Aircraft technical Basics: TM 1-406, Aircraft Electrical Systems, 1940: 5. Lighting Systems

TM 1-406, TECHNICAL MANUAL,  AIRCRAFT ELECTRICAL SYSTEMS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON, October 18, 1940.

SECTION V - LIGHTING SYSTEMS

38. General.-a. An aircraft lighting system is very similar to the conventional systems used in automotive lighting. It may employ a one or two wire system, therefore it is necessary that reference for details must be made to the wiring diagram in technical publications for the aircraft involved. The amount of equipment varies somewhat for different airplanes and the purpose for which it is used. In addition to a set of standard navigation lamps, the equipment may include landing, formation lamps, and a passing lamp. Figure 83 illustrates a complete external aircraft lighting system employing these units.

FIGURE 83. Typical external lighting system.

b. Illumination of the airplane instrument panel is accomplished by indirect, individual, or fluorescent lighting. Extension lamps and spotlight cockpit lamps are installed in the cockpits and cabins to provide selective lighting of instruments, maps, charts, etc. Wall panel and dome lamps are used for general illumination of cabins.

39. Navigation lamps.-A set of navigation lamps is the minimum lighting equipment for airplanes operating at night and consists of 1 red, 1 green, and 1 white unit. Each unit consists of two lamps streamlined into the edge of the surface to which they are attached. The green lamps are mounted on the right wing at the extreme tip; the red unit is similarly located on the left wing. The white lamps are usually located on the vertical stabilizer in a position that may be seen through a wide angle when viewed from the rear. The navigation lamp units are all similar with the exception of color and are controlled by a single switch in the pilot's compartment. Obviously the navigation lamps are connected in parallel and receive their current supply from the battery generator circuit. A suitable fuse is also incorporated in the system.

40. Landing lamps.-Two landing lamps are installed in all airplanes regularly used at night. These lamps are extremely powerful and are directed at such an angle as to illuminate an area sufficiently for safe landing in unlighted airports and emergency fields. They are generally located midway in the leading edge of each wing, streamlined into the airfoil section, although some installations employ retractable lamps set into the under surface of the wings. Suitable switches for operating the lamps independently are located within easy reach of the pilot, and the circuit is fused for safety.

41. Passing lamps.-A passing lamp is included in the lighting system of aircraft operating on or near the civil airways at night. This lamp is provided with a red lens and is located in the leading edge of the left wing. It is used only as a precaution against collision, when passing other aircraft, and is generally controlled by a simple toggle switch.

42. Formation lamps.-Formation or position lamps are used for night formation flights on certain military aircraft. The set consists of 7 blue lamps, 3 of which are installed in a straight line running fore and aft along the upper surface of the fuselage; the remaining 4 are positioned on the upper surface of the horizontal stabilizer, 2 on each side of the vertical stabilizer. Their location makes them invisible from the ground and at the same time provides for safe group maneuvers with all other lamps turned off. The current supply for formation lamps is controlled by a rheostat so that the pilot may increase or decrease their intensity.

43. Instrument, cabin, and cockpit lighting.-a. Indirect lighting of instrument panels is provided. by lamps in sockets set in the instrument panel, the light from these lamps being distributed over the entire panel by a reflector panel which has openings for observing the various instruments.

b. Individual lighting provides each instrument with a small self-contained lamp. The voltage for these lamps is obtained by reducing the battery supply voltage to approximately 3 volts through resistor boxes.

c. Fluorescent lighting completely eliminates the glare found in other lighting arrangements. The light assembly is comprised of a shell which has a special lens made to pass ultraviolet light, variable apertures for regulating the amount of light, and a cartridge type starting switch. The apertures are enlarged by an adjustment disk on the end of the assembly. There is also an aperture for passing visible light. Instruments used with this type of lighting have the figures on the dial painted with a material which is sensitive to ultra-violet light. When the invisible ultraviolet light is directed on the instruments, the figures are outlined in a soft glow which enables them to be seen distinctly although there is no visible light coming from the lamp itself. The supply voltage for these lamps is obtained from inverters which change the direct current of the airplane electrical system to alternating current and increase the voltage to the value required to operate the lamps.

44. Maintenance.-a. All visible wiring including connections, terminals, and switches should be checked periodically for condition and security. Lamp lenses and reflectors must be kept clean and highly polished. If a reflector is found to be cloudy, it. must be polished with a suitable compound or paste, such as a mixture of alcohol and lampblack. In cases of extreme cloudiness, the reflectors may require relacquering or even replating. As cloudy reflectors are usually due to an air leak around the lens, a new gasket should be installed when the lamp is reassembled. Care must be taken to see that proper focus and alinement are maintained.

b. Malfunctioning of the lighting equipment may be located and remedied by systematically testing each lamp and lamp circuit with the continuity meter tester, usually available for this purpose.