F-35 Test Jets to Undergo ‘Burn In’ for F135 Engine Fix

Amy Butler | Aerospace Daily & Defense Report

The Pentagon’s long-awaited deal with Pratt & Whitney to build the seventh low-rate production lot of F135 engines for the multinational F-35 fighter has finally been signed, though officials have not yet outlined a clear path forward to address the design issue that led to an engine fire that grounded the fleet of single-engine aircraft this summer.

Program officials have, however, approved a workaround to an engine retrofit that will get the test fleet back to unrestricted flight soon, according to a government source.

The average price for the 36 engines included in low-rate, initial production (LRIP) lot 7 is $18.8 million. The Oct. 14 contract is for $592 million and adds to earlier allocated funding. The total now for procurement is $680 million for this lot, according to Joe Dellavedova, F-35 spokesman for the Pentagon. Additionally, Pratt received a $263 million sustainment contract in December 2013 for LRIP 7 activities, bringing the total cost of this lot to $943 million.

Pratt declines to release its pricing for different engine variants, citing common practice in the engine market and competitive sensitivities.

The buy includes 19 engines for the U.S. Air Force’s conventional-takeoff-and-landing F-35A, six for the Marine Corps short-takeoff-and-landing (stovl) F-35B and four for the Navy’s carrier version. An additional seven engines, including one stovl version, are included for international partners.

Pratt referred requests for information on the contract to the F-35 Joint Program Office.

Though variant pricing is not released, the program office says the average cost of the engines was cut by 4.5% from LRIP 6 to 7. A similar reduction is expected in a forthcoming announcement for the next lot.

The Pentagon expects to sign the deal on LRIP 8 in the “near future,” Dellavedova says.

Lockheed Martin is the F-35 prime contractor; Pratt’s engine contracts are direct with the U.S. government. The government turns the engines over to Lockheed Martin as government furnished equipment for installation into F-35s as they are produced.

Meanwhile, the Pentagon has yet to announce a definitive path forward for dealing with a design problem in the F135 that led to an engine fire June 23 that temporarily grounded the fleet and kept the F-35 from making its international debut at the Royal International Air Tattoo and Farnborough air show in July.

Use of a brand new F-35A to conduct a ridge riding maneuver – requiring a combination of yaw, roll and Gs – prompted excessive friction inside the third-stage integrally bladed rotor (IBR). Three weeks later, the engine caught fire as a result of continued friction and heat – exceeding 1,900F, almost twice the expected temperature, inside the engine.

This problem was not detected on the F-35 earlier because the combination of low hours on an engine with such a maneuver had not been encountered, Bennett Croswell, president of Pratt & Whitney Military Engines, told reporters last month. Because the aircraft had so few flight hours, a suitable trench for the stator tips had not been “burned in” to the engine’s polyimide foam for a proper seal. Such a trench would normally be – and in the case of earlier development aircraft, had been – burned in as the aircraft was gradually introduced to more taxing maneuvers.

Pratt has been testing various densities of the polyimide foam used to form the plate seal between the stators and the IBR on a specially devised “rub rig” at its West Palm Beach, Florida, facility to select one for a retrofit. A government official says the program has not yet selected the retrofit fix; options are under review.

The system design and demonstration aircraft, however, will undergo a “burn in” process, whereby specific maneuvers are used to gradually wear in a proper trench between the stator and polyimide foam, the government source says. It is unclear how long this will take, but the process is expected to get the remainder of the flight test aircraft back into unrestricted operations.

The fleet continues to require borescope engine inspections after 3 hr. (or up to six hours if conducting aerial refueling or transits)of flight. Flying is limited to Mach 1.6 and 3.2g. The burn-in will eliminate these restrictions, which are causing continued delays in portions of the testing program.