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-   -   Police helicopter crashes onto Glasgow pub (https://www.pprune.org/rotorheads/528850-police-helicopter-crashes-onto-glasgow-pub.html)

Fortyodd2 3rd Dec 2013 10:37

Rotorspeed,
From my earlier post -
"Generally to others not in the know and especially the desparate media hounds - by the time the EC135 runs out of fuel, over the previous 30 minutes there would be 3 amber warning lights, 2 red warning lights with audio, 2 loudly complaining police air observers and number 2 engine will quit about 2 minutes before number 1 - that's the way the system is designed"

Police ops in Scotland, I believe, are still conducted under CAP 612 as they are not part of NPAS. As such they will have a minimum fuel load above that specified in the RFM. Prior to joining NPAS, our PAOM Pt 2 required a minimum landing fuel of 60kgs by day and 90kgs by night as against the aircraft's limit of 50kgs. For us, at night, it meant we could land with only the first 2 amber captions lit - if the 3rd was on or any red then I would be in the poo. The police crews I fly with know this and none of them would just sit there and say nothing whilst the panel starts to light up.

Feathers McGraw 3rd Dec 2013 10:46

Quick question chaps!

Does the EC135 have a fuel system 'g' crash switch, and if so what 'g' level is required to trigger it?

Fly_For_Fun 3rd Dec 2013 10:49

Hoping that this is just a one off seems like a pretty big gamble to me. It reminds me of the instance of the detached rotor blade on the Lynx in Germany, which Westlands insisted was not a fleet wide issue, which was indeed a massive issue.

cattletruck 3rd Dec 2013 10:58

Yes Tandem, that's been my gut feeling too, and you said it well. Thank you.

With the machine being at low level, if the MGB seized suddenly then it would have most likely departed the aircraft and the scene. If it seized slowly then the rotors would most likely still be turning during the short distance to the roof and there would have been evident rotor damage.

If the machine came down with an extremely high G landing you would also expect the rotors to slap down and end up slicing the throttle quadrant off the roof of the cabin and the tail boom off the back, damaging the rotors in the process.

But we cannot see any evidence of that.

So what if the roof is three layers thick, it's only designed to support itself, and like many old roofs it would be flush with many weak spots. 1 ton plus weight on each thin skid applied almost instantly to the roof and you can imagine it working like a knife to carve into these weak areas.

I feel the machine was skillfully controlled towards the landing spot on the roof, unfortunately the Fenestron shroud could have caught the aircon box and caused the nose to tilt down and roll the machine to the left with the left skid crunching into the front part of the roof first. The impacted roof could then have been prized from its supports from the front of the building, and with the masonry wall at the back being the only support left, the front of the roof eventually gave way and the machine fell into it.

As I said before, only those currently working at the scene of the accident are closer to the truth.

zorab64 3rd Dec 2013 11:20

SASless #416 - as mentioned, the broken bits are just inboard of where the main spar goes into the lifting part of the blade, the inner end (about 70-80 cm) is the control cuff that surrounds the spar. It's not surprising that in a low impact (unpowered) break of the blade that this is where it would snap, since the inner end widens to the attachment points. The nature of the material is that it looks shattered even when it's "just" snapped.

I'm afraid I don't have the ability/skill to post a picture of the workings (wrong word, as there aren't any, really) part of the blade - but a cutaway might help to explain how this fantastically simple, completely self-contained, rotor blade design does all the jobs required of it.

Fortyodd - Completely support your summary of Police Ops. Another pilot is not necessary when flying VFR, within the weather limits and with the extra training mandated. All some of them would do is reduce endurance by 30+ mins!

Ornis - there are nearly 1000 EC135s flying all over the world as we speak, nearly 1200 have been built. This is a mature, but still maturing, machine with all early build/design issues (FADEC & ARIS problems etc) ironed out by the manufacturers.
Obviously, all aircraft types have the potential to deliver unwelcome surprises, (just as in the motor industry) as the head crack issue did last year. This will have been the concern of all current operators in this case, although the continuity of the rotor system would say (to those who know the aircraft) that it's not, something effectively confirmed by the team from EC working with the AAIB, from their update yesterday. Whilst the head issue is under control, it's still not 100% understood but there's a big team at EC working on it, since no manufacturer wants a good reputation tarnished by any hint of un-reliability.

