B737 Pushback & After Start
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B737 Pushback & After Start
Do you wait for the bar to be removed before putting the generators on line?
I don't see how this could cause any harm, but some say that could cause steering movement, thus move the whole bar and hit the guy...
Do you believe that?
LEM
I don't see how this could cause any harm, but some say that could cause steering movement, thus move the whole bar and hit the guy...
Do you believe that?
LEM
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Several people were injured on the -200 series in the early days by the situation you describe.
The hydraulics started up and the nose wheel turned the tow-bar after the tractor had disconnected injuring the engineer/ground handler doing the push-back.
Shortly thereafter Boeing introduced the nosewheel steering lockout pin which isolated the nosewheel steering until the tow bar was removed.
Various changes have been made over the years and I don't know if this is still the case on the NG generation, but that was the origin of the story.
The hydraulics started up and the nose wheel turned the tow-bar after the tractor had disconnected injuring the engineer/ground handler doing the push-back.
Shortly thereafter Boeing introduced the nosewheel steering lockout pin which isolated the nosewheel steering until the tow bar was removed.
Various changes have been made over the years and I don't know if this is still the case on the NG generation, but that was the origin of the story.
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At least on the 737-200, and I suspect some later models, you would not want to select the generators on until you physically see the towbar being hauled away from the a/c. Remember that you will have the "A" hydraulic pumps (nose wheel steering) selected off. They are powered closed (off) and by momentarily interrupting their AC power source, you run the risk of powering the "A" system which could give the nose wheel, and anything attached to it, a nasty swing.
Our procedure was never touch anything until the bar was clear. There was no nosewheel steering bypass pin on the early 737s that I flew for 8,000 hours.
Our procedure was never touch anything until the bar was clear. There was no nosewheel steering bypass pin on the early 737s that I flew for 8,000 hours.
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From the 737 Flight Crew Training Manual....
.... 'Pushbacks present a serious hazard to ground personnel. The headset person, who is walking in the vicinity of the nose wheel, is usually the person injured or killed in the majority of the accidents.....Note: Pushback is normally accomplished with all hydraulic systems pressurized and the nose wheel steering locked out.'
In my experience...while the pin offers protection against the tow bar swinging and injuring a person during push back, it is all happening beneath the airplane and not in view of the skipper. While the bar is being disconnected and the pin is being removed, the crew are not in control and therefore having the A system off gives added protection to prevent an accident.
Your thoughts!!
.... 'Pushbacks present a serious hazard to ground personnel. The headset person, who is walking in the vicinity of the nose wheel, is usually the person injured or killed in the majority of the accidents.....Note: Pushback is normally accomplished with all hydraulic systems pressurized and the nose wheel steering locked out.'
In my experience...while the pin offers protection against the tow bar swinging and injuring a person during push back, it is all happening beneath the airplane and not in view of the skipper. While the bar is being disconnected and the pin is being removed, the crew are not in control and therefore having the A system off gives added protection to prevent an accident.
Your thoughts!!
Last edited by Global Pilot; 9th Feb 2005 at 08:13.
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My recollection- before pin, 'A' off, don't touch electrics until crew clear. Post-pin, as long as the pin comes out AFTER bar removal, no restrictions.
Any comment from p/back crews?
Any comment from p/back crews?
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Can only speak for the 737-NG's
These have engine driven and electrical hydraulic pumps. As soon as the engines are turning, hydraulic pressure will be available, so generator selection is irrelavant. You must fit the steering pin for a push back.
Like any system, this can be Murphied. I have read reports that ground crews have used the wrong diameter steering pin or placed it in a rigging hole instead of the lock out hole, thus failing to properly lock out the steering.
These have engine driven and electrical hydraulic pumps. As soon as the engines are turning, hydraulic pressure will be available, so generator selection is irrelavant. You must fit the steering pin for a push back.
Like any system, this can be Murphied. I have read reports that ground crews have used the wrong diameter steering pin or placed it in a rigging hole instead of the lock out hole, thus failing to properly lock out the steering.
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I can tell you from experience that not having the pin there (or having NWS system pressurised) breaks the shear pin on the tow bar when a turn is initiated. So if you have been pushed back and the shear pin hasn't broken then you can be sure that the NWS lockout pin is working. If its working, there is no reason to worry, so you should be able to reconfigure the systems without a problem.
