Aircraft and tug part company

[cornerstone]

The local news in Dublin reports that an aircraft made it's own way landside through a preimeter fence. No mention of the airline yet but, but it was an ATR and an engineer has been taken to hospital. As this is the second time this has happened here, think it may be time to beef up the tow bars!

[BN2A]

Quickest way to the local Guinness House, I guess!!

Shame about the Eng though....

:uhoh:

[Flightmech]

What was the brake-rider doing then, down the back making coffee? :ok:

Hope the engineer is ok if it was him on the brakes.

[Flame]

Aer Arann ATR42, Aircraft was EI-CVR, reports are saying the towbar snapped (?), it rolled just off the ramp into soft ground...but if you were to believe media reports in Dublin, you would think it flew straight into a perimeter fence..!!!!

[Sunfish]

I thought tow bars had a shear pin, if so, maybe it was a little too shearable?

[PIGDOG]

Isn't that the point of a shear pin?

[Irish Steve]

Isn't that the point of a shear pin?

Not 100% sure about the ATR bar, but most other bars I've had experience with, there are at least 2 pins. The Shear pin is designed to provide protection against going beyond the turning limit of the nosewheel, and if a tug driver goes beyond the limit, it breaks, protecting the aircraft, but there is still a (now non rigid) link between the aircraft and the tug. The other pin is not designed to break as such in the same way, it's a lot stronger.

On some bars, the shear pin also provides some protection against push against the brakes, but that's a secondary purpose.

In theory, and theory is a wonderful 20/20 thing that everyone else uses to hit people around the head after the event, if the shear pin breaks when towing, and if the tug driver realises, keeps going but slows down carefully, and the person on the aircraft brakes applies them, no damage will occur to either tug or aircraft, as the braking action will keep things in line. It's not quite that simple, especially on the larger aircraft, where there's a steering disable pin that means it's under control of the bar, but it should work. There may be an urgent requirement for a change of underwear, but that's lifr:E

If the bar breaks completely, (which is not supposed to happen :D ) then the tug should get out of the way PDQ, and let the man on the hot seat in the aircraft sort it out :E

On larger aircraft, on pushback rather than towing, if the shear pin breaks, and the engines are running, and the headset man is slow off the mark, and there are language problems, it can get "exciting" for a few seconds, but that's another story for another day;)

It helps if the tug and aircraft are both on R/T, but I know from experience at DUB that not all tug drivers are signed off for R/T, which can complicate things, and not all tugs are R/T equipped.

When towing, if the tug stops too fast after a broken shear pin, and the person on the flight deck is not aware of what's happened, there is also the risk of the aircraft keeping going, and of collision between aircraft and tug, depending on the speeds, length of bar, and all sorts of other issues, and damage can still be done to the nosewheel. That's expensive!

Based only on what's been reported here, and without accurate information, this sounds as if something else broke, as the aircraft "got away" from the tug.

OK, the nosewheel steering may well be inoperative (I think from memory, a CB has to be pulled on the flight deck to disable the steering, but it's a long time since I worked with ATR's), but the brakes should have been working, so something out of the ordinary has happened here, and we hopefully will find out from the AAIB site at some stage exactly what happened.

[Ranger 1]

Shear pins will shear from time to time through general wear & tear, the tug crew will normally be aware of this straight away, although the bar will not normally detach unless excessive load is applied, this is not always the tug crew/ flight decks fault, sometimes this happens when the aircraft has been pushed & then is then being pulled, on an apron/ramp with a gradient causing an overload at the point the tug changes direction due to the opposite direction of force of the aircraft, if a complete detachmet of a bar occurs usually an engineer is called to check the aircraft & the bar is removed from service for detailed examination.
:ok:
Shear pins as PIGDOG questioned are an overload safety device to prevent damage to aircraft, although there has been rare occurences where things have not gone according to plan & the nose gear has been severely damaged some aircraft, one such incident stands out well in my memory:uhoh:

[PIGDOG]

Irish Steve,

I have to admit I didn't think the shear pin was to protect against going beyond the turning limit. I was under the impression that the red line on the nose gear door and a bit of education were the only protection for that!!:rolleyes:

...and surely it wouldn't be called a 'shear' pin??
;)

Not that I'd ever want to contradict a fellow Meath man!:D

... although after checking on airliners.net it doesn't seem that many ATR 42's have said red line:O

It would appear that only Air New Zealands aircraft have it!

http://www.airliners.net/open.file/806963/L/

Oh well, we can't be right all the time.