The 135 has flown millions of trouble-free hours, in all conditions, with EC continuing to monitor the stats in every detail to see how things can be improved, be it parts, maintenance frequency or upgrades. I'm not in their pocket or employ, but have been regularly impressed with the continuous improvements seen over the last 10 years+ and it remains one of the best tools for the job, without the need for a blanket grounding, unless they discover a very good reason to, or a definitive cause - which, believe you me, both EC & AAIB will be looking for with the utmost urgency.


The other discussions about NVG and lasers have been fully debated before on earlier threads and, I would suggest, have no real relevance to this thread.

Ornis 3rd Dec 2013 11:23

It has been reported here the AAIB stated the helicopter descended vertically and the rotor blades were not turning. I struggle to see in what sense that is flight let alone controlled flight, but then I'm only a recreational pilot and I might have got the wrong end of the stick.

Edit: zorab64. Thanks.

Grenville Fortescue 3rd Dec 2013 11:29


Originally Posted by cattletruck (Post 8185982)

If the machine came down with an extremely high G landing you would also expect the rotors to slap down and end up slicing the throttle quadrant off the roof of the cabin and the tail boom off the back, damaging the rotors in the process.

Not if there was excessive blade coning prior to impact and not if there was minimal rrpm.

skadi 3rd Dec 2013 11:34

yellowbird135

UMS will be mandatory (without the 'Health' part)
Yes, your're right! It was too early in the morning ;)

Oldlae 3rd Dec 2013 11:47

Re grounding of the fleet, the AAIB must be careful to identify the exact fault which may mean stripping down the suspect component. The modification status may have a bearing on which other aircraft are at risk.

Bertie Thruster 3rd Dec 2013 12:32

I can't get my head round all this. I can only assume that the absence of a precautionary fleet check would indicate the evidence immediately available to the AAIB was of an empty but intact fuel tank?

rotorspeed 3rd Dec 2013 12:38

Fortyodd2

Yes I did read your post on all the warnings before running out out of fuel - very useful. And I don't believe for one minute this would have all occurred and the engines then stopped. As I said, given the heavy impact, I am very impressed with the integrity of the 135's fuel tank arrangement - and structure generally. However whatever caused this accident, it will be something extremely unlikely to happen. Maybe there was a failure of part of the low fuel warning system, or a fuel pump? I recall the Dyfed 109 crashing several years ago following a failed fuel pump, though that was from mis-management of the pump failure and cross-feed use when low on fuel. Fortunately that was also with forward speed and in daylight, and onto grass as far as I recall. However had it happened at night in a high hover over a built up area, I suspect it might not have looked too dissimilar to this.

Catastrophic main transmission failure is obviously possible, though one might have thought it would either freewheel and be possible to autorotate, or the abrupt deceleration would tear the MGB out, as with the Ian Shoebridge AS355 accident near Thruxton many years ago. TR drive failure would presumably have been more likely to result in the integrity of the aircraft being maintained until impact.

Finally I'm sure the roofing felt black roof would have looked very like an empty car park at night, so a 'least bad' place perhaps to attempt an auto to. Certainly sounds as though Dave Traill was a very skilled pilot and did his best to mitigate a desperate situation.

Fortyodd2 3rd Dec 2013 13:07

Rotorspeed,
There would have to be more than one failure of the fuel system. The "Amber" warning part has 4 sensors - any one of which will generate a caution, plus 2 transfer pump captions. The "Red + Audio" warnings are separate and independent systems.
If the fuel tanks display that there is plenty of fuel but the "Red + Audio" appears then I would believe it and land fairly sharply - you have no more than 8 mins at this point. Finally, the different "size" of the supply tanks and the 4kg difference between them means one engine shutting down before the other is designed as an un-missable clue that you really should be on the ground.

Cabby -
"Fuel limits are often flown to minima's on police ops. Its usually the police who want to stay on task longer with the pilot wanting to go back to base ten minutes earlier and fuel light activations do occur".
You are clearly not one of my pilots. If your "passengers" want to stay on scene with the fuel running low then you really do need to sort out your captaincy and get your crews into a simulator - and soon.

"The area was well lit, I don't think NVG's would have made any difference over a city".
Then clearly you have never used them.

"The police crew have their own airwave radios onboard which would have allowed them to push their personal emergency button which would have taken priority on the police comms".
Correct, but the emergency button has to be pressed - the personal radios are stowed and switched off. The aircraft Airwave head also has an emergency button but it requires to be pressed and held for 2 seconds and then confirmed before it activates. With engines out, the generators are offline so the mission bus drops out as both generators are required to run it - hence no camera, no searchlight, no mapping, no tac radios, no recorder.
In a fast developing emergency, the police crew, legally passengers, would be better off locking their harness and adopting the brace position than trying to find a button to press.

