Magnetism and Compasses

[Cron]

In my TRS notes I have:

'Magnetic compasses will become totally unreliable following a Lighting strike'.

What I can't make a connection with is that Magnetic compasses are all to do with magnetism and Lightning is all to do with electricity.

Is it that a lightening strike on an a/c turns the a/c into a magnet?

Can't be so shirly - a/c is aluminium. I guess I'm missing something. Grateful for some help.

Mod: I know this may be in the wrong place but r/heads seem to be very knowledgable and have better sense of humour than other forums. May it stay for a bit?

[Three Blades]

Very simplistic answer:

1) There will be some steel (or other magnetisable metal) in the aircraft.
2) Passing a current through magnetisable metal causes it to turn into a magnet (remember the experiments at school with a nail, battery and some wire)
3) Magnet + compass = error

[NickLappos]

Cron,

The tie between electricity and magnetism is what made 19th century physics!

If you pass a small current at a zillion volts thru a magnet, it will probably become re-magnetized along a new axis. If you pass that thru a non-magnet, it could become magnetized. Magnetics is a manifestation of the electron arrangement in a conductor, and is directly tied to the electrical environment around the magnet.

Can I suggest several different popular books on the development of this stuff? Faraday and others really captured the concepts. Try "Empires of Light" http://www.amazon.com/exec/obidos/tg/detail/-/0375507396/102-1678628-7275326?v=glance

[Cron]

I'm ok with the ferrous metals getting magnetised but all the magnets I've ever known well have little influence beyond a few inches.

Am i correct in thinking that only cockpit-present ferrous materials will influence the wet compass .. and then they have to be close?

[Matthew Parsons]

Following replacing a landing gear on the H46, a compass swing was required. That's because the gear was about 4 feet away from the flux valve. Quite often, the compasses couldn't be adjusted within limits, so the solution was to strike the landing gear with a mallet.

Ferrous material is made up of "domains" that each have strong natural magnetism. They usually sum up to a very small magnetic field. When the material is placed in a slowly increasing magnetic field, each domain tries to line up with the field. They're packed in quite tight, so they can't actually line up, but with enough energy they can rotate just a bit, bringing more domains closer to alignment with the magnetic fields. If you listen carefully you can hear ticks when the domains align (try this with some sound amplification). When the magnetic field is removed, there isn't energy to realign randomly so the material remains magnetized. The domains don't like to be lined up because then you get all those north poles lying beside north poles, trying to push apart. All you need to help them move is a little bit of energy....THE MALLET!!

The last bit didn't relate too well to the original question, except that small amounts of magnetism on the airframe can cause compass errors. Following a lightning strike, components can be magnetized, creating errors. Typically you use low voltage and high current to create an electromagnet, but the high energy in a lightning bolt with the relatively small magnetic field around it is enough to magnetize some material.

[Three Blades]

Two points, either of which will cause a deviation of the compass

1) Think about the compass itself.
If it is exposed to a vast magnetic field that a lightning strike will create, it may itself become remagnetised in a different orientation (as per Nick's comments).

2) The magnet in a simple cockpit speaker will effect compass in normal situations, hence the need to have a compass swung and deviation cards created whenever substantial new equipment is added. Therefore, if the gearbox suddenly becomes a magnet, even a weak one, this will have a marked effect.

A secondary effect of point 2 is that you need to make sure that gear components have not become magnetised or else they will render the chip detectors ineffective as the chips may cling to the gear itself and so not get picked up.

[cl12pv2s]

Hello,

In addition to what has been said already, I'd like to add / clarify a couple of points.

After a lightning strike or nearby, the actual compass is normally functioning fine...i.e. it is still magnetised along the normal axis of the 'needle'.
(Some older marine compasses have been known to have their polarity completely reversed, but the axis is still the same.)

The problem with the compass error after the lightning strike is usually due to the 'ferrous' metals around (in the cockpit and on the aircraft) being magnetised. The mechanism for this happing has been explained already.

To answer Cron's comments:

All magnetic objects near or far will affect your aircraft's compass.

Magnetic force between two objects is inversely proportional to distance between them. So if you increase the distance, the magnetic attraction / repulsion decreases. (I forget the formula.)

Bearing this in mind, the relationship between magnetic objects and the compass deflection is simply explained

Edited: Or rather....let me try to explain it simply!

If an object's magnetic strength is greater than that of the earth's, then it will cause a deviation from magnetic North. While individually, they may not be enough, an accumulation of weak magenetic objects in one area of the aircraft may be enough to influence the compass. Also, in particluar orientations, the lines of magnetic flux may be enough to cause an error.

This is a simple explanation, but might help.

Regards,

cl12pv2s

Nick: Thanks for the book suggestion...I'm going to order it tomorrow!

Some futher reading I found...

Lightning strikes are due to flow of charges between the ground and the clouds. The flow of current creates a magnetic field around it and the direction of the field is given by what we called the right-hand rule: point the thumb of your right hand in the direction of the current, and your fingers indicate the direction of the circular magnetic field lines around the current flow. If we imagine that the lightning flows through a straight wire that is connected from the clouds to the ground, we can use the Ampere's law or the Biot-Savart Law to find the magnetic field due to a steady current flowing in this very long straight wire. Let us assume that the wire is infinitely long. For an infinitely long straight wire, the magnetic field at a point a distance r from the wire carrying a current I has magnitude given by B = µ0 I / 2pr where B is the magnetic field strength (units in tesla) and the constant µ0 is the permeability of free space with a value of 1.26 x 10-6 tesla metre ampere-1. The current in an average lightning bolt is as high as 30,000 amperes and its short duration of about 0.1 seconds might just deflect the compass needle momentarily. The Earth's magnetic field strength is about 0.6 x 10-4 tesla on the Earth's surface at the poles. It is about 0.3 x 10-4 tesla on the Earth's surface at the Equator. Let us also assume that the compass is placed such that the magnetic field created in a lightning is perpendicular to the Earth's magnetic field and that a magnetic field strength of about 10% of the Earth's magnetic field strength is able to cause an observable deflection in the compass needle. Using the Earth's magnetic field strength of about 0.6 x 10-4 tesla and the formula for B given above, we get a value for r =1002 m. This implies that if the compass is at the position as assumed above, there should be an observable deflection of the needle even at a distance of about 1 km from the lightning bolt, though only momentarily.

Source Webpage (http://www.science.edu.sg/ssc/detailed.jsp?artid=6099&type=6&root=5&parent=5&cat=57)

[Cron]

I]If an object's magnetic strength is greater than that of the earth's, then it will cause a deviation from magnetic North. While individually, they may not be enough, an accumulation of weak magenetic objects in one area of the aircraft may be enough to influence the compass.[/I]

That's explained it .. they all gang up. I understand. Thank you all - much appreciated.

[ChopperSparks]

Am i correct in thinking that only cockpit-present ferrous materials will influence the wet compass .. and then they have to be close?

Standby compasses contains ferrous material.... the compensation adjustment magnets. I suppose a lighting strike could "remagnetize" them.