'Severe Vibratory Events'

NavyBlue007 · September 24, 2004

I'm a student pilot working on my private, planning on going all the way up to CFI (w/instrument.) This past June I was doing my second supervised solo flight when I encountered ground resonance that ended up destroying the helicopter. I was flying a 300CBi, and I was practicing pick ups, set downs, and pattern work. Fifteen minutes before I was supposed to head to the ramp I decided to practice a few more pick ups and set downs on the inactive runway. I landed twice in the same spot, picked up and everything was fine. I went to set down the third time and felt the skids touch gently, thinking I had made a perfect landing. Two seconds after my eyes moved to the center console to check my gauges, I felt a small vibration that began to grow more violent with each moment that passed. Before I realized what had happened the controls had vibrated out of my hands and all I could do was hang on for dear life. The helicopter rocked back and forth on the skids while turning to the left. The main rotor dipped low enough to chop the tailboom off and send it about 30 feet down the runway. I saw the main rotor blade then dip very close to the cockpit and pretty much thought "this is it" and hunched down in my seat and waited to be chopped liver. As the helicopter's main rotor blade made the last pass, it struck the runway and stopped, 180 degrees from the initial direction I was facing when I landed. When the rotor blades stopped I did what anyone in my situation would do, and that's an arm and leg check. Seeing that I had every part of my body and that I was not bleeding, I shut down the still running engine, shut the fuel valve, turned off the mags, and then unstrapped and booked my butt to as far away from the wreckage as I could run. I had bruises between my knees from where the cyclic beat them, and a bruise from where the shoulder harness had kept me strapped in, but otherwise I was uninjured. I can't remember if it was the attitude or altitude indicator that had shaken loose of the cockpit and was thrown onto the runway, I was just glad it didn't hit me on the way out.

To this day I don't know what exactly caused the ground resonance to set in. It's something we talk about in ground school, but not something that everyone gets practical experience with, especially a student pilot with .7 hrs solo time. Luckily the staff at the school I go to was very helpful and optimistic. They had me up in the air three days later for a "joy ride" so I wouldn't become too scared to fly again. I think I only took a week off before I was back in the saddle, and I've started my solo work again recently and hope to have my PPL within the next two months.

RDRickster · September 24, 2004

Two thumbs WAY UP for getting back into the saddle!

AllJeep · September 25, 2004

I can't find the NTSB report? When did this happen? What airport?

I'm not doubting your story but this is the only report that I found and the details don't match.

NTSB Identification: NYC04LA142

14 CFR Part 91: General Aviation

Accident occurred Tuesday, June 15, 2004 in Norfolk, VA

Aircraft: Schweizer 269C, registration: N206ST

Injuries: 1 Uninjured.

This is preliminary information, subject to change, and may contain errors. Any errors in this report will be corrected when the final report has been completed.

On June 15, 2004, about 1820 eastern daylight time, a Schweizer 269C, N206ST, was substantially damaged while landing at Hampton Roads Executive Airport, Norfolk, Virginia. The certificated student pilot was not injured. Visual meteorological conditions prevailed and no flight plan had been filed for the instructional flight conducted under 14 CFR Part 91.

The student pilot was conducting his third supervised solo flight, and had completed two takeoffs, and landings. During his third landing attempt, after the helicopter contacted the ground, it began to vibrate violently. The student pilot was not able to stop the vibration, before the helicopter began to break apart.

The student pilot had accumulated 22 hours of total flight experience, all in the same make and model as the accident helicopter.

NavyBlue007 · September 25, 2004

Well AllJeep, thats the report right there. Curious as to what you mean by the details not matching though? If you mean the 269C designation over the 300CBi, it was my understanding that the 300CBi was built on the frame of the 269C, so both are the same. Correct me if I'm wrong though, but trust me, that's not something I would make up.

AllJeep · September 25, 2004

Again, I'm not doubting you. Really sorry to hear the story but glad your here to tell the story.

But when I said the details don't match, it's because a 300 is usually listed as a 300 and the report said the student was on his 3rd solo. You said you were on your second. Was just curious and wanted to read the report.

Oh, and welcome to VR.

NavyBlue007 · September 25, 2004

I guess that would be right about the third solo. Guess the reason I thought it was the second was because I never actually finished that third solo, lol, or put it in my logbook. Prolly a case of my brain tryng to forget the incident. Either way, good catch. ::rotorhead::

Wally · September 25, 2004

I don't care what the Beach Boys sang- there ain't no such thing as "Good Vibrations!" Any change in vibration in a helo is cause for concern and should have your complete attention.

Taking off, throttle back. It's better to find out what's wrong after the helo's shut down.

Landing, take off, and figure out what's wrong.

Flying, figure out what's wrong as I head for the first survivable place to land.

