Every once in a while, a flight will be released with something on the airplane that is amiss. The minimum equipment list (MEL) is a document that is written by the airplane manufacturer in conjunction with the airlines and regulatory authorities. It spells out items that do not have to be in perfect working order in order to safely dispatch a flight. Likewise, an MEL can be pretty specific about items that must be working in order to operate a flight, or to conduct certain operations.

For example, every turbine airplane has multiple sources of producing electricity in the way of generators, usually at least three: on one each of the engines, and one on the APU. Some have more. There is almost always relief for flying with one generator that is inoperative, but for certain operations, such as long flights over water with only two engines, there may be a requirement that certain generators are functioning and available.

Certain low-weather approaches also have requirements with respect to equipment functionality and even basic maintenance. Category II and III ILS approaches require certain autopilot standards, and if the equipment isn’t tested or used on the proper schedule, the lower approaches can’t be legally flown until the mechanics can do their magic.

I recently had an airplane that had a problem with the plug for the external power cord that plugs into the jetway. This isn’t that big of a deal, but it can create some issues. With external power, the APU has to be run to provide electricity to the airplane, and APUs burn fuel, and fuel is expensive. It also means that between flights or crew changes, the APU has to be left running or the airplane has to be completely powered down. Some carriers—my first one, in fact—have strict rules about leaving an APU running unattended. Others have enough people trained to shut the APU down so that it isn’t that big of a deal, but it still costs money.

In our case, the deferral of the plug was on the release, and finding the airplane with the APU already running was no surprise. But we were doing a late flight to the outstation, so the company made sure that the outstation knew not to try to plug in the airplane (a maintenance sticker on the plug door would have theoretically alerted the ground crew). Further, the local mechanic was waiting for us, which meant that he could get his work done as quickly as possible and shut the airplane down for the night.

Some deferrals are relatively minor—like this one was—and some are far more complicated than they should be. But MELs are a part of daily airline life. The captain is responsible for being familiar with any deferrals, and both pilots need to know how to properly use the actual MEL document, which can run hundreds of pages. When in training, take the time to become familiar with the layout (it is standardized) and some of the restrictions specific to your company. Most important, know how to determine if an MEL is expired so that you don’t fly with something illegal.—Chip Wright