Ground resonance fails AS350 tail boom
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Ground resonance fails AS350 tail boom
Just seen this (abbreviated) report on the NTSB accident site. Unusual, to say the least. Anyone come across this sort of thing before?
"On November 18, 2006, a Eurocopter AS 350 BA separated its tail boom during a ground resonance occurrence at the Buckingham Field Airport, Fort Myers, Florida.
The pilot started the aircraft without any discrepancies. After about one minute, he began to slowly advance the fuel control to flight gate. During this process, he monitored the torque gage not to exceed 40%. He continued to advance the throttle, and observed a sudden spike in the torque that exceed 40% then went below 40%, at the same time he heard the engine begin to rev rapidly. He stopped the advancement of the throttle and again the torque gage indicated a spike that exceeded 40% as the engine rev extremely fast and noted a spike in the RPM as the helicopter started to shake violently. He retarded the throttle to the closed position; however, he still heard the engine revving high. The helicopter began to bounce on the ground. He applied downward pressure on the collective as the helicopter rocked back and forth extremely violently. He attempted to close the fuel shutoff valve; however, the collective rose when he released it and elected to keep his left hand on the collective to ensure the throttle was closed. The helicopter continued to shake and bounce violently on the ground for sometime prior to coming to a rest upright, 90 degrees from its original startup position.
A witness stated he observed the helicopter at flight idle. When the engine was advanced, it seemed like it advanced very quickly. He immediately thought that it was an overspeed, just by the way the blades sounded. The helicopter then started with a slight lateral bounce, which progressed into severe bouncing back and forth from skid to skid. The helicopter then rotated 90 degrees to the right and the tail boom separated, falling to the ground."
"On November 18, 2006, a Eurocopter AS 350 BA separated its tail boom during a ground resonance occurrence at the Buckingham Field Airport, Fort Myers, Florida.
The pilot started the aircraft without any discrepancies. After about one minute, he began to slowly advance the fuel control to flight gate. During this process, he monitored the torque gage not to exceed 40%. He continued to advance the throttle, and observed a sudden spike in the torque that exceed 40% then went below 40%, at the same time he heard the engine begin to rev rapidly. He stopped the advancement of the throttle and again the torque gage indicated a spike that exceeded 40% as the engine rev extremely fast and noted a spike in the RPM as the helicopter started to shake violently. He retarded the throttle to the closed position; however, he still heard the engine revving high. The helicopter began to bounce on the ground. He applied downward pressure on the collective as the helicopter rocked back and forth extremely violently. He attempted to close the fuel shutoff valve; however, the collective rose when he released it and elected to keep his left hand on the collective to ensure the throttle was closed. The helicopter continued to shake and bounce violently on the ground for sometime prior to coming to a rest upright, 90 degrees from its original startup position.
A witness stated he observed the helicopter at flight idle. When the engine was advanced, it seemed like it advanced very quickly. He immediately thought that it was an overspeed, just by the way the blades sounded. The helicopter then started with a slight lateral bounce, which progressed into severe bouncing back and forth from skid to skid. The helicopter then rotated 90 degrees to the right and the tail boom separated, falling to the ground."
Join Date: Jan 2006
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Have seen that a fair few times. The first time I observed it, it was like as if the tail boom attachment point is a "weak link" in the system. The good ol' AS350 tail boom does not like hard landings or Ground Resonance. Of note, if the tailboom has departed, have a look in the baggage compartments each side for any "abnormalities in the heaxadron covers" and therefore the hexadron itself.
Have had incipient ground reasonance quite a lot, if you have flight revs, goes away easy by taking off. Unfortunately if you don't it happens far too quick usually to prevent by shutdown and rotor brake.
The spare parts guys at the factory must have made good coin over the years making spare booms..............
Have had incipient ground reasonance quite a lot, if you have flight revs, goes away easy by taking off. Unfortunately if you don't it happens far too quick usually to prevent by shutdown and rotor brake.
The spare parts guys at the factory must have made good coin over the years making spare booms..............
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Did one of the star arms fail first?
Did one of the star arms fail first?
Another unexplained AS350 ground resonance type accident. One seemingly normal AS350 on the ground at flight idle without warning or prior rock&roll, abruptly goes into ground resonance. Statements from pilots of other accident aircraft, some experienced with the onset of ground resonance, stated theirs was not a normally caused ground resonance. The common factor in these cases was the fractured Star Arm found after the accident.
