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To: Pilots and mechanics of Bell 412, 214 or any Bell with Elastomeric teeter bearings to include pilots and mechanics that work on or operate helicopters with composite rotorheads and elastomeric pitch and lead lag bearings.

Please perform the following check at your earliest convenience.

1 Get a magnetic field indicator (Gaussimeter) from your shop. If you have a Magna Flux machine and a degausser this instrument is used to detect the level of residual magnetism after degaussing a part.

2) Take the instrument and place it near the top of the rotor shaft just under the rotor head and measure the level of magnetism. Perform the same check on the Jesus Nut. Record the level of magnetism at each point.

3) Check the overhaul manual for the level of residual magnetism allowed after degaussing a part that has been tested in a Magna Flux or Magna Glow machine. This is in the Bell overhaul manuals but I do not know if these limits are a part of the Eurocopter helicopter manuals.

4) If there is a significant difference between the recorded level and the allowable level check with your local tech rep. If he tells you to remove the mast and degauss it tell him it won’t do any good as one hour of flight will re magnetize the mast. Now, ask him once again what action should be taken.



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The Cat

Lu,

Forgive my ignorance - I don't know one end of a helipcopter from another. Whats a Jesus Nut - If it falls off do you say "Jeeeeeeeessssssssssus....." :) :)

GV

Thats exactly what it means!

I think you are allowed to say 'BUGGER' as well!

Come on guys, let's get back on thread which is about the potential for residual magnetisim in main rotor shafts.

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The Cat

You cannot be serious??

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TC

Yes, I'm very serious.

If this condition is detected on a helicopter it can reduce the overall reliability of the transmissions due to spark generated erosion of gears and bearings and, If the generated magnetic field is strong enough around the helicopter it can effect the operation/accuracy of the standby compasses and it could possibly effect the operation of your electronic navigation system.

One other way of checking for the presence of a strong magnetic field around the helicopter is to fly it at night and check for a glowing around the tip path plane. If you happen to be a military pilot this is a very bad condition as all your enemy has to do is shoot at the middle of the glowing ring.

What I am looking for is feedback from those of you that have made the check. After all, I could be wrong.

From a pilots perspective the helicopter has a decreased reliability and it may effect his instruments.

From a mechanics perspective he has to change trannies at a rate higher than anticipated.

And, from an operators perspective it is costing more to opwerate his fleet.

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The Cat

Lu,
Stdby compasses are normally checked for accuracy with the Helo running which would allow for any static electricity generated fields.
As for transmission wear, is this theoretical?..... or has it been determined?

Dear Sprocket,

What you say is true however on the 214 the magnetic field was so strong that the two standby compasses both pointed towards the centerline of the aircraft sort of looking cross eyed. They were also very slow to respond and they never would point in the correct direction. Another problem on the 214 was the pilots had difficulty in operating their VOR. Bell stated that it was due to rotor flicker interfering with the radio frequncies. They told the pilots that when they wanted to use the VOR they should slow the rotor down almost to the point that the helicopter would want to fall out of the sky. I don't know if Bell was correct or the problem was caused by the high magnetic field.

The problem stemmed from the fact that on the Bell heads that had elastomeric teeter bearings the rotor head was isolated from the mast by the rubber bearings and there were no bonding straps. It is normal for static to dissipate to the fuselage via the transmissions but with the highly magnetized masts the electrical currents are stronger.

We also found the problem on a 412 rotor head and mast, only in this case, the blades were isolated from the mast by the elastomeric elements in the rotorhead.

I presented this problem on another forum and one individual quoted an engineer friend who said that the oil was an insulator.

Technically it is but oil in a helicopter transmission is not pure oil as it has wear metals in it and that is why you do a SOAP analysis on the oil. Also the oil in its' pure form has what is called a puncture voltage. When the voltage differential is high enough the spark will travel from one point to another puncturing the oil to get there. In order to do that the voltage has to build up much like the spark plug in an engine only, not as high a voltage. This build-up causes the spark to be stronger and effecting the discharge wear to a greater extent.
I have seen an engineering demonstration where a bearing on a shaft suffered from static discharge erosion and the electrical potential was supplied by a small battery that could be used in a Sony Walkman.

Spark discharge erosion is common in transmissions

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The Cat

GulfStream V – for your info the Jesus nut is basically the one that keeps the main rotors connected to the rest of the helicopter, so as you suspect and as others have commented it is a question of “Jeeeeeesus…..” if it ever comes off!!

Lu – I have no experience of the 214 but when the 214ST was introduced into service in 1982 I can confirm that in my operation we had real concerns about both the VOR indications and also the problem of cross-eyed standby compasses. I can’t remember now how we solved the VOR problem but solve it we did not long after it came into service. I have never heard of the Bell recommendation to slow the main rotor down when pilots wanted to use the VOR – how could you do this when the normal operating rotor speed is 100% and the minimum permissible in powered flight is 99%?