Lemain 3rd Dec 2013 13:32


Finally, the different "size" of the supply tanks and the 4kg difference between them means one engine shutting down before the other is designed as an un-missable clue that you really should be on the ground.
Isn't 4kg around 1.5 mins? Always assuming that the consumption of both engines in-flight were perfectly matched leaving the larger tank with 4kg. Sounds a bit odd to me. Surely they would have incorporated a distinct inviolate reserve tankage if that was the purpose?

SASless 3rd Dec 2013 13:36

Bertie.....be careful about making "assumptions" not based upon definitive information. Nothing has been reported that would support a "Fuel Exhaustion" event.

The 135 has a well designed Bladder system that is crashworthy in its design. There would have been a reduced amount of fuel left in the Tanks due to the length of time the aircraft had been airborne.

There are multiple warning systems, as being described by several posters who are quite familiar with the Type and Tasks the Crew would undertake....and to put forth "Fuel Exhaustion" as a cause at this point is just premature. (IMHO anyway.)

Fortyodd2 3rd Dec 2013 14:04

Lemain,

By the time you get to the Red Captions, you should no longer be in the air - The RFM minimum landing allowance for this aircraft is 50Kgs. The Red + Audio warning appears at 26kgs in the Port Supply Tank and 22kgs in the Starboard. If those lights come on you are already in the poo.
I'll not argue the toss over figures but 200kgs per hour/3kgs per min is the twin engine fuel planning rate. 4 kgs on one engine may buy you 2 mins but, as I have already stated, you are already in the poo.

CharlieOneSix 3rd Dec 2013 14:11


Originally Posted by Ornis (Post 8186025)
It has been reported here the AAIB stated the helicopter descended vertically and the rotor blades were not turning.

The AAIB briefing that I saw on TV only said that the helicopter descended vertically. The possible scenario that the rotor blades were not turning was not mentioned by the AAIB, only by contributors here.

cenzo 3rd Dec 2013 14:18

I simply cannot reconcile in my own mind a 3 tonne helo falling out of the sky from a considerable height with no rotor rpm and causing, comparatively speaking, so little damage to both the airframe and the building. But, I'm not a structural engineer nor an aircraft designer.

SawMan 3rd Dec 2013 14:25

Much has been said since my first post regards the building. And since then I've seen pics of the machine's removal which widen my understanding of the building. "robdean", I am not an engineer, just a builder, but I have done quite a bit of self-study on calculated strengths of building materials and in failure analysis when they let go. I have much experience in construction and repairing buildings, and I have personally seen many damaged structures but none quite like this. I know what math to do and how to do it regarding strength and failure. It's complex, tedious, and coincides with my experience so I'd rather not go there myself since the investigators are going to do it anyway. All I post here will be based on my experience alone which I can prove through valid calculations if necessary. This is going to be a long post, my apologies for that but it's necessary. If you can't be troubled to read it all, please read the italicized parts.

The pics I saw showed the newest roof used lightweight bar joist construction. The strength of this kind of roof is well known to me, and it alone would not have held the static weight of this machine. "Technet101" notes this. I'll get back to this part later, it's important.

The pics showed another older roof structure underneath. "G-CPTN" noted this, and that makes it plausible that there would have been a delay between impact and the machine dropping to the floor. We know this happened. And with this being a wooden structure, my earlier post concerning staged slow failure seems to have been correct after all even though there was much more structure here than I anticipated. What would happen with and excessive load on this roof is the beams would deflect, the wood fibers stretch until they lost molecular adhesion with surrounding fibers, then they would slip apart. This would occur from the bottom of the beams upward, the upper sections initially being under compression then going to tensile extension as drop induced stretching. Everyone has seen this in the frayed ends of broken wood. Under slowly applied weight this would occur slowly, but under impact would occur almost instantaneously. More on this later too.

The floor beams under this was quite a surprise to me. They would have been the strongest framing members involved and they are what did the most work here. It is very likely that there was direct contact between this and the roof members above, causing this part of the structure to act and fail in unison with the old roof above. This structure together could possibly held this machines weight- I feel that it's likely. Yet the machine still broke through here and that seems to indicate some substantial downward velocity at impact with this part of the structure, causing partial failure of the wood, thus weakening it allowing further failure to occur. Said more simply, the wood cracked but did not initially break, and this explains the delay before the fall to the floor. This slow rate of failure is what saved the most lives below. (It's also the basis of why wooden wing spars work so well in light FW aircraft, within their ultimate limits they will 'take more punishment' without distress than metal does before ultimate failure occurs). Back to the end of my last paragraph, from this failure rate occurring as slowly as it did, one can reasonably ascertain the force needed to cause this failure, and one can reasonably calculate the rate at which this force was applied.

