- [Petter] How can an aircraft descend to six feet,
that's only 183 centimetres over the ground, almost a full mile away from the airport without the pilots noticing it? That's the mystery I will explain
to you in today's video so stay tuned. - [Radio Altimeter] 100, 50, 40, 30, 20, 10. - The story I'm gonna tell you today
is based on a very complete interim report by the French authorities, the BEA. Now the reason I've chosen
to use an interim report rather than a final report,
which I normally always do, is because this story highlights
some very important safety lessons that I wanna share with you guys. The incident took place
on the 23rd of May, 2022 on a flight between Stockholm-Arlanda in Sweden, and Paris Charles de Gaulle Airport in France. The aircraft that was being flown
was a 23-year-old Airbus A320 that was operated by a Maltese Airline
called Airhub Airlines. And it was operating on something called an ACMI wet lease agreement
with Norwegian Air Shuttle. An ACMI wet lease agreement basically means that the company that is leasing out the aircraft, in this case, Airhub Airlines is providing
both the aircraft, crew, maintenance and insurance, basically the entire operation to the lessee company. So the tickets that the
passengers would have bought, would have been with Norwegian Air Shuttle but the company that was actually
doing everything was Airhub airlines. And in the media report that was issued
after this interim report was released, that wasn't made very clear. Anyway, the two pilots that was going
to operate this flight were employed by Airhub Airlines and they started
the day just like any other day by going through the pre-flight preparation,
checking the weather, NOTAMs, en-route weather and fuel calculations. Now we don't know the age
and experience of the pilots because as I mentioned before,
this is based on an interim report, but what we do know is that the weather
they were presented with was quite nice. There was no problems forecasted en-route and at their destination airport, Paris,
it also looked quite okay with some scattered clouds at 1,500 feet,
good visibility, light winds from the west, but also mentioning of some temporary rain showers and cumulonimbus activity around the airport. Those types of clouds and rain showers
are very common in Europe this time of year and not normally of a huge concern to pilots, as long as they're not frequent
or forecasted together with thunderstorms or hail because they tend to be
of a very transitory nature. They tend to move away very quickly and even if they don't, as long as it's only rain,
since the aircraft are not made of sugar, we can just fly straight through them. So the pilots wouldn't
have thought much about this. But in this case, they're actually going
to play a fairly important role. When the pilots had finished
their pre-flight preparation, they went and they briefed their cabin crew and then they walked
all together out towards the aircraft. They decided that the captain
was going to be pilot flying for the first leg and the first officer,
pilot monitoring and then they would switch, something that we normally do
as we try to get equal amount of exposure to both roles during any given day. The captain checked the technical status of the aircraft which was found
to be perfectly in working order. And then he started doing
the pre-flight preparation as the first officer did the
walk-around of the aircraft. The cabin crew also finished
their safety inspection of the aircraft and then they started boarding all of the passengers. And once that was done,
the cabin crew gave cabin ready to the pilot. The pilots finished up their pre-flight preparation and then requested push and start clearance
from their gate in Stockholm-Arlanda. At this point, there was two pilots,
four cabin crew and 172 passengers on board the aircraft. The aircraft taxied out normally
and at time 0930 UTC, that's 11:30 local time,
Norwegian Air Shuttle Flight 4311 with callsign Red Nose 4311 took off for Runway 08 in Stockholm-Arlanda and started climbing towards
the southwest towards Paris. The flight proceeded completely normally
and as the aircraft flew in over Dutch airspace, close to Amsterdam, the captain handed over
the controls over to his first officer to start to prepare, set up
and brief for the arrival and approach into Paris Charles de Gaulle Airport. The first thing he did was to switch
his second radio box onto the ATIS frequency for Charles de Gaulle in order to listen in
to the latest arrival and weather information. Now the information
that he received sounded like this: This is Charles de Gaulle Information Oscar recorded at time 10:50 UTC. Runway in use for landing
is Runway 27 with an RNP approach, transition level seven zero. Wind: 280 degrees, 10 knots. Visibility: 10 kilometres, broken clouds
at 1,500 feet, few cumulonimbus at 5000 feet, temperature 19, dew point 14 and QNH 1001. This was a quite typical ATIS message
and it actually showed quite good weather at their destination aerodrome. The only real threat was the mentioning
of cumulonimbus or storm clouds at 5,000 feet. But what I want you to focus on
is the very last bit of this message, QNH 1001. This is a very important value because
it gives us the reference pressure in hectoPascal. In the United States,
they use inches of mercury instead. But we need to enter that value
into our barometric altimeters in order for them to show us the correct altitude
