Aircraft Lighting, Warning Systems, and Autopilot Interface

Aircraft Exterior Lighting Systems

Aircraft exterior lights serve two purposes: making the aircraft visible to other pilots and illuminating the ground ahead during taxi and landing. Every powered aircraft operated at night or in instrument conditions must carry lighting that meets applicable airworthiness standards. For small aircraft, 14 CFR Part 23 establishes the general design requirements, while 14 CFR Part 91 governs when each light must be on.

Navigation (position) lights consist of a red light on the left wingtip, a green light on the right wingtip, and a white light on the tail. This arrangement lets other pilots determine your direction of travel at a glance. Bulbs are typically sealed-beam or replaceable incandescent lamps, though many modern aircraft use LEDs. Always confirm the replacement part number against the aircraft's approved parts list — fitting the wrong lamp can change the light's intensity or beam angle and create an airworthiness discrepancy.

Anti-collision lights include red rotating beacons and/or white strobe lights. Strobes produce very high-intensity flashes through a xenon discharge tube energized by a high-voltage power supply (often several hundred volts). Treat strobe power supplies as a serious shock hazard even with aircraft power off — capacitors inside the unit can retain a lethal charge for minutes after shutdown. Follow the manufacturer's discharge procedure and observe lockout/tagout before touching internal components. Always wear safety glasses when handling xenon tubes, which can shatter under impact.

Landing and taxi lights are high-wattage sealed-beam units that draw significant current. Inspect the wiring for heat discoloration near connectors, because a poor connection in a high-current circuit creates resistance, heat, and potential fire. Verify the lens is uncracked and the reflector is undamaged. Record any discrepancy on the maintenance record; if a lamp is replaced, log the part number, date, your name, and certificate number in the aircraft maintenance logbook per 14 CFR Part 43 requirements.

Crew-Alerting and Warning Systems

Modern aircraft use a variety of crew-alerting system (CAS) circuits to alert pilots to abnormal conditions. Two of the most fundamental are the stall warning system and the fire/overheat warning system.

Stall warning systems sense the angle of attack (AOA) on the wing. The simplest design uses a vane or lift detector mounted near the leading edge. As the wing approaches the stall AOA, airflow lifts the vane, closing a switch that completes the circuit to an electric horn in the cockpit. More complex aircraft use a stick shaker, which is a small eccentric motor that vibrates the control column to give both an aural and tactile warning. When testing the stall warning on the ground, use only the manufacturer-approved test switch or procedure — never apply physical force to the vane in a way that could damage it.

To trace a stall warning circuit, start at the power source, follow the wire through the circuit breaker, through the AOA sensor switch, through any applicable control module or relay, and to the alerting device (horn or shaker motor). Comparing wire numbers, pin numbers, and circuit-breaker labels on the wiring diagram to what you find on the aircraft is the standard verification method.

Fire and overheat warning systems use loop-type or spot-type temperature sensors in engine compartments and other zones. A break or short in the sensing loop can illuminate a false warning; an open in the circuit can suppress a real one. Both failure modes are hazardous, which is why continuity and resistance checks are part of every inspection.

Autopilot Electrical Interface

An autopilot moves flight-control surfaces through servo motors — small, geared electric motors mechanically coupled to the control cables or pushrod system. Understanding the electrical interface is essential for safe maintenance.

Key circuit elements in a typical autopilot servo circuit include:

• Engage relay: When the pilot selects autopilot ON, the engage relay closes and allows the flight computer to command the servo. Removing power from this relay disconnects the computer from the servo.
• Servo motor: Receives variable voltage or pulse-width-modulated (PWM) signals to drive the control surface in the commanded direction.
• Servo disconnect switch (quick-disconnect): Located on the yoke or stick, this switch opens the engage relay instantly, declutching the autopilot from the flight controls. The pilot can always overpower the autopilot physically, but the disconnect switch is the primary electrical means of removal.
• Torque limiter: A mechanical or electromechanical device that limits the maximum force the servo can apply to the control system. This prevents a runaway or hardover servo from overpowering the pilot or causing structural damage.

Hardover fault isolation: If a servo drives to its mechanical stop and holds there (a hardover), the pilot presses the disconnect switch or master autopilot switch. This de-energizes the engage relay, removing all electrical drive from the servo. The torque limiter ensures the pilot can physically overpower the servo during the instant before the relay opens. After a suspected autopilot fault, the aircraft is grounded until the cause is identified and corrected per the manufacturer's maintenance manual.

Safety Summary

• Apply lockout/tagout before opening any avionics panel or working near high-voltage strobe power supplies.
• Treat strobe capacitors as energized until discharged per the manufacturer's procedure.
• Wear safety glasses when handling lamp assemblies.
• Never bypass a disconnect switch or torque limiter during autopilot maintenance — these are life-safety devices.
• All corrective actions must be recorded in the aircraft maintenance logbook with the required regulatory information.