When a critical component in a helicopter’s main rotor system fails in flight, the resulting accident is almost always fatal. How much warning, if any, does a pilot get with these kinds of failures? Unfortunately, helicopters typically do not have cockpit voice recorders and unless the pilot can provide ATC with details, it can be hard to understand exactly what happened.
Even if the pilot is in contact with an air traffic controller, an emergency situation leaves little time to completely explain a problem. 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-equiped Bell 212 crashed near Philadelphia, Miss. Visual meteorological conditions prevailed and no flight plan was filed. The helicopter was destroyed and the airline transport-rated pilot and one passenger (who was employed by the owner as a mechanic) were fatally injured.
The transcript of communications recorded on the cockpit voice recorder showed that about 18 min. before the accident, the passenger 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 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.
Witnesses in the area of the accident site said they observed the helicopter flying from east to west, just above the treetops. The helicopter began rolling back and forth. They then observed the main rotor blades contact the tail area, and the aft tail boom and tail rotor separate. Shortly after this, the main rotor separated and the helicopter descended and crashed to the ground.
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 tailboom. 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 sound safety practices dictates that any unexplained vibration should be investigated on the ground until the source is found and corrected. 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 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. Difficulty with track and balance should be considered highly suspect, and would warrant grounding the aircraft until the problem is completely resolved.
Two recent accidents involving Robinson R22 helicopters, one in Israel and one in Australia, prompted the U.S. FAA to issue an emergency airworthiness directive (2004-06-52). 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. In fact, the accident in Israel happened during one of the track-and-balance flights.
In these three accidents, adequate warnings were present that a serious failure could occur. If the people involved understood the seriousness of the clues, three fatal crashes would have been prevented. It is reasonable to assume that in other main rotor blade failures, there were probably some warning signs that could have averted a fatal accident as well.
One of the best methods 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 tool for preventing fatal accidents.