as we're arriving into the airport. When aircraft are flying at higher altitudes,
what we refer to as flight levels, we all set the same pressure reference, which is 1013.25 hectoPascals. And the reason we're doing that
is because when we're at higher levels, we are less interested in knowing exactly
how high we are above the sea level and we're more interested in making sure
that all of the aircraft are flying all around us are using the same pressure reference
so that we can maintain the altitude separation between each individual aircraft. Because the air pressure
is different geographically all over the globe, it will be impossible for air traffic control
to know which setting each individual aircraft has. So we will set the same. But, of course, as we are getting closer
to our destination, and as we're descending down, it becomes much more important
for us to know how high we are above obstacles and above our landing airport. And that's when we need
to be given the local pressure setting which in this case, was 1,001 hectoPascals. Now the way that we, pilots,
get this pressure reference officially is by air traffic control giving it to us
when they clear us from a flight level down to an altitude later on. But we typically preset the expected value
that we've gotten from the ATIS information, so that it'll be easier to just switch over
when we do get that clearance. Now the other thing that we need
to discuss before we go any further is the type of approach
that these pilots are now preparing for. Remember, the ATIS said that they could prepare for an RNP approach for Runway 27 Right? Well, RNP stands for
required navigational performance and these approaches are also sometimes
referred to as Baro RNAV approaches. And they're flown using the aircraft's GPS system together with an internal
navigational performance monitoring system that can notify the crew in case
something is going wrong with it. In other words, an RNP approach can be flown without any airport-based navigational aids. These approaches are becoming
more and more popular now because there are enabling smaller airports that might not be able
to afford big and expensive ILS system to be installed to have a very accurate
instrument approach system anyway. On bigger airports
like Charles de Gaulle, for example, the RNP approaches are typically used
as backups whenever the ILS systems are either not working or under maintenance. And that's exactly what
was going on on this particular day. Now, because RNP approaches
are flown using internal systems rather than external beams, it's very important that all
of those internal systems are properly set up prior to
the approach being flown. Now like I mentioned before,
the area navigation or the RNAV part of this type of approach has
a monitoring system built in that monitors the navigational accuracy of the GPS system. And on the approach segment,
it has a tolerance of about 0.3 nautical miles. And if the GPS system cannot maintain that,
it will be shown to the pilots. So we will stop the approach if that happens. Now that's just the horizontal navigation part of these type of approaches. The vertical path is built up
on altitudes that are set at given RNAV points, following typically
about a three degrees glide slope. In order for this type of approach
to be accurate, it's very important that the correct barometric pressure
is fed into the barometric altimeters. So the captain now briefed his first officer
on how he intended to fly this approach. The minimum descent altitude was 752 feet. That's about 250 feet above the ground, but the airline policy was
to add 50 feet on top of that. So the value that they actually put
into their minima selectors was 802 feet. The reason that the airline
had a policy to add 50 feet is because of a technicality that we have to follow when flying non-precision approaches. When we're flying a precision approach
like an Instrument Landing System, an ILS, we fly down to minima that is referred
to as a decision height. If we reach the decision height
and we don't have the visual references we need, we will then execute the go-around
but as part of the go-around, we are allowed to momentarily sink
through the decision height. This will happen every time
because the aircraft has a lot of inertia. So from the time that we decide to go around to when the aircraft actually starts climbing, takes a few seconds and you lose a couple of feet when doing so. Now when flying a non-precision approach because of the less exact nature
of the navigation involved with flying non-precision approaches,
we are not allowed to transgress the minimum descent altitude. So on a precision approach,
we have a decision height, on a non-precision approach, we have an MDA,
a minimum descent altitude. But because the airlines want the pilots
to execute the go-around using the same procedure,
they add 40 or 50 feet on to the MDA. That way, when the pilots hear
the minimums call on a non-precision approach, they can execute the go-around exactly
in the same way as they did on the ILS. And even though the aircraft continues to descend a little bit,
they will not transgress the MDA. Anyway, when the captain had completed
his briefing, he made a PA to the passengers. And after that, the first officer
requested descent from air traffic control. They received several descent clearances and changed frequency multiple times
until they eventually got into contact with Charles de Gaulle approach control. Now here is where things