(Just in case anyone is wondering - the NWS steering pin was inserted, but the housing had cracked sufficiently that it didn't hold the plunger far enough back to cut of the HYD supply - Thus it didn't work!)
(Just in case anyone is wondering - the NWS steering pin was inserted, but the housing had cracked sufficiently that it didn't hold the plunger far enough back to cut of the HYD supply - Thus it didn't work!)
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Now I see it more clearly.
In our company (737 classic) nobody turns the A pumps off for pushback.
In this case, waiting for the bar to be seen disconnected before turning the gens on, doesn't make any sense.
It's a ground personnel responsibility to remove the pin AFTER the bar is disconnected.
Should they mess up, their problem, legally speaking.
But of course, if you want to avoid troubles, you can adopt the most cautious procedure, by turning A system off before push.
In this case, yes, it makes sense to wait before selecting the generators on line, for the reason explained by Sawbones.
Now, it's our call... which of the two procedures we want to adopt?
Surely, if one operates in third world countries, the answer comes naturally.
Operating in Europe only, one might say depressurizing sys A is exaggerated... but I'm sure that if we had access to the whole record of pushback incidents/accidents, one would change his mind...
Thanks for your kind replies, Gentlemen.
LEM
In our company (737 classic) nobody turns the A pumps off for pushback.
In this case, waiting for the bar to be seen disconnected before turning the gens on, doesn't make any sense.
It's a ground personnel responsibility to remove the pin AFTER the bar is disconnected.
Should they mess up, their problem, legally speaking.
But of course, if you want to avoid troubles, you can adopt the most cautious procedure, by turning A system off before push.
In this case, yes, it makes sense to wait before selecting the generators on line, for the reason explained by Sawbones.
Now, it's our call... which of the two procedures we want to adopt?
Surely, if one operates in third world countries, the answer comes naturally.
Operating in Europe only, one might say depressurizing sys A is exaggerated... but I'm sure that if we had access to the whole record of pushback incidents/accidents, one would change his mind...
Thanks for your kind replies, Gentlemen.
LEM
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Surely, if one operates in third world countries, the answer comes naturally.
Operating in Europe only, one might say depressurizing sys A is exaggerated... but I'm sure that if we had access to the whole record of pushback incidents/accidents, one would change his mind...
Operating in Europe only, one might say depressurizing sys A is exaggerated... but I'm sure that if we had access to the whole record of pushback incidents/accidents, one would change his mind...
, or, to be fair, any country where the national language is not English, it is ALWAYS worth being careful with pushback crew. In my most memorable case, Montpellier:-"Hello bonjour [matelot] (note the linguistic flair) - your checks please"
" OK - 'eer we go"
Park brake off just in time before nosewheel was snapped off
Followed by "arretez arretez" (impressed?)
I remember also that DanAir had a nosewheel driven back into the fuselage on a 146 when pushback in AMS was started before the brakes were called off.
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Hi
Yes it's a good idea to have the A pumps when the personnel have moved out and the ground crew has shown you the red flag for the pin, we have had SYS A going up with the pin in during a push but we saved the tow-bar by pushing straight.
I guess the SAA procedures for push back are great, nothing you do until you see the engineer left or right of the aircraft and all ground equipment removed.
Zenj
Yes it's a good idea to have the A pumps when the personnel have moved out and the ground crew has shown you the red flag for the pin, we have had SYS A going up with the pin in during a push but we saved the tow-bar by pushing straight.
I guess the SAA procedures for push back are great, nothing you do until you see the engineer left or right of the aircraft and all ground equipment removed.
Zenj
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Early 737s did not have the pin and isolation valve for the nose wheel steering. So in that case the A pumps were selected off. The selenoid valves that close off the A pumps are power to close/isolate. When switching power sources (bringing the generators online) the concern is the A pump momentarily pressurizing A system.
So, we do it because that way it is standard with the early acft and also provides a back up to the pin. If you were at a carrier that never had old acft, they may not do it that way.
Lock out pin reliability? Should be good. But right now A 320s have some sort of problem with this causing un intended nose gear movement even with the lock out pin installed. So right now to start an A 320 with the towbar connected we have to turn off the BSCU which removes the hydraulic power to the steering. New plane, old problem.
So somewhat tradition, somewhat extra insurance. The old aircraft are long gone but it does provide extra protection.
So, we do it because that way it is standard with the early acft and also provides a back up to the pin. If you were at a carrier that never had old acft, they may not do it that way.
