This is the story of air france flight 66. It was a saturday the 30th of september 2017
and an air france A380 was flying from Charles De Gaulle international to los angeles international
airport. The plane had 24 crew members on board and
497 passengers. The plane took off at 9:50 am and it climbed
to 33,000 feet. Since this was quite a long flight the plane
had two first officer on board, the second first officer decided to take the first shift. As the plane made its way across the atlantic
the plane climbed and descended a bit by 11:14 am the plane was at 37,000 feet. With the plane about 100 Nm from the coast
of greenland the pilots got in touch with gander oceanic over the CPDLC or the Controller-Pilot
Data Link Communication which is a written form of communication between the pilot and
the controller, its mostly used for clearances and things like that. The controller allowed them to climb to 38000
feet and they were asked to report when they were at 38000 feet. As the plane climbed to its new cruising altitude
the outer casing of the number 4 engine of flight 66 was destroyed. The air inlet of the engine was ripped away
and it fell to the ground. The plane's number 4 engine was out of operation,
the wing of the plane was hit by small pieces of debris. On the flight deck they knew something was
wrong, the plane turned three degrees to the right in three seconds and the entire airframe
started vibrating. The crew had seen similar circumstances in
their simulator training and they assumed that an engine was surging. An Eng 4 stall message came up on the ecam
display, Ecam actions said the captain. He disconnected the autopilot and took over
control of the plane and he pulled back power on engine number 4. The engine shut itself down and the first
officer pushed the engine 4 fire push button. So far the crew had not seen what had happened
to engine number 4. A member of the cabin crew entered the cockpit
with a picture of engine number 4 taken by a passenger. First officer one went to the upper decks
to see the damage for himself. He saw that the leading edge slats were damaged
and that the flaps were vibrating a bit. The captain fell back on his training in this
situation air france pilots are trained to deal with issues using the FOR DEC method. Facts, Options, Risks & benefits, Decide,
Execution, Check. He saw that the planes speed had fallen from
277 knots to 258 knots. He knew that they couldnt stay at 37000 feet
for much longer, the captain decided to bring the plane down to a more reasonable altitude
step by step. They got down to 36000 feet then to 35000
feet and so on till they got the plane down to 27000 feet. ATC noticed that the plane was below its assigned
altitude and so a message came in saying ““ATC NOW SHOWS YOU FL330. IS THERE A PROBLEM?. Air france flight 66 declared a mayday, they
were too far out for voice transmissions and so their mayday had to be relayed by another
plane. At this point the crew needed an airport to
land their plane at and they had two options, goose bay airport in canada and Kangerlussuaq
in greenland, they studied the approaches and the environment of the two airports and
decided that goosebay was the better option even though it was further away. Flight 66 was cleared onto runway 26. As the plane was at 1000 feet the captain
disengaged the autopilot and then hand flew the airplane in. The plane landed at 3:42 pm. They rolled down the runway and they took
longer than usual to taxi as they had to stop taxing quite a few number of times as they
had to wait while ground services picked up their debris from the runway. At 4:22 pm the engines were shut down and
their ordeal was over. They had landed the worlds largest plane with
one engine out and 24 people suffered minor injuries, once on the ground they got to see
the engine number 4, the front half of the engine was all but gone. Even though engine number 4 was out, the crew
made a safe landing at goosebay. Something that really helped out the crew
in this situation was the training Air France and airbus gave them, specifically the FCTM
or the flight crew techniques manual. The FCTM helps pilots deal with contingencies
like this, the FCTM provides pilots a pathway to assimilate and act on situations when the
said situation does not fall within any predefined conditions. It basically asks pilots to think on their
feet. The next question that the pilots were asked
was their choice of goose bay as the diversion airport, at the start of the crisis they had
three airports that they could divert to. Kangerlussuaq , Iqaluit and goose bay. They soon narrowed their options to between
Kangerlussaq and Goosebay, Kangerlussaq has high mountains near the airport and the pilots
were not familiar with the runway 09 approach of Kangerlussuaq so for them goosebay was
the safer bet even though it was further away. After examining the engine that failed they
had deduced that the fan hub had failed. At the start of the investigation the investigators
did not have the fan hub of engine number 4, it was somewhere in the ice sheets of greenland,
so they did the next best thing and ran simulations. The simulations they ran helped them narrow
down reasons for the disintegration of the fan hub of the engine, it also helped them
model the trajectory of the ejected fan hub pieces improving their chances of finding
