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AS350 - Ground Resonance [video]
22/02/2012
:eek: |
Jeesh! For some reason it reminds me of that spoof video of the Australian politician and the oil tanker that the front fell off.
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Another couple of grams on the red blade should fix that.
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Several very lucky, very foolish numpties running around under the blades while that was going on.
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By the time those 'foolish numpties' got there to see if anybody was hurt it was all over.
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looks like pretty level ground. bad springs or oleos?
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.. bad springs or oleos? As with the New Zealand crash the driver seems to have been thrown about a bit and, as ever, the Eurocopter cabin stands-up to the trauma with amazing resilience - not! |
Sobering to say the least!!! :eek:
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I think before he even realised what was going on, it was all over. What a result. :eek:
The rush of adrenalin must have kept him wide awake for a long time. |
.... bad springs or oleos? |
I think before he even realised what was going on, it was all over. In the video (which begins with the onset of GR underway) there are a good 8 seconds (maybe more) before the resonance reaches catastrophic (ie. unrecoverable) proportions. The moment a driver feels any abnormal vibrations (especially of an escalatory nature) instant/immediate action must be taken. That's what you are trained to do! TRC put it well. By contrast .. in the clip below (which has done the rounds on PPRuNe I know) the onset occurs at 40 seconds. By 42 seconds the driver has the bird off the ground (probably delayed by the fact that he may have been considering the open door). |
I ask because I dont know , but what would be the next step when landed and everything gone quiet, would the airframe and such, need to be thoroughly examined with measurments and joints checked,?
Peter R-B |
....With Door Flying... |
Who's the Dumbass that did not latch the door upon exiting? are you sure that that the helicopter was at full power to lift off? It's the lack of either lift-off, or throttle chop and rotor brake (but too high an Nr for that probably) that prompted me to suggest the pilot just sat there and watched while his world - and job I expect - disintegrated around him. I have witnessed an Al II get into resonance and the pilot lifted off immediately as did the Gaz in the video. Maybe the 350 pilot hadn't been trained to recognise what was happening, or what to do about it. I would like to read the investigation report before blaming the pilot |
It's the lack of either lift-off, or throttle chop and rotor brake. .. throttle chop and rotor brake (but too high an Nr for that probably). What would you expect to find in the report that would exonerate the pilot? Lack of training I suppose. |
I'm sorry, I didn't reply to Eddie's comments:
.. are you sure that that the helicopter was at full power to lift off? When the resonance has put your blades out of symetrie it can be interested to lift off. Pitch lift regardless off rotor rpm? Has the pilot tried this or why is the helicopter turning before separating? |
Does it sound like the OP video was increasing NR?
The answer is simple: when it (ground resonance) starts, stop what you're doing. If you're running up, NR from ground to flight idle, go back to flight idle (I'd probably shut down...). If you've just landed and/or are still at flight NR, takeoff. It will fly with the doors open. Getting the helo off the ground with people under the disk is safer than letting the aircraft dynamically disassemble itself overhead/around them. I've seen guys try to "land through it", especially tempting in a multi with overhead engine condition levers to complicate an eventual hovering auto. That didn't work. I've seen guys try to run up, hoping "it" would stop with a change in NR. Ditto. I know a guy who tried to complete a crosswind running landing with a student, only to end up in a pile of parts. I'm probably preaching to the choir, but my recollection is that it's clearly a bad thing occurring when "dynamic divergent excitation" commences and one has to react quickly before it's fully developed and destroys the helo- something like 3 turns of he NR? I've been told that the AS350 could do this in flight... |
Its pitty that we are missing the first bit, it would be interesting to see the whole sequence.
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Personally, I'd apply a slight amount of rotor brake even if the rotor RPM was well outside the limits for the brake.
Anything to get the blades / fuselage to behave. |
As I understand it ground resonance is a temporary imbalance in the rotor system caused by external forces. But let's say something actually structural did depart the rotable components, a tip fairing or a blade damper or something, then I assume no amount of lifting off recovery is going to save you. Right?
