New problems disclosed on 2 models of F-35

The F-35 Lightning II strike fighter has previously undisclosed problems with its handling, avionics, afterburner and helmet-mounted display, according to a report by the Pentagon’s Director of Operational Test and Evaluation.

Both the Air Force F-35A variant and Marine Corps’ F-35B model experienced “transonic wing roll-off, [and] greater than expected sideslip during medium angle-of-attack testing,” the report said.

The report also says that various components are not as reliable as expected.

Additionally, the Pratt and Whitney F-135 engine has encountered an afterburner “screech,” in which airflow disruptions cause severe vibrations, preventing the engine from reaching maximum power. That problem has delayed some required testing.

According to the report, the program has already begun efforts to fix the problem. Pratt and Whitney officials were not immediately available for comment.

Further, the report indicates problems with the aircraft’s helmet-mounted display. Unlike many previous aircraft, the F-35 does not have a cockpit-mounted head-up display; the pilot instead views critical data projected on the helmet visor.

The report does not elaborate on the nature of the problems, but says they must be solved before the Block 2 mission systems software can be tested. Currently, the program is testing preliminary Block 0.5 and Block 1 mission systems software. Block 2 would incrementally increase the aircraft’s capabilities and would be followed by the fully mission-capable Block 3 software.

A Lockheed Martin official could not immediately describe the technical problems with the display.

“The F-35 air system advances Helmet Mounted Display technology to capabilities not flying today on any other tactical platform. With this advancement in technology come challenges that the program is actively managing. The challenges are being worked with the supplier,” said Lockheed Martin spokesman John Kent.

“While there are no current plans to change suppliers, options are being considered in parallel that mitigate the most stressing issues. Flight testing is proceeding with the HMD installed and used with no safety of flight concerns.”

The report also calls for the Block 3 mission system software to be tested on a simulated battlefield because existing test ranges are not adequate to test the F-35’s sensor arrays.

“Open-air testing is constrained by range limitations that are incapable of providing realistic testing of many key capabilities provided by Block 3 aircraft,” the report says.

The report also calls for the aircraft’s On-Board Inert Gas Generations System, which generates inert gases to prevent oxygen building up inside the fuel tanks, to be redesigned.

“The OBIGGS system fails to inert the fuel tank ullage spaces throughout the combat flight envelopes evaluated,” the report says.

The report recommends program officials redesign the OBIGGS system “to ensure that the fuel tank ullage volume oxygen concentrations are maintained below levels that sustain fire and/or explosion throughout the combat flight envelopes.”

These issues are in addition to the known difficulties with the B-model aircraft’s insufficiently strong structural bulkhead and problems with auxiliary air inlet doors on the aircraft’s top surface.