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Magnetized of Aircraft

Old 18th Oct 2019, 18:27
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Magnetized of Aircraft

Hi all,

Does anyone heard about magnetized of aircraft ?
but what causes magnetizing of aircraft ?
how it would affect the aircraft
and how to demagnetize the aircraft?
Thank you so much
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Old 19th Oct 2019, 10:38
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Certain steel parts of an aircraft can become magnetised such as the flap tracks or parts of the landing gears.
Very often this is cased by lightening strikes.
Dependant upon the location of the part/component it can sometimes have an effect on the magnetic (standby) compass, in other cases there is little or no effect.
I remember one case in a B737 where the standby compass would move by several degrees every time the nose gear was raised.
Dependant upon the size and location of the part that's magnetised you can either use a hand held magnetic yoke (also used for magnetic particle inspection) or in some cases a larger power pack with a large coil of cable.
Often, after demagnetisation a compass swing is required.
In the case of Boeing aircraft there is a demagnetisation procedure in the AMM.

I hope this helps.
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Old 19th Oct 2019, 11:52
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Originally Posted by Webby737 View Post
Certain steel parts of an aircraft can become magnetised such as the flap tracks or parts of the landing gears.
Very often this is cased by lightening strikes.
Dependant upon the location of the part/component it can sometimes have an effect on the magnetic (standby) compass, in other cases there is little or no effect.
I remember one case in a B737 where the standby compass would move by several degrees every time the nose gear was raised.
Dependant upon the size and location of the part that's magnetised you can either use a hand held magnetic yoke (also used for magnetic particle inspection) or in some cases a larger power pack with a large coil of cable.
Often, after demagnetisation a compass swing is required.
In the case of Boeing aircraft there is a demagnetisation procedure in the AMM.

I hope this helps.
Many, many moons ago I was involved in degaussing a whole herd of chipmunks and was surprised at the amount of residual magnetism that some of them had.
We naturally carried out a compass swing on them all after the degaussing process and some required a lot of twiddling on the compensating magnets to get correct compass readings.

One thing to bear in mind is that although the B737 nose gear example above is a good one, if the residual magnetism is in a static component then you will not see it as an obvious movement of the compass and the "deviation error" will be consistent.
So it is likely that the last compass swing and any adjustments carried out would have "dialled out" this deviation.
So simply degaussing alone will actually create an error in the compass readings.

Last edited by mikemmb; 19th Oct 2019 at 12:01. Reason: clarity improvement
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Old 20th Oct 2019, 00:43
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Originally Posted by mikemmb View Post
Many, many moons ago I was involved in degaussing a whole herd of chipmunks and was surprised at the amount of residual magnetism that some of them had.
We naturally carried out a compass swing on them all after the degaussing process and some required a lot of twiddling on the compensating magnets to get correct compass readings.

One thing to bear in mind is that although the B737 nose gear example above is a good one, if the residual magnetism is in a static component then you will not see it as an obvious movement of the compass and the "deviation error" will be consistent.
So it is likely that the last compass swing and any adjustments carried out would have "dialled out" this deviation.
So simply degaussing alone will actually create an error in the compass readings.
You're absolutely correct Mike, that's why I mentioned that after demagnetisation a compass swing is often required.
From memory, I think the Boeing procedure was to use a magnetic compass to check for magnetisation and provided it didn't move when within 3 feet of the demagnetised part, the part was considered acceptable.
The more general rule is that after demagnetisation (after a magnetic particle inspection for example) parts should read less than 3 Gauss.
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Old 20th Oct 2019, 00:52
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Old 20th Oct 2019, 02:03
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I once observed a DC-9 being de- magnitized due to a lightening strike. The airplane was placed on jacks and a very large oval shaped coil mounted on a movable frame was moved over the fuselage several times from the nose to the wing leading edges. I did not get to observe the complete job and did not see how the aft fuselage was dealt with.
I can tell you from experience it is also very important to use Non- Magnetic Fasteners in areas where they are specified in Manuals and drawings.
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Old 21st Oct 2019, 09:43
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a/c magnetized