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...two ways to stop ground resonance...get off ground...chop throttle...as ground resonance is a function of power... dissipates almost instantly when throttle is cut...no need for weird rotor brake action....have worked on squirrels for over 20 years..lamas as well
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The theory as I was taught is that articulated systems will diverge at some frequency as the blades 'hunt'. It's usually not noticed because it's transient and low energy anywhere except at operational NR. The airframe is tuned away from most energetic and maximum exposure time, flight NR. That's why the AStar has heel springs, to change the airframe's natural frequency to keep it from being a harmonic to the NR. There are also dampers in the system (forward crosstubes on AS350) to absorb energy and keep it from magnifying the issue. Everything has to be within parameters, from the disk to the ground and the the heel springs should have the actual skid heels clear, a reason to takeoff again and perhaps move and try another SMOOTHER landing so you don't bump a blade out of phase and clear the heel allowing the heel springs to do their thing.
If you stop the process before anything is damaged, a change in the NR arrests the divergence because the excitation is not sympathetic/harmonic. That's a trap to the unwary- you can't accelerate the rotor fast enough from ground to flight idle to keep the airframe intact. |
As I understand it ground resonance is a temporary imbalance in the rotor system caused by external forces. Ground resonance is a hazardous condition that can occur any time the rotor of a helicopter or gyroplane is turning while the aircraft is on the ground. Similar in concept to the behaviour of a washing machine when the clothes are concentrated at one point during the spin cycle, ground resonance can occur with a rotorcraft when the spacing between blades become irregular or the damping system, including drag hinge dampers, landing gear oleo struts, or wheel tire pressure, is operating out of limits. Ground resonance occurs at three rotor rpm bands, the first at about one-third of normal rotor speed, the second at a range including normal rotor speed, and the last in a range above normal rotor speed. Ground resonance is precipitated by a shock to the aircraft arising from excessive motion of a rotor blade in its plane of rotation thereby moving the rotor center of gravity from the axis of rotation. Inadequate damping allows the rotor center of gravity to spiral away from the rotor axis of rotation, causing the rotor to generate unbalanced rotating moments beyond the compensating ability of the damping system. In addition to damping system malfunctions mentioned above, such blade movement can occur as a result of taxying over rough ground with the rotor turning at a speed within a susceptible range, or takeoff/landing in which a shock occurs to one side of the landing gear. Under extreme conditions, the initial shock can cause violent oscillations that quickly build and result in catastrophic damage of the entire airframe. In some cases, complete destruction occurs, e.g. body panels, fuel tanks, and engines are all ripped about normal rotor speed. Recovery is possible in some cases. If sufficient rotor RPM exists, immediate takeoff can restore rotor balance. If rotor RPM is low, complete shutdown might be sufficient. Two bladed semi-rigid see-saw (underslung) type rotor systems are not susceptible to ground resonance because the blades do not lead and lag. But let's say something actually structural did depart the rotable components, a tip fairing or a blade damper or something, then I assume no amount of lifting off recovery is going to save you. Right? |
Wasn't there a French Squirrel in the Paris-Dakar rally that managed to get down after one MR blade departed, or something? Sounds like a tall tale, but I'm sure I heard it somewhere.
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Wasn't there a French Squirrel in the Paris-Dakar rally that managed to get down after one MR blade departed, or something? Sounds like a tall tale, but I'm sure I heard it somewhere. |
There was an EC 130 that had an accident in NY due to partial blade separation. Pilot was Tammy Fein. Landed safely on floats in the Hudson River.
On July 7, 2007, about 1651 EDT, a Eurocopter EC-130-B4 helicopter, N453AE, operated by Liberty Helicopters, Inc., experienced an in-flight separation of a section of one of the main rotor blades during flight and sustained substantial damage during an emergency descent and subsequent autorotation into the Hudson River in New York. The commercial pilot and seven passengers were uninjured. No flight plan was filed with the Federal Aviation Administration (FAA) for the 14 Code of Federal Regulations Part 135 and 136 sightseeing flight, nor was one required. Visual meteorological conditions prevailed at the time of the accident. |
There was a similar incident to an AS350 in Florida, 2006, discussed in Ground Resonance fails AS350 tail boom.
