A D-Check is accomplished every certain amount of hours and cycles. Every operator has a different maintenance program, which allows them to have the aircraft grounded for less time. When it comes to a D-Check, that is a Heavy Check! Now days, a D-Check is not common, because of the amount of time that the aircraft stays on the ground (usually from 30-45 days, and more). Basically what many airlines do is a C-Check (which takes less time) and finish the rest through a process called "Progressive Maintenance."
Every A, B, C, or D-Check is accomplished depending on the operator's program, which of course it needs to be approved by the FAA.
I can't wait to see UAL B744 in new colors!
Hope this helps!
[Edited 2004-12-01 22:43:41]