Lighting plays an important role in the operation of an aircraft and many of its systems, and in the main falls into two groups:
External Lights generally consist of the following lights
Navigation Lights or Position Lights
White and Red Flashing Light or Anti-Collision Light
Landing and Taxiing lights
Wing or Ice Inspection Light
AIRCRAFT EXTERIOR LIGHTS
Navigation Lights or Position Lights
The requirements and characteristics of navigation lights are agreed on an international basis and are set out in the statutory Rules of the Air and Orders for Air Navigation and Air Traffic Control regulations.
Briefly, these requirements are that every aircraft in flight or moving on the ground during the hours of darkness shall display:
A green light at or near the starboard (right) wing tip, visible in the horizontal plane from a point directly ahead through an arc of 110 degrees to starboard.
A red light at or near the port wing tip, with a arc of visibility of 110 degrees to port.
A white light visible from the rear of the aircraft in the horizontal plane through an arc of 140 degrees. The conventional location of this light is in the aircraft's tail, but in certain cases, white lights are mounted in the trailing edge sections of each wingtip.
AIRCARFT POSITION OR NAVIGATION LIGHTS
In general, they consist of a filament type lamp, appropriate fitting and transparent coloured screen or cap. The screen is specially shaped and, together with the method of arranging the filament of the lamp, a sharp cut-off of light at the required angle of visibility is obtained.
The electrical power required for the lights is normally 28 volts d.c. but In several current types of all a.c." aircraft, the lights are supplied with 28 volts a,c. via a step-down transformer.
AIRCARFT POSITION OR NAVIGATION LIGHTS
Anti-collision lighting also fulfills a position marking function and,in conjunction with navigation lights giving steady lighting conditions, permits the position of an aircraft to be more readily determined.
A lighting system may be of the type which emits a rotating beam of light, or of the strobe type from which short-duration of flashes of high-intensity light are emitted. ln some current types of aircraft both methods are used in combination, the strobe lighting forming what is termed "supplementary lighting".
Rotating beam lights
These tights or beacons as they are often called, consist of a enamel lamp unit and a motor, which in some cases drives a reflector and in others the lamp unit itself; the drive transmission system is usually of the gear and pinion type and of a specific reduction ratio. All components are contained within a mounting enclosed by a red glass cover.
The power required for beacon operation is normally 28-volts d.c. but a number of types are designed for operation from an a.c. supply, the motor requiring 115 volts and the lamp unit 28 volts supplied via a step-down transformer.
The motor speed and gear drive ratios of beacons are such that the reflector or lamp unit, as the case may be, is operated to establish a beam of light which rotates at a constant frequency.
Typical speeds are 40-45 rev/min giving a frequency of 80-90 cycles per minute.
AIRCRAFT ANTI-COLLISION LIGHT
This type of lighting system is based on the principle of a capacitor-discharge flash tube. Depending on the size of the aircraft, strobe lighting may be Installed in the wing tips to supplement the conventional red beacons, they may be used to function solely as beacons, or may be used in combination as a complete strobe type anti-collision hight intensity lighting system.
The light unit takes the form of a quartz or glass tube filled with Xenon gas, and this is connected to a power supply unit made up essentially of a capacitor, and which converts input power of 28 volts d.c. or 115 volts a.c. as the case may be, into a high d.c. output, usually 450 volts.
The capacitor is charged to this voltage and periodically discharged between two electrodes in the Xenon-filled tube, the energy producing an effective high,intensity flash of light having a characteristic blue white colour. A typical flashing frequency is 70 per minute.
When used as supplementary lighting or as a complete strobe anti-collision lighting system, they are installed in trailing positions in each wingtip (next to position light), and all lights are controlled in a flashing sequence by controllers and flasher timing units.
The number of flash depends on the type of aircraft.
AIRCRAFT STROBE LIGHT
Landing and Taxi Light
As their names indicate these lights provide essential illumination for the landing of an aircraft and for taxi-Ing it to and from runways and terminal areas at night and at other times when visibility conditions are poor.
Landing lights are so arranged that they illuminate the runway immediately ahead of the aircraft from such positions as wing leading edges, front fuselage sections and nose landing gear structure. The lights are of the sealed beam type and in some aircraft are mounted to direct beams of light at pre· determined and fixed angles.
In other types of aircraft, the lights may be extended to preselected angles, and retracted, by an electric motor and gear mechanism, or by a linear actuator. Micro-type limit switches are incorporated in the motor circuit and are actuated at the extreme limits of travel to interrupt motor operation.
A typical power rating for lights is 600 watts, and depending on the design the power supply required for operation may be either d.c. or a.c. at 28 volts, the latter being derived from a 115-volts supply via a step down transformer.
