Polish HEMS Agusta 109 incident
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Polish HEMS Agusta 109 incident
The only Agusta 109 of Polish HEMS was destroyed beyond repair in
probable ground resonance incident today about 14:30 CET in Warsaw (EPBC).
It was an annual check ride for one of the experienced pilots with
check examiner/instructor on board.
After landing and starting the shut down procedures the helicopter
"started to shake violently", according to witness, rotor blades hit and
cut of the tail.
The crew was taken to hospital for examination. One of the pilots
has been already released, the other with suspected spinal injury
still remains hospitalized.
Some images and details (in Polish) under the link below:
?mig?owiec LPR rozbi? si? na lotnisku Bemowo - Stolica - TVN Warszawa - 20.11.2009
probable ground resonance incident today about 14:30 CET in Warsaw (EPBC).
It was an annual check ride for one of the experienced pilots with
check examiner/instructor on board.
After landing and starting the shut down procedures the helicopter
"started to shake violently", according to witness, rotor blades hit and
cut of the tail.
The crew was taken to hospital for examination. One of the pilots
has been already released, the other with suspected spinal injury
still remains hospitalized.
Some images and details (in Polish) under the link below:
?mig?owiec LPR rozbi? si? na lotnisku Bemowo - Stolica - TVN Warszawa - 20.11.2009
Last edited by Senior Pilot; 21st Nov 2009 at 22:28. Reason: Add images
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And people were wandering what LPR will do with it when they get the EC135s... well, the problem solve itself 
The accident is weird though. Haven't heard of Ground Resonance incidents in A109s. BTW I heard it was straight from factory overhaul...
Just like that AW139 that "lost" its tail... hmmm...
The accident is weird though. Haven't heard of Ground Resonance incidents in A109s. BTW I heard it was straight from factory overhaul...
Just like that AW139 that "lost" its tail... hmmm...
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Strange. I'd be surprised if this was a straightforward "ground resonance" accident.
I wonder what the wind was doing (apart from going down trouser legs)..
I wonder what the wind was doing (apart from going down trouser legs)..
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Obviously, the main gearbox support appears to have failed in some way. Was this ground resonance in the true sense, seeing as the aircraft remained upright? I hope we get to know the results of the investigation of this one. 
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An observation from blakmax
OK guys, I have waited a while to see if the discussion progressed, principally because I don't want to be seen as an AW basher. I assure you I have no axe to grind, I simply comment on what I see irrespective of the OEM.
I am suspicious of the appearance of the blades. Do you notice the absence of the structure aft of the spar on several blades? I have seen this in a number of other helicopter accidents, and I have investigated failures on structures with a similar construction such as fixed wing aircraft rudders.
Now I am not familiar with the AW blade structure but usually the skin is bonded to the spar and also bonded at the trailing edge. Usually there is honeycomb bonded inside the skins to provide shear resistance and stiffness. That core is also bonded to the rear of the spar using a foaming adhesive to fill the gaps in the core. Now the analysis of the foaming adhesive bond between the core and the spar is usually trivial and often ignored because the adhesive usually has a strength above 1000psi and the core has a shear strength of 100-200psi, so the core should always fail before the bond.
Despite the trivial nature of the analysis, this bond is of importance to the structure because it transfers the shear loads from the core to the spar. In the rudder failure I mentioned above, the bond beteen the core and the mast of the rudder had not formed correctly at manufacture and an extensive injection repair was performed. Now if you have read any of my postings on other threads you will know that injection repairs have a zero chance of bonding anything. As a consequence, all of the shear loads had to be carried by the bond between the skin and the mast. The high stresses in the skin caused a fatigue crack which led to the rudder departing the aircraft after a high load event. In this case the bond between the skin and mast was backed up by fasteners, otherwise the adhesive would have peeled much earlier, probably even before the fatigue crack initiated.
Now there is an analogous situation with rotor blades because if the bond between the core and the spar is ineffective, the shear loads must be transferred thriugh the bond between the skin and the spar, resulting in high peel stresses. Such a condition may lead to failure of the bond between the blade and the spar, and that would almost certainly lead to a loss of lift, the blade(s) out of balance and consequent impacting the fuselage as shown in this example.
Now before anyone jumps down my throat, I have no evidence of this being a cause of the example shown. I am only suggesting that the investigators should look for this type of failure. One characteristic would be parts of the blade without evidence of fusealge strikes which would be located well away from the fuselage.
