On a sunny day off the coast of the island of Moorea in French Polynesia, a Twin Otter aircraft piloted by Captain Michel Santurenne with 19 passengers onboard crashed shortly after takeoff. In episode 75 of Take to the Sky: the Air Disaster Podcast, we explore the investigation that proved jet blast from another plane damaged an already worn-down elevator cable that snapped as the plane climbed toward 400 feet, causing it to go into a steep dive. The investigation also highlighted the importance of having jet blast deflectors (JBDs) at airports where both small and large aircraft takeoff and land, so the structural integrity of small aircraft is protected from the jet blast of larger aircraft.
On August 9, 2007, Captain Michel Santurenne was preparing for yet another flight on his usual route. He spent his days flying back and forth in a Twin Otter aircraft, taking passengers from the island of Moorea to Tahiti, and back again. The voyage was just a scant 7-minute flight each way known as Air Moorea Flight 1121.
Moorea and Tahiti are just two islands in French Polynesia that feature popular resort locations prized for stunning beaches and green hilly peaks. As many pilots in this location did, Captain Santurenne flew many times a day, having just 30 minutes in between flights until he was back in the air for the next one. In addition to having been with the airline for three months, the Captain had over 3500 total flight hours and prior experience working for a similarly scaled airline also flying short haul flights.
Air Moorea Flight 1121 was more like a taxi in the air than a standard short haul flight on a large carrier. And given the low altitude with a maximum cruising altitude of just 600 feet (183 m), the quick hop to the next island ensured that passengers would get a gorgeous view of the pristine beaches and sparkling water below. And for beach goers on the ground, they got to see the planes take off and land all day.
The DHC6 Twin Otter 300 series was a perfect aircraft for this exact type of island-hopping flights. Pilots regarded the aircraft as the “Jeep in the sky,” because it was a rugged plane that could fly into and takeoff from locations that many other planes could not. You could consider it a no-nonsense kind of aircraft. The 19-seat plane had twin turboprop engines and a high rate of climb, which made it a perfect commuter passenger airliner in this area for tourists and workers trying to get quickly from one island to the next. And, best of all, because it has little automation, pilots must fly this plane by hand.
And as it is a perfect day for flying, Captain Santurenne is already preparing for his next flight. In between flights, he always checks the aircraft over, and then helps situate his 19 passengers onboard, carefully arranging people and their items to ensure an equal distribution of weight across the plane. And as is typical for this flight, the Captain is a one-man operation. Even though he is flying an aircraft that belonged to a fleet owned by Air Moorea, Captain Santurenne is the only crew member onboard – there are no co-pilots or flight attendants assisting him.
At around noon local time, Air Moorea Flight 1121 is cleared for takeoff and the Twin Otter begins to sail up toward its cruising altitude. But just moments later, something is wrong. The plane begins to lose altitude – and fast. Very quickly, the plane falls into a steep dive. Just one minute and eight seconds from powering up the engines, Air Moorea Flight 1121 crashes into the ocean at a vertical rate of speed of 6,500 ft/min (1,980 m).
Onlookers at the nearby airport and beaches cannot believe what they have just seen. Shaken fishermen described seeing the plane crash so quickly that they did not understand what had happened. Soon after the crash, suitcases began to bob in the lagoon as helicopters circled above and rescue boats scoured the area.
It quickly became evident that all 20 people perished on impact. One official called it the worst-ever airplane accident in French Polynesia, and the disaster rocks the small island community where almost everyone knows somebody who died onboard.
Right after news of the crash spreads around the island, other Air Moorea pilots refuse to fly again until investigators find out the cause of the crash. None of the pilots believe for a moment that Captain Santurenne is to blame – they fear something may be wrong with plane. And, even more so, they fear that what caused this crash could happen again unless the cause is determined and rectified.
Because the crash happened within the boundaries of a French collectivity (as they call it), the BEA arrives to investigate immediately (they are the equivalent of the NTSB in the United States). But the investigation has encountered its first major challenge: the plane crashed about 1.1 miles (or 1.8 km) offshore and then sunk to the bottom of the sea floor at about a depth of 600 to 700 meters, or 1,970 to 2,300 feet, deep.
Investigators wish to urgently examine the wreckage for possible causes of the crash – and because there was a cockpit voice recorder (CVR) onboard. Even though the Twin Otter was not built to include a CVR, it was added to the plane by Air Moorea. Despiet having access to the CVR, investigators will not have any flight data recorders or even radar data about the plane’s location.