are starting to go wrong. But before I get into the details of that,
I just wanna share this short message from my sponsor who makes it possible
for me to create these videos. Before we move on, let me introduce you
to today's sponsor, Curiosity Stream from the founder of the Discovery Channel. Curiosity Stream offers you thousands
of documentary films and nonfiction series on demand however you stream video,
available all around the world. Content spans nature, history,
technology, music, sports and my favourite category, science. You get access to all of that plus original titles
for less than $20 a year and because of that, Curiosity Stream is definitely the most value-for-money streaming service I've ever come across. Right now I'm watching a great new series
called Proof of Concept hosted by scientist and fellow YouTuber Dianna Cowern. You might know her as Physics Girl. This show was recommended to me by Patrick, one of you guys and it explores complex concepts with a fun and engaging twist. Since you're a follower of this channel,
you can use curiositystream.com/mentourpilot with the promo code Mentourpilot to save 25% off. That's just $14.99 for the whole year
or $1.25 per month which is fantastic. So support me by supporting my sponsor
and go to curiositystream.com/mentourpilot and start enjoying the world's top documentaries and nonfictional series today. Now back to the video. At time 11:32:24, the first officer
called up and checked in with Charles de Gaulle approach control. The controller responded
with the following message: Red Nose 4311, bonjour.
Descend to 6,000 feet, QNH 1011. The first officer responded with 6,000 feet
QNH 1011, 1011, Red Nose 4311. Did you catch that? The pressure that they got
from the ATIS earlier on was 1001 but the pressure that they've now gotten, the official pressure from the controller was 1011 which is the incorrect pressure. From the read back, you can hear
that the first officer is a little bit hesitant about this value, that's probably
why he repeated it. It's likely that the pilots had preset 1001
in order to be able to quickly change it over. And now they had to change it up
with another 10 hectoPascals. But they didn't question it. And it's possible that he thought
that maybe we just misheard the ATIS report earlier on. In any case, two minutes later,
the same controller comes back and clears the aircraft to descend to 5,000 feet,
again repeating the incorrect pressure value. It's hard to know why this error occurred. But these things happen from time to time. People have a tendency to turn numbers around. So for example, QNH 1020 becomes QNH 1002. And in this case, the call sign Red Nose 4311 had 11 at the end of it,
which might be a possible explanation. But why is this such a big deal? Well, like I mentioned before,
the correct pressure setting is absolutely crucial for the pilots
to know what their correct altitude is. One hectoPascal represent about 28 feet. So if you input an error of 10 hectoPascals,
that represent an altitude error of 280 feet. And remember, the minimum descent altitude
on the approach they're about to fly is only 360 feet above ground. One minute after the controller has cleared Red Nose 43111
to descend to 5,000 feet, he calls up an EasyJet aircraft
that's ahead of the Red Nose on the approach queue. He tells the EasyJet aircraft to continue to a waypoint called Papa Golf 650 and to descend to 5,000 feet using QNH 1011. But the difference here is that
when the EasyJet aircraft reads back the clearance, they said, "We'll continue to Papa Golf 650, descending 5,000 feet, QNH 1001." So they actually read back the correct QNH even though the approach controller
had given them the wrong one. This difference in read back was not picked up neither by the approach controller
nor by the pilots on the Red Nose 4311. Another minute later, the same controller cleared an Air France crew to descend to 5,000 feet,
this time using the correct QNH of 1001. That was read back by the Air France crew but, unfortunately, that whole message
was done in French, so our pilots on the Red Nose 4311
did not have a chance to pick that up. Red Nose 4311 now received
their approach clearance. They start to configure their aircraft, they select flaps one, flaps two
and at time 11:36:55, the aircraft reaches its final descent point
for the RNP approach Runway 27 Right and the aircraft starts
descending using the autopilot, following the pre-programmed descent path. As they started their descent,
the indicated altitude that the pilots could see in the cockpit was about 4,900 feet, but the actual altitude the aircraft
was at was 4,623 feet. They were 14.3 nautical miles away from the runway and with a speed of about 180 knots. Another factor that we need to mention here
is that as the crew started their approach, a large rain cloud had moved in
over the extended centerline with quite moderate rain inside of it. This meant that the crew
was now completely inside cloud and they had no visual reference
neither with the runway, nor the ground below them. Because of this, the first officer was reading out distances
versus altitude checks as they were descending down the approach. Our approach plates would give
very accurate mile by a mile information about where the aircraft should be altitude-wise
as they're moving down the approach path. And as the crew was doing this,
these checks were perfectly okay. They showed that they were exactly
where they were supposed to be. Here is where the vulnerabilities
