The Federal Aviation Administration (FAA) has stringent mechanical restrictions an airplane must meet. This includes strict standards on the operating condition of the airplane as well as required regular maintenance checks. Any deviation from normal operations on an airplane must be resolved before the plane can depart.
Repairs could take anywhere from a few minutes to several hours. If you've already boarded the plane, you may be forced to wait on board during the repair. This could be particularly annoying if the plane has to cut its power to make the repair, leaving you stuck inside without light, air movement or temperature control.
In extreme cases, the airline may be unable to complete the repair while you're waiting on the tarmac. In those cases, the airline will let passengers deplane and wait at the gate, especially to avoid federal fines for tarmac delays exceeding three hours [source: Stoller]. Sometimes, the airline cancels the flight and reschedules passengers for later flights. If you're stranded by these extended mechanical delays, be patient and courteous to the airline staff members. Keep in mind that they're probably as frustrated as you are about the situation. Cooperate with them on finding your best option to get going again.