Dear friends and followers, welcome back to my channel and to a
technical video about the Boeing
747 landing gear and what that's got to do with a bulldozer
Just a bit of advice before watching this video
you might want to check out my previous video about how do
airplanes taxi showing you how to steer an Airbus
A320 on ground because to be fair the steering system of an Airbus
A320 is fairly much just saying as the one of the Boeing 747 except for one key difference
Which is the topic of today's video, so let's get started
Look at the
747-400 you can see that she is one a very long airplane
70 point seven metres in length and sixty four point nine meters of
Wingspan not to mention her younger and even longer sister the 747 - eight
But I come back to her in a minute. Now a problem. The 747 is facing from time to time at given airports
It's not just the length and wingspan
but her turning radius and with needed to perform a
180 degree turn now with all systems working normal and let's say we would have to do a
backtrack to the end of the runway and then initiate a
180 degree turn to face the departure runway the
747-400 needs a minimum pavement width of forty six point six meters
That none of her tires get stuck in the grass at the side that's already a problem as most standard airport
Runways the 747 flies into are only 45 meters wide
So a wide enough turning pad at the end of the wrong way is absolutely mandatory and no
Performing a three-point turn by using reverse thrust is a not adoption maybe with military planes
But that's a whole another story now
do you remember the last time he was standing at a crossroad and you a truck with a long trailer performing a
90-degree turn and you feared for your life as the trailer was about to clip the pavement
Probably the truck driver forgot to swing out to the left first to avoid his trailer cutting the corner now. Well a
747 can really swing out to either side due to its limited space instead
She comes with a pretty cool system
Which is a feature only a few other wide-body airplanes have the so called body gear steering
So the landing gear of the 747?
consists of four main landing gear
Assemblies the wing gear which is mounted in the wing route on either side and the body gear mounted in the belly
Or fuselage now if we look closely at the body gear you can see those two actuators
Those are the body gear steering actuators
Which can rotate the entire
Truck by 13 degrees to either side now
The body gear is electrically linked to the nose wheel steering
now meaning if a pilot moost and nose wheel tiller to steer the plane along the taxiway as
Soon as the nose wheel is deflected more than 20 degrees
Let's say to the left. For example, the body gear steering comes active and each body gear truck
Rotates in the opposite direction as the respective actuator in this case the right one
forces the forward part of the truck to the right and the aft to the left and
Obviously into the other direction
Placing the tiller to the right and doesn't that look like very much to the steering mechanism of a wheel bulldozer
Or one of those trailers with steered axles
The entire steering system nose, and body gear is powered by a hydraulic system
number one now interesting to know that prior to
Pushback we request to pressurize the hydraulic system in accordance with ground stuff. Now, why do we do that now?
There are three main reasons
Number one to pressurize the parking brake so ground staff can remove the chocks
second to have hydraulic pressure to perform the flight control check before engines start and
third to have pressure on the steering system because as we commenced pushback a so called
Steering pin is placed into the nosewheel strut which will deactivate in those rules steering
but the tiller will still move in the cockpit if the pushback driver initiates a turn and
rotates the nose wheel now once it's beyond 20 degrees the body steering comes
Active and more or less aides to driver during pushback
also interesting to know the system hydraulically sent us the body gear and
deactivates the system if the ground speed
Increases beyond 20 knots and comes active again when the speed decreases
Through 15 knots now besides that there were other pedals which are also connected to the nose wheel steering
Won't activate the body gear steering at any time as you can only deflect the nose wheel by seven degrees to either
Side not reaching the activation of 20 degrees like the tiller makes total sense as you
Shouldn't or can't use the nose wheel tiller and during takeoff roll
Give it some thought while you can't use it now
You might question how much difference can 13 degrees of body gear steering have on the turning radius?
You will be surprised now this animation here shows the center of turn point for a continuous
With minimum thrust on all engines and no differential braking
so let's perform a
180 degree turn with
Maximum nose wheel deflection of 70 degrees and body gear steering at 13 degrees
Gives a minimum turning radius of 46 point six meters now watch the same
animation with an inoperative body gear steering
No differential braking but added thrust. Otherwise, the turn radius becomes even larger
52 meters that's 5.4 meters more now
I know it doesn't look much but in some places that could ruin your day
Now even worse are the numbers on the sim for 7-8
The minimum turning radius is 52 point four meters and with an inoperative body gear the radius
Extends to 69 point five meters that's nearly 17 meters an increase
That's a lot of extra space that dash 8 needs plus the longer fuselage and wing span
Besides the better maneuver capability it also reduces the amount of tire scrubbing now if you think about it a fully loaded
747 dash 8 at a maximum taxi weight of
449 tons
18 tires equals nearly 25 tons. Each tire has to withstand
So imagine the abrasion forces if the tires weren't
Aiding during the turn now the cool thing about it
Is that Joe Sutter and his engineers were?
Aware that the layout of the landing gear and the weight it had to distribute over the tires
Would cause a lot of torsional stress and abrasion
So he and his team were one of the first to put the body gear steering into place so they did on the 747
Prototype, I mean imagine that we're talking to
1968 interesting to know on the Boeing
747-200 better known as the classic came with a body gear steering switch in the cockpit so prior to takeoff you had
To deactivate the body gear steering system by placing the switch to disarm at the same time the system
Checked if the body gear was locked and centered
Otherwise a warning horn would sound on
Advancing firstly the number 3 for takeoff
And even if he would have taken off the landing gear lever would have been
Mechanically locked so you couldn't retract the gear now other planes using a similar technique other Boeing triple7
But they can only rotate the most aft axle of the triplet
Also the Airbus a380 they call it the body wheel steering which also rotates the aft axle as the triple7 does
Please comment below if you know other great steering system on planes. I hope you enjoyed today's video
By the way, this is one of the new shirts in the captain Joe merchandise store
Which will only be available until the end of March before the next line of shirts will be released
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