Regards
Blakmax
I am suspicious of the appearance of the blades. Do you notice the absence of the structure aft of the spar on several blades? I have seen this in a number of other helicopter accidents, and I have investigated failures on structures with a similar construction such as fixed wing aircraft rudders.
Now I am not familiar with the AW blade structure but usually the skin is bonded to the spar and also bonded at the trailing edge. Usually there is honeycomb bonded inside the skins to provide shear resistance and stiffness. That core is also bonded to the rear of the spar using a foaming adhesive to fill the gaps in the core. Now the analysis of the foaming adhesive bond between the core and the spar is usually trivial and often ignored because the adhesive usually has a strength above 1000psi and the core has a shear strength of 100-200psi, so the core should always fail before the bond.
Despite the trivial nature of the analysis, this bond is of importance to the structure because it transfers the shear loads from the core to the spar. In the rudder failure I mentioned above, the bond beteen the core and the mast of the rudder had not formed correctly at manufacture and an extensive injection repair was performed. Now if you have read any of my postings on other threads you will know that injection repairs have a zero chance of bonding anything. As a consequence, all of the shear loads had to be carried by the bond between the skin and the mast. The high stresses in the skin caused a fatigue crack which led to the rudder departing the aircraft after a high load event. In this case the bond between the skin and mast was backed up by fasteners, otherwise the adhesive would have peeled much earlier, probably even before the fatigue crack initiated.
Now there is an analogous situation with rotor blades because if the bond between the core and the spar is ineffective, the shear loads must be transferred thriugh the bond between the skin and the spar, resulting in high peel stresses. Such a condition may lead to failure of the bond between the blade and the spar, and that would almost certainly lead to a loss of lift, the blade(s) out of balance and consequent impacting the fuselage as shown in this example.
Now before anyone jumps down my throat, I have no evidence of this being a cause of the example shown. I am only suggesting that the investigators should look for this type of failure. One characteristic would be parts of the blade without evidence of fusealge strikes which would be located well away from the fuselage.
Regards
Blakmax
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Ok, I remember. Old dogs, new tricks. I finally worked out how to post these photos.
This one shows a spar to adhesive failure which was NOT the cause of the crash. However, given that the adhesive is 10 times stronger than the core we should be seeing core attached.
Now here is a lower magnification of the matching surface.
Got a nice carpet in my house, yeah?
Note the adhesive has completely separated from adjacent to the spar. This is PROBABLY because of moisture (humidity) contamination during the bond curing process. Moisture at the interface has prevented adhesion.
Now given the evidence I have seen on the AW139 boom failures, it appears that at least some of the AW bonding is performed in less than optimum controlled environments which has probably led to moisture contamination of the bond and subsequent disbonding due to weakened core to adhesive bonds caused by micro-voids. Now I have worked out this photo-linking stuff, here is a photo of a good bond which fails by fracture of the core and tearing of the fillet bonds. Note the absence of micro-voids compared to the next photo.
Here is a photo of an AW139 boom disbond.
Note the number of small bubbles, and the number of shiny surfaces in the fractured core areas. These are also micro-voids. As I said on the AW139 thread, there is published data which shows that similar micro-void levels typically cause a 50% strength loss in overlap joints and I would expect a similar loss of flatwise tensile strength for core bonds, See my postings on the AW139 tail boom failure thread.
Hence, if similar core to spar bond failures as shown in the first two figures are evident in the AW109 event, there may be a common link. I hope that the investigators read this posting before assigning the event to "ground resonance". The possibility of a structural failure should not be excluded from consideration just yet. Again, I make no definitive conclusions. I simply suggest that this failure mode be investigated and if appropriate, eliminated.
Regards
Blakmax
This one shows a spar to adhesive failure which was NOT the cause of the crash. However, given that the adhesive is 10 times stronger than the core we should be seeing core attached.
Now here is a lower magnification of the matching surface.
Got a nice carpet in my house, yeah?
Note the adhesive has completely separated from adjacent to the spar. This is PROBABLY because of moisture (humidity) contamination during the bond curing process. Moisture at the interface has prevented adhesion.
Now given the evidence I have seen on the AW139 boom failures, it appears that at least some of the AW bonding is performed in less than optimum controlled environments which has probably led to moisture contamination of the bond and subsequent disbonding due to weakened core to adhesive bonds caused by micro-voids. Now I have worked out this photo-linking stuff, here is a photo of a good bond which fails by fracture of the core and tearing of the fillet bonds. Note the absence of micro-voids compared to the next photo.
Here is a photo of an AW139 boom disbond.
