"Operational suitability of the F-35 fleet remains below service expectations", Behler told the US House Armed Services Subcommittee on Readiness and Tactical Air and land Forces. "No F-35 variant meets these specified reliability or maintainability metrics".
Behler explained that the F-35 aircraft breaks down more often and the necessary repairs take more time than expected.
However, Behler pointed out that all participants in the US-led international F-35 program are making good progress.
"The Department of Defense is collaborating closely with its partners to evaluate the F-35 survivability and readiness", Behler said.
The fifth-generation jet has three variants: the F-35A conventional takeoff and landing variant, the F-35B short takeoff/vertical landing variant and the F-35C aircraft carrier variant.
Earlier in the day, the Government Accountability Office presented a report saying the F-35 fighter jet fleet has a lack of parts and the underperformance of the Autonomic Logistics Information System (ALIS).