Two Blade rotor systems + Neg G
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Two Blade rotor systems + Neg G
Good afternoon, to those of you who are Degree Trained please forgive any Gram/spelling mistakes(especially long words) that said my question is for you very long and ProRotor Pilots who would be kind enough to help us low timers: We read nearly every day on the Pprune about the problems of Neg G, we know what the dire results can be (that is we should)so as far as I can see 99% of us adhere to the rules, but as one who does not own his own I like many others rent SFH on such as the R22 R44 and B206 all of them fall into the model range that are suseptable to Neg G damage, apart from looking into and up underneath the actual center of the rotor stem/transmision shaft, are there any other warning signs of possible previous pilot /flight abuse, obviouse signs are marks on the shaft but if you look at that area of shafting on all of these types there are marks caused by the rocking motion whist the Helios are parked in windy conditions, and supposing you get airborne with such an abused machine are there any dynamic signs that would point to immediate return to terra-firma, or by then would it be to late ? I know Lu will love this one but I feel that there must be something more than marks on shafts to indicate this problem area, or is it open to last man holding the stick gets first prize.
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This one is pretty easy. If the Ac has been in a negative G environment and the mast has been touched (bumped) there is simply no way back, the mast will fold and fail within a second or two. Ergo you are unlikely to see a mast bumped Ac sitting on the pan as it will have already reverted to kit form!
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Yep, wishful (but sensible) thinking to imagine you could spot a 'bumped' a/c before you went flying in it.
The rate of crack propagation that would ensue following a bump means that the mast would separate in seconds. Unfortunately, unlike a blade being shed, you'd know all about it too!
There is a classic (true) tale of an instructor, in Iran at the Bell school, doing engine off landings. On one particular landing there is a strange noise on touchdown and after a moment they realise there are no blades. The mast had fractured at as they landed. If this seems to contradict the above, it was a quality problem in manufacturing and this was the first crew to live and tell the tale.
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Another day in paradise
The rate of crack propagation that would ensue following a bump means that the mast would separate in seconds. Unfortunately, unlike a blade being shed, you'd know all about it too!
There is a classic (true) tale of an instructor, in Iran at the Bell school, doing engine off landings. On one particular landing there is a strange noise on touchdown and after a moment they realise there are no blades. The mast had fractured at as they landed. If this seems to contradict the above, it was a quality problem in manufacturing and this was the first crew to live and tell the tale.
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Another day in paradise
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To: 212 Man
The incident you referenced was not the result of a quality problem. Here is the story and it fits right into the question raised by Vfrpilotpb regarding being the last person to fly the aircraft after a previous pilot screwed it up.
It was an Agusta-Bell 205 that was being piloted by An American pilot working for Bristow. Bristow had a contract with the Iranian Air Force to train their pilots. The American had an Iranian student and was teaching him to autorotate. The pilot chopped the power and told the Iranian student to take over. I won’t go into what the student did but he was very slow in taking over the controls. Rotor speed was bleeding off at a rapid rate so the pilot put the 205 into a dive to build up airspeed and rotor speed. At the same time, he tried to reengage the engine but it was as if there were no freewheeling unit installed. He tried several times and the engine needle oversped the rotor needle. The pilot elected to make a run-on engine out landing. The ground was very soft and undulating and offered no resistance to the skids. When the helicopter came to a stop the pilot who had already notified the base was filling out his flight report as the rotor speed bled down. He heard a loud crack and the helicopter lurched to the right accompanied by a loud bang. The rotor hit the pylon.
Immediately thereafter, there was another even louder bang and the stabilizer bar came crashing through the fuselage just behind the student.
There was a lot of infighting on the part of Bell in trying to say that the pilot had pulled his cyclic so far back that he hit the pylon. This is a physical impossibility unless the transmission support structure and primary airframe structure had been compromised. This was later proven false as the trannie was pulled out and put back in several times proving that the structure had not been deformed.
To make a long story a tad shorter when the trannie was disassembled the freewheeling unit was shown to be totally destroyed. The only thing that could cause this sort of damage was a compressor stall. The records were checked and there was no write-up of a compressor stall. If the helicopter had been subjected to a compressor stall the entire dynamic system and the driveline and engine had to be removed and checked for overstress. The airframe would also have to be checked.
The American pilot left the country to return to Bristow and the whole thing was swept under the carpet. Bell stuck with their story about pilot involvement.
Two weeks later a Bell techrep told the accident investigator that he was aware that the 205 had suffered at least two compressor stalls. He was a Bell techrep and he never reported it and just left it to chance. If the mast separation had taken place while in flight the helicopter would have crashed and the pilot would have been blamed for getting into a mast-bumping situation. The name of the accident investigator was Lu Zuckerman.
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The Cat
[This message has been edited by Lu Zuckerman (edited 04 April 2001).]
