In what circumstances could a 787 stay powered on continuously for 248 days? The FAA has issued an airworthiness directive for the 787:


  SUMMARY: We are adopting a new airworthiness directive (AD) for all
  The Boeing Company Model 787 airplanes. This AD requires a repetitive
  maintenance task for electrical power deactivation on Model 787
  airplanes. This AD was prompted by the determination that a Model 787
  airplane that has been powered continuously for 248 days can lose all
  alternating current (AC) electrical power due to the generator control
  units (GCUs) simultaneously going into failsafe mode. This condition
  is caused by a software counter internal to the GCUs that will
  overflow after 248 days of continuous power. We are issuing this AD to
  prevent loss of all AC electrical power, which could result in loss of
  control of the airplane.


I'm curious: Are there any circumstances under which a 787 would remain powered continuously for 248 days?

[Edited to add a follow-on question:]

Boeing has said "If there is a definitive record of a powercycle within the last 120 days, no operator action is immediately required."  Does that happen? 120 days?

What is the longest period a passenger aircraft might conceivably be continuously powered on?
Aircrews seldom have to power off an aircraft completely (also known as a cold and dark cockpit). Airliners usually stay powered on at the gate. This is known as a "short turn-around": engines are stopped, the APU is stopped, but electrical power and air conditioning is still supplied by ground equipment.

This has the advantage of minimizing turn-around time (after all, airliners are meant to stay in the air to make money). The cabin also has to be cleaned and prepared for the next flight at this time, so something must power the lights.

It is theoretically possible to stay on this way for 248 days. However, airliners have scheduled maintenance, which completely powers off the aircraft. They are also powered off if the expected time at the gate is long. So, in reality, it's very unlikely to have an airliner powered continuously for 248 days.