U.S. Code of law explicitly limits the total number of vice admirals that may be on active duty at any given time. The total number of active-duty flag officers is capped at 162 for the navy. For the Navy, no more than 16.7% of the service's active-duty flag officers may have more than two stars. Some of these slots can be reserved by statute. Officers serving in certain Defense Agency Director positions such as the Director of the Defense Logistics Agency, when filled by a naval officer, are vice admirals. The Superintendent of the United States Naval Academy is usually a vice admiral, either upon nomination or shortly thereafter. The President may also add vice admirals to the Navy if they are offset by removing an equivalent number of three-star officers from other services. Finally, all statutory limits may be waived at the President's discretion during time of war or national emergency.
Appointment and tour length
The three-star grade goes hand-in-hand with the position of office it is linked to, so the rank is temporary. Officers may only achieve three-star grade if they are appointed to positions that require the officer to hold such a rank. Their rank expires with the expiration of their term of office, which is usually set by statute. Vice admirals are nominated for appointment by the President from any eligible officers holding the rank of rear admiral or above, who also meet the requirements for the position, under the advice or suggestion of the Secretary of Defense, the applicable service secretary, or the Chairman of the Joint Chiefs of Staff. The nominee must be confirmed via majority vote by the Senate before the appointee can take office and thus assume the rank. The standard tour length for most vice admiral positions is three years but some are set four or more years by statute. Extensions of the standard tour length can be approved, within statutory limits, by their respective service secretaries, the Secretary of Defense, the President or Congress but these are rare, as they block other officers from being promoted. Some statutory limits under the U.S. Code can be waived in times of national emergency or war. Three-star ranks may also be given by act of Congress but this is extremely rare.
Retirement
Other than voluntary retirement, statute sets a number of mandates for retirement. Three-star officers must retire after 38 years of service unless appointed for promotion or reappointed to grade to serve longer. Otherwise all flag officers must retire the month after their 64th birthday. The Secretary of Defense, however, can defer a three-star officer's retirement until the officer's 66th birthday and the president can defer it until the officer's 68th birthday. Flag officers typically retire well in advance of the statutory age and service limits, so as not to impede the upward career mobility of their juniors. Since there is a finite number of three-star slots available to each service, typically one officer must leave office before another can be promoted. Maintaining a three-star rank is a game of musical chairs; once an officer vacates a position bearing that rank, he or she has no more than 60 days to be appointed or reappointed to a job of equal or higher importance before he or she must involuntarily retire. Historically, officers leaving three-star positions were allowed to revert to their permanent two-star ranks to mark time in lesser jobs until statutory retirement, but now such officers are expected to retire immediately to avoid obstructing the promotion flow.
