F-35 Fire: In Search Of A Solution
A version of this article appears in the September 8 issue of Aviation Week & Space Technology.
The long-delayed operational debut for the Lockheed Martin F-35 Joint Strike Fighter is at risk of further slippage due to a safety-critical problem with its Pratt & Whitney F135 engine for which no single root cause has yet been identified. Senior leaders initially dismissed the fire that damaged F-35 AF-27 at Eglin AFB, Florida, on June 23 as a “one-off incident,” as U.S. Marine Corps commandant Gen. James Amos called it in mid-July. However, Aviation Week has learned that at least five engines have been removed from F-35s after showing signs of premature wear.
The JSF flight-test team has until the end of this month to demonstrate and validate an operational work-around that will allow aircraft in the 21-strong systems development and demonstration (SDD) fleet to fly without the onerous flight restrictions that were imposed after the fire. If that is not done in time, the flight-test schedule that supports initial operational capability (IOC) dates will be in jeopardy, JSF Program Office Director Lt. Gen. Chris Bogdan said Sept. 3.
Meanwhile, Pratt & Whitney is working on redesigned engine components that are intended to prevent the problem from recurring, with the objective of lifting restrictions on the JSF training and tactics development fleet. While the investigation into the fire and related defects is ongoing, Bogdan says that 138 possible causes have been narrowed down to four. Pratt’s proposed design changes are planned on the assumption that further investigation will validate those findings and is intended to address any of the likely root causes.
No firm timelines or cost numbers have been established for redesign, production cut-in or retrofit, but Pentagon acquisition chief Frank Kendall said on Sept. 3: “I am getting more confident. I do not think it will be a huge cost [to fix the engine].”
The JSF program office declines to give details of the damage to AF-27 and says that it has not yet established whether the $205 million fighter (its program acquisition cost, according to the 2013 Selected Acquisition Report) can be restored to flyable condition. For Pratt & Whitney, the risk to the F-35 schedule emerges as another major test program—the Bombardier CSeries airliner—remains on hold due to a problem with a new P&W engine.
Five F135 engines have been pulled from F-35s after failing an inspection regime that was instituted after the June 23 fire. One of the failed engines was removed from F-35 CF-9 (the ninth production F-35C), an aircraft with fewer than 70 flight hours, according to a retired military officer and JSF program veteran.
Bogdan says the problem was triggered by an event three weeks before the fire, when AF-27 was flown in a benign maneuver, well within the normal envelope, that involved a specific combination of yaw, roll and vertical g loads. The engine “flexed” to an unexpected degree during the 2-sec. maneuver, Bogdan says. This caused a “hard rub” of a fan stage stator against the engine rotor. Frictional heating resulted in estimated material temperatures of 1,900F compared with a design temperature of 1,000F.
This temperature overrun led to -micro-cracking in the neighboring third-stage fan blades, according to Bogdan. The cracks subsequently grew in normal operations and caused the blades to separate from the disk. The engine casing failed to contain one or more of the failed blades, which punctured the adjacent left-and-aft fuel cell. The resultant mixing of jet fuel and superheated air caused the fire.
Signs of stator-to-rotor wear of the kind that is believed to have caused the failure are apparent in images provided to Aviation Week by the former program official. The images are of the engine removed from CF-9, he says. Acquired with a borescope—comprising a camera and flexible fiber-optic probe—they show the inner ends of cantilevered stator blades in the engine’s fan stage, and a knife-edge-type seal on the interstage section of the rotor. Aviation Week asked the JSF Program Office to confirm that the images were from CF‑9, but the office declined to do so.
Investigation has narrowed the root causes down to four key suspects, according to Bogdan. An important factor in the investigation was that at least two of the failed F135s were not high-time engines. AF-27 made its first flight in late April 2013, and CF-9 flew three months later. According to information provided by the former military officer, when CF-9 was grounded in late July, it had accumulated 66.7 engine flight hours and 146.5 engine operating hours since delivery. It had passed two borescope inspections after July 13, with only 1.3 engine flight hours between the previous inspection and the check it failed.
The F135 features seals at the tips of both static and rotating airfoils, with abradable material (“teeth”) on the blades and a knife-edge on the outside of the rotor (for stators) and on the inside of the case (for rotor blades). In a high-performance engine where each stage has a high pressure ratio, close tip sealing is essential to minimize air leakage from the rear of the engine to the front, which reduces performance and efficiency. The knife-edge cuts a channel when the engine is first run, but according to an engine industry source, “knife-edge seals in a fan are designed to take no rub or a very limited rub in normal operation.”
The reason why moderate-g maneuvers would be related to the “hard rub” and overheating has not been definitively identified, but a causal linkage between maneuver loads and clearances inside the engine points to distortion or displacement of engine components. The F135 is the heaviest high-performance fighter engine ever built, weighing 70% more than the P&W F100 and measuring 24% larger in diameter, with consequently larger inertial and gyroscopic forces. The Pegasus engine in the AV-8B Harrier II, with a similar diameter to the F135, suffered from blade rubbing in the event of departures from normal flight, and those engines had to be removed for inspection and overhaul.
Remedial work is proceeding along two tracks. The SDD fleet is conducting flight tests to develop and validate a “burn-in” procedure, comprising a prescribed sequence of maneuvers, which is intended to wear down the abradable materials in the seal gradually, without reaching off-design temperatures. The initial goal is to clear the SDD aircraft for a larger flight envelope in order to keep testing on schedule. It is possible, Bogdan says, that the burn-in procedure will suffice to prevent a recurrence of the problem.
Next month, however, Pratt & Whitney is due to start testing a modified fan section with a deeper abradable rub strip that should mitigate the effects of the sort of blade rub encountered in June, independent of the root cause of the rubbing. “The final fix is intended to meet the full specified life,” the manufacturer says. “We hope to be able to share more details at the conclusion of our tests.”
P&W will pick up the tab for fixes to the 156 engines already delivered. “Pratt and Whitney’s reaction to this problem, in my opinion, has been very, very good,” Bogdan says. “From a technical standpoint, they have put their A team on this. From a business standpoint, they realize this is a problem they need to solve.” Bogdan says the most critical item in meeting the Marine Corps’ IOC date is the need to modify aircraft to IOC standard.
Under the return-to-flight program that followed the July 3 grounding, borescope inspections were mandated at every three flight hours, maximum speed was limited to Mach 0.9, and aircraft were restricted to -1 g to +3 g. The fleet was also limited to 18-deg. angle of attack and half-stick deflection rolls. Test aircraft are now cleared to Mach 1.6 and 3.2 g, and can fly up to 6 hr. between inspections if engaged in air refueling missions or on transits to ranges for weapons testing. F-35 BF-3, which is instrumented for loads testing, is being used to help clear the flight envelope. Aircraft outside the test force are still subject to the original restrictions.
Neither the program office nor Pratt & Whitney have responded to a query concerning the reasons the F135’s diagnostic and prognostic systems failed to detect the incipient failure of AF-27’s engine or the excessive rubbing found on borescope inspections of other engines. During its campaign to advocate termination of General Electric’s F136 alternate engine, P&W stated that prognostic systems could detect problems early and “mitigate their potential impact on flight operations.”