This is the story of smartlynx 9001. On the 28th of february 2018, a smartlynx
A320 was over the skies of tallin, estonia. It wasnt flying anywhere it was just a flight
to train pilots for the airline. On that day the plane had a safety pilot,
4 training pilots and a safety inspector onboard and before the flight they had a safety briefing,
today theyd be a training session would have 5 touch and gos, for a touch and go you approach
the runway as you would for a normal landing and then you touch down and take the plane
back into the air, it's great practice for pilots. In addition to that they'd also have one go
around and one full stop landing. The first student pilot took the wheel, or
the sidestick maybe? As the student pilot did their touch and gos
a warning popped up on the screen. ELAC1 + ELAC2 PITCH FAULTs. The ELAC or the elevator aileron computer
had an issue, the ELACs as the name suggests controls the elevators and the ailerons, the
ELACs take in the sidestick inputs and then move the control surfaces as the pilot commanded. Think of it as a middle man in the chain. The pilot who was monitoring the flight didnt
think much of it and so they did what we all do, they reset the computer to clear the error. Weve all done it, turing it off and then back
on fixes a lot of problems but not today. The first student pilot landed his plane and
brought the plane to a stop and now the second student pilot assumed control of the plane. As the student number 2 took the plane to
the skies the ELAC1 pitch fault error popped up again, and like last time the system was
reset as per procedure. The same happened during the next touch and
go. On the fifth touch and go an ELAC2 pitch fault
error popped up. This happened again for the 4th students second
touch and go, these pitch fault errors kept coming and the resets werent helping. It was time for The third touch and go the
plane touched down and it was rolling down the runway. The plane then accelerated to 130 knots to
lift back into the skies. The instructor asked the student to rotate,
he said "rotate rotate" but the plane wouldn't budge, it was stuck to the ground firmly. The pilot replied that was trying his best
to raise the nose of the plane into the air. At this point a new warning was shown "manual
pitch trim only" the pilots would have to manage the pitch trim on their own. When you trim an airplane the airplane stays
in a particular state without any inputs, so let's say you trim a plane up, then the
plane will nose up on its own. But now the pilots had to manage that on its
own. The plane was on the ground. It was as if the pilots were locked out of
the controls of their plane. The plane gained so much speed that it lifted
off on its own. Sensing trouble, the instructor said " I have
control". As the plane was pitching up and flying away
from the runway the instructor retracted the flaps a bit. The pilots selected gear up, but the plane
started to descend, and the plane hit the ground, the plane hit the ground and bounced
and began to gain altitude at 6000 feet per minute. As the plane climbed out they got a flaps
locked warning and a warning that alerted them to a fire in engine number 2. They weren't out of the woods yet. The pilots kept the plane number control by
manually trimming the plane and by using engine thrust. It was hard to control the plane when they
pulled back on the power the plane would dive setting off the terrain alarm. But somehow the pilot stabilized the plane
at 1200 feet and he took stock of the situation. The pilot in control asked, do we have engines,
the safety pilot replied with "we have engine two fire". They were in dire straits, the elevators were
in mechanical backup mode, engines one and two were damaged from the bounce and engine
number two was on fire and the flaps were locked in place. The pilots declared a mayday and made a right
turn to make a beeline for the airport before anything else failed. As they headed towards the airport they let
the tower know of their intentions "Tallinn Tower, we are going for runway 26", they requested
for fire trucks as the right hand engine was on fire. They talked about shutting the burning engine
down but they decided to keep it running because theyd need all the help that they could get
to get this plane down on the ground. Soon after the landing gear was extended and
engine number two failed. On their final approach engine number one
failed killing power to most of the airplane. Only the basics still worked. They had the runway in sight they were on
the home stretch, the safety pilot stated. "Gear is down. We don't have engines". The safety pilot started counting down the
airplanes speed. "Speed 150" "Speed 130", the plane touched
down hard 150 meters short of the runway, it decelerated as it went along and soon it
was on the runway but after a harrowing touchdown the plane came to stop to the left hand side
of the runway. The plane was in one piece and all onboard
were alive, they had survived their airborne ordeal. Now before we go any further lets just get
some acronyms out of the way first. Theres a few so much so that I wanna call
this episode the alphabet soup. Okay so the ELAC is the elevator aileron computer,
the SEC stands for the Spoiler elevator computer, the THS is the trimmable horizontal stabilizer
which I'm gonna call the horizontal stabilizer , the THS-OVM is the horizontal stabilizer
override module and the PTA is the pitch trim actuator. With the airplane more or less intact and
with a bunch of pilots who lived to tell the tale, they investigators were able to know
that the plane lost pitch control during the flight and so thats where they focused their
efforts. The pitch of the airplane is controlled by
two elevators and the horizontal stabilizer, the two small wing like things at the back
of the plane. These are massive and are moved by hydraulics,
the hydraulic systems are controlled by electrical signals sent by a computer. Yeah if that computer were to fail it could
lead to a loss of pitch control and thats why airbus built an insane amount of redundancy
into this pipeline. Usually the horizontal stabilizer and the
elevators are controlled by ELAC 2 if that computer were to fail ELAC 1 would take over. If that failed the SEC 2 would take over and
if that failedSEC 1 would take over. If that failed only manual pitch trim would