of a fully internal approach system comes to light. Because the crew had the
incorrect QNH reference pressure set, it meant that the altitude and the altimeters
was indicating the correct altitude for each of these distance checks they were doing. But, in reality, they were actually
300 feet lower than they thought they were. If this same error would have happened
on an ILS approach, the physical location of the ILS glideslope beam
would have stayed the same. So no matter what pressure setting
the pilots would have set on their altimeters, they would have still locked on
to that glideslope beam which would have brought them down
towards the runway. In that case, if they would have done distance versus altitude checks, the error would have been shown very clearly. At 11:38:09, the aircraft is handed over
to the north tower controller in Charles de Gaulle. The first officer calls in and the tower controller greets them
and clears the aircraft to land Runway 27 Right with a surface wind of 260 degrees at 12 knots. From this point onwards,
there are no more calls regarding the pressure setting. The aircraft is still descending through the cloud. But unbeknownst to the crew,
the tower controller has forgotten to switch on the approach light system. This will further reduce the chance
of the crew establishing visual contact as they get closer. That might sound really strange. How could a tower controller forget to do this? But you have to remember
that this all happened during daylight hours. And up until this point, the weather had been very good and
it was still good over the airport. This rain shower and this dense cloud
was only really situated over the approach path which is where this aircraft now found itself. So are there no checks that the pilots
could have done during their approach that could have helped
them to identify this error? Yes, there actually is, but you have
to be really switched on as a pilot to notice it. At time 11:38:44, the aircraft's
radio altimeter activated and made a call out as it now sends
a height over the ground of 2,500 feet. That should normally happen
at about six and a half miles final. And if that happens earlier than that,
that's a clue to the pilots that maybe they are much lower than they should be. And it's a prompt to verify
that they have the correct QNH set. But in this case the pilots did not pick that up and from this point onwards the radial height was displayed on both pilots'
primary flight displays. The pilots continued
to configure the aircraft to flaps full and when they passed 1,000 feet
height over the ground and the radio altimeter called out-
- 1000. - They were completely stabilised. They had all of the flaps out,
they had completed the checklist, they had a speed of 139 knots
and, for the pilots, everything looked completely normal. The only thing that was out of the ordinary was a lower than normal radial height
at the distance they were from the runway, but none of the pilots have picked that up. So the aircraft just continued to descend down its pre-programmed descent path, down towards its minima. But now, unusual things
are starting to happen inside of the tower. The two air traffic control towers in Charles de Gaulle Airport are equipped
with a warning system called MSAW, stands for minimum safe altitude warning. What this system basically does
is that it has created a 64-nautical miles square around the airport. Inside of this square, it has modelled
all of the different obstacles and then it has applied a 300 feet safety margin on top of these obstacles. If the system feels that any aircraft
within this square is on a trajectory to get into that safety zone
and potentially hit a obstacle, it will issue a warning in the tower,
both a visual warning and an aural warning calling beep, beep, terrain alert. At time 11:41:32, the first MSAW warning went off in the tower. The aircraft was then at an indicated altitude
of 891 feet, which with the altitude error they were now flying with,
was actually an altitude of 601 feet. But remember, that's over sea level. And in reality, that was a radar height
of 200 feet at a distance of 153 nautical miles, that's 2.8 kilometres,
away from the landing threshold. The aircraft was still descending
following the descent path and that gave them a vertical speed
of about 700 feet per minute. Now the correct procedure
that the tower controller should have followed in case of an MSAW warning
would be to immediately contact the pilots, advice them of the warning,
ask them to verify position and altitude and give them the correct QNH setting. That did not happen. Instead, nine seconds later,
the aircraft had descended down to an indicated altitude of 802 feet,
which, if you remember, was the minima that the pilots had set earlier. And since the pilots had not established
visual contact with the landing runway, the captain now decided
to execute a go-around. The aircraft is now 102 feet above the ground at a distance of 1.2 nautical miles
away from the landing runway. But remember how I told you
that when the pilots start executing the go-around, the aircraft continues
to descend for a little bit longer? Yeah, that's happening now as well. So as the pilots are taking the decision
to go around and starting to both disconnect the autopilot in this case
and pitch up the aircraft, the aircraft continues to descend. And it is at this point that the tower controller,
finally calls the aircraft and says, "Red Nose 4311, I just had a terrain alert.
