DRC or standby compass Primary function: show magnetic heading Nowadays: heading reference instrument, relegated to standby role Its carriage in all types of aircraft is still a mandatory requirement of JAR
MAGNETIC PROPERTIES 3 principle properties of a simple permanent bar magnet: – It attracts other pieces of iron and steel – Its power of attraction is concentrated at each end of the bar – When freely suspended it always comes to rest in an approximately north-south direction (If displaced from that direction, it will return to same alignment always)
Two poles: RED : North seeking pole (south pole) BLUE: North pole– Like poles will repel and unlike will attract– A magnetic field of influence surrounds the magnet (reflected in lines of force that may never be broken and will never cross each other)– A magnetic field is strongest where the lines of force are closest together (close to poles)
Hard iron and soft iron HARD IRON: magnetic materials difficult to magnetise, but once in a magnetised state, retain magnetism for long periods of time – Coercive force: resistance to magnetisation and demagnetisation SOFT IRON: metals easily magnetised. They lose magnetised state once the magnetising force is removed.
Magnetosphere: shields the surface of the Earth from the chargedparticles of the solar wind. Generated by electric currents located inmany different parts of the Earth. Compressed on the day (Sun) sidedue to the force of the arriving particles, and extended on the nightside. (Image not to scale.)
Terrestrial magnetismEarly measurements of magnetism showed the existence of the Earth’s magnetic field (weak) and the existence of strong: – Magnetic Pole under surface on Northern H – Magnetic Pole on SHThe positions of this poles are not permanent, they are slowly moving. ( from 2007 it is proceeding quicker and to Siberia: 55km/year)
Terrestrial Magnetism The position of the Magnetic North Pole is 84.742ºN 129.077ºW, North of Canada Source: NOAA (2010) The Magnetic South Pole is 64.049ºS 137.227ºE , south of Australia Source: NOAA (2010)
The source of the terrestrial magnetism, based on observations in the space surrounding the Earth, seems to be a fairly short magnet, located close to the centre of the EarthA plane passing through the magnet and the centre of the Earth would trace and imaginary line on the Earth’s surface called a magnetic meridian
Terrestrial Magnetism The magnetic poles are not antipodes The lines S to N represent the direction of the Earths magnetic field The lines of force will be seen that they do not always coincide with the Earths horizontal.
Difference Earth’s MF – bar magnet Its points of maximum intensity are not at the magnetic poles, but at 4 other positions (magnetic foci) Two of them are close to the poles
VARIATION Variation is the angle between TN and MN and is measured in degrees East or West from the TN (0º-180º) Isogonals are pecked lines on a map or chart joining places of equal magnetic variation Agonic Line is the name given to isogonals joining places of zero variation
Magnetic field is not regular. There are a large number of local irregularities (magnetic anomalies) (ferrous nature of rocks disturbing Earth’s magnetic field) – Large changes in VAR over very short distances) Nowadays the most important anomaly being studied is the South Atlantic one. (area approx 5,000,000 km2) near Brazil coast – Region at which the magnetic field is being weakening.
ANGLE OF DIP The Earth’s lines of magnetic flux are not horizontal, don’t lie parallel to the Earth’s surface at all points. The Angle of Dip is the angle in the vertical plane between the horizontal and the Earths magnetic field at a point
The Magnetic North Pole is the position on the surface of the Earth where the dip (or inclination) is plus 90 degrees The Magnetic South Pole is the position on the surface of the Earth where the dip (or inclination) is minus 90 degrees
Angle of dip Isoclinals are lines on a map or chart joining places of equal magnetic dip Aclinic Lines is the name given to isoclinals joining places of zero dip. Dip may be described as magnetic latitude: – Zero at the magnetic equator – 90º at the magnetic poles
Earth’s total magnetic force The direction of the Earths magnetic field at any point may be split into its Vertical and Horizontal components If the value of dip angle is ϴ: H = F cos ϴ Z = F sin ϴ
When moving from the Magnetic Equatortowards one of the magnetic poles: - F will increase - H will decrease because of dip - Z will increase
Aircraft magnetism Challenge to designers: – DRC must be located where pilot can readily see it – DRC in the cockpit is surrounded by magnetic material and electrical circuits – Such magnetic influence provides a deviation force to the Earth’s magnetic field: compass needle will not point to the local meridian
Aircraft magnetismSuch magnetic influences may originate from:- components of the aeroplanes structure,- items of the traffic load, cargo and passengers baggage- items placed near to the compassDeviation caused by a/c magnetism can be analysed and errors can be minimised
Aircraft magnetism Deviation is the angle measured at a point between the direction indicated by a compass needle and the direction of Magnetic North It is termed East or West according to whether the Compass North lies to the East or West of Magnetic North.
