The US Department of Defense is lowering the performance bar for the Lockheed Martin F-35 Joint Strike Fighter according to a new report by the Pentagon's director of operational test and evaluation (DOT&E).
The specifications for all three variants pertaining to transonic acceleration and sustained turn rates have been reduced. Worst hit in terms of acceleration is the US Navy's F-35C carrier-based model.
"The program announced an intention to change performance specifications for the F-35C, reducing turn performance from 5.1 to 5.0 sustained g's and increasing the time for acceleration from 0.8 Mach to 1.2 Mach by at least 43 seconds," reads the report prepared by J Michael Gilmore, the Pentagon's DOT&E. "These changes were due to the results of air vehicle performance and flying qualities evaluations."
The US Air Force F-35A's time has slipped by eight seconds while the US Marine Corps short take-off vertical landing (STOVL) F-35B's time has slipped by 16 seconds. However, turn rates for both the A and B models have been impacted more severely than the USN variant. Sustained turning performance for the F-35B is being reduced from 5G to 4.5G while the F-35A sinks from 5.3G to 4.6G according to the report.
All three variants are having problems with their horizontal tails. "Horizontal tail surfaces are experiencing higher than expected temperatures during sustained high‑speed / high‑altitude flight, resulting in delamination and scorching of the surface coatings and structure," the report reads. "All variants were restricted from operations outside of a reduced envelope until the test team added instrumentation to the tailbooms to monitor temperatures on the tail surfaces."
Meanwhile, the F-35B and C variants continue to have issues with transonic roll-off and buffeting. On the F-35B, the program introduced vehicle systems software to reduce rudder and flaperon hinge moment in the transonic/supersonic region. "The program expected to see improvements in transonic wing roll-off with these changes, but results were not available at the end of November 2012," the report reads.
Transonic buffet is more severe on the F-35C compared to the other variants due to its larger wing. "The program is making plans for investigating how to reduce the impact of transonic roll-off in the F-35C with the use of wing spoilers; however, detailed test plans are not complete," the report reads.
Meanwhile, the aircraft's crucial helmet-mounted display still has problems with jittery images and is not meeting specifications for night vision acuity. Additionally, a new problem called "green glow" has been discovered where light from the cockpit avionics displays leak into the helmet-mounted display and degrade visual acuity. However, the image latency is now within tolerances. "Latency of the projected imagery from the DAS [distributed aperture system] is currently down to 133 milliseconds, below the human factors derived maximum of 150 milliseconds, but still requires additional testing to verify adequacy," the report reads.
Perhaps in worst shape is the F-35's software. According to the report, even the initial Block 1 software package is not complete, some 20% remains to be delivered and flight tested. An initial version of the more advanced, but still not combat capable, Block 2A software was delivered four months late to flight test. "In eight subsequent versions released to flight test, only a limited portion of the full, planned Block 2A capability (less than 50 percent) became available and delivered to production," the report reads. "The program made virtually no progress in the development, integration, and laboratory testing of any software beyond 2B. Block 3i software, required for delivery of Lot 6 aircraft and hosted on an upgraded processor, has lagged in integration and laboratory testing."
Meanwhile, structural durability testing continues, but the F-35B has hit a snag. "The program halted testing in December 2012 after multiple cracks were found in a bulkhead flange on the underside of the fuselage during the 7,000-hour inspection," the report reads. "Root cause analysis, correlation to previous model predictions, and corrective action planning were ongoing at the time of this report."
Lockheed could not immediately offer a substantive comment. "Our experts are going through it so it will be a while before we have detailed questions like yours answered," the company says, but adds, "From an Operational Test and Evaluation perspective, we fully expect to deliver a qualified product to OT&E as scheduled."