Pentagon reports estimated Global Hawks were able to bring back only about 40 percent of the information they needed to deliver when used "at low operational tempos," meaning when the drones were grounded for maintenance.
When operating at near-continuous operational tempos, the system provides less than half the required 55 percent Effective-Time-On-Station (ETOS) coverage over a 30-day period. Frequent failures of mission critical air vehicle components reduce takeoff reliability and increase mission abort rates, which reduces ETOS performance.
"Rapid depletion of available spare parts reduces air vehicle availability to support additional missions at near-continuous operational tempos. A Global Hawk CAP can consistently generate sorties at a lower operational tempo of up to three sorties per week, when sufficient spare parts are available. However, these individual sorties collectively produce only 42 percent of the “tasked” ISR coverage time due to poor takeoff reliability, maintenance ground aborts, and high air abort rates. Current and planned Air Force reliability improvement activities will improve system reliability. In the interim, operational commanders should anticipate low air vehicle mission capable rates, spare part shortages, and a heavy reliance on system contractor support to sustain operations when attempting to conduct operations at near-continuous operational tempos….
… "The RQ-4B Global Hawk Block 30 is not operationally suitable. " –RQ-4B Global Hawk Block 30 Operational Test and Evaluation Report, May 2011.
The inadequately explained crash of a Global Hawk in the Frontier territory of Pakistan near the Afghan border Aug. 20, 2011 didn't help the drone's reputation for reliability, predictability or stealth.
From a distance it does look as if the more-reliable, pilot-controlled U-2 simply outperformed the Global Hawk in both capability and operational cost, the truth appears to be that the Global Hawk shot down its own future by lolling around the hangar being repaired when it should have been out working.
It didn't help that, when it did get into the air, eyesight that was dimmer than it should be and an adamant refusal to effectively carry the big, heavy, ancient Optical Bar Camera, even if wet-film cameras should have been as obsolete as pilots who have to ride along with their aircraft to let either one do their jobs.
When budgets are being cut and patience is short, decision-makers (and, apparently, Congress) are simply not willing to wait years until new digital gadgetry reaches the quality level of old, heavy Cold War-era technology.
U.S. Air Force