Helicopter Crash in Goshen, Indiana Due to Ground Resonance

[ShyFlyGuy]

A friend of mine was the instructor in this helicopter.

GOSHEN, Ind. -- A helicopter shook itself apart after landing at a northern Indiana airport Friday, injuring all three people aboard.


"I could see them getting thrown around like rag dolls," said Gordon Miller, who was fueling planes at Goshen Municipal Airport when the accident occurred about 11 a.m. "I've never seen anything like that before."


Airport manager Randy Sharkey said the shaking apparently was caused by ground resonance, a phenomenon that occurs when a helicopter's rotors get out of synch while it is on the ground.


"It was a smooth touchdown, but then the helicopter came back up in the air and began to shake violently," Sharkey said. "It was out of control."


Miller said the Enstrom helicopter's tail section began bucking and broke off after it bounced back into the air. Witnesses said all three occupants were ejected from the aircraft.


"It was sickening to see that," Sharkey said. "It was horrible."


The pilot was airlifted to Memorial Hospital in South Bend, about 20 miles northwest of Goshen. His identity was not released and information about his condition was not immediately available.


The Truth of Elkhart identified the other two as flight instructor Kenny Keller of Burboun and Justin Dale of Mishawaka. They were taken to Goshen Hospital.


Sharkey said the passengers' injuries were not life-threatening.


The three were completing a cross-country flight from Grand Rapids, Iowa, to Goshen, said Harold Jones, an aviator safety inspector with the Federal Aviation Association.


Federal officials are investigating the accident.


Copyright 2005 Associated Press. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

What is ground resonance?

Shy

 

[FracCapt]

Ground resonance is caused when an outside force on the airframe(such as a hard touchdown) causes the blades to go out of sync on a fully articulated rotor system. This only occurs on 3 bladed rotors. With 3 blades, each blade is 120 degrees off from the other 2. Articulated rotors have drag hinges that allow the blades to "swing"(lead/lag) about 2 to 3 degrees either direction of center to allow for certain aerodynamic conditions and forces. When one of the blades gets out of sync, this creates an uneven disc. This happens when one of the blades leads or lags and there is not another blade doing something to counteract the force. The only time it poses a problem is on the ground. Imagine a spinning top. Push down on it on one side while it's spinning, and watch the wild oscillations. Same principle.


It's hard to describe without being able to draw diagrams....so go to the link below(the FAA Rotorcraft Flying Handbook) and go to page 11-7.

http://www.faa.gov/library/manuals/aircraft/media/faa-h-8083-21.pdf

The corrective action for ground resonance is to lift off again. The aerodynamic loads will normally sort the problem out immediately on its own. Chances are they didn't pick it back up....in which case the vibrations get worse and worse until the helicopter shakes itself apart.

 

[skiddriver]

Pretty good description by FracCapt, but a couple of minor quibbles.

It's not related to the number of blades. You could do it with a 5 bladed head as well as 3. The easiest way to imagine what is happening is to think about the center of mass of a rotor system. Normally, the center of mass is over or very near the center of the rotating mast. If you touch down hard with side or forward or aft drift, the blades in an articulated (or soft in plane rigid rotor system) can "sling" in the direction of drift. So the blades end up bunched up on one side of the rotating mast at the end of their lead/lag axis range of motion. This moves the center of mass of the rotor system off the center of rotation, and the rotation of the rotor system causes the mast to swing farther and farther from center until the helicopter shakes itself to death, or the pilot finds a way to stop it (very hard).

One way is to lift off the ground, but this can intensify the resonance and the aircraft can shake to pieces before it comes off the ground.

The other is to chop the throttles and apply the rotor brake, which should cause the blades to all move toward their foward lead stop and move the center of mass over the center of the rotaing mast.

This phenomena is highly affected by hinge stiffness or springs in the system, as well as the stiffness of the oleo struts and tires, which may become more obvious if you check out the following link.

MPG of a Chinook shaking itself to pieces: http://www.chinook-helicopter.com/video/Ground_Resonance_Rear_View.mpg

 

[FracCapt]

It's not related to the number of blades. You could do it with a 5 bladed head as well as 3.