the fan hub pieces. Three phases of searches were carried out,
phase one found bits and pieces of the fan but and the engine cowling. Phase two was not successful in recovering
the fan hub. By phase three they had a better idea of the
size of the fan hub fragments and their exit velocities and with this they made a new ballistic
model which narrowed down the search zone. This time they were armed with an electromagnetic
detection system developed by the university of Aarhus in Denmark, it could detect the
titanium fan hub below multiple feet of snow. With this new data they went out and they
did it. They found fragments of the fan hub and now
they could analyze the broken fan hub to see what had really torn the engine apart from
the inside. Upon analysis they found that the fan hub
had cracked due to LCF cracking or low cycle fatigue inside the metal itself. They looked at how the fan hub was manufactured
but first let's look at the material of the fan hub itself. The fan hub is made from a titanium alloy
called Ti 64. Titanium at the micro level has two phases,
an alpha and beta phase. In the alpha phase titanium has a hexagonal
structure and in the beta phase it has a body centered cubic structure and as you'd expect
both phases have different physical properties. These phases occur in texture zones and depending
upon the size they are categorized as micro or macro zones. In the case of the failed fan hub the cracks
started in a macro zone and the orientation of this micro texture zone was so that if
force was applied on it in just the right way the alloy might fail. The macro zone in question was bigger than
any other macro zone that they had seen and the force was applied on it perpendicularly
exactly what's needed for crack formation. The parts of the engine are put under great
stress, specifically cyclic stresses and this cyclic stress can cause parts of the engine
to fatigue crack at lower stress levels when compared to normal tensile stress. Exposing a part to the types of stress found
in a jet engine can radically shorten its life time, this is known as dwell fatigue. Dwell fatigue refers to the reduction in the
fatigue life-time of a component as a result of exposing the component to a constant high
mean stress during cruising, between the ramping up of the load during take-off and the ramping
down of the load on landing and when this dwell fatigue happens at relatively cold temperatures
of the engine its called cold dwell fatigue. Now cold dwell fatigue is nothing new. It has been linked to the failure of a few
engines in the past and we've known about it since the 1970s, but what really surprised
everyone was that ti64 was susceptible to cold dwell fatigue. Remember those alpha and beta phases of titanium
that we talked about, well when there are large regions of the alpha phase titanium
it can increase the probability of cold dwell fatigue because of how the stress is applied
and how the material handles stress. Cold dwell fatigue in the ti 64 alloy was
very very unusual .Other alloys of titanium are known to be susceptible to cold dwell
fatigue. At the time the engine was certified the scientific
community thought that ti 64 was insensitive to the cold dwell fatigue phenomenon, they
also found that depending on the magnitude of the stress that the part was under the
parts operational lifespan would be drastically shortened. So this caught the aviation and the scientific
community off guard. In the case of this fan hub the cracks started
inside the fan hub about .76 millimeters from the surface. Over the course of about 1600 cycles the cracks
grew and when the cracks reached its breaking point the fan hub shattered. The fan hub was rated for at least 15000 cycles. As of right now we don't have a non destructive
way to test for these kinds of faults just yet. If its there when the hub is manufactured
it'll stay there. After the accident the FAA and the EASA asked
airlines to inspect the fan hubs of their A380s. About 19 hubs had damage because they were
struck by something most likely due to a faulty repair, after a few revisions they started
looking at fan hubs with more than 3500 cycles on them and one damaged hub was found, it
was repaired. Before they found the fan hub in Greenland
they considered the idea that a faulty repair might have damaged the fan hub causing it
to fail. While doing maintenance it was hard to remove
the fan blade lock ring and so they designed a new fan blade lock ring that was easier
to remove and so would result in lesser unintentional damage to the fan hub. Right now the EASA and the FAA are trying
to bring about changes to the manufacturing process to reduce the likelihood of cold dwell
fatigue.they are also batting for an inspection program to check for fatigue cracks in parts
already in service. Since this incident is pretty new well have
to see how they tackle this cold dwell fatigue cracking issue. Thank you for watching this episode of mini
air crash investigation, this report was loaded with scientific data a big thank you to Professor
James C. Williams of OSU for helping me understand the scientific data and the basics of material
science. His inputs made the final report infinitely
more readable for me. I would not have been able to make this video
without his help.
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