Aircraft Electrical Load Circuits: Nav Light, Landing & Taxi Lights, and Landing Gear Circuits

Nav. Light Circuit
Position lights, also called navigation lights, are used for aircraft identification and collision avoidance. Variations exist across aircraft; traditional position lights used incandescent bulbs, while modern aircraft increasingly use LED lights due to better visibility, reliability, and lower power consumption. Some position-light units contain two lamps and are often integrated into the aircraft surface. The green light unit is mounted at the extreme tip of the right wing, and the red unit on the left wing. Exterior lights support night operations, including landing and midair collision avoidance; anticollision lights are also employed in many installations.

Reading Schematic Diagrams
Aircraft maintenance schematics show the flow of electricity and the placement of components. To read them: start at the battery or bus, trace every wire to ground (the zero-voltage point). When a relay or solenoid coil is involved, follow the contacts as they change state, and when a circuit splits, trace one path to ground before tracing the other path. These diagrams depict the circuit topology, not the physical installation.

Landing & Taxi Light Circuit
Landing lights are high-intensity exterior lights used to illuminate runways at night, directed by a parabolic reflector to maximize range. They are very powerful and aimed for maximum illumination. Taxi lights provide ground illumination while taxiing or towing; they are not designed to substitute for landing lights. Taxi lights are mounted at an oblique angle to the centerline to illuminate directly in front of the aircraft and also toward the sides to aid maneuvering on the ground.

Landing Gear Actuation & Indicating Circuit
The landing gear system uses a sequence of switches, relays, and indicators to control gear extension/retraction and to indicate gear status. Key tracing example: current flows through a 5-amp circuit breaker, wire #6, the nose-gear-down switch, wire #5, left-gear-down switch, wire #4, right-gear-down switch, wire #3, then through the green indicator light to ground. With gear down, the down-limit switch opens, preventing motor operation in the gear-down direction. The gear-up relay is open in this condition, preventing retraction.

Bulb test and indicators
Both red and green bulbs can be checked by pressing their lenses; closing an internal switch allows current through the bulbs via the 5-amp breaker and return path through the ground. When airborne (weight off the gear), the gear switch can be set to Gear-Up, allowing current to reach the motor via a path that includes a 20-amp circuit breaker, wire #1, the gear switch, wire #13, wire #10, the up-limit switch, the gear safety (squat) switch, and wire #12 to ground through the coil of the gear-up relay. Energizing the coil closes the relay, permitting current through the relay contacts and wire #14 to ground through the motor to retract the gear. As the gear leaves the locked-down condition, the green light turns off and the down switches reconfigure.

If the gear is up and locked, the up-limit switch conducts, and current can flow through the 5-amp breaker, wire #19, the up-limit switch, wire #8, and to ground via the red light, indicating gear up and locked. If throttle is retarded while the nose gear is not down and locked, current can flow through the 5-amp breaker, wire #6, nose-gear-down switch, throttle switch, wire #16, down-limit switch, and wire #11 to ground through the horn to indicate an unsafe condition.

Alternative circuits exist
Other landing-gear actuation circuits show variations, such as a Neutral position in which the control valve must be Neutral to prevent horn during ground, or a configuration where green indicates both gears down and locked via grounding through both gear switches. In alternate designs, the red light illuminates if either gear is not down and locked, while the green light only illuminates when both gears are down and locked. The warning horn may sound if a throttle switch is closed and any gear is not down and locked, or if the control valve is not in Neutral when both gears are down and locked.

Summary
This lesson covered Nav. Light, Landing & Taxi Light, and Landing Gear Actuation & Indicating Circuits, focusing on how lights indicate status, how to trace circuits on schematics, and how gear movement is interlocked with indicators and alarms to ensure safe operation during ground and flight conditions.