n710ps
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747 Steering Bypass During Push

Sun Sep 23, 2007 12:57 pm

So I was chatting with a pal of mine from my days working the ramp today on the phone and we got to talking about the bypass systems in various diffrent airplanes and how on the CRJ at some carriers you disconnect the torsion link and at our carrier we disengage it from the flight deck instead of the ramp disconnecting the torsion links.

Somehow we managed to the topic of the 747 and how their system works and neither of us was sure about it so here goes.

When you put the bypass pin in (I am assumeing the 747 is done with a bypass pin like the other Boeing birds) does this also disengage the body gear stearing systems and also is the body gear system directly linked with nose steering or are there seperate systems that control it with a diffrent set of bypasses. When pushing can you control the bypass system or is it locked for the push?

Thanks in advance.


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PhilSquares
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RE: 747 Steering Bypass During Push

Sun Sep 23, 2007 1:45 pm

The by-pass pin only unlocks the nose gear. During pushback, the body gear steering is pressurized so as the nose wheel is turned beyond 20 degrees the body gear steering is activated.

Here's the description from Vol 2:

Nose Wheel and Body Gear Steering
Nose wheel and body gear steering is powered by hydraulic system 1.
Primary low speed steering control is provided by a nose wheel steering tiller for
each pilot. Limited steering control is available through the rudder pedals. The
tillers can turn the nose wheels up to 70 degrees in either direction. A pointer on
the tiller assembly shows tiller position relative to the neutral setting. The rudder
pedals can be used to turn the nose wheels up to 7 degrees in either direction. Tiller
inputs override rudder pedal inputs.
Body gear steering operates when the nose wheel steering angle exceeds 20
degrees. This reduces tire scrubbing and lets the airplane turn in a minimum
radius. Body gear steering is activated when ground speed decreases through 15
knots. As speed increases through 20 knots, the body gear is hydraulically
centered and body gear steering is deactivated.
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fr8mech
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RE: 747 Steering Bypass During Push

Sun Sep 23, 2007 1:51 pm

The bypass pin only bypasses the nose gear steering. As long as #1 hydraulics is pressurized BGS will become active when the nose gear goes passed 20 degrees.

Pushing a jumbo is a task with the BGS not active. Nice wide turns, usually on narrow ramps.

Edit: That's what I get for getting up for a drink in the middle of a post. PhilSquares beats me.

[Edited 2007-09-23 06:53:02]
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n710ps
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RE: 747 Steering Bypass During Push

Sun Sep 23, 2007 2:09 pm

Thanks for the info folks. It was something in my time on the ramp I never got to experince.
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HAWK21M
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RE: 747 Steering Bypass During Push

Sun Sep 23, 2007 5:58 pm

Quoting N710PS (Thread starter):
and at our carrier we disengage it from the flight deck instead of the ramp disconnecting the torsion links.

Pls Explain this about Torsion link disconnection from Flight deck/Cockpit.

Quoting N710PS (Thread starter):
When you put the bypass pin in (I am assumeing the 747 is done with a bypass pin like the other Boeing birds) does this also disengage the body gear stearing systems and also is the body gear system directly linked with nose steering or are there seperate systems that control it with a diffrent set of bypasses

Most Steering bypass disconnect pins,are placed in a slot created by moving the steering bypass lever/plunger to the Bypass position,thereby creating a hydraulic runaround so the NLG can move with the system pressurised.

regds
MEL
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jetmech
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RE: 747 Steering Bypass During Push

Sun Sep 23, 2007 11:15 pm

Quoting N710PS (Thread starter):
Somehow we managed to the topic of the 747 and how their system works and neither of us was sure about it so here goes.

Basically with the 747, you are locking the steering summing lever in the neutral position when you insert the bypass pin. The way Boeing designed the 747 steering hydraulic system is that when the summing lever is in the neutral position, the spool valve is dimensioned such that there is just enough clearance to allow hydraulic fluid to flow freely in and out of the steering cylinders.

The summing lever is mechanically acted upon via cables by two inputs. One input is from the pilot's steering tillers, the other input being from the rotating part of the steering yoke. Say the pilots wish to make a turn. They manipulate the tillers in the appropriate direction which mechanically moves the steering tiller away from neutral. This ports fluid to the appropriate side of each steering cylinder, and via the torque links, forces the nose wheels to move in the commanded direction.

