WASHINGTON — It was a hot day aboard the amphibious assault ship Essex when a pilot brought his F-35B in for what is known as a “mode four” flight operation, where the jet enters hover mode near a landing spot, slides over to the target area and then vertically lands onto the ship.

It’s a key part of the F-35B’s short-takeoff-and-vertical-landing capability, known as STOVL. And normally, everything in a “mode four” landing goes smoothly. But on this day, when the pilot triggered the thrust to slow his descent, something went wrong.

The engine, working hard on a day that temperatures cracked 90 degrees Fahrenheit while trying to lift a plane that was heavier than most returning to base, wouldn’t generate the needed thrust for a safe, ideal landing.

The pilot got the plane down, but was shaken enough by the situation to write up an incident report that would eventually be marked as “high” concern by the F-35 program office.

“May result in unanticipated and uncontrolled sink, leading to hard landing or potential ejection/loss of aircraft, particularly in the presence of HGI [hot gas ingestion],” reads a summary of the issue, which was obtained by Defense News as part of a cache of “for official use only” documents that detail major concerns with the F-35 Joint Strike Fighter.

The issue could impact future F-35B operations in the Middle East, where temperatures are climbing as summer approaches.

This could also be bad news for industry, as F-35 program head Vice Adm. Mat Winter indicated his belief that the fix, which he called the Marine Corps’ “No. 1 priority” for the F-35 program, should be paid for, at least in part, by the big contractors who designed the aircraft.

Still, Winter expressed confidence that the landing issue, which has so far proven to be a one-off incident, will be addressed by a series of fixes that should be in place by April 2020.

“We’re not done yet, and the Marine Corps will tell you we’re not done yet until we see the fix in the fleet, because that’s where we are,” Winter said. “That is the only way a STOVL aircraft lands on an L-class [amphibious] ship, so it’s important.”

Technical details

The issue seems to stem from two factors: the heat, and the fact that much of the testing for the “mode four” maneuver was done with planes that were lighter, as they weren’t armed with heavy stores of weaponry.

Feedback from the Marine Corps highlighted that while the average engine should not see this issue until around 750 flight hours, “several engines are at/near the point of concern,” and that number will continue to grow with the extended use of older planes.

Rebecca Grant, an analyst with IRIS Independent Research, said the issue seems to be a variant of the traditional “high-hot” problem, where hot temperatures make the air less dense.

“All engines are less efficient when super hot days reduce engine power and lift. Think of the helicopters and even tankers flying ‘high-hot’ mission parameters in Afghanistan or commercial jets out of Mexico City,” she explained. “Although air is dense at sea level, the heat surge slightly changed engine power.”

Winter said engineers have identified an issue in the design of the control software that the pilot uses to generate demand for thrust from the propulsion system.

“There’s no redesign of the engine [necessary]. The engine is doing what the engine is supposed to do,” Winter emphasized, before acknowledging that in addition to the software fix, the program office has worked with Honeywell to change how the company calibrates the throttle valve on the engine.

“We’ve identified the software fix for the control system, the calibration fix to the throttle valve and some near-term fleet actions that could be taken for very hot days to ensure that the pilot gets the performance he or she needs on those hot days,” he said.

That software fix will be a rolling target, as the first increment of the software release is due in June, followed by another at the end of this year or early next year.

“We’ve given them tighter tolerances to tune them more precisely, so that when it goes on the engine it’s no longer not giving the command the way it’s supposed to be,” Winter explained. “It wasn’t tuned correctly for this high-demand phase of flight. Now, we fixed that. That’s fixed. The software is going in to make sure that the pilot can command that thrust and understand the heat and the loading.”

Those fixes won’t be cheap, and when asked who would pay for them, Winter was blunt, saying it is his office’s belief the thrust issue is a “design deficiency” that merits “consideration” from industry.

“In this case it doesn’t matter that the design was done back in 2002, it’s still pragmatistic, so you owe consideration because we’re fixing it right now,” Winter said of industry.

A U.S. Marine Corps pilot from the first forward-deployed F-35 unit tells Defense News what it's like to vertically land an F-35B, at the 2018 Singapore Airshow. (Jeff Martin/Staff)

Temporary solutions

The real test is going to be how the fixes perform in the field, given the F-35B’s 2018 deployment into the Middle East shows the jet will be used in a region known for lacking cool summer days.

When asked if the issue could impact operations in the region, Winter acknowledged it could during “very hot days.”

“I will not go on the record to say that there hasn’t been [an effect on operations]. There has been operational impact — that’s how we found this, and now we are implementing the fix to eliminate that operational impact, and the war fighter right now is mitigating that operational impact through the mechanisms and techniques we’ve provided them,” he said.

And until the fix is fully in place, pilots operating the F-35B can do a few things to mitigate the risk of a hard landing. First, make sure to wash the blades on the engine more frequently to avoid the buildup of salt or dirt that can make the system less efficient. Second, the squadron commander will need to think about load management, making sure aircraft aren’t returning too heavy with fuel and weapons.

“It’s wind over the deck. It’s aircraft stores loading. It’s those types of operational activities that a war fighter already takes into account,” Winter explained.

Richard Aboulafia, an aviation expert with the Teal Group, agreed weight matters, saying that high-hot issues can often “be dealt with easily, but often at the expense of weight, which can impact range and payload.”

Grant also noted that Marine pilots will be able to adjust how they land, now that the issue is a known problem, adding that in comparison to the old Harrier Jump Jets, “the F-35B actually does way better than the Harrier in controlling its heat downwash.”

Valerie Insinna in Washington contributed to this report.