Aircraft and tug part company
Thread Starter
Join Date: Feb 2005
Location: misc.
Posts: 14
Likes: 0
Received 0 Likes
on
0 Posts
Aircraft and tug part company
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!
Join Date: Sep 1999
Location: EIDW
Posts: 271
Likes: 0
Received 0 Likes
on
0 Posts
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..!!!!
Join Date: Mar 1999
Location: Ashbourne Co Meath Ireland
Age: 73
Posts: 470
Likes: 0
Received 0 Likes
on
0 Posts
Isn't that the point of a shear pin?
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
If the bar breaks completely, (which is not supposed to happen
) then the tug should get out of the way PDQ, and let the man on the hot seat in the aircraft sort it out 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.
Forewarned is Forearmed
Join Date: Nov 2003
Location: uk
Age: 60
Posts: 227
Likes: 0
Received 0 Likes
on
0 Posts
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.
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
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
Join Date: Apr 2005
Location: Ireland
Age: 43
Posts: 90
Likes: 0
Received 0 Likes
on
0 Posts
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!!
...and surely it wouldn't be called a 'shear' pin??
Not that I'd ever want to contradict a fellow Meath man!
... although after checking on airliners.net it doesn't seem that many ATR 42's have said red line
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.
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!!
...and surely it wouldn't be called a 'shear' pin??
Not that I'd ever want to contradict a fellow Meath man!
... although after checking on airliners.net it doesn't seem that many ATR 42's have said red line
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.
Last edited by PIGDOG; 6th May 2005 at 15:14.
Thought police antagonist
Join Date: Jul 2003
Location: Where I always have been...firmly in the real world
Posts: 1,381
Received 144 Likes
on
103 Posts
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.
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.
Join Date: Mar 1999
Location: Ashbourne Co Meath Ireland
Age: 73
Posts: 470
Likes: 0
Received 0 Likes
on
0 Posts
I have to admit I didn't think the shear pin was to protect against going beyond the turning limit.
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 .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
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
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.
Join Date: Mar 1999
Location: Ashbourne Co Meath Ireland
Age: 73
Posts: 470
Likes: 0
Received 0 Likes
on
0 Posts
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.
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
Join Date: Jul 2003
Location: uk
Posts: 36
Likes: 0
Received 0 Likes
on
0 Posts
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.
Join Date: May 2002
Location: Who can say?
Posts: 1,700
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jul 2003
Location: uk
Posts: 36
Likes: 0
Received 0 Likes
on
0 Posts
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.
Join Date: May 2002
Location: Who can say?
Posts: 1,700
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jul 2003
Location: uk
Posts: 36
Likes: 0
Received 0 Likes
on
0 Posts
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.
Join Date: May 2002
Location: Who can say?
Posts: 1,700
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.