Are you okay? Do you see the runway?" The pilots did not hear
or respond to this message. They were likely too focused on the go-around
that they had just decided to execute. Three seconds later, as the captain
has finished moving the thrust lever into the TOGA position, the aircraft arrives at its lowest indicated height
of six feet above the ground, that's 183 centimetres,
basically two centimetres lower than my full height, at a distance
of 0.8 nautical mile from the runway, one and a half kilometre. That is incredibly low and it, mercifully,
now starts to climb away. But this story is not finished yet. Remember, they're still not down on the ground. Now one thing that's curious here
is that the pilots later stated that they didn't get any type of GPWS warning, nor any radio altimeter call-outs,
which they would normally get at those lower radial heights. Now, we won't know why that was the case. That's actually part of the investigation,
and we will likely hear more details about that once the final report is released
but my personal theory is that this has something to do
with a part of the GPWS system called the terrain clearance floor. The way that this part of the
enhanced ground proximity warning system works is that it reduces the margin
to where you will get a ground proximity warning, the closer you get to the active landing runway. And this is logical if you think about it,
that this is made to avoid nuisance warnings as you're coming in for landing. But the fact that this incident happened,
0.8 of of a mile away from the landing threshold could potentially be the reason
for these warnings not being activated. Now when it comes to the lack
of the radio altimeter call-outs, that is indeed strange and we're gonna have to take the word of the pilots
that they didn't hear it here because sadly, the cockpit voice recorder was not to saved after this incident
but I'll tell you more about that in a second. 10 seconds after the lowest radio height was recorded, the first officer called up the tower controller and advised him that they
were executing a go-around. And the tower controller responded,
"Roger, Red Nose 4311. Turn right heading 360 degrees
and climb to 5,000 feet QNH 1,001. Finally, the correct QNH value
was given to the crew but, unfortunately, the first officer now read back,
"Heading 360, degrees climbing 5,000 feet on QNH 1011, Red Nose 4311," just reading back the incorrectly set QNH that they already had set. And the tower controller
did not pick up that faulty read back. So the aircraft just completed the turn
heading north, continued to climb up to 5,000 feet, still on
the incorrect pressure setting. The autopilot was engaged
and the after takeoff checklist was completed and the crew started preparing
for another RNP approach into the same runway. At the same time, in the north tower
down on Charles de Gaule Airport, the assistant controller in the south tower
that was taking care of the other runways had noticed that the approach light
for Runway 27 Right had not been activated. He contacted his colleague,
the assistant in the north tower and told him about this. And when the north tower controller
realised that they had missed switching on the approach light system
and that they had had an MSAW warning as well, he was replaced and the
assistant took over his place and a new tower assistant was called in. Meanwhile, the aircraft had been handed over to the approach controller
and was now being lined up for its next approach. Remember, the pilots are, at this point,
completely unaware of the severe incident that they had been part of. In their minds, they had just flown
a normal RNP approach with all values stabilised and then executed a normal go-around
when they didn't get the visual references. They had no idea how close
they had been to the ground. At this point, the aircraft was already
maintaining 5,000 feet which was the initial altitude
to start another RNP approach. And during the time they were receiving vectors,
there was no more radio communication that was pertaining to the QNH setting. So there was no point from this point onward
for the crew to pick up that they, once again, were sitting with the wrong QNH. Instead, the crew now found
themselves in exactly the same position like they had been on the last approach, 300 feet below profile
as they initiated the descent. They were still inside of cloud at this point
and they were descending down the glide path, as they were once again handed over
to the north tower controller. At time 11:53:40, the aircraft
was once again cleared to land, Runway 27 Right. And this time, the first officer called up
the tower and asked him to confirm that the approach lighting was actually activated. The tower controller confirmed this. Two minutes later, the next MSAW warning
went off in the tower. The tower controller who was