In order to discuss the causes of deviation in detail, the magnetic properties of an aircraft are divided into those that are caused by: – Hard iron magnetism – Soft iron magnetism
Hard magnetism in aircraft Is permanent in nature Caused by steel components used in its construction Such components are difficult to magnetise but once magnetised, hold their magnetic field for a long time.
Hard magnetism in aircraft This magnetism has its origin in components permanently installed in the aircraft. So: Direction and force of hard magnetism relative to the compass position will be the same for all attitudes and headingsHardened steel materials used in the engines, the fuselage and in bolts and nuts all over the a/c
Hard magnetism in aircraft Distance between magnetic steel component and sensitive part of the compass is importantThe force of the field is reduced by the square of the distance from the magnetic source
Hard magnetism in aircraft To study their influence on the sensitive part of the compass, all hard iron sources are considered simultaneously, based on the direction and force of the magnetic field they produce in the position where the sensitive unit of the compass is installed.
Hard magnetism in aircraft HARD IRON DEVIATION FIELD COMPONENTSComponent Aircraft axis along Positive direction which the component of component acts P Fore-aft axis Forward Q Lateral axis Starboard Z Vertical axis Downward
Vertical hard iron magnetism In straight and level flight: If compass needle is kept horizontal, the directive force from the vertical magnetic field will not cause deviation. It will cause dip or raise of one of its ends If not: turn/nose up or nose down the vertical magnetic field no more vertical with respect to compass card Its horizontal component will cause deviation.
Vertical hard iron magnetism Whenever the aircraft is banked or pitched, the deviation values are likely to change These changes are predictable but as they vary rapidly, it is not common to record them for cockpit use. Deviation effects in a turn are more complex due to sideways acceleration making the compass card to leave the horizontal
Soft magnetism in aircraft Soft iron magnetism is referred to metal easily magnetised but that will lose its magnetism with same facility. It is temporal induced magnetism due to the Earth´s magnetic field, acting as a focus for it and causing a localised intensifying of that field.
Soft magnetism in aircraft It is due to the Earth´s field which gives the components of the soft metal a variable magnetic value which depends on the forces H and Z
Soft magnetism in aircraft The strength will therefore vary with the relative direction of the Earth’s field and the strength of the Earth’s field. So: It will vary with heading, attitude and position Its effects are analysed by considering the equivalent effect from a series of imaginary soft iron bars aligned horizontally (h bars) or vertically (z bars)
Horizontal soft magnetism Magnetic intensity and polarity of the h bars will vary directly with the strength of H (Deviation force will increase as H increases) As magnetic latitude varies, the directive force of the compass also varies directly with H. both effects tend to cancel each other and deviation doesn’t change with latitude
Soft magnetism in aircraft Magnetic intensity and polarity of z bars vary with Z component of the Earth’s field.(As Z increase, the strength of the deflecting force will increase) As magnetic latitude varies, the directive force of the compass also varies directly with H. Deviation due to z bars changes as Z/H Polarity inverts when crossing the magnetic equator
X’ = X + aX + bY + cZ + P Y’ = Y + dX + eY + fZ + Q Z’ = Z + gX + hY+ kZ +R
Soft magnetism in aircraft Effects and strength of their magnetic field around the compass are easy to calculate but difficult to compensate for in isolation. They are most often only recorded as a part of the total deviation registered during a compass swing.
