When you board an airplane, you probably don’t
expect it to break in flight. You assume that a certain standard of maintenance has been carried
out that would keep the plane you’re flying on in an airworthy state. Airplanes are made of upwards
of millions of individual parts. The failure of any one of them could result in outcomes
ranging from negligible and un-noticeable, all the way up catastrophic meaning certain
fatal injury. When Air Moorea Fight 1121 left for a very short flight in French
Polynesia, its occupants certainly couldn’t have been prepared for what would happen to
their plane just moments into the flight.
Aviation History is littered with disasters
and incidents which stem from the failure of critical aircraft elements. Whether these
be in the area poor maintenance or just general wear and tear is down to the individual
case. For this video we’re going to examine a disaster which occurred with a particular
plane which is known for its toughness.
The Canadian built Dehavilland DHC-6 Twin
Otter is a plane which suits many different roles. Officially classed as a “utility
plane” they can be found pretty much all over the world. Around 1000 of these planes
built over the years serve hard to reach airports, airports with unusual terrain or simply serving
short routes carrying just a handful of people at a time. It was this purpose that the airline
Air Moorea saw the Twin Otter to be useful.
Air Moorea was an airline based in French
Polynesia, operating out of the famous island of Tahiti but taking the name of the
neighboring island of Moorea. Tahiti’s Faa’a Airport was the home of the airline between the
years of 1968 and 2010 when it eventually ceased operations. The Airline provided a network of
flights in and around the Polynesian Islands. Many of the airline’ passengers connected
through Tahiti on International flights. Often holiday makers looking for the quiet
getaway that these islands can provide.
For the vast majority of the airline’s
life, it enjoyed a reputable safety record. On August 9th, 2007, things would change for the
airline as the disaster of Air Moorea flight 1121 occurred. The Accident occurred on a short
flight between Moorea and Tahiti. To call this flight short would be an understatement,
it’s one of the shortest commercial flights in the world with a distance of roughly 10
miles or 18 kilometers between the airports. Roughly 7 minutes of flying time. The cruising
altitude on such flight was just 600 feet.
Given the number of seats of the planes which
performed such route and the demand for this trip, flights between Tahiti and Moorea
were performed up to 50 times per day. This meant that Air Moorea’s planes were
in service more frequently than usual. This amounted to the accident Twin Otter
plane accumulating 55,000 thousand take-offs and landings across 30,000 flight hours.
Flight 1121 was expected to depart at just before midday. The flight was packed with every
seat fully booked amounting to 19 passengers. On these short flights there was only one
[pilot flying the plane. On this occasion, that pilot’s name was Michel Santeurenne aged 53.
From French Polynesia himself he had accumulated over 3500 flight hours total and was hired by Air
Moorea just three months prior to the accident.
At 11:53am, Captain Santeurenne began his
preparations for the flight to Tahiti once his passengers had boarded. With just the one
member of crew he was also responsible for giving the safety briefing which was done over the PA.
Four minutes later, local air traffic control cleared the small propeller plane to taxi down
to runway 12. Over the next three minutes, flight 1121 would make its way out to
the runway and line up for take-off.
It was exactly midday when flight 1121 was given
its take-off clearance and the Twin Otter’s engines were powered up. Such a small plane does
not require much speed or runway to get airborne. Operating in tight spaces is one
thing the Twin Otter is known for.
As expected, the plane lifted off the ground
and began climbing. Shortly after take-off, the Captain retracted the planes flaps. It was in
this moment as the plane passed through just 400 feet of altitude that a catastrophic failure
would occur. Were going to pause right here and examine in a bit more detail what component
actually failed and to do that we need a closer look at the Twin Otter plane itself.
The Twin Otter has a very distinctive design with its wings mounted on top of the fuselage and
the Horizontal Stabilizer mounted on the tailfin. Just like any other plane, pilots of the
Twin Otter control the plane with the standard control surfaces of Ailerons, Rudder
and Elevator. Many commercial passenger planes have these control surfaces powered by a
hydraulic system. The Hydraulic fluid is often referred to as the “blood” of the plane.
The Twin Otter however works a bit differently. According to a specifications publication
from Viking, one of the plane’s manufacturers, the only hydraulic system on the plane
powers the flaps and nose wheel steering. The typical Control Surfaces here are
connected to the pilot’s control wheel via a complicated system of cables and pulleys.
