A 2015 Pentagon report found these issues:
The Joint Program Office is re-categorizing or failing to count aircraft failures to try to boost maintainability and reliability statistics;
Testing is continuing to reveal the need for more tests, but the majority of the fixes and for capability deficiencies being discovered are being deferred to later blocks rather than being resolved;
The F-35 has a significant risk of fire due to extensive fuel tank vulnerability, lightning vulnerability and an OBIGGS system unable to sufficiently reduce fire-sustaining oxygen, despite redesigns;
Wing drop concerns are still not resolved after six years, and may only be mitigated or solved at the expense of combat maneuverability and stealth;
The June engine problems are seriously impeding or preventing the completion of key test points, including ensuring that the F-35B delivered to the Marine Corps for IOC meets critical safety requirements; no redesign, schedule, or cost estimate for a long-term fix has been defined yet, thereby further impeding g testing;
Even in its third iteration, the F-35’s helmet continues to show high false-alarm rates and computer stability concerns, seriously reducing pilots’ situational awareness and endangering their lives in combat;
The number of Block 2B’s already limited combat capabilities being deferred to later blocks means that the Marine Corps’ FY2015 IOC squadron will be even less combat capable than originally planned;
ALIS software failures continue to impede operation, mission planning, and maintenance of the F-35, forcing the Services to be overly reliant on contractors and "unacceptable workarounds";
Deficiencies in Block 2B software, and deferring those capabilities to later blocks, is undermining combat suitability for all three variants of the F-35;
The program’s attempts to save money now by reducing test points and deferring crucial combat capabilities will result in costly retrofits and fixes later down the line, creating a future unaffordable bow wave that, based on F-22 experience, will add at least an additional $67 billion in acquisition costs; and
Low availability and reliability of the F-35 is driven by inherent design problems that are only becoming more obvious and difficult to fix.
Three different types of data "massaging" are identified in the DOT&E report:[202] moving failures from one category to another, less important one; ignoring repetitive failures, thus inflating numbers of failure-free hours; and improper scoring of reliability.[203] Maintenance problems were determined to be so severe that the F-35 is only able to fly twice a week. To address the issue of wing drop and buffet maneuvering, the required control law modifications will reduce the maneuverability of the F-35, "only exacerbating the plane’s performance problems in this area". The F-35C's wing drop problem is "worse than other variants". Testing to investigate the impact of buffet and transonic roll-off (TRO or "wing drop") on the helmet-mounted display and offensive and defensive maneuvering found that "buffet affected display symbology, and would have the greatest impact in scenarios where a pilot was maneuvering to defeat a missile shot." Buffeting also degrades the gyroscopes in the inertial platforms which are essential for flight control, navigation, and weapons aiming. DOT&E explained that this was an ongoing issue: "In heavy buffet conditions, which occur between 20 and 26 degrees angle of attack, faults occurred in the inertial measurement units (IMUs) in the aircraft that degraded the flight control system (two of three flight control channels become disabled), requiring a flight abort."[204]

In early 2015 the AF-2 F-35A, the primary flight sciences loads and flutter evaluation aircraft, was flown by Lockheed Martin F-35 site lead test pilot David "Doc" Nelson in air-to-air combat maneuvers against F-16s for the first time and, based on the results of these and earlier flight-envelope evaluations, said the aircraft can be cleared for greater agility as a growth option. AF-2 was the first F-35 to be flown to 9g+ and -3g, and to roll at design-load factor. Departure/spin resistance was also proven during high angle-of-attack (AOA) testing which eventually went as high as 110 deg. AOA. "When we did the first dogfight in January, they said, ‘you have no limits,’" says Nelson. "It was loads monitoring, so they could tell if we ever broke something. It was a confidence builder for the rest of the fleet because there is no real difference structurally between AF-2 and the rest of the airplanes." "Pilots really like maneuverability, and the fact that the aircraft recovers so well from a departure allows us to say [to the designers of the flight control system laws], ‘you don’t have to clamp down so tight,’" says Nelson.[205]

With the full flight envelope now opened to an altitude of 50,000 ft, speeds of Mach 1.6/700 KCAS and loads of 9 g, test pilots also say improvements to the flight control system have rendered the transonic roll-off (TRO) issue tactically irrelevant. Highlighted as a "program concern" in the Defense Department’s Director of Operational Test and Evaluation (DOT&E) 2014 report, initial flight tests showed that all three F-35 variants experienced some form of wing drop in high-speed turns associated with asymmetrical movements of shock waves. However, TRO "has evolved into a non-factor," says Nelson, who likens the effect to a momentary "tug" on one shoulder harness. "You have to pull high-g to even find it." The roll-off phenomena exhibits itself as "less than 10 deg./sec. for a fraction of a second. We have been looking for a task it affects and we can’t find one."[205]

In July 2015, Lockheed Martin confirmed the authenticity of a leaked report showing the F-35 to be less maneuverable than an older F-16D with wing tanks.[206][207] The pilot who flew the mission reported inferior energy maneuverability, a limited pitch rate and flying qualities that were "not intuitive or favorable" in a major part of the air-combat regime gave the F-16 the tactical advantage. In general the high AoA capabilities of the jet could not be used in an effective way without significantly reducing follow-on maneuvering potential. In an interview with CBC Radio broadcast 2 July 2015, military journalist David Axe claimed to have read the leaked report and stated: "Against a determined foe, the F-35 is in very big trouble."[208] However, the F-35 used was a flight test aircraft with a restricted flight envelope and lacked some features present on the operational aircraft.[209][210] The Pentagon, JPO, and defense analysts have defended the F-35's utility in spite of the report's assertion that it lacks maneuverability by saying it was designed primarily to disrupt the kill chain of advanced air defenses while the F-22 would handle close-in dogfighting, it poses advanced sensor and information fusion capabilities to detect and engage enemy aircraft at long ranges before it can be seen and merged with, and that most air combat in recent decades has focused on sensors and weapons that achieved long-range kills rather than close combat.[211][212]

In the report's conclusions and recommendations it was noted that loads remained below limits, which implied there may be more maneuverability available to the airframe. There were five recommendations made: to increase pitch rate and available Nz (Normal Acceleration g) to provide the pilot with more maneuverability options given the inherent energy deficit; consider increasing alpha onset to also help offset the energy maneuverability deficit; consider increasing the beginning of the high AoA blended region to 30 degrees or greater to make high AoA maneuvering more predictable and intuitive; consider increasing pilot yaw rate to remove the gradual sluggish yaw response; and improve HMD Boresight performance to account for dynamic maneuvers and consider improving rearward visibility by creating more space for helmet motion.[206][213]