Rescue workers, helped by French Navy ships and local fishing boats, recovered the bodies of 14 passengers and the pilot. Five bodies are never recovered. Some airplane debris, including parts of the right main gear and seat cushions were recovered by fishermen and the rescue team. Some days later, at a depth of seven hundred meters, searchers recover both engines, the instrument panel, the front part of the cockpit including engine and flaps controls, the flaps jackscrews, and the tail section.
To plot out the plane’s trajectory from takeoff to impact, investigators will need the see the wreckage and collect witness accounts. Investigators began to interview two different groups of witnesses: people who saw the airplane from behind and people who saw the airplane from the side. The first group of people was on the airfield and on the beach. They described a normal take-off and climb, a short stabilization then a rather pronounced descent. Those who heard the engines added that they were functioning up until impact, though some people noticed a variation in power. Most described a straight trajectory. One did, however, point out that, just before impact, the airplane’s pitch attitude and bank were at about 45°. A luggage handler who was on the ramp area saw the airplane’s flight path deflect towards the left during its descent.
The second group was made up of fishermen who were a few hundred meters north of the point of impact. They saw the airplane on a slightly pronounced descending trajectory and one of them indicated that it was the front landing gear that first touched the water. They were the first to arrive at the accident zone, saw the rear part of the airplane that was sinking rapidly, and they smelt a strong smell of kerosene.
Using this information from the witness accounts to gain a clearer understanding of the crash sequence, investigators have one more ask of witnesses. Investigators try a reenactment to identify how high the plane had flown before it began to fall. They take a similar plane on a similar flight path to help jog the memories of witnesses who were on the beach and at sea the moment the plane crashed. If investigators can tell how high the plane flew before falling, investigators will be able to track the pilots’ actions up to that moment since pilots on this type of plane – which they had to hand fly – will have tasks to perform at very specific altitude milestones. After flying a few routes and adjusting the altitude each time, witnesses help investigators answer how high the plane was before it crashed. It was at almost 400 feet (122 m).
Now that investigators know how high the plane was flying, they turn their attention to the airplane parts that have been retrieved from the sea. Those parts also include the tail section, which contained the CVR. After examining the wreckage and listening to the CVR, investigators eventually rule out several causes of the crash:
- Weight and balance issues. The reason for considering this as a cause was due to the way that the plane crashed – a steep dive into the ocean. If the plane weighed too much, this factor certainly would have caused a crash just like how Flight 1121 crashed. We did explore a plane crash where the weight of the plane was a contributor, Air Midwest Flight 5481, in episode 5 on Patreon. But this is not the case for Air Moorea Flight 1121. Investigators can confirm that there were no weight or balance issues that led to the crash.
- Dual engine failure. Now that investigators could examine plane parts for malfunctioning systems, they immediately examine the engines. But the engines show that they were powered on all the way until the moment of impact. This finding also corroborates witness accounts of hearing the engines running as the plane fell to the sea. This cause would have been a very rare, unlikely cause anyway.
- Pilot medical issue. The investigators were concerned that the pilot may have experienced some sort of medical issue inflight, like a heart attack, that rendered him incapacitated and left no one at the controls. In this scenario, they imagined that if the Captain had had a heart attack, he likely would have slumped forward onto the controls, which also would have caused this same type of sudden dive. But fortunately, once the autopsy comes back, investigators confirm that Captain Santurenne was completely medically fit to fly.
- Sabotage. Lastly, another important factor they can rule out is sabotage. Again, based on the type of crash, it would have been plausible that someone might have intruded the cockpit and pushed the control column all the way forward, which would certainly have caused a crash like this one. But once investigators listen to the CVR, they can tell no one was in the cockpit except for Captain Santurenne. (We did explore instances of sabotage in past episodes on the podcast, including episode 5, episode 9, episode 12, episode 25, and episode 69.)
What is on the CVR ultimately leads them to what took the plane down. On the recording, the flight is progressing normally from lift off, and investigators can hear Captain Santurenne maneuvering his controls exactly how they would expect a pilot to do. Then, the Captain retracts his flaps, and right after he does, they hear the pilot cry out. And just moments later, the plane slams into the sea. Investigators are positive that the cause of the crash is somehow related to the flaps.
