The Defense Department has relaxed some of its flight restrictions on F-35 test aircraft after an engine issue in June led the Pentagon to ground its fleet.

F-35 Joint Program Office (JPO) spokesman Joe DellaVedova said Thursday F-35 test aircraft are now allowed to fly six hours between flights only in two specific scenarios: transit to ranges for weapons testing and for in-flight refueling tests. All other flights, he said, remain on the original inspection regimen, which he did not specify and did not respond to emails for clarification.

The Marine Corps' F-35B. Photo: Lockheed Martin.
The Marine Corps’ F-35B. Photo: Lockheed Martin.

DellaVedova said test aircraft are cleared to fly to speeds up to Mach 1.6 and to G-forces of 3.2 Gs. He added that the F-35 “angle-of-attack” remains at 18 degrees and flight time is still restricted to three hours for most other test missions.

There have been no changes to engine inspections and flight restrictions for the operational fleet, DellaVedova said. The news was first reported by Reuters.

A fire to an Air Force F-35A conventional variant June 23 forced the Pentagon to eventually ground its entire fleet for 11 days. A preliminary investigation revealed an “extensive rub” of a third-stage fan blade in the F135 engine as a key factor in the runway fire. The Air Force’s fleet had no-fly orders for 22 days. Marine Corps and Navy versions of the Joint Strike Fighter (JSF) went out of service and returned to flight before a formal Pentagon-wide grounding was announced. (Defense Daily, July 15).

The Marine Corps’ F-35B variant also recently completed wet runway and crosswind testing, which prime contractor Lockheed Martin [LMT] called an important milestone toward F-35B initial operational capability (IOC) certification. J.D. McFarlan, Lockheed Martin vice president for F-35 test and verification, said in a statement this testing was critical to block 2B flight software fleet release.

Lockheed Martin spokesman Eric Schnaible said recently two separate testing regimes were conducted by Naval Air Station Patuxent River, Md.,-based aircraft, called “BF-4,” that were deployed to California for testing between April and June. Schnaible said the wet runway testing was to validate F-35B braking performance, anti-skid system operation and aircraft handling characteristics.

Different amounts of water were applied on the main runway at Edwards AFB, Calif., to attain specific surface friction conditions to evaluate both conventional and short takeoff and vertical landing (STOVL) mode operations capable on the F-35B. All test points were successful with no skidding nor hydroplaning events experienced, Schnaible said. No significant crosswind was present for this phase of testing, he added.

Schnaible said crosswind testing was also performed at Edwards and Naval Air Weapons Station China Lake, Calif., where F-35Bs landed in crosswinds of up to 25 knots, validating flight handling qualities in both conventional and STOVL modes. Crosswind and wet runway testing validate the F-35B’s ability to operate in a wide range of weather conditions, Schnaible said.

Wet runway and crosswind testing is important to F-35B software, Schnaible said, because the vehicle systems software used for both test phases represent the capabilities that will be delivered to the Marine Corps as the block 2B fleet release and the IOC configuration. He added the testing also completed all planned wet runway testing for the entire system design and development (SDD) program for the F-35B and demonstrated overall software maturity.

While the JPO has struggled with software development, Schnaible said it believes delays will not preclude the program office from meeting any IOC dates. Though Schnaible said a report to Congress projected a delay to delivering software block 3F to flight test, JPO believes this schedule risk is approximately six months, “unmitigated,” if the F-35 enterprise takes no actions to improve software delivery. The Government Accountability Office (GAO) said in a March report that F-35 developmental test flight delays could hold back F-35B IOC by 13 months (Defense Daily, March 24).

Schnaible said the report also included eight specific recommendations for improvements in systems and software development and test, all of which the program office concurs with and is in process of implementing. DellaVedova said in June multi-sensor fusion involving the information the F-35 collects is delaying software block 3F testing. He said testing for block 3F, the full warfighting capability for all variants, is scheduled to be completed by late 2017/early 2018 (Defense Daily, June 12).

The F-35 is developed by Lockheed Martin with subcontractors BAE Systems and Northrop Grumman [NOC]. The F135 engine is developed by Pratt & Whitney of United Technologies Corp. [UTX]