The U.S. Air Force grounded its U-2 intelligence aircraft worldwide last week due to fuel leaks found in at least three of the high-flying aircraft.

The grounding disrupted intelligence collection operations around the globe, including surveillance activities in Iraq. The U-2 is being augmented by the Global Hawk there, though the comparatively young unmanned aerial vehicle is not able to collect the same imagery and signals intelligence as the U-2.

The first aircraft was found to have a "pinhole" sized leak in its sump tank, a common tank that sends fuel directly to the U-2's engine. Later, two aircraft at Beale Air Force Base, Calif., the U-2's home base, and an additional aircraft at Lockheed Martin's Palmdale, Calif., maintenance facility were found to have similar problems. The problems were caused by a chafed wiring bundle next to the fuel tank. The wiring arched, causing the hole to emerge which posed a danger of fire, according to Air Force officials.

The decision was made to ground the fleet to ensure the safety of the pilots, they say. A message on the subject was sent from Air Combat Command at Langley AFB, Va., on March 13. Each aircraft is being inspected, and last week Air Combat Command expected the problem to occur in most of its 35 U-2s. A fix that requires at least three days is being implemented on the aircraft found to have problems.

Some of the aircraft have been returned to service, and one Pentagon source says that at least one U-2 is now operational at every foreign operating location. The aircraft support activities in the Pacific, Middle East and Europe.

Air Force Secretary Michael Wynne cited the U-2 wiring problem as an example of its trouble managing its aging air fleet. However, the U-2 is not among its oldest. Many B-52s, F-15s and KC-135s are older.