[Krystal n chips]

Glad to hear the Eng is OK--however--from the facts presented here some rather basic errors it would seem.

Given that I have had the rather disturbing experience of applying the brakes whilst on tow--on several occassions over the years as a result of the shear pin failing---the question has to be where was he when the pin / tow bar failed and why were the brakes not applied at that time ?. That, after all, is the reason you sit there is it not ?.

I am curious--ignoring the media reports of course--as to why and how this a/c seemingly covered such a lot of ground--or was it parked near the fence anyway?.

Bit of a salutory lesson here I feel, sadly, in that some rather basic airmanship seems to have been negelected--not for the first time either. Always amazed me as to how nonchalant some people could be when riding the brakes "cos it never goes wrong and it's boring"---yeah, right---it is for 99 out of a 100--then comes the 1.

Hope though, that all works out well in the end. And by the way, this is NOT intended as a criticism of those involved--merely a commentary on the facts ? as published so far. There but for the grace of God etc.

[BOAC]

...always assuming the hydraulics WERE pressurised, naturally:eek:

It would not be the first time in history.....

[Irish Steve]

I have to admit I didn't think the shear pin was to protect against going beyond the turning limit.

I can assure you, having watched some of my less capable former colleagues break them on a regular basis, (One memorable a:mad:e manage to break 2 on the same aircraft in the space of less than 10 minutes one night) they are most definitely there to protect against both rotation and power overloads:D :D .

There are absolute mechanical limits to any aircraft that has a steerable nosewheel, and with the length of some towbars (the Tu144 was the worst, I think that one was over 25 Ft long!), the potential for damage at that radius is massive, and something is going to give if a 35+ Tonne tug decides to fight with a 300+ tonne aircraft:D

When a pin lets go without warning, which is not hard at DUB, due to the design of some of the drainage channels, life can get quite exciting for a few moments, especially if the aircraft concerned is something like a 767-300 or similar that's already wound up both engines during the push.


If the nose wheel is at an angle, and the shear lets go, the aircraft is going to start moving forward quite quickly, which requires the headset man to recognise what's happened, and call the flight deck for emergency brakes, before the aircraft hits the tug, or does other damage by still going beyond the steering limit. If the flight deck are busy, and they usually are, getting their attention can be challenging!

Strangely enough, the worst aircraft in that respect is the 757, as the engines on them spool up to a much higher power setting during the start, which means that there's a lot more thrust generated for the few seconds before it drops back to ground idle, though the 777 is also problematic in that respect, as it's got enough bleed air to be able to start both engines at the same time. When they get going, if it's a long tortuous push, like off stand 33, if the tug is a little underpowered, and most of them are, SA didn't spend the money to get decent ones, pushing a heavy 777 round a 90 degree turn with both engines running is also challenging;)

Back to the ATR. I am guessing that there could be several possible explanations for what happened. I don't know the ATR, but I reckon that the brakes will be assisted with hydraulic power, so there's a possibility that if it had been on stand for a while, or was coming out of maintenance, there may not have been enough (or any) pressure in the system to be able to apply the brakes. I might be 100% off beam here, I never got to tow anything that small, whenever I was towing things around, which I did for quite a while, as I was the only one with an RT approval, they were larger, 737, A300's and 767's on an almost daily basis, and 747's and A330's quite often as well.

There are other less likely and maybe questionable reasons why this might have happened, best left to the official inquiry:E :E

[Clandestino]

On the ATR you have to have at least 70% propeller RPM in order to have normal braking availlable. So if a/c is towed with engines off (or props feathered) only brake you can use is emergency/parking brake powered by its own hyd accu and it's easily depletable.

[Irish Steve]

So if a/c is towed with engines off (or props feathered) only brake you can use is emergency/parking brake powered by its own hyd accu and it's easily depletable.

That's what I suspected. Thanks for the confirmation. It will be interesting to see what the accident investigation report recommends, and even more interesting to see if the handling companies take the recommendations on board:E

I know what I'd like to see brought in, but the chances of that are about as good as the chance of green snow:E

[Hand Shandy]

The braking system on the ATR is run off blue system hyds , which is also supplied by the dc aux pump , its probably the easiest aircraft i`ve ever rode brakes on .You don`t need power on as the aux pump runs off the hot bat bus via a switch on the centre pedestal , the hyds come up pretty quickly, obviously not quick enough in this case thats if there was any one in the left seat.