Also, stories of AS350’s experiencing severe rotor vibrations in-flight and upon landing going into ground resonance. Fractured Star Arm found after the fact.
The aircraft ends up totaled in about 20 seconds.
Claims were filed against Eurocopter linking the failed Star Arm as the cause of the ground resonance. Eurocopter holds that the aircraft was allowed to enter ground resonance and the severity of the resonance caused the Star Arm Failure.
I did not think much of this Star Arm failure alibi before, but the numbers of stories made me take note.
Another unexplained AS350 ground resonance type accident. One seemingly normal AS350 on the ground at flight idle without warning or prior rock&roll, abruptly goes into ground resonance. Statements from pilots of other accident aircraft, some experienced with the onset of ground resonance, stated theirs was not a normally caused ground resonance. The common factor in these cases was the fractured Star Arm found after the accident.
Also, stories of AS350’s experiencing severe rotor vibrations in-flight and upon landing going into ground resonance. Fractured Star Arm found after the fact.
The aircraft ends up totaled in about 20 seconds.
Claims were filed against Eurocopter linking the failed Star Arm as the cause of the ground resonance. Eurocopter holds that the aircraft was allowed to enter ground resonance and the severity of the resonance caused the Star Arm Failure.
I did not think much of this Star Arm failure alibi before, but the numbers of stories made me take note.
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I thought the Star Arms failed as a result of the Ground Reasonance, not the cause.
U/S Shock absorbers and incorrectly aligned stingers on the rear of the skids are apparently good accelerants for ground reasonance.
U/S Shock absorbers and incorrectly aligned stingers on the rear of the skids are apparently good accelerants for ground reasonance.
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I had experienced several evolving ground resonance, as well as Star Arm fractures on the AS350B and here are my observations..
1. On the old Star versions, we've always found the bushings cracked at the end of the arm. I believe this was corrected in later versions.
2. The Ball joints have always been found to have excessive play whenever vibrations not related to blade track is encountered and symptoms of evolving resonance. My observation is that this type of vibration is the one that one sees in the center post of the windshield and the instrument panel.
3. Observations on 350s that had a hard landing and failed tailboom showed the drive shaft splines gets detached due to flexing of the boom. I've seen this happen.
4. I've seen and flown a 350 with a transverse fractured star arm in a way that if you flex the M/R Blade downwards-- you see the gap on the fracture gaping. It was cleared to go by the manufacturer while awaiting parts due to within tolerance limits. No tendency to go into ground resonance noted. Aircraft remained in service until replacement arrived.
5. Item 1 & 2 were the ones always present whenever we encountered evolving resonance. We've tried to replace the shocks, anti-resonance strips to no avail. It's always #1 and #2 or other elastomeric bearings. Also noticed within 300 hrs one encounters this if the ball joints have play in them.
just my .10 cents
1. On the old Star versions, we've always found the bushings cracked at the end of the arm. I believe this was corrected in later versions.
2. The Ball joints have always been found to have excessive play whenever vibrations not related to blade track is encountered and symptoms of evolving resonance. My observation is that this type of vibration is the one that one sees in the center post of the windshield and the instrument panel.
3. Observations on 350s that had a hard landing and failed tailboom showed the drive shaft splines gets detached due to flexing of the boom. I've seen this happen.
4. I've seen and flown a 350 with a transverse fractured star arm in a way that if you flex the M/R Blade downwards-- you see the gap on the fracture gaping. It was cleared to go by the manufacturer while awaiting parts due to within tolerance limits. No tendency to go into ground resonance noted. Aircraft remained in service until replacement arrived.
5. Item 1 & 2 were the ones always present whenever we encountered evolving resonance. We've tried to replace the shocks, anti-resonance strips to no avail. It's always #1 and #2 or other elastomeric bearings. Also noticed within 300 hrs one encounters this if the ball joints have play in them.
just my .10 cents
Make sure your landing gear dampers are overhauled every 2500 hours, as they are supposed to be. Ask for the records from the timing check on the landing gear dampers post overhaul. Best if they are within 1/2 a second apart. Do not accept them back if they are more than 1 second apart.
Check the skid extension springs too, they flatten out with age.
Dampers can be a pain in the a
to set up properly, but can make all the difference.
Check the skid extension springs too, they flatten out with age.
Dampers can be a pain in the a
to set up properly, but can make all the difference.