Note the number of small bubbles, and the number of shiny surfaces in the fractured core areas. These are also micro-voids. As I said on the AW139 thread, there is published data which shows that similar micro-void levels typically cause a 50% strength loss in overlap joints and I would expect a similar loss of flatwise tensile strength for core bonds, See my postings on the AW139 tail boom failure thread.
Hence, if similar core to spar bond failures as shown in the first two figures are evident in the AW109 event, there may be a common link. I hope that the investigators read this posting before assigning the event to "ground resonance". The possibility of a structural failure should not be excluded from consideration just yet. Again, I make no definitive conclusions. I simply suggest that this failure mode be investigated and if appropriate, eliminated.
Regards
Blakmax
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Wire cutter involvement???
Hi 9Aplus, your response came in as I was posting the photos.
If the wire cutter impacted the blades, then there would be damage on the leading edge of the blades and I see no evidence of that on the two blades visible on that web site. Further, I suggest that a sideways swipe on the wire cutters by a blade at even moderate rotational speeds would have at least produced some significant bending of the wire cutter in the direction of rotation. I don't see that either.
I don't think that the wire cutters played any part in this event. It is pure coincidence that the blade came to rest against the wire cutter.
Just to be clear, I am not saying that my theory is the cause. I just request that investigators eliminate it by appropriate engineering assessment of the failure surfaces. I'd be happy to discuss the matter with them (at no charge) if they wish to contact me.
Regards
blakmax
If the wire cutter impacted the blades, then there would be damage on the leading edge of the blades and I see no evidence of that on the two blades visible on that web site. Further, I suggest that a sideways swipe on the wire cutters by a blade at even moderate rotational speeds would have at least produced some significant bending of the wire cutter in the direction of rotation. I don't see that either.
I don't think that the wire cutters played any part in this event. It is pure coincidence that the blade came to rest against the wire cutter.
Just to be clear, I am not saying that my theory is the cause. I just request that investigators eliminate it by appropriate engineering assessment of the failure surfaces. I'd be happy to discuss the matter with them (at no charge) if they wish to contact me.
Regards
blakmax
Last edited by blakmax; 24th Nov 2009 at 11:36. Reason: Late night. Poor typing. Red wine. Take your pick.
Shiny tip on wire cutter leads me to consider in that directions, please
take care that leading spar is something like titanium and we can not see
possible marks from bellow of both remaining blades, tip can jump over
and remain soft structure than ripped off. This naturally can not be considered
like primary problem...
take care that leading spar is something like titanium and we can not see
possible marks from bellow of both remaining blades, tip can jump over
and remain soft structure than ripped off. This naturally can not be considered
like primary problem...
This is not unusual. The blade has pitch so the trailing edge is lower than the leading edge. The blade is not strong in this area as it does not need to be. As you can see by the black stuff showing through the paint the skins are carbon and the core is only foam in this area. Touching trees with composite blades has varying rates of success depending on the blade manufacturer. i.e an AS 350 blade will survive more than an EC 135 and a alloy Huey blade is serviceable until you see the dings coming through on the TOP skin in the outer areas! The centrifugal force far exceeds the lift forces involved, work it out.
blackmax - I think you are barking up the wrong tree.
My guess is a possible gearbox strut failure leading to resonance. It has happened before. Problem now is trying to work out which happened first as the struts will be pretty sad.
How about the rusty old shed in the background with "Eurocopter" above the door? Class!
blackmax - I think you are barking up the wrong tree.
My guess is a possible gearbox strut failure leading to resonance. It has happened before. Problem now is trying to work out which happened first as the struts will be pretty sad.
How about the rusty old shed in the background with "Eurocopter" above the door? Class!
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I can't see a fourth blade, or its cuff.
Or a third either! But what's left of it is in the previous pix. Both damper rod ends you can see, appear to have broken as well. The "sleeves" for want of a better term are carbon as well. If you pick one up when it is off the aircraft you wouldn't believe how light they are.
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Guys, I am not saying that this IS the cause. All I am saying is that not many investigators are aware of this form of failure and it needs to be considered in cases where the core and skins separate from the spar, even if only to eliminate it from the list of potential causes.
blakmax
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I can't believe it...
I had wrote a long post to answer you guys, but when I've finished I decided to delete it because was not appropriate, like yours expertize comments on this accident, based on a couple of pictures....
AUTOCENSORED !!!
I had wrote a long post to answer you guys, but when I've finished I decided to delete it because was not appropriate, like yours expertize comments on this accident, based on a couple of pictures....
AUTOCENSORED !!!