You're correct. My post should have read 3 or more blades. Of course, this only occurs on fully articulated rotor systems.

The other is to chop the throttles and apply the rotor brake, which should cause the blades to all move toward their foward lead stop and move the center of mass over the center of the rotaing mast.

Most light training helicopters that I'm familiar with do not have rotor brakes. The specified recovery action is generally....1) If rotor RPM is normal, lift off immediately....or...2) If rotor RPM is below the normal range, close the throttle and lower the collective.

I have no experience with the larger, multi-engine helicopters as you do...so, of course, I can't comment on corrective actions for them.

I also have no Enstrom time...so I don't know if it's equipped with a rotor brake or not.

 

[erj-145mech]

I'm also under the impression that ground resonance can't happen unless the helicopter has tires or shock struts. Very unlikely to happen if the aircraft has skids? Please correct me if I'm wrong, I'm going back to A&P school 35 years ago.


Thanks

 

[mattpilot]

Of course it can also happen with skids. If the skid hits hard, it will 'jolt' the rotor system, causing the above explained problems.

Helos with wheels are affected differently than skid helo's. WHeels have struts in them, and if one of those malfunctions, or doesn't produce the same dampening effects as the other wheels under the helo, the helo will start to oscillate. No hard jolt needed.

 

[FracCapt]

I'm also under the impression that ground resonance can't happen unless the helicopter has tires or shock struts. Very unlikely to happen if the aircraft has skids?

Actually, the primary reason for oleo dampers(which is pretty common on fully articulated, skid equipped helicopters) on skid equipped helicopters is to dampen the shocks or jolts so they're not transmitted to the main rotor system. Thus, the dampers(and shocks on wheel equipped helicopters) help prevent ground resonance...not contribute to it.

 

[Phrogboy]

The best analogy I give is when you wash all your jeans at once in the washing machine and they all end up on one side of the washing drum. I am sure you are familiar with the noises the machine makes when that happens. Same for a helo. Struts and thick tires do wonders for dampening it out. Skids on the other hand have nothing to absorb the vibrations and undulations. That is the biggest reason why you will never see a fully artuculated head on an aircraft with skids. If anyone knows of one, please let me know.

 

[Flying Illini]

It is threads like this that make me really appreciate FI.com. I'm not a helo pilot (would love to learn) but I've learned an incredible amount just listening to people who know what they are talking about.

Impressive video, btw.

 

[helimech]

ok, let's straiten a few things out....Ground resonance can happen with ANY number of blades, 2 blades as well.... and there are some 2 bladed systems that are also "soft-in-plane", also it can happen with wheels, skids, skis, floats, whatever the aircraft happens to have.... yes they each have their own little quirks about them, and when they can induce the resonance. I actually saw this exact aircraft a week ago yesterday, the day before the incident, in South Bend, In. when we were working on our helicopter. The new Enstrom's, the turbine ones, have had a refit to the lead-lag dampers by replacing the normal grease/oil dampers with elastomerics. This accident is the fourth that has happened due to this change. As you all know, I think from Fraccapt description, ground resonance is when one of the blades leads and the adjacent blade lags creating an imbalance in the rotor disc. What is happening with these new elastomeric bearings is that after sitting for awhile they tend to stick at their shut-down positions and sometimes they don't go into their normal positions. Bell helicopter instituted the elastomerics a long time ago and now everyone in the industry is catching up, most of the time without proper testing. Now as I said before, there are a "couple" of 2 bladed rotor systems that have the same problem and they are the Bell 222, Ut, B and 230. all because elastomerics are pretty much rubber and metal and that allows for a little bit of movementin the lead-lag axis. The A and B model 222's really only have this problem if they are on wheels. I know most books, including the FAA's say that ground resonance only happens with "fully-articulated" rotor heads, which is always 3 or more blades, but it has been proven wrong a couple of times. One of which was with an FAA check airman on board. Enstrom knows about the problems with their design and are actively looking into it. If you guys want a couple picks of the aircraft at Goshen go to here: http://www.wsbt.com/main.asp?Search=1&ArticleID=2914&SectionID=1&SubSectionID=53&S=1