Lock out pin reliability? Should be good. But right now A 320s have some sort of problem with this causing un intended nose gear movement even with the lock out pin installed. So right now to start an A 320 with the towbar connected we have to turn off the BSCU which removes the hydraulic power to the steering. New plane, old problem.
So somewhat tradition, somewhat extra insurance. The old aircraft are long gone but it does provide extra protection.
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I read about an incident involving a B737-200 pushback at night at Taipei. The A pumps had been selected off prior to engine start.
Engines were started during pushback. In accordance with procedures described earlier the generators had not been selected on after both engines were running. No problem so far.
As the tug turned the aircraft the tug driver was momentarily blinded by the nosewheel light which the captain had for some reason switched during the push.
Pilot says Oops - so sorry! and goes to switch off the nose wheel light switch which is conveniently situated next to the APU switch. He turned off the wrong switch and the APU died causing the A Pumps to supply hydraulic pressure to the nosewheel and the tug driver was injured when the tow bar broke.
Engines were started during pushback. In accordance with procedures described earlier the generators had not been selected on after both engines were running. No problem so far.
As the tug turned the aircraft the tug driver was momentarily blinded by the nosewheel light which the captain had for some reason switched during the push.
Pilot says Oops - so sorry! and goes to switch off the nose wheel light switch which is conveniently situated next to the APU switch. He turned off the wrong switch and the APU died causing the A Pumps to supply hydraulic pressure to the nosewheel and the tug driver was injured when the tow bar broke.
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You raise an interesting point, Hudson.
Since the blocking valves on the Hyd engine driven pumps are powered to close, this means that should the APU fail, the A system would be suddenly pressurized (the engine is turning at that stage)....
Intersting
Eventually, the only really safe way to push back is with the engines shut down!
You stop, then you start up!!!
Since the blocking valves on the Hyd engine driven pumps are powered to close, this means that should the APU fail, the A system would be suddenly pressurized (the engine is turning at that stage)....
Intersting
Eventually, the only really safe way to push back is with the engines shut down!
You stop, then you start up!!!
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LEM
Yes indeed eventually thats the only solution to push back safely.
Even for us in the airline when we are suspecting an aircraft to pressurize on engine start with HYD A off, we ask to be pushed straight and start and if it's worse scenarion , to push and start when the tow bar has been removed and all ground equipment removed.
Zenj
Yes indeed eventually thats the only solution to push back safely.
Even for us in the airline when we are suspecting an aircraft to pressurize on engine start with HYD A off, we ask to be pushed straight and start and if it's worse scenarion , to push and start when the tow bar has been removed and all ground equipment removed.
Zenj
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According to our (ATA Airlines) B737-800 CFM, vol.1 page NP.3.6 - NP.3.8 we don't touch anything until the ground crew has disconnected from the aircraft, held the pin up in plain view, and issued a salute.
I have no clue if this is a Boeing procedure or just ours.
I have no clue if this is a Boeing procedure or just ours.
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LEM;
That is exactly why a previous company I used to fly for did not start engines on the old 200 series until push back was complete and tow bar disconnected and tug driven away with towbar in sight. This was their SOP... (However a subsequent company I flew for did not see the need to adopt this procedure... )
That is exactly why a previous company I used to fly for did not start engines on the old 200 series until push back was complete and tow bar disconnected and tug driven away with towbar in sight. This was their SOP... (However a subsequent company I flew for did not see the need to adopt this procedure... )
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you either have a PIN that way have pumps On and save solenoid
or you don t have a PIN,then select Pumps OFF.
you could say it's only a backup to have them off,OK i agree but why should you select Gens online during pushback and cause troubles if you are looking for backup??
IT really doesn't make any sense to select Gens online while on pushback.
to save time (and solenoid) : Hydraulics Normal.PIN IN
start pushback,start engine 2.wait till end of pushback and then start engine 1. doesn't take long time. (for elec 2 you can select it OFF)
or you don t have a PIN,then select Pumps OFF.
you could say it's only a backup to have them off,OK i agree but why should you select Gens online during pushback and cause troubles if you are looking for backup??
IT really doesn't make any sense to select Gens online while on pushback.
to save time (and solenoid) : Hydraulics Normal.PIN IN
start pushback,start engine 2.wait till end of pushback and then start engine 1. doesn't take long time. (for elec 2 you can select it OFF)