"Chronus" has posted some figures regarding the force of impact - I'll take them as being accurate. In this we can see that if the entire structure could have held the static weight of the machine as I believe it would have, then there had to be more energy involved- the added force from mass in motion. Now we get to the tricky part- the bar joists I first mentioned. In the pics, I saw where these were perpendicular to the machine and also appeared to be severed. On impact they would have twisted under load, sagged downward as strength was lost with their structure deforming, then snapped through necking when the ultimate tensile strength of the components they are made of was exceeded. This would have slowed the downward momentum, acting as slings under the machine, until they parted. And with this support being applied only at intervals and not across the whole area under the machine, it is probable that one of them was under the tailboom at it's point of separation, causing it to part from the cabin section at that point. With the downward momentum slowed as this all happened, the bar joists would have abated some of the force which was thereafter applied to the structure underneath, allowing it to hold where it might have instantly failed otherwise. Had these joists been parallel to the machine, it would have slipped right through immediately. The roof members under the bar joists were oriented similarly, also reducing the chance of this 'slip-through' scenario.

Knowing the strength of wooden floor and roof joists which I saw some of in the pics, knowing how the bar joists affected the impact, and knowing the approximate weight of the machine, it seems clear to me that there was a free-fall drop from considerable height involved here, perhaps not from the 300ft "Chronus" calculated but still quite a lot regardless. It would probably have held intact even with a free-fall of say 10-20 feet. This seems to agree with there being no significant MR blade damage from rotation and witnesses to the event saying there was no rotation. This would all imply that nothing was done to stop the fall, which would certainly have happened had there been enough time and any ability left to do that.

The practice of doing a roof-over is purely economic, but in this case it also gave a cushioning effect and prevented a larger catastrophe. I'll never call them a cheap way out again. This was critical in saving lives underneath, but it also likely prevented the crew from leaving the cabin were they otherwise able to do that. Much had been said of the designed resistance of g-force on impact of these machines, but we need to understand that this is calculated based on a flat, relatively unyielding surface like the ground and this was an entirely different scenario where the forces were applied to the machine locally and not generally. With these impact forces being concentrated, unless a seat was directly over a joist the designed impact resistance could not have functioned as well as it was designed to, and there being more space than joist here, this arrangement would be very unlikely. No fault to the safety design engineers for you can't foresee every possibility and this was a very unlikely one.

Sorry to have bored you with all this, but it does explain and seemingly clear up several points of contention being discussed here, which I hope will bring about a better understanding of this tragedy. In my analysis of the bar joists and flat roof construction in general, I hope the pilots here now understand their danger to you should you ever be so unlucky as to need somewhere to set down right away. As buildings go, the Clutha was a great anomaly, not at all usual by any stretch of the imagination, and this along with the orientation of the machine on impact certainly saved many lives inside, even as it may have also doomed the crew. Having nothing more I can add I'll drop back to read-only mode in this thread now. Thanks for allowing me in your presence.

falcon900 3rd Dec 2013 15:14

Could it be the roof structure of the pub which is contributing to the confusion here? Multi layered and wooden, it might have actually absorbed quite a lot of energy in a reasonably progressive (relatively speaking) way before actually failing. Just a thought.

Seloco 3rd Dec 2013 15:36

Like many others I've been struggling to make sense of what happened in this accident, with its range of apparently conflicting evidence. As it happens I know a bit more about buildings than I do about helicopters, and I've been wondering just how the rather unusual roof structure of the pub might react to a 3 tonne object falling almost vertically on it. Given that the roof appears in the crash area to be a combination of timber joists laid between two very large rolled steel joists (RSJ), this might be expected, at least initially, to be somewhat "elastic" to something falling on it.

Could it possibly be that the 135 initially landed predominantly above one of the RSJs, damaging its underside but bouncing back into the air briefly before landing back slightly further ahead over the timber section, which it perforated this time?

This sequence might explain those few seconds of silence described by witnesses in the pub as occurring between the initial bang and the main collapse of the ceiling/roof.


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