As the nose-wheels move, the summing lever is also acted upon mechanically by the portion of the cable wrapped around the steering yoke. This gradually returns the summing lever to neutral, and thus, acts as a proportional feedback to physically stop steering movement of the nose-wheels once they reach the position commanded by the pilots.

Once again, as the summing lever reaches neutral - this time with the nose-wheels rotated away from straight ahead - fluid is free to flow in and out of the steering cylinders. The secret with the system is that fluid is only free to flow in and out of the steering cylinders in the immediate region around the neutral position of the summing lever.

Imagine the pilots have the nose-wheels commanded to a certain position (straight ahead or displaced), and the summing lever has returned to the neutral position. If there was some external disturbance that tried to move the nose-wheels away from the commanded position, the portion of the cable wrapped around the steering yoke will transmit a signal to the summing lever. This would cause the summing lever to displace slightly away from neutral.

The slight displacement of the summing lever would immediately cause a hydraulic lock to form in the steering cylinders. This would also thus stop movement of the nose-wheels, and prevent the external disturbance from moving the nose-wheels away from the position commanded by the pilots.

Quoting N710PS (Thread starter):
When pushing can you control the bypass system or is it locked for the push?

The bypass system can only be accessed from the nose-wheel, and is thus locked for the push. Because the summing lever is locked in neutral by the bypass pin, fluid can freely move in and out of the steering cylinders. This allows the tow-bar and tow vehicle to freely steer the nose-wheels. If you are riding the brakes in the cockpit for the tow, you will also notice that the steering tiller follows the movement of the nose-wheels as they are steered.

The one thing that is always drilled home when you are riding the brakes is to never touch the steering tiller during the tow. The reason being that if you apply enough force, you may cause the summing lever to move away from neutral, which would form a hydraulic lock in the steering cylinders and snap the tow-bar.

Regards, JetMech

[Edited 2007-09-23 16:21:08]
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n710ps
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RE: 747 Steering Bypass During Push

Mon Sep 24, 2007 2:16 am

Quoting HAWK21M (Reply 4):
Pls Explain this about Torsion link disconnection from Flight deck/Cockpit

Sterring is disengageable from the flight deck, there is a method that I know Comair employs though where they disconnect the torsion link to the nose gear instead though. Our method is the equivlant of a bypass pin if you will.
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Jetlagged
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RE: 747 Steering Bypass During Push

Mon Sep 24, 2007 2:22 am

Quoting N710PS (Reply 6):
there is a method that I know Comair employs though where they disconnect the torsion link to the nose gear instead though

Disconnecting the torsion link sounds a bit drastic. Some kind of hydraulic bypass would be more likely, especially if this can be selected from the flightdeck too (as it can on some aircraft).
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jetmech
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RE: 747 Steering Bypass During Push

Mon Sep 24, 2007 3:25 am

Quoting JetMech (Reply 5):
They manipulate the tillers in the appropriate direction which mechanically moves the steering tiller away from neutral.

Whoops; the term "steering tiller" in the above except from the second paragraph of my original post should read "summing lever". Apologies.

Regards, JetMech
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n710ps
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RE: 747 Steering Bypass During Push

Mon Sep 24, 2007 3:33 am

From wehat I have been told by a friend who works for Comair they disconnect the torsion link on the nose gear. I agree though Jetlagged that it is easier to do it they way we do at the carrier I am with. We bypass the system from the flight deck. They are the only CRJ company I know that doesthat. Most carriers perfer to use a form of by pass whenther it be a hydraluc bypass switch or a pin. For some reason they disconnect the torsion link on the push though.
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HAWK21M
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RE: 747 Steering Bypass During Push

Mon Sep 24, 2007 7:01 pm

Quoting N710PS (Reply 9):
We bypass the system from the flight deck.

Is there a pic of the unit used for the Disconnection.
regds
MEL
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Jetlagged
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RE: 747 Steering Bypass During Push

Tue Sep 25, 2007 8:51 am

Quoting HAWK21M (Reply 10):
Is there a pic of the unit used for the Disconnection.

The N/W STRG switch is seen at extreme left
View Large View Medium
Click here for bigger photo!

Photo © Jorgos Tsambikakis


Another example: the A320 has a combined with A/SKID & N/W STRG switch near the clock which bypasses the NWS
View Large View Medium
Click here for bigger photo!

