Most people are unaware of airline maintenance procedures. One of the most common procedures used by airline maintenance, and pilots involves using a mel . A mel stands for minimum equipment list. This is a list of items on the airplane that have been approved by the FAA to allow the crew to fly with broken. The purpose of this is to allow the airlines temporary relief to defer a broken item to fix for later so it doesn't disrupt airline operations by grounding airplanes all over the place. However,their are some restrictions. The FAA will never allow you to mel something very critical to safety. To give you an example of a mel would be something like a windshield wiper in the cockpit. If one is broken, you can certainly fly the airplane safely without it. Plus, there are two in the cockpit. So one can be broken temporary. The FAA would normally put restrictions on that item being mel. For example, they may say you can't fly with a broken windshield wiper if there is rain in the forecast at your destination, or alternate airport. They may also say that the item must be fixed within three days. Mel's are perfectly legal & safe to use. It also allows airline operations to run more efficiently. Without mel's, it would be chaos in airline operations. Just imagine if a flight was canceled overseas because of broken windshield wiper, or cabin tray table. Obviously, these items are not a major safety item critical to flight,.

(Neat maintenance video)

http://www.youtube.com/watch?v=PNs9pKoYcXk