Aircraft will inevitably have maintenance checks (A, B, C, D) as well as tyre changes and other more minor checks. The standard C-D checks involve hangar time and will be scheduled by the airline in advance and the other checks can be done at the gate or on stand overnight.
Most airlines will have more aircraft than routes and there will be some spare capacity. In the worst case, too many poorly aircraft, or too many routes, aircraft will be leased in. E.g. BMI
and their Arkefly B767 and FI
once had a UA
B767 fly into MAN
after an A330 went tech.
An example on utilisation is the BA
B767 that does the MAN
route and can do so for weeks or months, if not years on end before being rotated with another LHR
aircraft. Presently BNWT since 19th Feb. Before that BNWN from 19 Jan, 'WM
from 24 Nov, 'WW
from 28th Sept. The BA
767s have being going through a cabin upgrade so there's been more of a change in the aircraft on the route than in the past, e.g. in 2003-2005 when 'WH did the route pretty well all the time.