Deadly Ground Resonance

[TheShadow]

Interesting Reading
http://www.iasa-intl.com/folders/pdf/Magazine_resonance9.pdf

[[email protected]]

I had ground resonance in a Wessex with a perfectly set up rotor head during rotor start - the tyre pressures hadn't been checked after changing all 3 wheels and the two main wheels were at significantly different pressures.

After releasing the rotor brake and starting to advance the speed select lever, the aircraft started to rock and then very quickly build up into resonance. Fortunately, retarding the SSL and applying the rotor brake firmly got it under control but the groundcrew saw the main wheels alternately over a foot from the ground - had I tried to continue with the rotor start we would have certainly rolled over.

So, despite the good info in the linked report - it is possible for something as simple as tyre pressures to cause DR in an otherwise serviceable aircraft.

[CYHeli]

I like some of their solutions, although not every helicopter has a rotor brake. I sometimes fly an MD500, without a rotor brake. I mentally go through the 'what if's', but without a rotor brake on start-up there are few options.:{ At least on touch down I can lift it up, hopefully smoothly and quickly... :ouch: not snatching it up.

I think that I've actually flown more helicopters fitted with two blades and a rotor brake, than the fully articulated heads which did not have a brake.:eek:

So far, I haven't had the displeasure...

[Big Bucks Bernie]

Ingesting foreign objects through the rotor system whilst the helicopter is still on the ground, can also lead to very severe ground resonance (see the following major incident which occurred in southern Germany at the end of last year shortly after an EMS helicopter had come in to land at a call-out scene).

According to the crew, as the helicopter was setting down at the emergency scene, two building panels of the building next to the chosen landing spot came loose. The two loose panels, each about 2,5 x 1 m in size, were pulled through the main rotor system just after the aircraft already had touched down on the ground. As a result, the main rotor system and transmission then got severed off the aircraft, eventually ending up about 3 to 4 m to the left of where the aircraft came to rest.

Judging by the looks of the bent skids in the photos, it would appear as if the aircraft must have gotten airborne again at the point when the main rotor and transmission separated from the airframe. It would seem fair to say that the ground ressonance at that point must have been extremely excessive .

Well anyway, I thought it was pretty disturbing to see that two loose panels on the side of a building can cause the main rotor system and the entire gearbox of a helicopter to get separated from the aircraft with such violent force.

The good news is, nobody was seriously injured in the accident.

Link to photos:

Rotor & transmission separation #1 (http://www.feuerwehr-regensburg.de/einsaetze/bilder/2005/hubsch_unf_dez05_1.JPG)
Rotor & transmission separation #2 (http://www.feuerwehr-regensburg.de/einsaetze/bilder/2005/hubsch_unf_dez05_4.JPG)
Rotor & transmission separation #3 (http://www.feuerwehr-regensburg.de/einsaetze/bilder/2005/hubsch_unf_dez05_3.JPG)

Source and Copyright: Berufsfeuerwehr Regensburg (http://www.feuerwehr-regensburg.de/einsaetze/2005.html)

Rotor & transmission separation #4 (http://www.rotor-rescue.info/library/luftrettung/Bilder/DHIMU.jpg)
Rotor & transmission separation #5 (http://www.rotor-rescue.info/library/luftrettung/Bilder/DHIMU1.jpg)

Source and Copyright: luftrettung-info.de.vu (http://www.luftrettung-info.de.vu/aktuell2005.htm)

[CYHeli]

In photo #3 you can dents in the top of the wall of the building. I wonder if that is from the helicopter during break up (could be a blade) or if it was caused by the loose stuff that went into the blades? If it was from the helicopter, it shows how far bits will fly.:eek:
Another photo from the same site shows damage to a car in the car park. And to a tree next to it.

http://www.feuerwehr-regensburg.de/einsaetze/bilder/2005/hubsch_unf_dez05_2.JPG


They say there were no serious injuries, you'ld have to be wearing a helmet to not get hurt in that! :ok:

I wonder if they had prior permission to land in the factory area? Was this a road accident or a factory accident they were attending? My German's not that good for reading the article...

[SASless]

After a couple of panels went through the blades....I would suggest it was not "ground resonance" but more like a blade or two disentegrating and causing a massive out of balance condition that lead to the Main Gear box and Rotor Head leaving the aircraft in a heck of a hurry. It would have been far more than a mere angular out of balance situation with divergent oscillations. More like a loud bang or two and then bits went bye-bye!

BK heads and blades are tough but even they have their limits.

[diginagain]

Perhaps more suitable on AH&N, but wasn't the Skeeter prone to ground resonance too? Three blades, and three wheels, as well as poor lead-lag damping, from what I can recall of the discussion on the pilots course?

[Hilico]

Also happened to an H300 that had one shock-absorber pumped to a different value than the others.

[Gaseous]

I have come across this in older Enstroms.

They tend to live outside and water gets past the dust seal at the top of the oleo (shock absorber).

The corrosion that goes on between the dust seal and the pressure seal causes them to gradually get stiffer. Even with proper inflation this stiffness can cause severe ground resonance. If there is any tendancy to rock about when on the ground the oleos should be depressurised and the damping action and free movement checked.
Lead/lag damper problems can also cause resonance but in my experience siezed up oleos are much, much worse.

[HelihunterG]

Hello,
In photo #3 you can dents in the top of the wall of the building. I wonder if that is from the helicopter during break up (could be a blade) or if it was caused by the loose stuff that went into the blades? If it was from the helicopter, it shows how far bits will fly.:eek:
Another photo from the same site shows damage to a car in the car park. And to a tree next to it.

http://www.feuerwehr-regensburg.de/einsaetze/bilder/2005/hubsch_unf_dez05_2.JPG


They say there were no serious injuries, you'ld have to be wearing a helmet to not get hurt in that! :ok:

I wonder if they had prior permission to land in the factory area? Was this a road accident or a factory accident they were attending? My German's not that good for reading the article...
More pics: http://mephis.to/gallery2/v/helocrash05/?g2_page=1

The damage at the building is from the thing that went through the rotor. You can see it here: http://mephis.to/gallery2/v/helocrash05/IMGP9600.JPG.html right of the hor. stabilizer. The second panel is "unhurt" (next to the helicopter).
At this photo:http://mephis.to/gallery2/v/helocrash05/DSCN2880.JPG.html you can see the missing panels and the damage to the building. Note the hole at the upper right.
From the inside: http://mephis.to/gallery2/v/helocrash05/h1.jpg.html a "Bulls Egg" changes into a bullet.

They didn't wear helmets and it was an accident at the factory.

SASless is absolutly right. This happend "only" because of the balance thing...

HG