In lights of the retractable type which require a.c. for their operation, the motor is driven directly from the 115-volts supply. The supplies to the light and motor are controlled by switches on the appropriate control panel in the cockpit. To indicate the retracted position, and so the "retract" and "extend limit switches controlling the motor, are open and closed respectively. The supply circuit to the light itself is automatically interrupted when it is retracted
When the control switch is placed in the "extend" position, the 115-volt supply passes through the corresponding field winding of the motor until interrupted by the opening of the extend limit switch. The retract limit switch closes soon after the motor starts extending the light. The switch in the supply circuit to the light also closes but the light is not illuminated until it is fully extended and the control switch placed in the ''on" position. The power supply to the light is reduced from 115 to 15 volts by a step-down transformer.
In some aircraft, a fixed type landing light is located in the leading edge of each wing near the fuselage, and an extending/retracting type is located in the fairing of each outboard landing flap track.
In lights located in flap track fairings, additional switches are included in the "retract'' and "extend" circuits, The switches are actuated by a mechanical coupling between the wing and flap track fairings. Thus, when the landing flaps are lowered, and the landing lights extended, the circuits of the motor will be signalled to adjust the positions of the lights so that their beams remain parallel to a known fore and aft datum regardless of flap positions.
AIRCRAFT LANDING LIGHT
Taxi lights are also of the sealed beam type and are located In the fuselage nose section, in most cases on the nose landing gear assembly. The power rating of the lights is normally lower than that of landing lights (250 watts is typical) and the supply required is either d.c. or a.c. at 28 volts.
In certain cases the function of a taxi light is combined with that of a landing light. The light has two filaments, one rated at 600 watts and the other at 400 watts; both filaments provide the illumination for landing, while for taxiing only the 400 watt filament is used.
AIRCRAFT LANDING LIGHT
In addition to taxi lights some of the larger types of transport aircraft are equipped with lights which direct beams of light to the sides of the runway (see Fig. 10.1). These are known as runway turnoff lights, their primary function being to illuminate the points along the runway at which an aircraft must turn to leave the runway after landing.
AIRCRAFT RUNWAY TURN OFF LIGHT
Wing-Scan or Ice inspection Light
Ice inspection or wing-scan lights are fitted to most types of transport aircraft, to detect the formation of ice on the leading edges of wings and also at the air intakes of turbine engines.
Lights are also of the sealed beam d.c. or a,c. type and with power ratings varying from 60 watts to 250 watts depending on the lighting intensity required for a particular aircraft type. They are recessed into the sides of the fuselage and are preset to direct beams of light at the required angles. In some aircraft having rear-mounted engines lights are also recessed into the trailing edge sections of the wings.
WING-SCAN OR ICE INSPECTION LIGHT
The Internal lighting of aircraft can be broadly divided into three categories ; cockpit or operational lighting, passenger cabin lighting, and servicing lighting which includes galleys, toilet compartments, freight compartments and equipment bays.
illumination of cockpit instruments and control panels.
illumination of passenger cabins and passenger information signs.
Indication and warning of system operating conditions.
The· most Important requirements for cockpit lighting are those necessary to ensure adequate illumination of all instruments, switches, controls, etc., and of the panels to which these items are fitted.
Some of the methods adopted to meet these requirements are as follows;
integral lighting, ie. one in which the light source is within each instrument;
pillar and bridge lighting, in which a number of lights are positioned on panels to illuminate small adjacent areas, and to provide flood-lighting of individual instruments;
flood-lighting, whereby lamps are positioned around the cockpit to mood-light specific panels or a general area.
trans-illuminated panels which permit etched inscriptions related to various controls, notices and instructions to be read under night or poor visibility conditions.
The principal form of integral lighting for instruments is that known as wedge or front lighting; a form deriving its name from the shape of the two portions which together make up the instrument cover glass. lt relies for its operation upon the physical law that the angle at which light leaves a reflecting surface equals the angle at which will strikes that surface.
The two wedges are mounted opposite to each other and with a narrow airspace separating them. Light is introduced into wedge "A" from two 6-volt lamps set into recesses in its wide end. A certain amount of light passes directly through this wedge and on to the face of the dial while the remainder is reflected back into the wedge by its polished surfaces. The angle at which the light rays strike the wedge surfaces governs the amount of light reflected; the lower the angle, the more fight is reflected.
The double wedge mechanically changes the angle at which the light rays strike one of the reflecting surfaces of each wedge, thus distributing the light evenly across the dial and also limiting the amount of light given off by the instrument.
Since the source of light is a radial one, the initial angle of some light rays with respect to the polished surfaces ofwedge "A" is Jess than that of the others. The low-angle light rays progress further down the wedge before they leave and spread light across the entire dial. Light escaping into wedge "B" is confronted with constantly decreasing angles, and this has the effect of trapping the light within the wedge and directing it to its wide end. Absorption of light reflected into the wide end of wedge "B" is ensured by painting its outer part black.