standby compass uses none steel case lamp, not standard 387
have known the knifes and forks in the trolleys become magnetised due to vibration and direction of travel.
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Old 21st Oct 2019, 14:37
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Originally Posted by B727223Fan View Post
I once observed a DC-9 being de- magnitized due to a lightening strike. The airplane was placed on jacks and a very large oval shaped coil mounted on a movable frame was moved over the fuselage several times from the nose to the wing leading edges. I did not get to observe the complete job and did not see how the aft fuselage was dealt with.
I can tell you from experience it is also very important to use Non- Magnetic Fasteners in areas where they are specified in Manuals and drawings.
If you think that was a big coil consider that the US navy demagnetizes entire ships.
Long time ago heard about a discussion of whether computer disk cartrides were safe inside a ship undergoing the process, consensus was they were but they did take backups on shore during the process.
No affect on the disks, think about all the steel surrounding them deep in the hull.
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Old 22nd Oct 2019, 06:53
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One more question guys
how do you all know the aircraft is magnetized
without a gaussmeter
thank you for all those reply
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Old 22nd Oct 2019, 20:59
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Originally Posted by Danial9874 View Post
One more question guys
how do you all know the aircraft is magnetized
without a gaussmeter
thank you for all those reply
Very often you don't, although the B737 I mentioned is a good example.
You may get a compass deviation, but as mikemmb mentioned this error is very often adjusted out during a compass swing.
Many of the steel parts on an aircraft are subject to routine inspections during overhaul (flap tracks, gears, etc.) Part of this overhaul process is a Magnetic Particle Inspection (for cracking), part of this inspection would be to check for residual magnetism before starting the inspection, and of course would be demagnetised post inspection.
Sometimes if the engineers suspect that a part is magnetised they would ask their NDT guy to have a look as most NDT shops would have a Gaussmeter.
You can check for magnetisation using an off the shelf magnetic compass, or the cheap method (that works surprisingly well) is to string 3 paperclips together and drag the last one over the part you suspect is magnetised, if you see it "stick" to the part it's magnetised.
A basic Gaussmeter only costs around 150 USD for a calibrated example and about 80 USD for an uncalibrated one so it's not a particularly expensive process.
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Old 22nd Oct 2019, 23:58
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Originally Posted by Danial9874 View Post
One more question guys
how do you all know the aircraft is magnetized without a gaussmeter
Usually there is a discrepancy with a compass based navigation system which leads to finding a magnetized parts. Unless you're inspecting an aircraft after a known lightening strike I know of no requirement to check aircraft regularly for magnetism.

For example, we a BO-105 helicopter with an ongoing nav issue that could not be fixed.
It wasn't until we removed an entire working compass system from one aircraft and put it into the suspect aircraft that we found the steel tail skid was highly magnetized. The remote compass (flux valve) for the nav system was mounted in the back of the tailboom to prevent interference from the aircraft structure. Unfortunately, that flux valve location was right above the tail skid.
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Old 23rd Oct 2019, 22:46
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Some 35-40 years ago we had a Gardan Horizon GY80 in the hangar with a massive compass indication problem. The chief engineer wrapped the fuselage with the extension cables of two 2kw heaters all day (and it was summertime) in a bid to degauss it. Didn't work as far as I remember.
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Old 24th Oct 2019, 22:57
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Originally Posted by stevef View Post
Some 35-40 years ago we had a Gardan Horizon GY80 in the hangar with a massive compass indication problem. The chief engineer wrapped the fuselage with the extension cables of two 2kw heaters all day (and it was summertime) in a bid to degauss it. Didn't work as far as I remember.
That would only have a (slim) chance of working if the cable/path was split so only one current carrying leg was wrapped around the fuselage, else the current induced magnetic fields cancel.

The other problem is that degaussing works by saturating the steel (both ways due to AC) then gradually reducing the strength, in the case of passing an item through the coil this happens as the distance increases.

More than a few seconds at full intensity is unlikely to change things, a sudden switching off could leave parts more magnetised than at the start depending on where in the 60/50Hz cycle the break happened.
BTW: Degaus would not work with DC in the coils.
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