Because UAVs are not burdened with the physiological limitations of human pilots, they can be designed for maximized on-station times. The maximum flight duration of unmanned, aerial vehicles varies widely. Internal-combustion-engine aircraft endurance depends strongly on the percentage of fuel burned as a fraction of total weight (the Breguet endurance equation), and so is largely independent of aircraft size. Solar-electric UAVs hold potential for unlimited flight, a concept originally championed by the AstroFlight Sunrise in 1974 and the much later Aerovironment Helios Prototype, which was destroyed in a 2003 crash.
Electric UAVs kept aloft indefinitely by laser power-beaming  technology represent another proposed solution to the endurance challenge. This approach is advocated by Jordin Kare and Thomas Nugent.
One of the major problems with UAVs is no capability for inflight refueling. Currently the US Air Force is promoting research that should end in an inflight UAV refueling capability, which should be available by 2010.
One of the uses for a high endurance UAV would be to “stare” at the battlefield for a long period of time to produce a record of events that could then be played backwards to track where improvised explosive devices (IEDs) came from. Air Force Chief of Staff John P. Jumper started a program to create these persistent UAVs, but this was stopped once he was replaced.
The Defense Advanced Research Projects Agency (DARPA) is to sign a contract on building an UAV which should have an enormous endurance capability of about 5 years. The project is entitled “Vulture” and a September 15, 2010 news release indicated DARPA’s Vulture Program Enters Phase II. The developers are certain neither on the design of the UAV nor on what fuel it should run to be able to stay in air without any maintenance for such a long period of time.
|QinetiQ Zephyr Solar Electric||82 hours 37 minutes||28–31 July 2008|||
|Boeing Condor||58 hours, 11 minutes||1989||The aircraft is currently in the Hiller Aviation Museum, CA.|
|QinetiQ Zephyr Solar Electric||54 hours||September 2007|||
|IAI Heron||52 hours||?|||
|AC Propulsion Solar Electric||48 hours, 11 minutes||June 3, 2005|||
|MQ-1 Predator||40 hours, 5 minutes||?|||
|TAM-5||38 hours, 52 minutes||August 11, 2003||Smallest UAV to cross the Atlantic|
|Aerosonde||38 hours, 48 minutes||May 3, 2006|||
|I-GNAT||38 hours, landed with 10-hour reserve||?|||
|RQ-4 Global Hawk||36 hours||?|||
|Aerosonde “Laima”||26 hours, 45 minutes||August 21, 1998||First UAV to cross the Atlantic|
|TAI Anka||24 hours||March 2010 ||[verification needed]|
|Vulture (UAV)||Has not flown. Potential endurance 5 years||?||A DARPA project|
|RQ-2 Pioneer||5.5 Hours||?||The US Navy retrieves the Pioneer by flying it into a large net.|
|RQ-7 Shadow||4 Hours||?||Used by both the US Army and the US Marine Corps.|
|RQ-11 Raven||80 min.||?||The Raven UAV can be launched by throwing it into the air.|
|MQ-8 Fire Scout||+5 hours||?||The MQ-8 is designed to autonomously take off and land on any aviation-capable ship or confined land area.|