remain. The horizontal stabilizer is fed by 3 hydraulic
lines and there are three motors and each of these computers is wired to use a different
hydraulic line - motor combo, so no matter what the computers can control the pitch of
the plane, or that's the idea. Now you might be wondering, how does the pilot
control the horizontal stabilizer? In this chain, right before the trimmable
horizontal stabilizer actuator moves the flight control surfaces, there's something known
as the THS-OVM or the override module. The OVM allows the pilot to as the name suggests
override the computer at any point. If youve watched a cockpit video of youve
probably noticed a spinning wheel in the cockpit between both pilots. That's the trim wheel. The pilot can rotate it to trim the plane
up or down. When the pilot does that a signal is sent
to the override module. This causes a disc in the override module
to move downwards, this means that the computer's commands can no longer reach the actuator
as it is physically separated. The pilot now has all the control. But when this happens a small piston in the
override module moves downwards and triggers three PTA microswitches, when these switches
are triggered it sends a signal to the computers saying that they've been overridden. On the accident airplane as the plane touched
down, the computer would bring the horizontal stabilizer to 0 degrees or neutral. But since they were practicing touch and gos,
the pilot grabbed the trim wheel and he trimmed the plane to nose up, making it easier for
them to take back off again. When he did this everything that we talked
about before happened the OVM gave control to the pilot and the piston went down but
the micro switches didnt engage correctly, this meant that the computers were not told
that they were overridden. So the computer is commanding all sorts of
stuff from the horizontal stabilizer but its not gonna carry those commands out because
the pilot is the one controlling it. So as far as the computer is concerned the
horizontal stabilizer is moving uncommanded and so the ELAC 2 computer by design fails,
the computer is essentially programmed to go " wait i didnt command that something wrong
in the system, i should probably step down and let someone else take over" . And this
is how we got the ELAC 2 pitch fault warning. But the pilots trimmed the plane on the ground
why did they get these warnings in the air as they were climbing out, well these warnings
are inhibited during takeoff so as to not distract the pilots. Since they were being warned about something
that happened a while ago. It was hard for them to piece together what
was happening. In this way both the ELAC 1 and 2 computers
failed, taking away half their redundancy. But the question still remained. Why did the PTA micro switches fail, why did
they fail to inform the computers that they had been overridden? They sent all of the hardware to france where
they tested the switches, they found that the activation of the switches were erratic
and was highly dependent on the temperature. The reason for this? They found that the oil used in the THSA and
the OVM was not the oil that the manufacturer called for, this oil had a different viscosity
value and so the piston encountered a different amount of friction than what it was designed
for and this meant that the piston did not activate the switches as intended and thats
why the computers had no idea that they werent in control. They couldnt figure out where the oil of the
incorrect specification came from, they guessed that it was from an earlier maintenance check. Now if you're still watching you might be
wondering, okay that explains the failure of ELAC 1 and 2, what happened to SEC 1 and
2? Well to understand that we need to look at
a moment in time, no literally a 1.02 second silver of time where something seemingly insignificant
happened.First lets look at the SEC, the spoiler elevator computer can control the horizontal
stabilizer when needed and much like the ELACs it too has a command sub computer and a monitoring
sub computer. It works as youd think the command computer
sends out commands and the monitoring computer makes sure that everythings alright. These computers take in all sorts of data
from all over the plane, part of what they do is figuring out if the plane is on the
ground or if its in the air. It takes in data from the landing gears to
figure this out. On the accident flight the one of the landing
gears was compressed for 1.02 seconds, it basically bounced. the monitoring computer picked this up but
the command computer didnt because they work asynchronously and on data with different
time stamps. When the landing gear is compressed for a
second it sends the monitoring computer into flight mode. But since the SEC's command computer had not
detected this it stayed in ground mode. This meant that the command computer and the
monitoring computer disagreed with each other. Over time they diverged from each other. Remember how i told you that both halves work
together, and within a short amount of time both halves of the SEC disagreed with the
other by a large margin. Due to this internal disagreement both SECs
failed. And with that the pilots had lost ELAC 1 and
2 and SEC 1 and 2, they no longer had pitch control , they only had manual pitch trim. In a way they were locked out of the controls
of their plane. No one had expected such a series of unforeseeable
events to occur, these events were so unpredictable, from the wrong oil to the precisely timed
bounce of 1.02 seconds to a lot of other things. It was a perfect storm, but these pilots were
lucky. They managed to walk away from something no
one could have foreseen. Before we wrap up for the day on the scale
of 1-10 how much automation do you want in your plane,
where 1 is the least and 10 represents the most amount of automation. Im just curious because Whenever I post a
video about automation going wrong i get a lot of comments lamenting the rise of automation
and when I put out a video about a pilot making a mistake I get a lot of comments calling
for more automation. Where do you lie? Let me know in the comments below.
Mini AIr Crash investigation makes some of the best videos.
Well I've just discovered a new Youtube channel. Thanks!
What a wild story!