When we receive take off clearance, "cleared for take off," the captain will turn on the landing lights. They will remain on until 10,000 feet. The pilot flying will call "10 thousand" prompting the PNF (Pilot Not Flying) to turn off the lights. Many aircraft have the lights embedded in the wing. The lights on the Airbus 320 fleet extend downwards from under the wing. They cause some drag and we pilots liken them to "mini" speed brakes. We would stow these lights if we plan on exceeding 250 knots below ten thousand feet during climb.
When descending through 10,000 the lights will go on as part of the "in-range" checklist. The in-range checks should be done while descending through 10 thousand feet or about ten minutes from landing.
In a private plane,, at least, it's legal to land with the landing light off (or broken), as long as you're not carrying passengers.
There's some evidence that leaving landing lights on below 3,000 feet helps avoid bird strikes, at least for light aircraft. I'm not sure how much that applies to airliners, with the higher speeds.
If you're intercepted by police or military aircraft and can't make radio contact, turning on your landing light is part of the procedure for signalling that you're ready and willing to follow the interceptors.
Light single-engine aircraft often have only one landing light, which also serves as a taxi light -- it's normally on a wing (e.g. Cessna) or right under the propeller (e.g. Piper). Some owners have two or three landing lights installed, so that they'll look more like airliners on a night approach :-)
The foregoing was found at:
http://www.fromtheflightdeckbook.com/2009/04/question-on-landing-lights.htmlMy personal experience with light aircrafts I have flown from C152 to AC680 is that you are supposed to turn the lights on whenever your are inside tower controlled airspace which is normally 10 nm around.
