Promotion to squadron leader thereafter is strictly upon merit; officers promoted beyond flight lieutenant are appointed to a career commission, or service to age 60. Resigning a commission is generally dependent on the needs of the service, although an officer who has completed their return of service (service the RAF requires to justify its expense in originally training the officer) could leave after as little as four years. For aircrew, given the large expense required for training, this return of service is generally the length of their initial commission anyway, unless they re-role to a different branch having failed an element of flying training.