Dear friends and followers, welcome back to my channel and today we'll be looking at this picture in more detail.
Why do airplanes have a split rudder and others don't have this unique feature?
And, An incident with this rudder system.
And I'll show you which plane has the "coolest" split rudder of all!
So, let's get started!
Remember the last time you sat at the gate and an Airbus A380 or a Boeing 747
Taxied by and you spotted this. Now what you see here is the so-called
Split rudder and obviously there is a reason why it is split in two.
Now, But before we look at the rudder,
We have to get a better understanding of the hydraulic layout of planes, in particular, the Boeing 747.
The primary flight controls such as the
ailerons,
elevator
and rudder are hydraulically powered.
Now in case of the Boeing 747,
which has 4 independent hydraulic systems
1, 2, 3 and 4 are each powered and pressurized by its respective engine.
Now if we look at this schematic. you see that the tail fin rudder or the vertical stabilizer is
Split into the upper and lower rudder, now
The upper rudder is deflected to either side by three actuators,
two of them are powered by hydraulic system number 3,
and one by system number 1.
Now, the lower rudder comes with two actuators,
one powered by system number 2, and the other by system number 4.
Now as you apply force into the rudder pedals to either side
both the upper and lower rudder will deflect
simultaneously towards the given input.
Now, I'm sure many of you have
steered a little boat before and you might remember as
you were maneuvering the boat into a harbor or more in position
You need a lot of rudder deflection to actually steer the boat.
Now because the inputs are less effective due to the low speed
and similar physics apply to airplanes,
the slower you fly the more deflection you need on the rudder to have an effect,
and the faster you fly the less deflection you need.
So for example, as she comes in for landing if the pilots apply full left or right rudder
It will go to the maximum deflection of 31.5 degrees,
which sometimes can be necessary in strong crosswind conditions.
Now during cruise or speeds beyond
350 knots with the same amount of input into the pedals the rudder will only deflect
7.6 degrees and less to either side.
But as an airline pilot you always expect the same reply of your plane,
no matter to speed
and therefore airplane engineers have fitted the flight control system with a so-called :
"Rudder Ratio Changer"
Now the general purpose of the "Rudder Ratio Changer"
is gradually reduce the surface deflection of the rudder by the pedals
with increasing air speed
Now, please do not mistake this with the yaw damper
That's a whole another system and video.
But to be fair you barely used a rudder during cruise flight
except in non-normal situations like engine failures etc.
but in case you need to center the slip skid
indicator which shows if your plane is yawing to either side for instance due to
Unbalanced loading you can use the rudder trim to even out the yaw
Which saves power and fuel.
Now if we quickly take a look at the Airbus A380 she has a very similar system
But one thing I can't really wrap my head around
is why the Airbus A380 rudder sometimes points in either direction
when being parked at the gate position.
I did a whole video on that subject as the rudder is being deflected by the wind
But how can the A380 lower rudder point to the LEFT and the upper rudder point to the RIGHT?
It makes no sense to me. Please comment below if you know the answer to that question.
But that still doesn't answer the question of why they are split in the first place
Now, let's say hydraulic system number 1 would fail for some reason.
Now, not yet to worry about the rudder
as we still have system number 3 powering the two other actuators
now let's say the situation is becoming even worse and system number three fails
Also, trust me if that were to happen
You are really having a bad day as all hydraulic systems also have a backup feature
So it is very unlikely. But let's imagine the worst-case scenario.
Meaning you have now lost complete control over the upper rudder -- so if it weren't for the split rudder,
Hydraulic system 2 and 4 backing up the lower rudder, you would have no more rudder-authority
Which would make your landing very hazardous
in case you've had engine failures leading to the hydraulic system losses for example.
So, you see the primary reason for the split rudder as ever so often is redundancy
and the rudder isn't the only flight control which has a backup,
the horizontal stabilizer comes also with inboard and outboard
Elevators so technically there are also split and so are the inboard and outboard ailerons.
Redundancy is key especially in terms of flight controls.
But also the split rudders provide a finer high speed control and that only the lower one moves at high speed,
reducing the exposed surface area and therefore the control effects as
There is a structural benefit.
By only using the lower rudder, when the aircraft is at high speed
it reduces the twisting moment and
transfers the load to a bigger stronger part of the airframe
and that's the principle is the same why the Outboard ailerons are disabled at high speed.
Now, as I said the chances of multiple hydraulic system failures is very rare
Nevertheless there was an incident in 2002 on a Boeing 747-400,
Which experienced a so-called lower rudder hardover event.
The lower rudder suddenly went into the maximum deflection to the left
Causing the plane to abruptly bank 30 to 40 degrees to the left.
The pilots acted fast and applied full right rudder
Unfortunately, that was still not enough to prevent the plane from losing altitude
yawing and rolling to the left,
So the right aileron was also needed and due to the limited
Controllability the pilots immediately declared an emergency and landed at the next suitable Airport
Which FYI was two hours away.
So for two hours, the pilots were fighting with extreme forces on the control column and with
Decreasing speed the rudder-authority of the slightly smaller
Upper rudder was not enough to fly the plane in a straight line
the pilots then used
Differential thrust to counteract the yawing moment on approach. The plane safely landed and later
Investigation showed that the lower rudder control module had a broken housing due to metal fatigue
causing the hard over, but for more details read the
NTSB report on Northwest flight 85.
Now you question: What about smaller jet airliners such as the Airbus A320 or the Boeing 737
which do not have a split rudder?
Now, on the Airbus A320
She has a 3 hydraulic system. The rudder is moved by 3 actuators each individually powered
and as a backup
you have the hydraulic power transfer unit
as a 2nd backup the ram air turbine
and as a 3rd the mechanical connection.
So the necessary redundancy is given,
but please may Boeing 737 pilots comment below, on how their rudder redundancy is given
or just ask DutchPilotGirl.
And the most unique split rudder you can find, is on the B-2 bomber,
as she has no vertical stabilizer/rudder,
She still needs to be controllable along the yaw axis.
So the engineers placed these panels far out on her wing. They look like ailerons, which they are
Not as the secret is on the lower side
Another panel on the lower side deflects downwards, as the top one goes upwards,
creating the necessary drag,
forcing the plane to yaw to either side making it the most unique split rudder for yaw control and
The panels you see here are the so-called
Elevons for roll control of the plane. That is so clever.
And if you now want to know why the rudder very often points to the right or left
Whilst the plane is parked at the gate click onto the video link
which will pop up here in a few seconds
and also check out my new t-shirt designs just above the description box.
If you are a Boeing or Airbus lover, you definitely want to get one of my new shirts.
That's it for today
Thank you very much for your time
And don't forget to hit that subscribe button and activate the notification bell,
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and don't forget "A good pilot is always learning".
See you next week
Wishing you all the best!
Your Captain, Joe!