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Tear along the dotted line comes to mind,
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Speaking from personal experience. I once landed AS350 carefully on the flat strip at the rear of the starboard skids, then applied a little too much downward pressure on the collective and ground resonance kicked in immediately - it happens too easily. I lifted off at once and repeated the landing, this time with micro-millimeters of pressure easing on the collective for an uneventful setdown. Never to be repeated again.
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devil49. I think you are wrong . I don't believe you have ever seen anyone try to "land through " this and I don't believe it has ever occurred in flight .
I have been around 350,s and flown them for circa 30 yrs and have never come across this happening in flight . I believe it can ONLY occur when in contact with the ground ...sorry !!! |
What would you expect to find in the report that would exonerate the pilot? Lack of training I suppose. |
The sound track on the video gave me the impression that the Nr was increasing but not up to flying speed as the resonance began. It continued after it began decreasing again after the throttle was retarded. I don't therefore think he had a chance to lift off.
(One thing that I'm absolutely sure about is that ground resonance can't occur in the air, btw. The answer's in the name). |
As one suspected (#7) - induced by the landing.
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What caught my eye is the way the cabin roof separated....then the tail boom. The 350/355 roof has always been a weak link....and if the tail boom will fail as it did.....well....not very impressed! They are plastic helicopters.
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They are plastic helicopters To get resonance in a 350 something is busted or worn out. They do not just decide to do it just for something to do............. The more we learn, the more we forget.............. |
Aerospatiale (now Eurocopter) claims (or did) that the AS350 cannot experience ground resonance. It can, however, experience dynamic divergence. A fart by any other name....
I've had it start many times, and lifting to a hover stops it immediately, assuming the throttle is full open and Nr is sufficient. I once had one go into dynamic divergence after landing. Everything was fine until I reduced the throttle to idle, and then it went bonkers. The bystanders (pilots and mechanics) who didn't run and hide said the skids were coming almost a foot off the ground, and the aircraft turned about 45 degrees on the pad. When it started, I had the throttle at idle, and I wasn't about to try to bring the rpm back up, so I put the throttle in cutoff and pulled the rotor brake full on. It only took a few seconds to stop the movement, and there was no further damage. The inspection revealed only a slight amount of wear in the spherical bearings, nothing else. The springs on the heels of the AS350 can be a real problem, especially on wavy concrete. I don't know if that was the problem, but I've had dynamic divergence start many times on concrete pads in the marsh. I always waited a few seconds after landing before retarding the throttle, and that almost always was sufficient to prevent bouncing, but not that time. |
It would appear that those springy leaf things need to be very precisely fitted for them to work properly.
Phil |
Gomer, thank you for this input verifying the correct action when not using the lift-off option.
I always waited a few seconds after landing before retarding the throttle, .. It would appear that those springy leaf things need to be very precisely fitted for them to work properly. ps: I was going to drop you a PM after Christmas - I was exposed to non-stop Abba and Boney-M during the festive season the former bringing back memories of John Broome. Boy he was keen on that group. Did you also have to suffer the tape deck in the Towers LongRanger playing 'Dancing Queen' hour after hour, lol! Them' were the days! |
those springy leaf things Watched someone fit "one" new item opposite a worn one due to supply issues from EC on a 355. Bad move! The other issue that people miss is a worn/loose DU sleeve on the end of the star although there shouldn't be many of those left by now as the new star has "carbide" bushes which are an "improvement" or "not" depending on your point of view. |
I know a guy that had a very similar incident in a Twin Squirrel, on landing the machine went berserk and started thrashing around. Fortunately it was on a large gravel parking lot and it slid around the show while he was chasing the throttles to close them off.
By the time he got them closed the machine had moved about 15 meters on the pad in eccentric circles. It turned out the starflex had delaminated and threw the blades out of sync once he landed. A LOT of bits had to be replaced on that ship. |
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