One important method for determining a helicopter’s health is vibration analysis. All helicopters have an inherent vibration. The type and intensity varies as a function of rotor design and isolation systems. Understanding basic vibration levels and being alert to changes is an important safety tool for helicopter pilots.
This is exemplified by the crash of a Bell 212 helicopter equipped with a cockpit voice recorder. About 18 minutes before the accident, the passenger (who was also a 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 one minute 30 seconds before the accident, the pilot asked the passenger, “Has this vertical 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. About 20 seconds 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.
According to the National Transportation Safety Board, 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.
Many components on a helicopter can fail and still allow the pilot to make a safe landing. The main rotor system is 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 and investigated on the ground until the source is found and corrected.
Another telltale sign of a potential problem is difficulty with tracking and balancing the rotor system. Two accidents involving Robinson R22 helicopters, one in Israel and one in Australia, should never had happened. In both cases, investigations revealed that corrosion from water penetration initiated a fatigue crack in the main rotor blades. Both helicopters experienced an increase in main rotor vibration prior to final blade failure. In both aircraft, the vibrations were corrected with track and balance, only to reappear a short time later.
It’s normal for a helicopter to require periodic tracking and balancing as paint and bearings wear over time. However, when a vibration reappears or abruptly changes, helicopter pilots need to take notice.
Fortunately, failure of a critical component in the main rotor system is rare–it would be like a wing spar failure in an airplane. The good news is that a lot of times the helicopter will try to tell you. Many sharp pilots and maintainers have been alert to this and corrected an issue before anything happened. I think vibration awareness and analysis should be an important part of the helicopter private pilot curriculum. If everyone understood the importance of vibrations, we could virtually eliminate these kinds of accidents.
Tags: Tim McAdams