Main rotor vibrations

November 21, 2011 by Tim McAdams

When a critical component in a helicopter’s main rotor system fails in flight, how much warning, if any, does a pilot get with these kinds of failures? Unfortunately, helicopters typically do not have cockpit voice recorders (CVR) so it can be hard to understand exactly what happened. Consequently, the following accident is unique in that it provides some insight as to what the flight crew knew. 

On Nov. 27, 1999, a CVR equipped Bell 212 crashed near Philadelphia, Miss. The transcript of communications recorded on the cockpit voice recorder showed that about 18 min. before the accident, the passenger (who was also the aircraft’s mechanic) stated to the pilot, “Boy, those catfish are going crazy down there, aren’t they?” 

“Yep,” the pilot responded, “must have been the vibrations from the helicopter.” 

About 2 min. later, the passenger and pilot discuss sighting deer in a field. About 1 min., 30 sec. before the accident, the pilot asked the passenger, “Has this vertical (a term used to describe a vibration that moves up and down) just gotten in here or has it been here for a while?” 

“We haven’t had any verticals at all,” the passenger replied. 

“We do now,” the pilot said. 

“Yeah, well it started right after we left back there,” the passenger said. “I think it maybe, ah, that’s why I was thinking it was the air.” 

About 20 sec. later, the passenger stated that another person had tracked the helicopter’s blades before they left and that he was commenting on how smooth it was. Forty seconds after that, the pilot said, “This stuff is getting worse.” 

The recording then ended. 

The National Transportation Safety Board determined the probable cause of this accident was the failure of the pilot and company maintenance personnel during preflight and periodic inspections to identify the signs of fretting and looseness in the red main rotor blade pitch change horn to main rotor blade grip attachment. As a result, the NTSB found, the helicopter was allowed to continue in service with a loose pitch change horn, which led to separation of the pitch change horn from the blade grip and the in-flight breakup of the helicopter after the main rotor struck the tail boom. Contributing to the accident, the safety board said was the pilot’s failure to respond to increased vibration in the main rotor system and land immediately. 

Interesting to note is that the pilot and mechanic were aware of the vibration, but apparently never considered a precautionary landing. Any pilot would land immediately when a sudden and severe vibration occurs. But any unexplained vibration should warrant a precautionary landing. Some parts and bearings that become loose can experience exponential wearing and fretting and quickly reach a failure point. 

Many components on a helicopter can fail and still allow the pilot to make a safe landing. The main rotor system is normally not one of them. Thus, any abnormal low-frequency vibration felt in the airframe or through the flight controls should be treated with extreme caution.

7 Responses to “Main rotor vibrations”

  1. Rocky Lee Says:

    Good article. Glad to see AOPA discussing helicopter topics.

  2. Carl Schultz Says:

    At about 300 to 400 hours, I experianced unusual vibration in my R44. It was flight tested and the mechanic determined that the blades needed rebalancing and would be taken care of at next scheduled service. I flew off but returned a few flight hours later when the vibration increased to a point where it realy alarmed me…something was not right!
    I should have detected the problem on my preflight, but this being my first helicopter, I didnt realize that the loosness (play) was not normal until I landed and compareed it to a another ship. The pitch horn bearings had failed and the bolt was partailly worn through,
    When ships talk to you, you had better listen up… and in hind sight, after detecting the loosness, I still didnt realize how serious this was and should not have flown it back to the FBO mechanic that was located on the other side of the airport.

  3. Brett Reeder Says:

    Great article. Thanks for the helicopter related articles

  4. wendy Says:

    I was flying a R-22 up to Long Island from PA. Near Teteboro, I heard a whistling sound coming from the main rotor blade. It got worse by the minute, and I made a precautionary landing. Having read about blade delaminations, I was worried it was now happening to me. A three minute flight to the nearest airport seemed like an hour. After landing, I discovered the “never pull down” sicker had started to peel off. I reached up and peeled it all the way off, and the flight continued without problems. I was very impressed how such a small object near the tip of the blade could cause such a loud whistling noise, and increased vibrations. It was a lesson I will never forget. When in doubt set it down, don’t assume the problem will “correct itself” with continued flight.

    P.S. Thanks for the great website!

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