Deviation coefficients•Before compass swing (check on compassaccuracy and documentation of deviation values)deviations caused by aircraft magnetism onvarious headings must be determined•Values of deviations are analysed intocoefficients of deviation
Deviation coefficients•Coefficient A: constant on all headings. Due to misalignment of the compass lubber line.•Coefficient B: results from deviations caused by P+cz (deviations vary as the sine of the heading. Max on E/W•Coefficient C: results from deviations caused by Q+fz (deviations vary as the sine of the heading. Max on N/S
• C.A = DevN + DNE+DE+DSE+DS+DSW+DW+DNW 8• C.B = Dev East – Dev West 2• C.C = Dev North – Dev South 2• Total deviation = A + B sin (hdg) + C cos (hdg)
Compass swing Special calibration procedure To determine to which extent compass readings are affected by aircraft hard and soft iron magnetism. – Deviations may be determined – Coefficients calculated – Deviations compensated
When must a compass be swung New a/c from manufacture or New compass fitted Periodically or when specified in maintenance schedule After major inspection change of magnetic material in the a/c a/c moved involving a large change in latitude Lightning strike Same heading for more than 4weeks Carrying ferrous freight For issue of a C of A Any time when compass or residual deviation in compass card is in doubt
JAR LIMITS JAR 25 for large a/c requires a deviation card (level flight with engines running) near the instrument Placard must show each MH readings not greater than 45º steps Deviations greater than 10º not allowed in any heading after compensation Distance compass-mag. material: no dev > 1º (same for electrical equipment when ON)
JAR LIMITS Use of undercarriage or flight controls: no dev >1º Coefficient B/C not >15º(DRC) or 5º(RIC) Greatest dev on any heading after correction: • DRC:3º • RIC:1º
Compass swing includes:Preparations: including finding suitable location Correction swing: establishing the presentdeviation on selected headings and correcting as much as possible. Check swing: registration of residual deviation on as many headings as required. Producing a deviation curve: based on the residual deviation values found on check swing Filling out the deviation card: for presentation in the cockpit
Deviation Card This Card shows the pilot what corrections need to be made to the actual magnetic compass reading in order to obtain the desired magnetic direction This correction usually involves no more than a few degrees
Dynamic Errors of the Compass Turning and Acceleration Errors
Dynamic Errors of the Compass• Dynamic errors of the magnetic compass will occur when the aircraft turns, accelerate or decelerate• This type of errors will occur because the compass card CG is located below the compass card suspension point
Dynamic Errors of the Compass• When accelerating, decelerating or turning the aircraft, the compass card CG will be displaced and the compass card will be tilted• When tilted, the vertical force will cause the compass card to rotate and present a false heading
Dynamic Errors of the Compass• The magnetic compass card CG is below the point of suspension in order to keep the compass card horizontally stable during level flight• The vertical force of the Earth’s magnetic field will pull the north pole of the compass needle down on the northern hemisphere and up on the southern
Dynamic Errors of the Compass• This effect will partly be balanced by the low position of the centre of gravity• The exception is at the Magnetic Equator, were there will be no such effect
Dynamic Errors of the Compass• A swirl error will occur when the liquid in the compass is not turning at the some rate as the aircraft• The liquid will indicate a too large heading change after the turn has been completed
Turning Errors• Turning error is most apparent when turning to or from a heading of north or south• This error increases near the poles, due to the magnetic dip and the vertical component of the Earth’s magnetic field• There is no turning error flying near the equator
Turning Errors• When turning from a northerly heading in the northern hemisphere, the compass gives an initial indication of a turn in the opposite direction• It then begins to show the turn in the proper direction, but it lags behind the actual heading
Turning Errors• The amount of lag decreases as the turn continues, then disappears as the aircraft reaches a heading of east or west• When turning from a southern heading, the compass gives an indication of a turn in the correct direction, but it leads the actual heading
Turning Errors• This error disappears at east and west headings• You must lead the roll-out for turns to north• And lag the roll-outs for turns to south
Turning Errors• To compute lead or lag in the roll-outs, use the local latitude plus or minus the normal roll-out lead (one-half the bank angle)• Example: At 35ºN a right turn to north requires a roll-out point of 318º(360-35-7) (42º for LT)( and a right turn to south 208º(180+35-7) (152º for LT)
Acceleration and Deceleration Errors• Magnetic dip causes the acceleration and deceleration errors, which are fluctuations in the compass during changes in speed• In the northern hemisphere, the compass swings toward the north during acceleration and toward the south during deceleration
Acceleration and Deceleration Errors• This error is most pronounced when you are flying on headings of east or west and decreases gradually as you fly closer to a north or south heading• The error doesn’t occur when flying directly north or south headings
Acceleration and Deceleration Errors• The memory aid: ANDS (Accelerate North, Decelerate South)• In the southern hemisphere, the error occurs in the opposite direction (accelerate south, decelerate north)