"Good vibrations" and "friendly fire..." Whatever, deal with it right now- "Roger, returning friendly fire."

fulldownauto · October 7, 2004

Was just looking through the NTSB database, and came accross two incidents that involved Schweizers self destructing. Do they happen all that often? Apparently yes, at Group 3, two S269's destroyed from ground resonance within 6 months? N61415 and N61413.

Anyone have any info on these? How hard is it to get into resonance in the Schweizer?

Thanks,

James

IgorS · October 7, 2004

It's happened to me a couple of times practicing set-downs.

It has only happened to me in higher-time ships. Coincidence? Maybe, maybe not. If I ever own one, I'd tend to replace the oleos more often than TBO.

I think at least one of these incidents (maybe both) involved primary students on their first solo. Maybe it's easier to induce resonance when then ship is lighter, and of course the student would be slow to react.

I've also read of a shake-apart during the magneto check, which is performed at 2500 rpm in the 300C. My instructor thinks he could pick up at 2500 (flight rpm is 3100) and arrest the resonance, I'd like to see him do that. If it happened to me during a solo, I'd at least try to get light on the skids. It might be the only way to save it.

It is certainly on my mind when I set down.

NavyBlue007 · October 7, 2004

I was involved in an incident with ground resonance in a 300 CBI. It was weird too, because I was on my third solo flight making my third set down from a hover and I don't remember doing anything different from the first two set downs. I actually have a post in the helo stories section about that incident. I also managed to get one of our other Schweizers into ground resonance, but my instructor and I pulled it up as soon as we felt the vibes. Again, neither he nor I could see if it was something I was doing to cause it. Later that day two of the instructors and the owner all flew the helicopter to try to get it to go into ground resonance, but they couldn't. Go figure. I've only got about 46 hrs flying time though, but thats my experience with it.

Wally · October 8, 2004

Years ago, with "wet" dampers it wan't hard at all. One could induce it with poor technique. If the oleos were properly inflated, and the ship had good blade dampers properly phased, it seldom was more than a transient issue: If you're increasing revs, decrease. If you're landing, takeoff again. In a nutshell- stop what you're doing and do it differently, sometimes as simple as moving off the stone that was bottoming an oleo.

It is an issue that must be trained. Recognition of the circumstances that allow it, and quick and proper response to onset is the answer. It's not an issue unique to the 269- it is possible in any fully articulated system. I've seen the remains of a Twinstar that suffered the event.

If you want a nightmare, what the French term "dynamic divergent excitation" (Analgous to what we usually train for as ground resonance.) is it. The throttles are on the ceiling and there two of them. I'm kinda average as a pilot and I'm short at least one left arm for this scenario.

Bottom line- train for it and it's a managable issue in the 269. Ignore it and you'll have a nice pile of formerly helicopter parts in approximately three main rotor rotations.

Heliport · October 9, 2004

Merged the "Ground Resonance" thread into this discussion.

fliboy · October 10, 2004

one of our machines is a H269C used mainly for ag spraying. It has the old friction type dampers which make it a bit more prone to GR we also land on trolly's that have quite small castor wheels and the lay out of the pad means final touch down is usually with a tail wind. Having operated like this means our pilots are used to dealing with the problem. On landing banging one heel down can start it off as can jerky control inputs with the machine light on the skids in any case get the heli airborne asap. You may need to re phase the dampers by doing a few tight turns or stir the stick in the hover. another trick is to apply a little more weight on the front oleos as you lower the colective right down. If you are still having problems land the grass. For take off make sure the blades are all in phase hold the tail rotor still move all the blades to the most forward position -lead lag hinge (for friction type dampers).

Smoothness of all control inputs including revs is the best preventitive.

Maybe of some help to somebody??

TH55 · October 10, 2004

I have over 400hrs in the 269A and have never experienced ground resonance, knock on wood. EVERY TIME I set down I prepare myself for it but it has never shown its ugly face. I fly an older machine but in (way) above average condition.

As a CFI it is a big concern with my students because all you can do is talk about it. It is not demonstrable!

I still feel this is the best trainer ever devoloped. Remember the military used it for decades.

Whirllybird8008 · October 24, 2004

I two have experienced ground resonance. Low time students will most likely experience the intital symptoms at one point it is usually caused by two factors.

One: stuck lead lag dampeners or oleo's that have not been serviced correctly or are not fucntioning and pilot technique, a very soft landing, say bouncing very lightly from skid to skid can cuase it. most of the time it just takes a little nudge to throw one of the main rotor blades out of phase and resonance issue.

Just remember, maintain rotor RPM until the helicopter is down and completely settled so the oleo's can do their jobs and dampens vibration.

I also agree with the individual that said their instructor could lift off at 2500 RPM an emergency. There is a maintenance hovering check that requires you to hover at low RPM and check left pedal travel limits. it can be done.