Photo © Antti Kostia

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HAWK21M
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RE: 747 Steering Bypass During Push

Tue Sep 25, 2007 7:35 pm

Thanks for the Pics.
regds
MEL
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Tristarsteve
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RE: 747 Steering Bypass During Push

Thu Sep 27, 2007 12:16 am

Just to add for those who have not pushed back a B747.
The steering lock out pin is the same size pin as a B757/B767/B777, but is not the same.
The lock pin is in the middle of the pin, not at the end. The B747 lock out pin is inserted upwards into the steering mech (good design Boeing!!) and if you use a B767 pin, it might fall out during the tow!
Make sure you have a B747 pin.
 
lowrider
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RE: 747 Steering Bypass During Push

Thu Sep 27, 2007 1:56 am

Quoting Jetlagged (Reply 7):
Disconnecting the torsion link sounds a bit drastic

Its not, it only sounds that way. The scissors are have quick disconnects that allow the halves to be seperated for push. A far simpler solution than any sort of hydraulic bypass. After the tow bar is disconnected and the halves are re attached, the nose gear steering is turned on and normal steering resumes.
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jetmech
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RE: 747 Steering Bypass During Push

Thu Sep 27, 2007 2:12 am

Quoting TristarSteve (Reply 13):
Make sure you have a B747 pin.

I also make sure I give the 747 bypass pin a sharp downwards tug to ensure the balls have locked. Many times I have inserted the pin to only find that due to wear and dirt in the pin, that the locking balls have not engaged properly.

Regards, JetMech
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Jetlagged
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RE: 747 Steering Bypass During Push

Thu Sep 27, 2007 2:57 am

Quoting Lowrider (Reply 14):
After the tow bar is disconnected and the halves are re attached, the nose gear steering is turned on and normal steering resumes.

If the N/W steering is switched off, isn't disconnecting the torque link unecessary apart from as a belt and braces safety thing, in case the flight crew switch the steering on to soon? Sounds like some operators only switch off the steering.

It's important to ensure the tow bar is removed before steering hydraulics is re-established.
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Tristarsteve
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RE: 747 Steering Bypass During Push

Thu Sep 27, 2007 3:13 am

Quoting Jetlagged (Reply 16):
If the N/W steering is switched off, isn't disconnecting the torque link unecessary apart from as a belt and braces safety thing, in case the flight crew switch the steering on to soon? Sounds like some operators only switch off the steering.

There is a very good reason why you disconnect the torque links, The nose gear can then be turned to any angle. Some aircraft have quite low turning limits with the links connected, and going beyond the limit can be very expensive. We always used to disconnect the torque liknks when putting the aircraft in the hangar as there were some very tight turns to do.
Many years ago it was very common to disconnect the links, but for an ordinary pushback it is very rare nowadays.
 
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Jetlagged
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RE: 747 Steering Bypass During Push

Thu Sep 27, 2007 3:35 am

Quoting TristarSteve (Reply 17):
The nose gear can then be turned to any angle.


IIRC some aircraft NWS can be turned some way beyond the normal steering limit by the towbar when in hydraulic bypass before hitting the towing limit. Probably not far enough for extremely tight manoeuvres though.

If you disconnected the link on a 747, there would be nothing to command the body gear steering to operate.
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lowrider
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RE: 747 Steering Bypass During Push

Thu Sep 27, 2007 3:50 am

Quoting Jetlagged (Reply 18):
IIRC some aircraft NWS can be turned some way beyond the normal steering limit by the towbar when in hydraulic bypass before hitting the towing limit. Probably not far enough for extremely tight manoeuvres though.

But if the aircraft is not equiped with a hydraulic bypass, you disconnect the torque link. It is quite common with many smaller aircraft, such as business jets.
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VC-10
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RE: 747 Steering Bypass During Push

Thu Sep 27, 2007 4:18 am

Quoting Jetlagged (Reply 11):

Another example: the A320 has a combined with A/SKID & N/W STRG switch near the clock which bypasses the NWS

That is not to be used for isolating the steering for pushback.
 
FlyASAGuy2005
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RE: 747 Steering Bypass During Push

Thu Sep 27, 2007 4:59 am

Quoting N710PS (Reply 9):
They are the only CRJ company I know that doesthat. Most carriers perfer to use a form of by pass whenther it be a hydraluc bypass switch or a pin. For some reason they disconnect the torsion link on the push though.