When the pilot influences the control wheel, the cable system pushes and pulls in the appropriate
way to deflect the control surfaces as needed.
These cables themselves are made of 132
individual strands of wrapping wiring which gives the cable its high tensile strength.
When the investigation into flight 1121 looked into the wreckage of the plane, it was
deduced that the critical failure occurred in the cable which connects the elevator or pitch
of the plane with the pilot’s control wheel.
The cable wirings themselves were originally made
from a Carbon Steel, but these were replaced on the accident aircraft with stainless Steel wiring.
This was suggested to help with corrosion effects. The plane’s manufacturer recommended the Stainless
Steel cables for tropical climates and higher saline environments. What the Airline failed
to account for, was the necessary maintenance needed to upkeep these planes given the number of
flights they performed. Air Moorea’s operations of their planes were unusually high which led
to a greater level of simple wear and tear over the years. No adjustments to the maintenance
schedule was made to account for this.
The investigation also found another factor
contributing to the wearing down of these cables. For this we should redirect our attention
from Moorea airport to that of Tahiti’s Faa’a airport. The airport here is only small. Air
Moorea parked and operated from this section of the apron. It is believed that jet blasts
from larger airliners whilst the accident plane was parked here contributed to the disaster.
Though served by many small planes on the daily, Tahiti is also served by multiple international
air carriers. At the time, Air New Zealand operated a Boeing 767 to the airport, Air France
a 747. Hawaiian Airlines also operated a 767 but also for a time operated a McDonnell Douglas DC10
here. Not least of which deserves a mention is the Tahitian Airline itself, Air Tahiti Nui which
operated numerous Airbus A340 planes out of Faa’a. Point is the airport despite being
small was no stranger to large planes.
To quote the own words of the
investigation as detailed in the analysis section of the accident report…
“The accident plane was parked near A340 type wide-body airplanes. Calculations showed that the
jet blasts from the engines of these airplanes could result in a load above the stress
failure level for a worn cable.”
These factors all culminated in the cables
chafing against the internal harnesses and buckles of the plane, wearing them down
over time. On the subject of the chafing, here is the accident report quote…
“The wear on the cable where it failed was due to its chafing on the polyamide bush located
in the cable guide. This wear was significant: due to the structure of the cable it had affected
all of the strands except the central strand and had led to the failure or the almost
destruction in cross-section of 72 wires out of the 132 that made up the cable.”
At just after midday on August 9th, 2007, The Cable wiring which allows the
pilot to manipulate the pitch axis, or more specifically as the accident report puts
it quote “The Pitch up Cable” failed and broke. In this moment, the Captain was heard on
the cockpit voice recorder giving a verbal indication that he knew something was immediately
wrong. As many eyewitnesses who watched the plane take-off from Moorea that day noted, the
aircraft then suddenly pitched downward.
There was nothing that Captain Santeurenne
could have done to save the plane. Air Moorea Flight 1121 impacted the water
at precisely 12:01 and 20 seconds, taking the lives of all 20 people on board.
As French Polynesia is an overseas French Territory, the Crash of flight 1121 was
investigated by European Authorities.
It was concluded in the investigation that the
cause of the crash was the breaking of the plane own internal control wiring for reasons
we’ve already discussed. The investigation also looked towards the airline Air Moorea.
Saying the did not provide adequate training to pilots with regards to a sudden loss of
pitch also going on further to highlight the lack of amendments to maintenance schedules
given the frequency of the plane’s usage.
Going forward, all Twin Otter planes were to
be inspected for chafing of the control cables. It’s highlighted that inspects of the
planes should be done on a calendar basis in the context of an operator like
Air Moorea. The European Aviation Safety Agency or EASA was recommended
to take jet blasts into account when aircraft are to be certified.
Patreon Outro
Hello Everyone. Thank you so much for watching
this video. I had a bit of an issue with trying to find the correct livery that Air Moorea
used for this plane in the flight simulator. I sort of had to improvise. If you found
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Anyway, I was really happy with how this video
turned out. I though this was very interesting. I have a list of my incidents to cover, I take note
of a lot of your suggestions or recommendations. Research is always on going, so if you don’t see
yours or multiple people’s request straight away, its probably because I just
haven’t gotten round to it yet.
Anyway with all that said, thank you so much for
watching, I’m going to sign off on this video now. Have a great day and I will
see you next week. Goodbye!