The incident you referenced was not the result of a quality problem. Here is the story and it fits right into the question raised by Vfrpilotpb regarding being the last person to fly the aircraft after a previous pilot screwed it up.
It was an Agusta-Bell 205 that was being piloted by An American pilot working for Bristow. Bristow had a contract with the Iranian Air Force to train their pilots. The American had an Iranian student and was teaching him to autorotate. The pilot chopped the power and told the Iranian student to take over. I won’t go into what the student did but he was very slow in taking over the controls. Rotor speed was bleeding off at a rapid rate so the pilot put the 205 into a dive to build up airspeed and rotor speed. At the same time, he tried to reengage the engine but it was as if there were no freewheeling unit installed. He tried several times and the engine needle oversped the rotor needle. The pilot elected to make a run-on engine out landing. The ground was very soft and undulating and offered no resistance to the skids. When the helicopter came to a stop the pilot who had already notified the base was filling out his flight report as the rotor speed bled down. He heard a loud crack and the helicopter lurched to the right accompanied by a loud bang. The rotor hit the pylon.
Immediately thereafter, there was another even louder bang and the stabilizer bar came crashing through the fuselage just behind the student.
There was a lot of infighting on the part of Bell in trying to say that the pilot had pulled his cyclic so far back that he hit the pylon. This is a physical impossibility unless the transmission support structure and primary airframe structure had been compromised. This was later proven false as the trannie was pulled out and put back in several times proving that the structure had not been deformed.
To make a long story a tad shorter when the trannie was disassembled the freewheeling unit was shown to be totally destroyed. The only thing that could cause this sort of damage was a compressor stall. The records were checked and there was no write-up of a compressor stall. If the helicopter had been subjected to a compressor stall the entire dynamic system and the driveline and engine had to be removed and checked for overstress. The airframe would also have to be checked.
The American pilot left the country to return to Bristow and the whole thing was swept under the carpet. Bell stuck with their story about pilot involvement.
Two weeks later a Bell techrep told the accident investigator that he was aware that the 205 had suffered at least two compressor stalls. He was a Bell techrep and he never reported it and just left it to chance. If the mast separation had taken place while in flight the helicopter would have crashed and the pilot would have been blamed for getting into a mast-bumping situation. The name of the accident investigator was Lu Zuckerman.
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The Cat
[This message has been edited by Lu Zuckerman (edited 04 April 2001).]
Joined: Jul 2000
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From: The Daylight Saving Free Zone
Vfrpilotpb , I recall an incident about eighteen years ago, when a pilot of a B206 reported, after some VFR night flying, that he got into a situation where he temporarily lost sight of the horizon and in those few moments the nose of the helicopter started to go up. He soon realised what was happening and pushed the cyclic forward, fairly quickly to bring the nose down.
A loud banging noise on the cabin roof with severe vibration occurred.
It soon settled down leaving him and his passengers somewhat unnerved. I don’t recall what he did to get out of that situation, but I think he was lucky to get back alive.
My first impression after an external inspection of the helicopter was that there was no obvious damage. The mast had indications of dynamic stop contact just under the head, but not dented and had not exceeded any serviceable damage limits.
A look under the transmission showed that there had definitely been contact, with the spigot plate on the deck being sheared off its mounting rivets. This was the only physical indication of damage that I could find.
This was my first, and to date, last experience of negative G incidents.
A loud banging noise on the cabin roof with severe vibration occurred.
It soon settled down leaving him and his passengers somewhat unnerved. I don’t recall what he did to get out of that situation, but I think he was lucky to get back alive.
My first impression after an external inspection of the helicopter was that there was no obvious damage. The mast had indications of dynamic stop contact just under the head, but not dented and had not exceeded any serviceable damage limits.
A look under the transmission showed that there had definitely been contact, with the spigot plate on the deck being sheared off its mounting rivets. This was the only physical indication of damage that I could find.
This was my first, and to date, last experience of negative G incidents.
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I misread Lu's post at first. I thought it said "This was later proven false as the trainee was pulled out and put back in several times proving that the structure had not been deformed" which put a slightly different light on it.
I once called an instructor over to an R22 I was pre-flighting ( I tend to do a check A every time unless I was the last person to fly the machine ) because there were score marks visible in the paint just above the ring which holds the scissors. It turned out to be scoring made by removal and refitting of the ring, but I felt it was safer to ask.
I once called an instructor over to an R22 I was pre-flighting ( I tend to do a check A every time unless I was the last person to fly the machine ) because there were score marks visible in the paint just above the ring which holds the scissors. It turned out to be scoring made by removal and refitting of the ring, but I felt it was safer to ask.
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Hey Lu,
When you say it's a physical impossibility for the rotor to hit the pylon without the compromised structures you mentioned: when you say pylon, do you mean the mast?
