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AS350 - Ground Resonance [video]

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AS350 - Ground Resonance [video]

Old 23rd Feb 2012, 08:26
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AS350 - Ground Resonance [video]

22/02/2012




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Old 23rd Feb 2012, 09:17
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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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Old 23rd Feb 2012, 09:36
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Another couple of grams on the red blade should fix that.
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Old 23rd Feb 2012, 10:22
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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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Old 23rd Feb 2012, 10:34
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looks like pretty level ground. bad springs or oleos?
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Old 23rd Feb 2012, 10:41
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.. bad springs or oleos?
Or a firm landing and which begs the question (if he was at flight idle) why the heck he didn't pull into the hover!

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!
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Old 23rd Feb 2012, 11:13
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Sobering to say the least!!!
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Old 23rd Feb 2012, 11:40
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I think before he even realised what was going on, it was all over. What a result.

The rush of adrenalin must have kept him wide awake for a long time.
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Old 23rd Feb 2012, 11:43
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.... bad springs or oleos?
And a pilot sitting there like a stunned mullet.
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Old 23rd Feb 2012, 11:52
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I think before he even realised what was going on, it was all over.
Yes, if he was on drugs perhaps!

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).

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Old 23rd Feb 2012, 12:50
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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
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Old 23rd Feb 2012, 13:08
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....With Door Flying...
Who's the Dumbass that did not latch the door upon exiting?
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Old 23rd Feb 2012, 13:54
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Who's the Dumbass that did not latch the door upon exiting?
It was an actor, would we expect otherwise?

are you sure that that the helicopter was at full power to lift off?
As Savoia said, the video starts after the resonance started. There seems to be plenty of Nr so it seems to be at or near t/o RPM.

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
What would you expect to find in the report that would exonerate the pilot? Lack of training I suppose.
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Old 23rd Feb 2012, 14:43
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It's the lack of either lift-off, or throttle chop and rotor brake.
Precisely. He did nothing and which inaction appears to be corroborated by the video footage.

.. throttle chop and rotor brake (but too high an Nr for that probably).
Correct. The throttle chop may have brought things back into balance but, to be effective, would need to be followed by rapid deceleration of the MRB's and which would have meant waiting until Nr came into range before one could apply the brake. Hence the take-off option.

What would you expect to find in the report that would exonerate the pilot? Lack of training I suppose.
Highly probable that this statement will also turn out to be correct.
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Old 23rd Feb 2012, 14:59
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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?
No I am not sure at all but .. the Nr does (as TRC said) appear to be nearer to flight RPM.

When the resonance has put your blades out of symetrie it can be interested to lift off.
Yes it can but, if I remember correctly, the moment the vehicle is taken out of contact with the ground and suspended by the disk, the 'new' loading on the blades combined with the normal dynamic of centrifugal force eliminates the dissymmetry.

Pitch lift regardless off rotor rpm?
If he opted for chopping the throttle then, if it were me personally, I would probably introduce pitch (as a form of braking) until I could use the brake. But .. one needs to know what one is doing. Collective braking can introduce its own risks and one needs to be familiar with the sensitivities peculiar to each type in this area.

Has the pilot tried this or why is the helicopter turning before separating?
To me, there is very little videographic evidence to suggest that pitch was introduced. His 'turning' may well be due to the fact that he is starting to be bouned around in his seat. Conjecture I admit. The yaw does however saying something about the Nr.

Last edited by Savoia; 23rd Feb 2012 at 18:05.
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Old 23rd Feb 2012, 15:08
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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...
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Old 23rd Feb 2012, 15:32
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Its pitty that we are missing the first bit, it would be interesting to see the whole sequence.
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Old 23rd Feb 2012, 15:34
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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.
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Old 23rd Feb 2012, 15:52
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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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Old 23rd Feb 2012, 16:19
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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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