the assistant on the previous approach, now immediately called up the aircraft saying, "Red Nose 4311,
we have a terrain warning, are you okay?? Once again, not following
the correct MSAW phraseology. But at this point, the rain cloud and
the rain shower had finally started moving away from the final approach path. So the aircraft broke through the clouds
and became visual with the landing runway. So the first officer responded, "Red Nose 4311, we are on path
and we are visual now." Around this time, the flight data recorder
indicated that the autopilot was disconnected and that the captain's side stick
was moved backwards in order to pitch the aircraft back,
probably to climb back up to a correct approach profile,
because from this point, if they just became visual,
the visual picture would have definitely told them that they were low together with the PAPIs
that most certainly also indicated this. Eight seconds later, the flight directors
were also switched off, and that's likely because the flight directors
would have indicated a path that was much lower because
of the incorrect pressure setting. At this point, the pilot also later testified
that they saw the PAPIs as being two red, one pink and one white,
indicating that they were almost back on path here. Less than one minute later,
Norwegian Air Shuttle Flight 4311 landed safely on Runway 27 Right. The pilots taxied the aircraft
into gate, they shut down. And, unfortunately, they did not pull
the CVR circuit breaker in order to preserve the cockpit voice recording. And that's likely because, at this point,
the pilots still did not understand what a serious incident
they had just flown through. As I mentioned before, this incident
is still under investigation and it's likely that we're gonna get even more details once the final report is released
by the BEA in France who, by the way, write some of the absolute
best incidents and accidents reports that I've ever come across. The reason I decided to make this video
based on the interim report, which I normally never do was because of how well it was written
but also because in this interim report, there are actually some really
good safety recommendations that I wanna share with you
before the final report comes out. The error that this crew experienced
is something that my airline referred to as QNH blunder error,
and the potentially disastrous consequences of that was identified a few years back
when we started flying these RNP approaches. There are several great mitigation techniques against this error, and one of them is for the pilot monitoring
to set an anticipated QNH value on his or her primary flight display
that they're getting from a separate source like the dispatch weather that they got
during the pre-flight preparation, for example. This way, if they're getting a wildly different value from air traffic control
when they're being cleared to an altitude, they can challenge that by using
the correct phrase, "Confirm QNH." Don't say confirm QNH with a value,
just say confirm QNH because that way, there won't be any confirmation bias in there. Another thing that we tend to do
is no matter how recent the latest QNH value was given to us, we tend to always ask
to confirm the QNH before we start the approach. That way, we can pick up
any potential error at that point. In the Boeing, we also have an instrument
on our navigation display called a vertical situation display,
which is basically the only instrument that can pick up a faulty QNH value
by showing the runway depicted either below like a coffin,
indicating that we actually have a too high QNH value set
or above the ground level, showing that we have a too low QNH value set, which has its own potential risks to it. But lastly, and probably the most important point I wanna get across with this video
is how important it is to be actively monitoring the communication that goes on between pilots and air traffic control and vice versa. If you come across a clearance
that sounds weird or a value that you don't recognise, query it, it's always better with one call too many
than one call too few. The report also highlighted
that there were some misunderstanding among air traffic control personnel
about the potential risks of flying RNP approaches and also the correct procedures
to follow in case of an MSAW warning. Now if you wanna see a crazy story
about a flight where the pilots ended up almost crashing into the seat twice,
check out this video or if you wanna continue to binge on accidents
and incidents, use this playlist. If you wanna support the work
that we do here on the channel, then consider becoming part
of my lovely Patreon family. We have weekly hangouts
and I'd love to see you there or you can also buy yourself some merch. Have an absolutely fantastic day
and I'll see you next time, bye bye.