We did it at ASA in Atlanta. It was company policy. It didn't matter if we were doing a straight push back or not (the CRJ-200 only). We never disconnected on the 70 seater, it was always stright back with them. anybody that knows ATL pretty well knows that ASA parks crazy. On the 45 spot down at C-35 was the worse. Imagine looging towrads the nose of the aircraft as if you are ready to push. There is the covered walk way just to the left (about 10 feet from the wing, an aircaft to your right and the guard rail right behind the airplane. We would have to start turning as soon as we start rolling. It was insane...but fun.
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n710ps
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RE: 747 Steering Bypass During Push

Thu Sep 27, 2007 5:07 am

We get pushed some crazy ways at some of the airports in the Northeast. CVG as well. For some reason you cannot get a strait back at that airport if you beg for it.
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DeltaGuy
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RE: 747 Steering Bypass During Push

Thu Sep 27, 2007 5:08 am

Quoting N710PS (Reply 6):
Sterring is disengageable from the flight deck, there is a method that I know Comair employs though where they disconnect the torsion link to the nose gear instead though.

Indeedy...you need two rampers, one on each side, to pull a spring-loaded handle outwards, which pulls the pins out from the link...let the link fall the few inches, then connect the towbar. Do the opposite once push is complete.

And like FlyASA said, we never unhooked the CR7...made for an easier p/b.

Alas, I don't miss my days on the ramp..but fun to talk about now.

DeltaGuy
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n710ps
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RE: 747 Steering Bypass During Push

Thu Sep 27, 2007 5:49 am

I know that feeling Delta lol. Do you ever feel guilty when you ask for a lav service? LOL I
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Jetlagged
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RE: 747 Steering Bypass During Push

Thu Sep 27, 2007 6:01 am

Quoting VC-10 (Reply 20):
That is not to be used for isolating the steering for pushback.

I thought depressurising the NWS would be sufficient, what is the reason for not using it for pushback?
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twal1011727
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RE: 747 Steering Bypass During Push

Thu Sep 27, 2007 7:31 am

Torsion links.......aaahhh those were the days...

The best one was the B727. ....It was chore to disconnect but could be worse to reconnect..
If the torsion links weren't lined up correctly by the tug driver then you had to "adjust it" by moving the
steering cables to get them lined up.

But nobody heard that from me.......

Only a few airlines had bypass pins installed for the B727s

KD
 
VC-10
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RE: 747 Steering Bypass During Push

Thu Sep 27, 2007 9:06 pm

Quoting Jetlagged (Reply 25):
Quoting VC-10 (Reply 20):
That is not to be used for isolating the steering for pushback.

I thought depressurising the NWS would be sufficient, what is the reason for not using it for pushback?

One word - Safety

Whoever is in the cockpit cannot see :-

1) If the gear is centred

2) If the tug has been disconnected

3) If the tug has been driven away, if the tow bar is still connected to the a/c

4) If the area is clear of personell

If that switch was used and the gear wasn't centred when selected to 'on' the nose gear would centre and damage/injury would ensue to any equipment/personell in the nose gear area if the area wasn't clear.

By using the electrical steering disconnect box on the located on the nose leg the people in the danger area are in control.

When the electrical disconnect box is in the by-pass position there is an ECAM memo message to advise the Flt Crew
 
A10WARTHOG
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RE: 747 Steering Bypass During Push

Thu Sep 27, 2007 11:49 pm

Quoting FlyASAGuy2005 (Reply 21):
We did it at ASA in Atlanta. It was company policy. It didn't matter if we were doing a straight push back or not (the CRJ-200 only). We never disconnected on the 70 seater

I believe the big difference between the CRJ 200 and 700 is that the 200 NWS steering is cable operated and the 700 is steer by-wire. I do not think that there is a NWS switch in the cockpit of the 200 as in the 700.

On the ERJ you have a switch aft of the ground power door that you flip or pull the E20 CB when you go to tow or push. The switch or breaker disables the NWS all together. The ERJ is steer by-wire.

On the 777 we disconnect the torque links when ever we are going into the hanger.
 
B747FE
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RE: 747 Steering Bypass During Push

Fri Sep 28, 2007 12:29 am

Quoting A10WARTHOG (Reply 28):
I believe the big difference between the CRJ 200 and 700 is that the 200 NWS steering is cable operated and the 700 is steer by-wire. I do not think that there is a NWS switch in the cockpit of the 200 as in the 700.

If I recall correctly, the NWS System in the CRJ 200 is steer-by-wire and for sure there is a cockpit switch to armed the system.

Regards,
B747FE.
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