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While the card is in the banked attitude, the vertical component of the earth’s magnetic field causes the compass to dip to the low side of the turn.. In the Northern Hemisphere where t

Permanent Magnetism

► Certain materials can be magnetized, By putting them in a magnetic field After removing the field remnant magnetism will be kept

S

Electro Magnetism

► Around a conductor that conducts electrical current a magnetic field is also generated.

Earth magnetism

The earth can be seen as

a giant magnet The magnetic poles

however do not align exactly with the

geographic poles.

The exact place of the

True North Magnetic north

Variation

The variation is not a function of the geographical position, But can vary due to changes in the earths rock formations Lines of equal variation around the globe are so called

isogonics lines.

Variation must always be

added to the magnetic

heading to get the true

heading!

Variation is positive if the

true north is to the

right (east) of the

magnetic north Else it

Inclination (magnetic dip)

The direction of the magnetic force

at a certain point on earth is not

always exactly horizontal Only

at the equator At the poles

they are nearly vertical The

angle of force line with the

horizontal is called the

inclination or dip angle.

A magnetic compass uses only the

horizontal component of the

Inclination

Inclination card

The strength off the earth magnetic field is not constant on all places on earth The strength of the magnetic field is measured in oersed In the Netherlands the earths magnetic field is about 0.471 oersted

Compasses

Aircraft compasses

There are tree kinds of aircraft compasses.

1 Direct-reading compasses or standby

of the scale and the magnet assembly is well below the pivot point So that the compass will mainly react to the horizontal component off the earth magnetic field The liquid is sealed off by a membrane to compensate for volume differences due to temperature changes Underneath the compass is a compensator unit

to allow adjustments to the compass

The compensator unit

There are two compensators inside the compensator unit One for

Longitudinal axis off the aircraft and one for the cross-axis Each

compensator consists of two bars that can be turned in opposite direction In each bar two small compensator magnets are mounted.

In neutral position the magnetic forces

of these magnets oppose each other, giving a resultant field of zero When the bars are turned, the resultant force will be added to the

N

Earths field Compensator field

Direct-reading compasses errors

There are two main errors involved in using a

direct-reading compass in an aircraft.

1 Northern turning error.

2 Acceleration error

Northern turning error

The compass card is mounted so that its center of gravity is well below the pivot point When the aircraft is in a banked turn, the card also

banks because of centrifugal forces While the card is in the banked attitude, the vertical component of the earth’s magnetic field causes the compass to dip to the low side of the turn The error is most apparent when turning trough headings close to north and south When the aircraft makes a turn from a heading of north, the compass briefly

indicates a turn in the opposite direction

Passing trough south, the compass leads the turn considerably As the aircraft nose passes though the west, the compass should

approximate the correct heading Then, as the aircraft nose approaches north again the

compass lags.

Acceleration error

A second way to tilt the compass card out of the horizontal plane is to accelerate or decelerate the airplane With the card mounted with its

CG below the pivot, acceleration causes the card to tip forward In the Northern

Hemisphere where the magnetic field has a downward component, this causes the north- seeking tip of the compass needle to swing downward.

Other compass errors

Besides the dynamic errors (northern turning error and acceleration error.) and the errors caused by the aircraft it self (for witch we will calibrate the compass.) there are some minor faults in direct-reading compasses.

► Scale faults caused by an unjust calibration of the scale.

► Collimination faults, caused by a difference between the magnetic axis of the compass and the north

of the scale.

► Lubber line alignment error.

► Liquid swirl error If there is not enough room in the compass housing, the scale will be turned with the aircraft, due to the viscosity of the compass fluid

The first 3 errors will be eliminated when the A error is adjusted during the compass swing.

Checking a direct reading compass:

•Pivot friction test: Pivot friction test: With the use of a small magnet, make the compass rotate

10° keep it in this position for 10 seconds Remove the magnet an let the

compass come to a rest Note the heading Do the same towards the other side Again note the heading The difference between the two observations should not

be more than 2 °.

•Damping test: Damping test: With the use of a small magnet make the compass rotate 90 °

Keep it in this position for one minute Take the magnet away The compass

should turn back with in 5 ° of its original heading with in 10 seconds.

The remote reading compass

The remote reading compass, in Fokker aircraft called the AHRS (or IRS, but they do not work with the

earths magnetic field), is basically a directional gyro system connected to a flux valve To adjust the gyro

heading indication automatically.

The flux valve is normally mounted in an area as free as possible Magnetic disturbances Mostly in the wing tips.

On Fokker aircraft the flux valve is mostly indexed That means that the flux valve can not be turned, to remove

a possible A error This indexing is a factory setting.

The flux valve consists of a double 3 spoke transformer with in each Spoke a pick off coil and one exciter coil per pair of spokes The exciter coil saturates the spokes, so that static magnetic field

in each of the spokes is transformed to a alternating field.

The strength of this field is a function of the earths magnetic field lines passing trough the length of the spoke Hence the amplitude of the induced voltage is a function of the position of the spoke in the earths magnetic field (and the intensity

off the field)

The electric compensator

The flux valve can be electrically compensated for B and C errors If a small DC current is

Applied in the sense windings of the flux valve This current should generate a magnetic field that is equal in strength as the field that caused the disturbance but opposite in direction.