[Captain Stable]

Hand, if the props are in feather you have no AC power to the engine-driven HYD pumps. The aux HYD pump is only a very small affair, and can never bring up the pressure in time if you have a towbar shear in a case like this. In some cases it can take as long as 30 seconds.

The main intent and use of the aux HYD pump is simply to bring up pressure enough to apply or release the prop brake.

Usual recommended procedure when taxying in with one prop in feather (hence only one AC Gen) is to keep one hand on the emergency brake. It won't take much to remove all capability of using the foot brakes.

[Hand Shandy]

I`d have to have to disagree , the aux pump brings 3000 psi up almost immediately , i`ve changed plenty of wheels and brakes to attest to that,the 30 seconds run time on the pump is actioned by a timed relay , in taxi as you point out there would be no need to use the aux pump as the blue and green systems would be online , and as a matter of point whenever i`ve been on brakes i would action the pump prior to any hangar `input` although the accumulator will hold the px for a good few stamps on the brakes.

[Captain Stable]

HS, you may be an engineer, but I still have to disagree. I have flown plenty of different models of ATR's, and the vast majority take quite a while to bring pressure up. I can guarantee you that I have used that pump and have more experience taxying and being towed/pushed than you have! ;) On a brand new aircraft the pressure may come up quickly - but why bother? Why waste time on something that may not work in time instead of taking action that takes less time and is sure to work?

It strikes me that hoping that a tiny little unit like that will work in time to stop you departing the airfield in a non-standard direction should a towbar break is not sensible. Far better to be towed (and taxy onto stand) with one hand on the emergency brake than waste time hitting the push button on the pedestal, wait for pressure to come up so you can use your feet. That does not smack of good airmanship or professionalism to me.

I have also seen a system fail with one engine in feather, (this combined with an MFC failure) so it requires the F/O to check BTC is made and hydraulic pressure good each time you feather while still taxying.

[Hand Shandy]

Twenty years man and boy [pull up a sand bag] i`ve been involved in moving aircraft and from my pov and most probably the aer arran guys involved the, aux pump is the only available way of stopping an aircraft in this situation , bearing in mind when we tow power is generally off , there is no way i`d suggest to a driver to use the aux pump to brake the aircraft what would be the point with 2 engines out of feather its a no brainer.

[Captain Stable]

Okay - I'm confused - are you saying you do advocate using the aux pump in this way or not?

Because either way, I suggest that whether being towed or pushed, engines running or not, the best way of stopping the aircraft in a hurry when the towbar snaps is to use the emergency brake. One action does it - pull - instead of pushing the button, waiting for hydraulic pressure and then applying the foot brakes.

PS Can I correct you on one detail? The Emergency brake is powered by the blue system. Normal braking is on the green. So no amount of use of the aux hyd pump will bring the normal brake accumulator up unless the crossfeed valve on the overhead panel is on - which is not a normal position.

[XXTSGR]

HS, are you by any chance a tug driver? Because I get the very slight impression that Capt. S. is an ATR-rated pilot. He therefore probably knows the systems quite well. I get the impression that you're not quite a clued-up on the machine as you have led us to believe.

[Hand Shandy]

Sorry for the delay I`ve been playing with my tug.I`m perfectly aware of the hydraulic system architecture it`s written on 26vu in black and white [well green and blue so even old duffers like me can understand it], as for bringing up normal brake px thats routed via the crossfeed valve as you`d know , maybe i should have clarified, this will give you about 40seconds worth of brake use before px decays , although you may have hit the nail on the head if the brakeman was not trained correctly.Two incidents i`ve seen with aircraft deciding to go their own way have all been down to lack of hydralic pressure and not even size 10 safety boots would stop them . As for your comment captain that the aux pump is only there to just to take off the prop brake why have they designed it to run off the hot bat bus , to take the prop off you`d have to switch on the battery , a good idea from the French from a maintenance point of view , and if the small affair of an aux pump can repeatedly carry out undecarraige swings in the same time as an acw pump i`m sure it`ll stop a trundling ATR. It seems we`ve drifted from the original thread though as to why this happened .
Oh and xxtsgr both hyd pumps are of the conventional axial piston type , with a cylinder barrel containing 9 pistons the only difference being the flow rate of the acw is higher than the dc i learn`t that tugging around