When flaps are extended, they increase drag, so as a pilot is flying toward a higher altitude, as Captain Santurenne was doing on Air Moorea Flight 1121, flaps are retracted to decrease drag and enable the plane to ascend.
And on this type of plane and for this type of flight, a pilot would retract the flaps right around when they reach 400 feet, which is consistent with witness accounts of how high the plane was right before it began to fall.
Investigators examine the flaps from the Twin Otter for this flight and can tell that the flaps were in the correct position (retracted) and with no sign of malfunction. They begin to sort through the cables that control the various fight surfaces on the plane, which are all found in the tail.
On the Twin Otter, the primary flight controls (ailerons, rudder, and elevator) are mechanical, controlled by a series of stainless-steel cables. Two cables move the plane’s rudder left and right and two cables move the plane’s elevators up and down. 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. Obviously, all the cables have been damaged when the plane impacted the sea. But despite the damage from the impact, one cable looks very different from all others – they can tell that the pitch up cable, which pulls the nose up, snapped before the plane impacted the sea.
Investigators came to this conclusion because this cable looks different from all the others in terms of its damage. The pitch up cable’s wires showed wear and tear on the exterior of the filaments that make up the cable while the inner wires seemed to have snapped. And if this kind of cable snapped while the plane was climbing, it would have caused the pilot to lose control of the plane and cause a steep, uncontrollable dive.
The wear on the exterior of the cables made investigators think that something had rubbed against the wires of the cable, causing it to wear and come unthreaded. And they were right. 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. Every time the pilot moved the elevator cables, it moved them through the guide hole. The wear due to this friction is anticipated, and pilots would always check their controls before taking off to ensure everything was working properly. But, as investigators uncovered, the airline did not have the right maintenance plan when it came to the stainless-steel cables.
Here is where we begin the technical portion of our program. Most Twin Otters had cables that were made of carbon steel instead of stainless steel. In fact, the plane that was Flight 1121 was one of the only ones in the fleet with stainless steel cables. The original reason for using stainless steel was that it suffered much less corrosion than carbon steel, especially in a high-salinity environment. But there was a trade-off: the stainless-steel cables suffered more frictional wear than the carbon steel ones. And the Twin Otter is a plane that is mostly hand flown by the pilots. Every time a pilot moves the control surfaces, the cables rub against various pulleys and guide holes, causing them to wear down over time.
At the airline, the carbon steel and stainless-steel cables were treated as being interchangeable, so that the maintenance of the accident plane was performed the same way as the rest of the fleet. Because the manufacturer offered no guidance on the difference between the two types of cables, Air Moorea replaced all its control cables on the interval specified for carbon steel cables — about once a year. While some reports stated that the airline was never made aware of this important difference, the tendency of stainless-steel cables to wear down more quickly seemed to have been understood in some aeronautical circles.
In 2001, about 6 years before this crash, a Special Airworthiness Information Bulletin (SAIB) was issued by the FAA that recommended Piper airplane owners and operators should carry out stainless steel cable inspections every hundred hours because of their much shorter life expectancy than carbon steel cables.
Investigators found no awareness campaign for operators on the risks of stainless-steel cable wear. Some operators reduced the maintenance intervals between special checks down to every fifty hours, but that was only based on their own experience and not based on any regulatory authority. During the investigation, it was revealed that anomalies had been discovered on several occasions with the stainless steel cables, but the operators simply changed the cables without informing the cable manufacturer. As there is no process for following up with the manufacturer when these types of anomalies are spotted, there was also no established procedure for systematically researching the causes of a cable’s wear and tear or even failure and determining the corrective measures to take.
To summarize, it is clear today that this wear phenomenon had been known disparately for a long time but that no study appeared to have ever been conducted to understand the process of wear and tear (appearance, speed, evolution in resistance), nor to determine what the consequences could be. This was a major systemic failure on the part of Air Moorea and the manufacturer.
And this brings us to the second part of the equation that brought down the flight. When investigators tested the strength of the cable like the one on 1121, they found that even if this cable had been worn down by 50%, it could still withstand the force needed to move the cables on this flight. This means that something else must have pushed the cable from Flight 1121 to the literal breaking point.