PILLAR AND BRIDGE LIGHTING
Pillar lighting, so called after the method of construction and attachment of the lamp, provides Illumination for individual instruments and controls on the various cockpit panels. A typical assembly, consists of a miniature centre-contact filament lamp inside a housing, which is a push fit in to the body of the assembly.
The body is threaded externally for attachment to the panel and has a hole running through its length to accommodate a cable which connects the positive supply to the centre contact. The circuit through the lamp is completed by a ground tag connected to the negative cable.
Light is distributed through a filter and an aperture in the lamp housing. The shape of the aperture distributes a sector of light which extends downwards over an arc of approximately 90 degrees to a depth slightly less than 2 in from the mounting point.
The bridge-type of lighting is a multi-lamp development of the individual pillar lamp. Two or four lamps are fitted to a bridge structure designed to fit over a variety of the standardised instrument cases.
The bridge fitting is composed of two light alloy pressing secured to, gether by rivets and spacers, and carrying the requisite number of centre contact assemblies above which the lamp housings are mounted. Wiring arrangements provide for two separate supplies to the lamps thus ensuring that total loss of illumination cannot occur as a result of failure of one circuit.
These panels or ' 'light plates", provide for the illumination of system nomenclature, switch positions etc. They are of plastic through which light from many very small incandescent bulbs is passed. The light can only be seen where appropriate characters have been etched through a painted surface of a panel. The bulbs are soldered in place and are not replaceable when installed . More than one bulb provides illumination in each relevant area so that failure of a bulb will not impair illumination.
Flood-lighting is used for the general illumination of instruments, control panels, pedestals, side consoles and areas of cockpit floors . The lights usually take the form of incandescent lamp units and fluorescent tube units and depending on the type of aircraft, both forms may be used in combination.
This form of lighting is employed in a number of aircraft as passenger information signs and also, in some cases, for the illumination of instrument dials and selective positions of valves or switches.
An electro luminescent light consists of a thin laminate structure in which a layer of phosphor is sandwiched between two electrodes, one of which is transparent.
The light requires a.c. for its operation, and when this is applied to the electrodes the phosphor particles Luminesce, i.e. visible light is emitted through the transparent electrode.
The luminescent intensity·depends on the voltage and frequency of the a.c. supply. The area of the phosphor layer which becomes "electroluminescent" when the current is applied is that actually sandwiched between the electrodes; consequently if the back electrode is shaped in the ion of a letter or a figure the pattern of light emitted through the transparent electrode is an image of the back electrode
Passenger Cabin Lights
The extent to which lighting is used in a passenger cabin depends on the size of a cabin and largely on the interior decor adopted for the type of aircraft; thus, it can vary from a small number of roof-mounted incandescent lamp fittings to a large number of fluorescent fitting located in ceilings and hat racks so as to give concealed, pleasing and functional lighting effects.
PASSERNER CABIN LIGHTS
The power supplies required are d.c. or a.c. as appropriate, and in all commercial passenger transport aircraft the lights are controlled from panels at cabin attendant stations.
PASSENGER CABIN LIGHTS CONTROL PANEL
In addition to main cabin lighting, lights are also provided for passenger service panels and are required for the illumination of essential passenger information signs, e.g. "Fasten Seat Belts" and "Return to Cabin". The lights for
these signs may be of the incandescent type or, in a number of aircraft, of the electroluminescent type.
They are controlled by switches on cockpit overhead panel.
PASSENGER CABIN LIGHTS CONTROL PANEL
In addition to the passenger cabin light, modern aircrafts have light for passenger convenience, such as passenger reading light and crew call light.
PASSENGER READING LIGHT AND CREW CALL LIGHT
In larger aircrafts it is important to provide mean of indication to crew about passenger calls or call from the lavatory or calls from the other cabin or cockpit crew, the same is provided by means of fixed colour lights above the crew seats.
CALL LIGHT INDICATIONS
Additional internal light include passenger entry and exit door light and crew work or galley lights.
An essential requirement concerning lighting is that adequate illumination of the cockpit and the various sections of the cabin, exits, escape hatches, chutes etc., must be provided under emergency conditions, e.g. a crash-landing at night.
The illumination is normally at a lower level than that provided by the standard lighting systems, since the light units are directly powered from an emergency battery pack or direct from the aircraft battery in some cases. The batteries are normally of the nickel-cadmium type although in some aircraft silver-zinc batteries are employed,
Under normal operating conditions of the aircraft, emergency battery packs are maintained in a fully charged condition by a trickle charge from the aircraft's main busbar system.
Primary control of the lights is by means of a switch on a cockpit overhead panel
Light Control Panel
The control panel for control of external and cockpit and some internal cabin lights is located in the cockipt and depends on the type of aicraft.