Description
Object description
Training film for pilots and navigators, with animations, photographs and commentary. The film starts with the theory of the earth's magnetic field and how it is utilised to establish the position of an aircraft by means of the magnetic compass. As an alternative, celestial navigation employing the Astro compass (in conjunction with the Sextant if required) may be employed. The film then proceeds to show how the theory is put into practice.
Content description
Reel 1
"Introduction and theory": The film opens to view of a Chinese compass, invented four thousand years ago, when it was realised a freely pivoted magnet will always point in one direction. The two poles of a magnet seek north (red), and south (blue). The forces of attraction and repulsion are explained: red/blue attraction, blue/blue repulsion. The earth acts like a gigantic magnet: description magnetic lines of force – diagram of globe and lines of magnetic force emanating from the north (N) and south (S) poles. A freely pivoted magnet will align itself with one of these lines. In reality these lines of force are much more complicated due to external distortive effects, as seen in the diagram. The magnetic meridian is explained: a freely mounted magnet will align itself with the magnetic meridian. The magnetic heading for an aircraft is determined by a compass to locate the lines of force. Cut to view of aeronautical compass located in the cockpit. The lubber line is explained, and ideally is the direction of flight of the aircraft. A diagram shows the magnetic heading as the angle between the lubber line and the north pole of the compass.
The magnetic heading is distorted by the metal used in the construction of the aircraft. Additionally, the lubber line may not be true if the compass was incorrectly installed and not aligned, thus the bearing read off the compass may not be the true heading, instead it is called the compass heading. Deviation is the term used to indicate the sum of these two errors – illustrated in a diagram. The application of the deviation correction is explained and numerical examples given. This value of deviation is constant on all aircraft headings, and may be corrected by shifting the lubber line to match the fore and aft axis of the aircraft. More diagrams seen during the explanations. Because the deviation due to the compass needle is different for a change in aircraft heading, it is necessary to determine the effect of the aircraft itself upon the compass needle.
The compass needle may be made from hard or soft iron. Explanation given with the aid of a diagram. Hard iron produces magnets that are permanent magnets and do not require an external field for their magnetisation. Soft iron depends upon it's position relative to the lines of force of the earth's field for it's magnetisation; these are temporary magnets. There are both permanent and temporary magnets in an aircraft. The permanent magnetisation may be regarded as fixed permanent magnet within the aircraft: with the aid of a vector diagram, the field is resolved into three forces, P, Q, and R. Explanation given. The lines of force for temporary magnets cannot be resolved as before because the induced magnetisation varies as the aircraft's heading. This soft iron 'bar' (the aircraft) or magnet can be replaced by, two adjustable, soft iron bars within the aircraft, one horizontal and one vertical. Explained with another vector diagram introducing three forces, k, c, and f.
The effect of these forces upon the aircraft's compass is further explained – it is necessary to study the animated diagrams closely as the narrative proceeds.
"The concept of co-efficient C": A hypothetical permanent magnet lies along the axis of the wings: as the aircraft rotates (around the four compass points) the deviation is a maximum when flying north and south, and zero flying east and west. This deviation is symmetrical about the fore and aft axis of the aircraft and is known as co-efficient C.
Content description
Reel 2:
"Co-efficient B": With reference to the earlier vector diagram, the concept of co-efficient B is explained. Coefficient C plus coefficient P gives coefficient B. A very detailed and extensive narrative explains – with the essential animated diagrams – of what is a complex rather than a complicated system.
"The micro-adjuster": The purpose of the micro adjuster is to eliminate coefficients B and C, the fore and aft magnetisation of the aircraft. A detailed explanation of the principles of the micro-adjuster and it's operation is given.
"Correction for co-efficient C": To correct for athwartship magnetisation. The adjustment is explained. This method also applies when eliminating co-efficient B.
"Correction for co-efficient B": To correct for fore and aft magnetisation: explained.
"Correction for coefficient A": The effect of a displaced lubber line is explained. The lubber line must coincide with the fore and aft magnet axis of the aircraft, if not an error will be introduced in each heading. The movement of the lubber line to correct for the error is known as coefficient A and is constant for all headings.
"Determination of coefficient A": Explained; magnetic headings are taken on the four cardinal points and the four quadrilaterals; coefficient A is the mean value of the eight headings. Camera shows tradesmen at work adjusting the compass housing.
Content description
Reel 3:
"Compass swinging (number 2) . Ground swinging procedure": The operation of swinging an aeroplane is to determine the deviation on the four cardinal points, and calculate the value of coefficient A.
"Ground swinging using a land compass": Film cuts to the office of the navigating officer who issues the necessary equipment to two tradesmen. They check the items, collect the log sheets and proceed to the hard standing where a Wellington (BJ 986) aircraft is waiting with engines running. The tradesmen and the Wellington move away to a magnetically quiet area of the aerodrome, away from hangars etc. The ground compass is set up about three hundred feet from the aircraft, which the pilot has manoeuvred to magnetic north and normalised the controls to their mid position. He liaises with the tradesmen as the swinging commences.
