The Pentagon’s top weapons tester is once again sounding the alarm over the F-35 Joint Strike Fighter (JSF), warning that significant deficiencies with the aircraft’s gun, challenges integrating the short-range AIM-9X missile and unresolved software bugs could delay fielding of the fighter’s full capability.
On the heels of the U.S. Air Force’s milestone decision to declare the F-35A ready for war, the Defense Department’s director of operational test and evaluation (DOT&E) is raising new concerns about Lockheed Martin’s fifth-generation fighter.
In an internal memo to Defense Department leadership on August 9, DOT&E warned that the jet still has a long way to go before full combat capability and may run out of funds to fix significant performance problems on time if late discoveries delay the end of the program’s development phase.
Before kicking off a final test period that will put the F-35 in its final warfighting configuration through its paces, known as initial operational test and evaluation (IOT&E), the Joint Program Office (JPO) still has a significant amount of development testing to complete and a number of problems to fix, DOT&E tells Aviation Week.
Still, the JPO is confident it will complete the F-35 program on time and budget, JPO Chief Lt. Gen. Christopher Bogdan tells Aviation Week.  
“There were absolutely no surprises in the recent memo from the OSD Director of Operational Test and Evaluation,” Bogdan says. “Specific to the memo, the JPO has been and is currently acting on all the recommendations.”
Weapons Integration Challenges
At the top of DOT&E’s list of concerns is the Air Force F-35A’s 25mm Gatling gun, which will be the jet’s primary means of delivering close-air support to soldiers on the battlefield. Most recently, May testing revealed the small door that opens when the gun is fired induces yaw, or sideslip, resulting in aiming errors, according to DOT&E spokesman Maj. Roger Cabiness. Software changes may be necessary to fix the problem, he says.
This news comes late in the game for the gun system, which is scheduled to begin accuracy testing later this year to prepare it for fielding in 2018. These modifications and subsequent testing to ensure the fixes work, as well as additional changes to correct deficiencies found during early tests in 2015, could delay the start of accuracy testing, DOT&E warns. The U.S. Marine Corps F-35B and U.S. Navy F-35C gun pods are even further behind in testing, so new discoveries that could require late fixes are also possible.
Given the ongoing challenges and risk to the start of accuracy testing, the F-35 may not be able to field its gun on time, DOT&E concludes.
Weapons integration, including the main gun pod, is central to the final Block 3F software load, which is meant to give the F-35 its full warfighting capability. Block 3F also will incorporate external weapons like the precision-guided Small Diameter Bomb (SDB 1) and the short-range air-to-air AIM-9X Sidewinder missile.
But the program is discovering integration problems with both SDB 1 and AIM-9X. DOT&E is particularly concerned with December testing of the AIM-9X, which revealed “load exceedances,” or excess stress, on the Navy F-35C variant’s wing structure during landings and certain maneuvers. This will either limit the F-35C’s ability to carry AIM-9X or require a redesign and testing of the supporting wing structure, DOT&E says.
“Planned weapons testing is behind schedule for SDD [the System Development and Demonstration period] due to software and structural discoveries that delayed testing,” Cabiness warns. “Weapons testing is one of several risk areas affecting completion of flight testing and the start of IOT&E.”
The program recently completed a record-breaking 25 weapons tests during a month-long surge effort with the newest 3F software, a marked jump from the previous high of three in one month, according to the JPO. However, the data has not yet been analyzed, DOT&E says.
Software Problems Still Unresolved
The F-35 JPO has corrected a bug in a previous iteration of software—Block 3i required for Air Force initial operating capability (IOC)—that caused the jet’s systems to stall out in mid-flight and need to be rebooted. But Aviation Week reported in July that at Edwards AFB, California, the team in charge of testing each new increment of software is now seeing the same stability events and shutdown issues with the new 3F.
Where test pilots were seeing shutdown events every five flight hours, a new 3F build improved performance to a point where the team is only experiencing an issue once every 9 hr., according to DOT&E. But that performance is still much worse than it should be; the final builds of 3i and 2B, which supported the Marine Corps’ IOC in 2015, experienced inflight stability events every 25 hr., according to DOT&E.
In addition, testers are continuing to see problems booting up the jet. Over a test period that ended July 6, none of the startups with the latest 3F build occurred without some type of reset or power recycle, Cabiness says.
Despite these setbacks, the JPO is pushing forward with 3F testing. The program elected to create “quick reaction capability” (QRC) test builds of software to finish the surge of weapons tests planned for this summer, according to DOT&E.
DOT&E deemed QRC a success: “Although analyses of the weapons events are ongoing, this process involving the QRC builds enabled the program to address deficiencies that were blocking completion of many of the [weapons delivery accuracy] events,” Cabiness says.
Problems Remain With USAF’s 3i Configuration
Top Air Force leaders have said repeatedly they are prepared to send the F-35 to war today if combatant commanders ask for it. But DOT&E cautions that the 3i software currently in use with the Air Force fleet is “an interim software build” and hence has limited capability. 
Cabiness detailed a long list of the limitations of aircraft flying in the 3i configuration. The jets can carry only two bombs and two missiles, and currently have no gun capability or standoff weapons as the 25mm cannon and external weapons won’t be introduced until 3F. Further, the F-35As have no target marking capability for close-air support and other missions, require voice communications to verify certain messages, and have poor geolocation capability, relying on off-board sources to locate threats and acquire targets.
Finally, the 3i jets have limited night vision capability, as the $400,000 Generation III helmet is still experiencing issues with light leakage and “green glow” that obscures pilots’ vision during very dark night flights.
DOT&E also pointed to deficiencies in 3i’s sensor fusion, electronic warfare, datalinks and pilot vehicle interfaces “that will impact mission effectiveness and suitability in combat.”
The JPO is aware of the limitations of 3i, according to a spokesman. Still, Bogdan has lauded the Air Force’s decision to declare IOC on Aug. 2.
“The U.S. Air Force decision to make the 15 F-35As at Hill Air Force Base combat ready sends a simple and powerful message to America’s friends and foes alike—the F-35 can do its mission,” Bogdan said in an Aug. 2 statement.
Test Aircraft May Not Be Ready For IOT&E
The JPO needs a certain number of production-representative operational test aircraft, retrofitted with all the modifications since the development program began, for IOT&E. But the program’s current schedule for providing these jets shows that the modifications won’t be completed until February 2020—three years after the scheduled August 2017 start of IOT&E, Cabiness says.
The JPO is considering actions to accelerate this schedule, but has yet to reach an agreement with the U.S. military services and put a plan on contract, he says.
In responses to Aviation Week’s queries, DOT&E reiterated concerns that the program’s official start date for IOT&E is “not realistic,” repeating the agency’s public assessment that the F-35 won’t be ready to begin the test period until the middle of calendar year 2018 at the earliest, assuming the JPO can provide production-representative test aircraft by then.
Given all the challenges the JPO has left to surmount, funding for the F-35 development effort will likely run out before the end of the program, Cabiness says.
“In light of the remaining challenges [and] the demonstrated rate of progress to date, DOT&E assesses that the program will likely need additional funding to complete SDD,” Cabiness says.