800Hz Output

DC source Flux

valve

The Remote Compensator unit

The variable DC power source in the previous slide is

located in the remote magnetic compensator unit.

On the unit, you find also some test points to measure the

DC voltage applied to the flux valve and a

potentiometer to adjust the voltages After successful

adjustment of the compass system, the voltages

should be measured and written down on a sticker on

the remote compensator unit.

This way a remote compensator unit or flux valve can be

replaced without the need to do a new compass

swing Only the voltages should be taken over from

the old compensator unit.

Inside the remote compensator unit is also a circuit that

transforms the tree wire output of the flux valve into

2-wire sine/cosine signals And a monitor circuit that’s

checks the functioning of the flux valves and the

remote compensator unit.

Flux valve errors

Principally the flux valve suffers also from the northern turning error and acceleration errors just as the standby compass does But because the reading of the flux valve output is stabilized with a directional gyro, The readout is much more reliable

Aircraft errors

Due to the fact that aircrafts contain a certain amount of iron and other magnetic material and there are

a lot of life wires in an aircraft The aircraft itself will generate a magnetic field that will disturb the precession of the compass systems These aircraft errors or static errors we can divide into tree categories; Index errors, one cycle errors and two cycle errors.

Static errors

Index errors One cycle errors Two cycle errors

Longitudinal Cross

Adjusting the compass

We have to adjust an compass for A, B and C faults E and F faults are generally minor and are not compensated

The A fault can be compensated by turning the compass or flux-valve in horizontal direction.The B fault is compensated by the E-W adjustment of the compensator

And the C fault is compensated with the N-S adjustment of the compensator

C

B

Total deviation

The total deviation caused by the aircraft should be

compensated by means of a

► A-fault correction that is linear over the whole compass range

► B-fault correction, that acts as a cosine function over the compass range

► C-fault correction that’s a sine function

With the B and C correctors we only adjust the amplitudes of the sine and Cosine curves The total Compensation is the sum of the A-fault, B- fault and C fault corrections This total correction should compensate for

0° 90° 180° 270° 360 °

The compass swing location

A compass swing must be carried out on an approved site!

buildings containing electrical power generation

There are two compass swing site classifications

Periodic re-survey

Swing site’s at Schiphol

There is only one compass swing site at

Amsterdam! This is the fairway near the Martinair hangar’s.

The site near the old Fokker buildings is not surveyed and contains a lot of iron in the ground.

The Compass swing

A compass swing contains two parts

adjusted

deviation table that is mounted in the aircraft

Note that with a Fokker 100 with IRS systems, the IRS may be used as reference

Else a approved prismatic landing compass mounted on a tripod should be used as a

reference

Keep in mind that, when standing in front of the aircraft with a reference compass, the

Window stile Tail

To read the reference compass, position the compass so that the window style and the vertical stabilizer line up with the compasses lubber line Make sure the

reference compass is adjusted horizontally before reading.

Prism

Preparing the compass swing

Before a compass swing is undertaken make sure the following preconditions are met:

Make sure you have the following equipment with you

The readout on the aircraft compass

Start at North

Example calculation

The calibration swing

After the correction swing is performed, a calibration swing should be made The calibration swing is to verify the correct adjustment of the compass and to make a deviation list and deviation cards, to make the crew aware of the remaining deviation errors

We are making a full 360 degrees swing with 30 degrees interval On every interval we note the deviation of the standby compass and the two AHRS systems and the standby

compass in emergency power condition

For the standby compass the maximal deviation may be 10 degrees but between successive (30°) steps the difference in deviation may be 4° maximum!

The maximum deviation for the AHRS may be 2°!

After the calibration swing a deviation form should be filled in for tech records and deviation cards should be made for in the cockpit

The pylorus method

With a normal compass swing a master compass is

used as reference As an alternative a pylorus

can also be used if the compass swing site is

approved for this method.

A pylorus is an instrument with witch we can very

precisely measure a angle between two

points In our case the longitudinal axis of the

aircraft and a known point at some distance

away.

The angle between this known point and the

aircraft heading is predetermined for all

compass headings used during the compass

swing

Before we start using the pylorus, we have

to align the instrument with the aircrafts

longitudinal axis This is done by putting

the instrument on a known spot (cockpit

side window), Level the instrument and

then point the instrument exactly to the

wing tip and adjust the bottom

reference ring of the pylorus to 240.25°

(F50) as given on the job card

Now the zero should be exactly in line with

the aircrafts longitudinal axis

After this we can shoot for every 30°

heading a suitable target from the

target list By subtracting the value

given on the target list , With the actual

readout on the pylorus scale, the datum

heading can be calculated

Subtract the readout heading from the

datum and you get the deviation

Fixed distance 32cm

N 330

α

α Value on target list

Actual value

Fine adjust

Horizontal adjustment

Deviation scale

longitudinal axis adjustment ring

longitudinal axis adjustment ring Fixing Bolt

Clamps to fix

pylorus to aircraft

window structure

Leveling bubble

The End