Virtually all aircraft types are fitted with external lights of some description. The type, purpose and complexity of the lighting systems installed on the exterior of a particular aircraft vary in accordance with its size, role and normal flight environment. External lights, in general terms, serve one of three purposes:
to make the aircraft more visible to other aircraft,
to improve pilot visiblity during critcal phases of flight or
to provide illumination for some other specific purpose.
Note that there is often overlap in the utility of some of the external lights. As an example, landing lights greatly improve a pilot's ability to see the runway during takeoff and landing but also enable the aircraft to be seen by ground personnel and by other airborne traffic.
Some of the lights of the first two categories are a regulatory requirement during hours of darkness. Minimum Equipment List (MEL) relief may be available under some circumstances.
The following lighting systems are intended to reduce the potential of collision, by making the aircraft more visible to other aircraft while in flight and to ground traffic while manoeuvering on an aerodrome:
Navigation Lights - Navigation lights consist of a red light on the left/port wing tip, a green light on the right/starboard wing tip and a white light on the aircraft tail. Dual systems are often installed to provide redundancy in the event of a bulb failure. In aircraft equiped with a beacon, the navigation lights will normally burn steadily whereas in aircraft without a beacon, the navigation lights will flash.
Beacon - The aircraft beacon lights are red in colour and either flash or rotate to provide a pulsating warning light. They are normally installed in pairs with one on the top of the fuselage and the other on the bottom. The beacon is normally turned on prior to engine start and turned off after engines have been shut down.
Strobe Lights - When installed, strobe lights are usually positioned near the trailing edge of the wing tips and may also be installed on the tail of the aircraft. Strobes are high intensity white lights which flash at a regular interval. They are normally turned on when entering an active runway for takeoff and turned off when leaving the runway after landing. In many cases, the strobe light switch will have an AUTO position which will result in activation and deactivation of the lights based on weight on wheels. Strobes can also be used to provide additional visibility when crossing an active runway during ground manoevering.
Pilot visibility through the flight deck windows is greatly improved by the following external lights:
Taxi Lights - Taxi lights are medium intensity lights which may be mounted on the nose landing gear strut, in the aircraft nose or at the wing roots. They are used to provide illumination of the taxiway during ground operations.
Runway Turnoff Lights - Turnoff lights are similar to taxi lights except they are mounted on an angle pointing to the left and right of the aircraft nose. These lights will illuminate taxiways or obstacles on either side of the aircraft.
Landing Lights - Landing lights are high intensity lights used to illuminate the runway surface for takeoff and landing and also to facilitate the aircraft being seen by other pilots. These lights may be wing, landing gear strut or fuselage mounted. In some installations, they are mounted in the leading edge of the wing behind a clear fairing while in others, they are extended and retracted from a cavity in the wing or fuselage. Depending upon the regional norm, landing lights are either switched on when entering the active runway or upon receipt of takeoff clearance. Where the AOM allows, landing lights are normally turned off as the aircraft climbs through 10,000' and are turned on again as it descends through 10,000' on approach.
Wing Inspection Lights - Wing inspection lights are mounted in the fuselage and are aimed to illuminate the leading edge of the wing and the engine pylons. They are often used during hours of darkness for aircraft preflight inspection and for illumination during engine start. They can be used in flight on an "as required" basis, often to see if there is any accretion of ice on the leading edge of the wing.
Ice Detection Probe Light - Some aircraft are fitted with a visual ice detection probe which is normally mounted between the windshields. In some installations, the probe is internally or externally lit to enable the pilots to see any ice accumulation occurring during hours of darkness.
Specific Purpose Lighting
Logo Lights - Logo lights are generally mounted on the upper surface of the horizontal stabilizer and are aimed to illuminate the Company markings on the tail of the aircraft.
Search Lights - Enforcement aircraft, such as police helicopters, and search and rescue aircraft may be equiped with high intensity search lights.
Formation Lights - Some military aircraft have variable intensity lights installed on the upper surface of the wings to facilitate night formation flights, These lights may be in the visible or infrared spectrum.