Three broken tow bars, all because of bypass pin

Servisair · Post by **Servisair** » Tue Mar 21, 2006 2:18 pm

So yesterday at Pearson T3, I was watching someone try to push a B757 off a gate with a TPX. For those that dont know what a TPX is, its a towing tug that lifts the nose gear off the ground. So he couldnt get the aircraft to turn so he decided to push it back w/ a tug and bar. After 3 attempts and 3 broken tow bars, he made the assumption that there was something wrong w/ the aircraft. After a closer examination by management the problem was the wrong bypass pin was installed in the nose gear by the push driver.
If any of you are familiar with the T3 ramp, the airline was North American and was pushing off of gate B18.

FREEFALL · Post by **FREEFALL** » Tue Mar 21, 2006 2:45 pm

Hmmmmmmmm.......

let me guess. You work with the person in question and you can't stand him or her, right?

flyin' fish · Post by **flyin' fish** » Tue Mar 21, 2006 4:25 pm

first of all, the bypass pin is just extra protection from the hydraulic system activating during start up (at least for the B737). The only time I've towbars broken is with the park brake set and the shear pin gives.
Were they starting engines in the process??

Pete · Post by **Pete** » Tue Mar 21, 2006 5:04 pm

HMY? I bet they were pissed. Then again a body is a body to a ground company, not a brain. Sometimes I felt like I was taking part in a drive by raid then loading a plane with all those little ghetto rampies. Kind of sad, and I bet they could care less about how many tow bars they break.

hazatude · Post by **hazatude** » Tue Mar 21, 2006 5:15 pm

Sounds like a clear case of pilot error to me

boeingboy · Post by **boeingboy** » Tue Mar 21, 2006 5:17 pm

first of all, the bypass pin is just extra protection from the hydraulic system activating during start up

Umm.... No.

The bypass pin allows hyd fluid to bypass the steering mechanism so that you can turn the nosewheel when towing. The hydralics are already turned on and pressurized before the aircraft moves.

If the hydralics are pressurized and you try to turn the aircraft (without a bypass pin) - you break the towbar shear pin.

hazatude · Post by **hazatude** » Tue Mar 21, 2006 5:18 pm

hazatude wrote:Sounds like a clear case of pilot error to me

flyin' fish · Post by **flyin' fish** » Tue Mar 21, 2006 6:18 pm

boeing boy.... just curious because I don't fly 757's but why would you pressurize the hydraulic system before disconnecting the towbar?
On the 37 we keep the A sys off as a safeguard until the bar is long gone.

asdfasd · Post by **asdfasd** » Wed Mar 22, 2006 3:54 pm

of course every operator has their own procedures, some do turn off the hydraulics during startup and some don’t. Ground ops should always assume the hydraulics are pressurized (which they are in most cases).

Mitch Cronin · Post by **Mitch Cronin** » Wed Mar 22, 2006 4:04 pm

hazatude wrote:Sounds like a clear case of pilot error to me

Mitch Cronin · Post by **Mitch Cronin** » Wed Mar 22, 2006 4:04 pm

Mitch Cronin wrote:
hazatude wrote:Sounds like a clear case of pilot error to me

I second that.

Big Pratt · Post by **Big Pratt** » Wed Mar 22, 2006 9:04 pm

Reminds me of an incident back in the last century.
I was working on the ramp and a chain of events nearly ended in an accident.

-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.

A badly swollen ankle and elbow taught me a lesson to never rush things and screw the delay, always do it by the book. A cheap lesson mind you, some paid a higher price to learn...

Be safe out there

fmrc3ame · Post by **fmrc3ame** » Wed Mar 22, 2006 11:05 pm

If the hydralics are pressurized and you try to turn the aircraft (without a bypass pin) - you break the towbar shear pin.[/quote]

True but the steering system on some aircraft uses an engine oil pressure signal to activate. So in other words you can tow without a bypass pin, however it isn't recomended.

fmrc3ame · Post by **fmrc3ame** » Wed Mar 22, 2006 11:07 pm

flyin' fish wrote:boeing boy.... just curious because I don't fly 757's but why would you pressurize the hydraulic system before disconnecting the towbar?
On the 37 we keep the A sys off as a safeguard until the bar is long gone.

Hydraulics are turned on for keeping brake pressure up.

wsguy · Post by **wsguy** » Wed Mar 22, 2006 11:22 pm

Flying fish,

Good question on the hydraulic pressure and "A" pumps. I have time on the B727 and older B737... we used to turn "A" pumps off with one company. With another company we used to delay engine start until tow bar disconnected as a powere bump could cause the "A" solinoid values to presurize the "A" system even with the "A" pumps selected off. You will recall from your hydraulic schematics that the "A" pumps (engine driven) actually pressurize during engine start and the "A" switches just cause the Hydraulic pressure to bypass when selected off. As I said, there is still the chance of the "A" system pressurizing. This is why one B737-200 operator used to just wait til the tow bar was disconnected before engine start/

As mentioned, the B757 is fitted with a by-pass pin to avoid nose wheel steering to be pressurized while the tow bar is connected. Other types also have similiar systems. The A310 and the A300-600 also have this type of system. If the by-pass pin is in place there should not be a problem. Having said this I recall having a tug driver break a tow bar on a push back when flying the A310. It happens.

Point? No much really. Just that different types and also different companies have different procedures. Right now operating the B757 we use Boeing procedures. Same as the said company which had the tow bars break.

flyin' fish · Post by **flyin' fish** » Thu Mar 23, 2006 7:26 pm

thanks for the input