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Richard McDonald Woods

Steering pin effects

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When the ground crew insert the steering pin for pushback, what actually happens to the steering mechanisms? Does it prevent the flight crew from having any steering capabilities?


Cheers, Richard

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Hi Richard,

 

Think about it this way,

 

The ground crew need to turn the aircraft off the stand and they need to move; turning the nose gear.  (Bypassing the hydraulics)

 

Without the pin the nose gear wood remain in a neutral position causing damage when the tug tries to turn the gear.

 

Does  that make sense?

 

Alex


Alex Ridge

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On the flight deck, the tiller stays neutral while the nose gears is turning with the steering lockout pin inserted.

You could even move the tiller around, without any effects "down there".

In real life however, that wouldn't be a good idea. The moment the ground crew removes the pin, the nose wheels would steer to where the tiller is pointing.

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Actually there are some type of Aircraft (i.E. Embraer 170-195, Fokker 70/100) that dont have a Pin. We have to flip a little

switch on the Ground Com panel. But that switch does exactly the same as the pin would do. The switch then flips back to

normal position when the panel is closed.

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Does it prevent the flight crew from having any steering capabilities?

 

Yes.  See below.

 

 

 


Without the pin the nose gear wood remain in a neutral position causing damage when the tug tries to turn the gear.
 
Does  that make sense?

 

That's not entirely true.  Well...it is, and it isn't, and I'm sure there's gotta be a plane out there that's built in a way that would take damage from not using the lockout...

 

...but...

 

The lockout - be it a pin, nosewheel steering switch (common on Bombarier aircraft like the CRJ and Dashes), a procedure of depressurizing the hydraulics, or something else - simply prevents the system from providing inputs to the nosewheel steering.  This allows the tug to turn the wheel more easily, and also helps to prevent damage should signals conflict (crew accidentally hits the tiller in one direction while the crew is trying to turn the gear in the opposite direction.  The whole concept of hydraulics is simply allowing larger forces to be applied than by human mechanical action alone.  As there is no direct mechanical connection, forcing the gear out of place only changes the pressures of the fluid until the pressures equalize at new position.

 

As an example, a car's power steering allows you to steer more easily.  Exerting a force on the wheel directly instead of on the steering wheel while the system was active would not harm it (excepting potentially knocking the alignment off some, but that's a different story), however, and you'd see the steering wheel eventually settle at the new, turned position.  The only difference is that you wouldn't be met with any hydraulic resistance from the system in trying to turn the wheel if there were a bypass pin like in an aircraft.

 

Similarly, the 744's main gear is actually hydraulically activated to turn in addition to the nose wheel to assist in turns (under something like 12 knots).  During pushback, only the nose gear is locked out; the main gear is not.  It's simply forced to follow along by the forces of the ground acting back on the main gear.  This is met by resistance in the hydraulics because they're pressurized, but fluids seek equilibrium, so despite the resistance, they eventually turn.  Granted, the main gear steering will kick in and assist the turn when the nose gear is turned past 20 degrees (if I remember my UAL training correctly), but up to that 20 degrees of nose gear deflection, the main gear offers no assistance.  Again, since the main gear is still pressurized, it's just like having a nose gear without the lockout pin installed.  It's not as much a damage thing as much as it is making the turns easier on all the equipment involved.  I'm sure you could make a long term damage case, though, as having to fight the hydraulics over time would eventually wear all the equipment involved out faster because higher forces are exerted to get it to do the same thing.

 

If you're ever at an airport serviced by a lot of CRJs (particularly those where the operator has the ground crew disengage the torque links), and see two or three rampies trying to pull the upper bar back into position to match the lower bar, what you're seeing is the crew had left (or jumped the gun on turning) the nose gear steering switch on.  It'll move to a new equilibrium position if you exert enough pressure (and if the crew isn't holding the tiller in place) without damaging anything, but it takes a lot of effort.  The clever rampie will just ask the crew to check the steering switch.  Once it's back off, it'll move with ease to the required position.

 

...or in my case, when I knew the crew:

"Verify prior crew's compliance with the shutdown checklist."

(The CRJ's nosewheel steering on our checklist was killed on shutdown, and then reactivated on the before taxi check)


Kyle Rodgers

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Its the same deal with the older buses, before the brake steering control unit update you have to flip the Nose wheel steering switch prior to pushback.

 

If you didn't switch it off you could potentially cause sever damage once the green hydraulic system pressurised during engine start.

 

On newer aircraft you follow this sop

 

NW STRG DISC.......................................................................................................CHECK AS RQRD

In case of pushback (conventional or towbarless), the nosewheel steering selector bypass pin must be in the tow position. The ECAM ’s NW STRG DISC, or N WHEEL STEERG DISC memos indicate this to the flight crew.


Rob Prest

 

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Hi Kyle,

 

Many thanks for the lengthyy reply. I posted in the PMDG forums because I am particularly interested in the Boeing aicraft.

 

I understand that the idea is to temporarily prevent the hydraulics from steering the nose gear. What I am struggling with is what is the action of the steering pin. I can understand a switch to turn off hydraulic pressure or pressure changes. I find it difficult to imagine a pin having some form of effect. What is it specifically doing?