If so, at low Nr I would think it would be easy to make the blade support assembly (ie the box section carry through structure) hit the mast with a big cyclic input. It has that freedom of travel anyway due to the teetering head setup.
If you're talking about the tailboom, with a bit of blade flexing as you would experience in a bouncy run-on landing, it would also be possible to hit that with the main rotor, especially at low Nr, I'd imagine.
In a nasty inflight incident a few years ago, an Australian UH-1H put a big scar in the main rotor blades when they hit the upper wire cutter assembly during some visual night flying when the pilot became disoriented.
Didn't mast bump, but the main rotor blades obviously came a hell of a lot closer to the rest of the airframe than they usually do.
When you say it's a physical impossibility for the rotor to hit the pylon without the compromised structures you mentioned: when you say pylon, do you mean the mast?
If so, at low Nr I would think it would be easy to make the blade support assembly (ie the box section carry through structure) hit the mast with a big cyclic input. It has that freedom of travel anyway due to the teetering head setup.
If you're talking about the tailboom, with a bit of blade flexing as you would experience in a bouncy run-on landing, it would also be possible to hit that with the main rotor, especially at low Nr, I'd imagine.
In a nasty inflight incident a few years ago, an Australian UH-1H put a big scar in the main rotor blades when they hit the upper wire cutter assembly during some visual night flying when the pilot became disoriented.
Didn't mast bump, but the main rotor blades obviously came a hell of a lot closer to the rest of the airframe than they usually do.
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PB - if you get to Neg G you have a real problem - It should be noted that low G is where things start going wrong (IAS dependent)
Surely to say that every mast bumping incident results in blade separation is extreme.
There must be a magnitude of 'bump'.
On the subject of evidence of low G.
R22/R44/206 M/R blade just clipping tailcone could be evidence.
Damage of the droop stop/stops.
As sprocket said, on the 206, damage on the drag pin or striker plate missing is evidence of excessive mast vs airframe angle after mast contact.
Surely to say that every mast bumping incident results in blade separation is extreme.
There must be a magnitude of 'bump'.
On the subject of evidence of low G.
R22/R44/206 M/R blade just clipping tailcone could be evidence.
Damage of the droop stop/stops.
As sprocket said, on the 206, damage on the drag pin or striker plate missing is evidence of excessive mast vs airframe angle after mast contact.
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Thank you all for your answers, in my humble opinion you cannot better first hand information,and it seems that you all have lots to offer low timers like me, Thank you! not one of you critisised my gramm/spelling , thanks for that also.
My Best regards
Peter RB, Vfrpilotpb
My Best regards
Peter RB, Vfrpilotpb
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To: Arm out the window
The pylon is the bit that attaches to the rear of the tailboom and extends upward and backward and supports the tail rotor gearbox placing it well out of reach of the blades as they rotate. In order to hit the pylon the transmission would have to rock rearwards and even then there would be no contact. In order to make contact the support structure for the trannie (transmission) would have to become distorted. By removing and replacing the transmission we proved that there is no structural involvement. The blade hit the pylon and in order to do so they had to have broken away from the mast. Bell didn’t want to accept that because it would mean that they had a rotor separation and at that time there were reports of over 53 incidents involving 206s and Hueys. It was the only survivable rotor separation and Bell just swept it under the carpet.
Agusta was also uncooperative in the metallurgical examination of the mast stub and we found out that the mast on the subject aircraft was originally installed on another 205 and there were no records of the original removal and installation on the subject aircraft. This meant that there was no record of the total time accumulated by the mast since manufacture. This was topped off by the non reporting of at least two compressor stalls on the subject aircraft.
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The Cat
[This message has been edited by Lu Zuckerman (edited 04 April 2001).]
The pylon is the bit that attaches to the rear of the tailboom and extends upward and backward and supports the tail rotor gearbox placing it well out of reach of the blades as they rotate. In order to hit the pylon the transmission would have to rock rearwards and even then there would be no contact. In order to make contact the support structure for the trannie (transmission) would have to become distorted. By removing and replacing the transmission we proved that there is no structural involvement. The blade hit the pylon and in order to do so they had to have broken away from the mast. Bell didn’t want to accept that because it would mean that they had a rotor separation and at that time there were reports of over 53 incidents involving 206s and Hueys. It was the only survivable rotor separation and Bell just swept it under the carpet.
Agusta was also uncooperative in the metallurgical examination of the mast stub and we found out that the mast on the subject aircraft was originally installed on another 205 and there were no records of the original removal and installation on the subject aircraft. This meant that there was no record of the total time accumulated by the mast since manufacture. This was topped off by the non reporting of at least two compressor stalls on the subject aircraft.
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The Cat
[This message has been edited by Lu Zuckerman (edited 04 April 2001).]
