass this on to Viking Air or other airlines, preventing this critical discovery from disseminating to everyone who needed to know about it. Air Moorea regularly inspected its control cables too, but because the damage to the elevator pitch up cable was in a location that was difficult to see, it was not discovered in time. It was clear that a system based on finding and replacing damaged cables during routine inspections was insufficient, and that a shorter mandatory replacement interval was necessary. If Air Moorea had replaced its stainless steel cables on this Twin Otter on the same interval as airlines that knew about the problem, the crash would never have happened.

Wreckage from flight 1121 after being brought ashore. Image source: La Dépêche

One final tragic element to the story was that, had Michel Santeurenne known what he was facing, he could have saved his plane. Live tests in a real Twin Otter showed that if Santeurenne had used the stabilizer trim to pitch the plane up within three seconds of the failure, flight 1121 would have recovered before hitting the water. But it was unreasonable to expect him to be able to act so quickly, especially considering that he had not been trained on how to react to failures of the primary flight controls. In their final report, investigators recommended that Twin Otter pilots be trained to react to such failures.

A monument commemorates the victims of the crash. Image source: Werner Fischdick

After narrowing down the cause, the French BEA learned of similar wear on other Twin Otters with stainless steel control cables and issued an urgent recommendation to Transport Canada and the European Aviation Safety Agency calling for inspections of all such cables. In its final report, the BEA took this one step further, recommending that stainless steel control cables be banned on the Twin Otter until research into wear was performed and new maintenance guidelines created. It also called for studies of other aircraft with stainless steel control cables to see if they too could be vulnerable. They also recommended that the French Directorate General for Civil Aviation encourage communication between airlines and manufacturers regarding recurring maintenance issues and that airports be informed of the risks of jet blasts to parked aircraft. Finally, the BEA took an opportunity to correct another shortcoming before it became a problem. In France, small aircraft like the Twin Otter were not required to have cockpit voice recorders, but Air Moorea had installed one anyway. This proved invaluable to investigators, so for the sake of future investigations, they recommended that all planes with capacity for 9 or more passengers be equipped with a CVR.

This crash illustrated several areas in which safety regulations for small planes fell short of those required of large jets. To those familiar with large aircraft, it might seem inconceivable that the manufacturer didn’t know the risks associated with its own control cables, or that a passenger plane in 2007 was not be required to have any black boxes. But these sorts of deficiencies extended, and to a certain degree still do extend, far beyond Air Moorea and the Twin Otter. Thankfully, the BEA took several steps to ensure that this safety gap is closed as quickly as possible.

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Analyzer of plane crashes and author of upcoming book (soon™). Contact me via @Admiral_Cloudberg on Reddit or by email at kylanddempsey@gmail.com.