[Clandestino]

ATR's "Both main hyd pumps loss" checklist calls for closing (or checking closed) hyd crosfeed, so after the landing gear lever is lovered, dc aux pump kicks in and gives you some pressure to lover flaps but gear goes down with a little help from gravity. There's warning in FCOM that in case of go-around you can not retract the gear, therefore it seems to me that opening crosfeed to pressurize green system by using dc pump is not ATR approved procedure, at least for flyers. I couldn't find the reason for it written explicitly in the FCOM but our French instructors told us that DC pump is too weak to supply both hyd systems.

[GotTheTshirt]

Irish Steve,
On large aircraft the shear pin will shear with any abnormal load not specifically steering turn limits.
In fact some tow bars can be used on different aircraft with different turn limits.
Many towbars have an interchangeable head so the bar can be used on different aircraft. I have used some tow bars that have a shear pin and then the head is connected to the bar with a steel pin in a slot. When the shear pin goes the head is still connected to the tow bar and flag is raised to show the shear pin has gone.
Another type that I have had shear on me had just the shear pin and when it sheared ( on an L1011) the head dropped on the tarmac and before the brake guy could react ( Moi:O ) ! the head dug into the tarmac and lifted the 2 nose wheels clear of the ground :rolleyes:
Now whas that fun getting the nose gears back on the ground with no jacks:cool:

[Irish Steve]

On large aircraft the shear pin will shear with any abnormal load not specifically steering turn limits.

Correct. Even hitting a drainage ditch at the wrong speed and angle can take a shear out, I've seen it done several times, though it never happened to me. Where real care has to be taken is when one bar fits a number of aircraft, if the wrong bar is used, the shear pin can be too strong, and damage can result.

Very much the case with 767/777, where one bar fits everything from the 200 up to the 400 on the 767, and also fits the 777, as well as (If I recall correctly) the DC10/MD11/A330 and a couple of other heavies) Delta provided a couple of bars, identical to look at, one for the 767's and the other for the MD11. Use the MD11 bar on a 767-200, and damage can result, as the shear pin is just too darn strong.


Many towbars have an interchangeable head so the bar can be used on different aircraft.

Tends to be on the smaller aircraft, especially executive jets and small turboprops, where the head is the only thing that's different. I've not seen an interchangeable head on the larger jets. We had interchangeable heads for the Dash 8's, the 300 & 400 needed different heads, but the operator only provided one bar.

Another type that I have had shear on me had just the shear pin and when it sheared ( on an L1011) the head dropped on the tarmac and before the brake guy could react ( Moi ) ! the head dug into the tarmac and lifted the 2 nose wheels clear of the ground

Never come across one like that, and I'm not going to complain about it.

I think the worst bar I ever came across that I had to handle was an abomination that was provided to Dublin by Aeroflot for handling the TU154 aircraft. Due to the nose length, the bar was about 25 Ft long, and about 15" diameter, with a horrendously complicated hydraulic system in the middle that allowed the wheels to be used to connect and angle the bar so that it could be coupled up. Problems started when one of the tyres punctured, and went soft, and no one could do anything to repair it, and as a result, the bar became unstable, which meant it could fall over very easily. That did the hydraulic system no favours, and the end result was a bar that was a nightmare to tow, and even worse to connect to the aircraft and tug. Eventually, the problem was solved when another 154 operator "forgot" to take their bar on one trip, as theirs was much shorter, lighter and easier to use, as long as you could push in reverse, as with some of the tugs, it was too short to get the tug under the nose unless it was reversed in, so that the cab was still clear. Great fun if you knew how to push in reverse, a nightmare if you didn't <g> . The other bar that was a pig was the 747 bar we had, it was so short, trying to do a reverse push before a tow had to be done with great care, as it was very easy to get out of line, and because the bar was so short, getting it back in line wasn't always easy.


Biggest bar ever I never had to deal with, the flight crew always fitted and removed the bar on the AN124, it needed a winch to get it back in the aircraft, and they carried it with them, as it was so large and specialised, due to the double nose wheel. I did hear that it weighed in at about 5 tonnes, and after seeing it in use one day, I can well believe it.