 

Cheers, R


Cheers, Richard

Intel Core i7-7700K @ 4.2 GHz, 16 GB memory, 1 TB SSD, GTX 1080 Ti, 28" 4K display

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The pin isolates the pressure and return lines to the steering actuators.. Doesn't matter if it is a Airbus/Boeing or MD, they all basically do the same thing.

 

During engine start you are powering up the hydraulics, the last thing you need is the NWS becoming pressurised while it is attached to a push back truck

Here's a little story I read a while back..

 

-A USAir DC9 flight came in a a bit late and we tried to push it out on sched.

-We were short staffed that day (what ground handling company isn't?)

-There were a couple of newbies working the flight and we tried to make sure they didn't kill themselves before they learned what not to do (training consisteb basically of telling them no to walk around by themselves before getting a "red" pass)

-To makeup time engines were started during pushback.

-While pushing, apron changed their mind and wanted the plane facing in the other direction resulting in the pushback tractor being at an angle to the A/C at the end of pushback.

-I showed the newbie how to disconect the towbar from the tractor (waiting for brakes set, etc.) and while turning away to give the thumbs up to confirm to the tractor driver to back up a bit the newbie proceeded to the NLG to remove the bypass pin. I saw him in slow motion and yelled but with the earmuffs and the engines running he did not hear me. With the engines running, and the hydraulics pressurized, the steering knob centered in the cockpit the gear did what it's supposed to do, it centered itself, FAST! I saw it swing with the towbar still attached. I had no time to move and it just wiped out the legs from under me.


Rob Prest

 

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Many thanks for the lengthyy reply.

 

You're welcome.

 

 

 


I find it difficult to imagine a pin having some form of effect. What is it specifically doing?

 

In addition to what Rob mentioned, you have to think of it in a different way:

 

Next time you're at an airport, have a look out on the ramp at the equipment they're using.  The belt loaders, bag carts and other equipment will generally look pretty beaten up.  The ramp is a really rough place, and rampies are not known for delicate care.  Ask anyone about their opinion on how the airline handled their bag.  Who was the one who damaged that bag?  A rampie.

 

So, when it came down to putting a switch in the nose gear bay; or a long, approximately 10mm metal pin that then actuates a switch that the rampie can't quite get to, in order to make it more "ramp proof," they went with the latter.  Just think of it as a nosewheel steering switch outside, but with some "childproofing," if you will.

 

Granted, different aircraft have different pin options.  A SAAB 340, as an example, has a pin (mainly to prevent the nose gear from retracting, I believe) and a switch depressor attached to it via a string.  The switch, of course, locks out the steering.  CRJs don't have a pin at all (most operators rely on the crew to turn the steering off, or the ramp to disconnect the torque links, which physically disconnects the nosewheel from the hydraulics, instead of bypassing).

 

...and I'll stop now before writing another novel.

 

 

 

This is all the side effect of working on the ramp for 5ish years.


Kyle Rodgers

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Thanks for your replies, gentelmen. I am now very happy.

 

Cheers, R


Cheers, Richard

Intel Core i7-7700K @ 4.2 GHz, 16 GB memory, 1 TB SSD, GTX 1080 Ti, 28" 4K display

Win10-64, P3Dv5, PMDG 748 & 777, Milviz KA350i, ASP3D, vPilot, Navigraph, PFPX, ChasePlane, Orbx 

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I'm confused by all the replies!

 

Some years ago I was boarding a 737-500 at my local airport and the identical aircraft at the next gate was being pushed back. I could see quite clearly that the push truck's mechanism had lifted the nosewheels clear of the ground by a couple of inches. So I assumed that the pin was to lock the nosewheel in the straight ahead position as you wouldn't want both the nosewheel and the trucks articulation working at the same time as this would make it very difficult to control.

 

Has this changed? Is the nosewheel not jacked up anymore?

 

Iain Smith

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Hi Ian,

 

There is more than one way to push back an aircraft, the method you witnessed is just one of them.


Rob Prest

 

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Has this changed? Is the nosewheel not jacked up anymore?

 

As I always say:

Aviation isn't as regimented or standardized as many believe.  The main two are cradle type (the type you saw), and tug and towbar.

 

Remember, just because you saw it one way doesn't mean it's not somehow different somewhere else.


Kyle Rodgers

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Here's another way to think of it. Some aircraft steering systems are limited to a certain amount of steering travel left or right. Every jet I've flown, DC-10 or GV for example, you have to remove the steering mechanism pin and disconnect the steering scissor before towing. The GV steering is limited to 80 degrees of travel left or right. At 82 degrees a pop up over travel indicator displays above the collar requiring internal inspection. At 84 degrees damage occurs. To prevent this, we disconnect the steering mechanism allowing the nose wheel to rotate freely past the steering limits during tow. Even though some tow vehicles lift the nose wheel, I still disconnect it to stay consistent.  So in a sense, you don't want the nose wheel to turn past the the steering system limits with the steering system connected to prevent damage. It seems in the aircraft discussed above, a pin is inserted to bypass the steering mechanism.

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