As far as the cross-eyed standby compasses are concerned we tied this down to two problems, one of which was acknowledged by Bell in the Flight Manual and was that the standby compasses would be in error if windshield heat was on – and the amount was considerable, about 30-35 degrees. We found out ourselves that the other cause was that the normal operation of the battery charger monitors affected the readings. As these cycled on and off there were wild fluctuations in standby compass readings, again in the order of 30-35 degrees. When we were trying to eliminate the problem, for test purposes we deselected the battery charging system and then the problem went away. IMHO the standby compass problem has nothing to do with residual magnetism in the main rotor shaft. I have never seen any problems with instruments which have not been cured by means other than any anti-magnetism remedy.

I can confirm that the 214ST had a static problem because the elastomeric bearings isolate the rotor head from the mast and that there are no bonding straps. This static caused endless problems with the standard factory fitted GNS 500 VLF area nav system – and this was why in the North Sea with its very regulated track system the 214ST was seen by radar controllers to meander vaguely toward its destination whilst Decca equipped aircraft drove (usually anyway) in straight lines. We attempted to fit Decca but the static problem precluded this – thankfully the Trimble GPS worked like a dream when it eventually replaced GNS 500.

I have never seen the tip path plane glowing at night in the way you describe, but on many occasions at night, if atmospheric conditions were right, you would see the whole rotor plane flickering with what I believed to be St Elmo’s fire. At least no-one was shooting at us though!!


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Quickly; bring me a beaker of wine, so that I may wet my mind and say something clever. (Aristophanes)

[This message has been edited by Speechless Two (edited 05 October 2000).]

Dear Speechless,

We discovered the problem on the very first 214s that went into service in Iran. I was manager of Technical Assistance for Bell Helicopter International. On of my service engineers told me of an unusual happening in the hanger. A mechanic was working on the roof of a brand new 214 that had just been delivered from the states. He was carrying a tray of tools and as he neared the mast a small tool was pulled from the tray and it stuck on the rotor mast. At this time, the rotorhead had not been installed. We made several checks with other magnetic metallic components and the same thing happened. We checked the overhaul manual for the shaft and there was a general note regarding residual magnetism on any part run through a Magnaflux or MagnaGlow machine. If A part were magnetized for the check it had to be demagnetized to a specific level. I assumed that they had not performed the degaussing so I had them remove the mast and take it into the shop for degaussing.

The mast was replaced and the rotor installed. They did a track and balance check and took it out for a test flight. It was gone for about 35 minutes and when it returned the mechanic went back up and tried the tool test again. The tool stuck. We brought out a gaussimeter from the shop and it checked out in excess of 10, which is the limit of the instrument. We had about 18 or 20 aircraft that had been recently delivered and they all had the problem. We found the same problem on our AH1-Js The only time on any of them was production flight test.

It is not the magnetized mast that causes the problem of the compasses and possibly other electronic problems. The magnetized mast is only an indication of the real problem. Just imagine if you were flying in the hole in a doughnut. The doughnut is representative of the torroidal magnetic field that surrounds the entire aircraft. The mast sits in the center of this field and as a result it becomes magnetized. This magnetization causes electrical flow problems in the trannie that are not normal. Normally if the rotor is bonded to the trannie there are electrical flows from the rotor to the airframe, which pass through the trannie. In this case the static field can’t pass from the rotor, which causes the high strength torroidal magnetic field resulting in the problems I described above.

Regarding the VOR problem, our pilots tried it several times and they in effect placed the helicopter into aurotation and even in that condition, the blades were rotating bellow the limits for autoratation. Needless to say they never used the VOR. You can imagine if they were flying too low they would have to gain altitude in order to lose altitude which is not a smart thing to do in a military helicopter. Or, for that matter in any helicopter.

The flickering on the rotor disc was most likely caused by excessive static buildup on the rotor blades. On several occasions I have seen late model Cobras that actually had a ring of light at the tip path. The Israelis’ invented a system that took advantage of this static buildup and used it to create formation lights on the top of the blades. Anyone in the formation could see the other guy due to a light ring on the upper blade surface but with no lights under the blade and no light ring around the tip path the helos could not be seen at night. Of course, they could be heard from twenty miles away.

The only thing that will solve the problem is when Bell bonds the blades to the Jesus nut. Of course, I don’t think they will, because I told them about the problem in 1976.

On the other helicopters mentioned in my introductory post the problems wiil also be solved by bonding.


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The Cat

[This message has been edited by Lu Zuckerman (edited 06 October 2000).]

Thanks for that, As I said before I know nothing about Helicopters, but am learning some interesting bits, from these debates.

Keep it up.

GV

To all of you that have responded:

Have you performed the check yet? If so, what were your findings?

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The Cat

OK Lu, I had the opportunity to check a 412 with a Magnaflux field indicator, which I presumed was an oersted/gauss meter.

The helicopter had not flown for app. 8hrs. The readings ranged from about .75 to 1 gauss at all points accessible on the mast from the mast nut to the top of the collective sleeve

The limits indicated in the Bell SPM were 3 gauss or less. So it does not appear to be a real problem for this particular machine.