Content description
Reel 4:
The swinging proceeds; as the Wellington turns to the east, south, west and back to north, coincidentally with the tradesmen and their Ground compass. The difference between the pilots and the tradesmen compass readings is the deviation. Component C is calculated from the north and south headings, component B from the east and west headings. The whole process is repeated for all eight cardinal points from which coefficient A can be calculated. The Wellington's compass is adjusted accordingly. The data from the exercise are tabulated and the residual deviations – explained – are thus obtained, entered on a circular card (form 316) and issued to the pilot – or navigator – to enable them to apply the correct deviation to the aircraft's compass reading.
"Occasions when a compass should be swung": Cut to Wellington on remote grass area; swinging is necessary when a compass has been repaired, a newly arrived aircraft, or new hardware is fitted.
Content description
Reel 5:
"Compass swinging (number 3) . Component R":
Component R acts vertically downwards and may be regarded as a hypothetical magnet either below or above the aircraft's compass housing. Explanation of the compass card mounted on a pivot. The card can be affected by component R: when the aircraft is tilted, the compass housing also tilts with the aircraft but gravity causes the card to remain horizontal and parallel with the fore and aft axis of the aircraft. Component R has no effect when flying level; when climbing component R has an effect which also occurs when the swinging process takes place. For this reason the tail of the aircraft must be raised so the fore and aft axis of the aircraft is parallel to the ground.
"Determination and correction of deviation due to component R": A series of animated diagrams and a detailed narrative explain the procedure. Component C must first be obtained in the usual manner. The process is similar to that for components A, B, and C with the aircraft's tail down. Cut to close up of micro adjuster mounted underneath the aircraft's compass housing; the effect of component R is completely neutralised by the installation of small cylindrical hard iron magnetic pellets inside the base of the compass housing.
Content description
Reel 6:
"Compass swinging (number 4) . Theory of the Astro compass": Compass swinging in the air (i. e, while the aircraft is flying) requires the use of the astro compass. The principles on which the astro compass is based are explained by a very technical but clear and lengthy narrative, with a series of animated diagrams. The concept of latitude, longitude, meridians, Greenwich prime meridian, the celestial sphere, Greenwich hour angle, sidereal hour angle and declination of the stars, are introduced. Earth is considered to be at the centre of the celestial sphere. Cut to film of the astro compass; use of the compass explained in detail.
"Obtaining a true heading with the astro compass": Setting up the astro compass, explained step by step.
Content description
Reel 7:
"Compass swinging (number 5) . Swinging in the air": Film opens to a Wellington aircraft in flight. During flight the magnetic behaviour of an aircraft can be quite different from that on the ground; undercarriage, engines and control surfaces cannot be allowed for on the ground. In the air the movement of control levers and the electrical circuits will have an effect. These effects can be allowed for if the aircraft is swung in the air. The astro compass is used to define the heading of an aircraft, as described in reel 6. This is the true heading. By applying the variation correction the magnetic heading can be deduced.
"The astro compass . Air swinging method . Conditions necessary": Sun at an angle of less than 450 degrees, steady flying conditions, flying on automatic pilot to maintain a constant and steady flight line.
"Pre flight procedure": As before, the equipment, data sheets and form 316 are collected from the navigators office, together with a series of pin point locations over which the aircraft has to fly. Procedure is the same as for ground swinging. Cut to several views of the work underway in the cockpit. The astro compass, when correctly set up will show the true heading of the lubber line. Before the true heading of the aircraft can be determined, the lubber line must be aligned with, or parallel, to the aircraft's fore and aft axis line. Other criteria that must be observed when setting up the astro compass are explained.
"Alignment of the 0.5 standard": The 0.5 standard is the mounting frame for the astro compass body. Each aircraft is allocated it's own astro compass and unique mounting frame that remains fixed in the fuselage and incorporates all the adjustments determined, thus the compass body may be removed and replaced without disturbing the adjusted values. Cut to close up view of the compass and frame.
"Alternative method of aligning the 0.5 standard": In the absence of the sun the astro compass may be aligned to the true heading as determined by a corrected ground compass. Explanation given.
Content description
Reel 8:
"Flight and after flight procedures": The narrator describes how the swinging is carried out. The aircraft flies a series of horizontal loops, at the four quadrants, centred on the pin point, creating four lines each about ten miles long yielding two N – S and two E – W legs. The astro compass is set up and simultaneous readings are taken with the astro compass and the aircraft's compass along each leg. Cut to close up views of both compasses, supplemented with a clear explanation. The deviation is thus determined. The aircraft then flies a series of eight headings from which coefficient A is determined. The coefficients B, C, are then determined, coefficient A is re-determined, with the aircraft on the ground.
"Other methods of finding the heading of an aeroplane":
1. Camera obscurer method.
2. Hill's mirror method.
3. Long railway lines and photographic methods.
Each method is explained with animated diagrams and photo clips.
The film closes as the tradesmen complete the compass swinging of the Wellington aircraft.