And investigators believe they may know what that “something else” was. They notice something about the layout of the runways and parking lots at the airport in Moorea that gives them concern. Both large and small planes land and takeoff there, and the smaller fleet of planes are parked very close to the runway. But they notice there is no barrier wall present to help protect the smaller aircraft from the jet blast generated by larger aircraft.
Jet blast is the phenomenon of rapid air movement produced by the jet engines of aircraft, particularly on or before takeoff. A large jet-engine aircraft can produce winds of up to 100 knots (190 km/h; 120 mph). Jet blast can cause serious damage – the force is on par with hurricane-level winds, and it can be a hazard to people or other unsecured objects behind the aircraft. It can flatten buildings and destroy vehicles. To prevent this from happening, most airports place a barrier wall known as a jet blast deflector (JBD), or blast fence, between the smaller planes and the runway near where the larger aircraft takeoff. This barrier wall is a safety device that redirects the high energy exhaust from a jet engine to prevent damage and injury to people and planes. But the airport in Moorea did not have a barrier wall surrounding its fleet of Twin Otters. Investigators discovered that the wall had been removed to make it easier to move between parking lots.
The Twin Otter fleet that were parked near the runway were typically parked in a nose down position with the elevators locked, which would make it impossible for the plane to be moved by the force of the jet blast. But investigators wonder if having the small planes parked in this position meant that the elevators caught most of the brunt of the blast.
This hypothesis leads investigators to conduct more tests on the cables, this time taking a cable that has been worn down by 50% (like the one they found on Air Moorea Flight 1121) and then subjecting it to jet blast – and the cable snaps. Essentially, investigators ultimately determine that the cable onboard Flight 1121 was worn in such a way that it could withstand the force of the takeoff, but not enough strength to last a 7-minute flight with additional maneuvering of the cable.
And here is what is so heartbreaking. If the cable had severed before the flight left the ground, it likely would have been discovered, and Flight 1121 never would have left the ground. And, had the plane been at a higher altitude, the pilot may have been able to use the plane’s trim to maintain flight control. But investigators doubt this second possibility only because Air Moorea did not train its pilots on upset recovery. In essence, Captain Santurenne and his 19 passengers were doomed.
In the final probable cause report issued by the BEA, they made several safety recommendations:
- Require operators to inspect as soon as possible all stainless-steel elevators cables on the Twin Otters.
- Recommend that stainless-steel cables be forbidden to use on this model aircraft until improved knowledge made it possible for the BEA to determine new regulatory requirements and appropriate maintenance procedures.
- Modify in-flight training programs for Twin Otter pilots to include awareness training for flying an airplane in case of a failure of one of the primary flight controls.
- Organize an information campaign on the risks associated with jet blast from airplanes.
- For the EASA, consider the appropriateness of taking jet blast into account in the process of aircraft certification.
Let’s not forget that 20 people lost their lives for no reason that day. The passengers included two Australian tourists, two European Union officials, and a group of Polynesian environmental and tourism officials.
A memorial was erected in memory of the 20 victims onboard Air Moorea Flight 1121 and can be found near the airport. A single large stone stands at the top of three rows of steps, angled toward the final resting place of Flight 1121. On a plaque are the names of all 20 people onboard.
In 2017, the families of the victims of Air Moorea Flight 1121 made a renewed effort to take aviation officials to court over the crash. The case first went to court in 2018 when at the insistence of the victims' families the airline employees and aviation officials were charged with involuntary homicide.
Also in 2018, in a separate decision the court of appeals ruled that $3.3 million had to be paid in compensation to the families of the victims. Finally, in January of 2021, after a long legal proceeding and appeals process, The French Polynesia Court of Appeal confirmed the prison sentences of three former executives and one other employee of Air Moorea for the deaths of the 20 passengers.
The former general director of Air Moorea, Freddy Chanseau, was sentenced to 36 months in prison, 18 of which were suspended. The airline itself, which ceased operations in 2010 and merged into Air Tahiti subsidiary Air Archipels the following year, was subjected to a fine of around EUR200,000 euros (USD220,000). Air Moorea’s former technical director, Jacques Gobin, was given a three-year prison sentence of which two years were suspended. The former head of the French Polynesian government's air safety unit was also given a three-year prison sentence, 18 months of which were suspended. One more Air Moorea employee was sentenced, while two others were acquitted on appeal. As of early 2021, the lawyer for the company and the convicted General Director of Air Moorea pledged to appeal.