Although making progress, F-35 flight testing is not delivering required capabilities on schedule
Amassing flight hours and test points at an accelerating pace, the Joint Strike Fighter program faces the question of whether it is completing the right tests at the right time. As it enters 2013 with a focus on high angle-of-attack and weapons testing, the program is still falling behind in clearing the capabilities its customers require.
exceeded its development flight-test goals for 2012. But a report by the Pentagon's director of operational test and evaluation ( &E) reveals this was helped by bringing forward test tasks from future years. Aircraft deficiencies and software delays prevented the program from achieving some test objectives set for 2012 and required to deliver capabilities to the services now beginning to train pilots on the F-35.
Lockheed says it logged 1,167 flights and 9,319 test points in 2012, against the plan of 988 flights and 8,458 points. Of the flights completed, 926 were by flight-sciences test aircraft expanding the flight envelopes of the three F-35 variants, while 241 were by the smaller number of mission-systems aircraft assigned to testing avionics and sensors.
But according to the DOT&E report, through November 2012, the program had completed only 78% of the test points planned for the year. The addition of test points to investigate new problems, for regression testing of aircraft and software fixes, and for tasks brought forward from future years pushed the test points accumulated to 35% above the total planned (see F-35 Flight Testing table).
The report says horizontal-tail scorching and delamination and higher than expected airloads on open weapons-bay doors has restricted testing of all variants. Afterburner operating restrictions and delayed aerial-refueling disconnects have affected the conventional-takeoff-and-landing (CTOL) F-35A. Testing continues on a redesigned clutch, driveshaft and doors for the short-takeoff-and-vertical-landing (Stovl) F-35B, and the tailhook is being redesigned for the F-35C carrier variant (CV).
Sustained-maneuver and transonic-acceleration performance has been reduced for all three variants based on flight-test results, the DOT&E says. And live-fire ballistic testing has confirmed vulnerabilities resulting from the 2008 decision to delete shutoff systems for the flammable avionics coolant and nozzle fueldraulic actuation systems to save weight, the report reveals.
“The DOT&E report offers 10 recommendations. The F-35 Joint Program Office has already taken action on six of the 10 recommendations,” says the JSF program office, noting all of the issues highlighted in the report were known. “Of the remaining recommendations, three involve vulnerability concerns and are being reviewed.”
While the program continues to evaluate fixes for problems already identified, and to test multiple blocks of mission-system software concurrently, it is pushing ahead into new areas of testing. “From a flight-test perspective, there are two priorities this year: complete weapons verification and high alpha [angle of attack],” says Tom Burbage, Lockheed Martin F-35 program integration executive vice president and general manager.
“We are using 18 aircraft to conduct the equivalent of three development flight-test programs and one mission-system program,” says J.D. McFarlan, F-35 test and verification vice president. “We expect it to eventually include 8,000 flights and 60,000 test points. So far we've collected 22,000 of those, so we are around 36% through the test program.”
Across the three variants, there are 12 aircraft in the flight-sciences test fleet. Another four are assigned to mission-systems testing, with two more—Stovl production aircraft BF-17 and -18—about to join and complete the flight-test stable based at Edwards AFB, Calif., and NAS Patuxent River, Md. Together they had logged 4,243 flight hours by Jan. 7, with the Stovl aircraft logging 1,309 of those hours and conducting 381 vertical landings.
Tests to clear the F-35A “clean wing” flight envelope for the Block 2B initial combat capability—to Mach 1.6/700 kt airspeed, 9g maneuvers and 40,000-ft. altitude (see F-35 Capability Plan table)—are wrapping up. The next step will be to clear the envelope with internal weapons-bay doors open, he says. Release of the Block 2B envelope is planned for mid-2015. Expanding the flight envelope to 50,000 ft. for the Block 3F full combat capability is planned for 2016, when development testing is scheduled to finish.
The flight-sciences sortie rate is ahead of plan for the F-35B and C, but behind for the A, says the DOT&E. There are high-speed/high-altitude restrictions on all three variants caused by the tail scorching. New surface coatings have been flown unsuccessfully, so a new skin design will be tested on CTOL aircraft AF-2 early this year, according to the report.
High angle-of-attack testing is underway at Edwards AFB using aircraft AF-4 equipped with a spin-recovery parachute. Where theis limited to 26 deg. alpha by its fly-by-wire flight-control system, the F-35's limiter is set at 50 deg. and the aircraft has been flown to 73 deg. to ensure there is sufficient pitch authority. “We have to really slow down to get those pitch conditions—100 kt at 40,000 ft.,” says McFarlan. “Pilots are pleased with the ability to get the nose down from high angle of attack.”
The control system is designed to prevent departure from controlled flight at high alpha. In tests now beginning, the prevention feature is turned off, the aircraft forced to depart and the system turned back on to ensure it recovers the aircraft. The ultimate goal is to demonstrate safe recovery from a flat spin, with the chute as a backup. After completing these tests, they will be repeated with the spin chute removed. High-alpha testing on the F-35B will begin this year.
Test flights to investigate transonic roll-off, caused by slight differences in left and right wing stalls during maneuvers, have shown that control-law software changes are sufficient to mitigate the phenomenon, says Burbage. As a result, pop-up spoilers added to the larger wing of the F-35C have been removed in production aircraft.
The DOT&E report says alternative trailing-edge flap settings are being explored to improve flying qualities during the approach to the carrier. Pilot surveys show handling qualities improve with 15-deg. flap deflection, but flight tests indicate 30-deg. flaps are needed to meet the 145-kt. maximum-approach-speed requirement. Burbage says the F-35C's flight-control system uses the flaps to keep the aircraft stable on the approach glideslope, taking out the lag from the pilot's inputs.
A new tailhook for the CV will undergo its critical design review within a month and is scheduled to be installed in test aircraft at Pax River by year-end. After the F-35C had problems catching the arrester cables, the hook point, shank and dampers were redesigned. The concept was demonstrated last August at NAS Lakehurst, N.J., when aircraft CF-3 made five successful engagements, McFarlan says.
While live-fire testing in the first F-35, aircraft AA-1, at China Lake, Calif., has confirmed the redundancies and reconfiguration capabilities of the flight-critical systems, it has also shown that ballistic damage to polyalphaolephine (PAO) coolant and fueldraulic lines can cause fires. The report says the program office may reconsider installing the PAO shutoff valve, but not the fueldraulic fuses removed in 2008. The onboard inert-gas-generating system already is being redesigned to address deficiencies in fuel-tank inerting.
Weapons testing is underway for the limited F-35A Block 2B clearance envelope, which comprises internal carriage of the GBU-31/32 joint direct attack munitions, GBU-12 laser-guided bomb and AIM-120 air-to-air missile. “This year, we are focusing on ramping up to 15 weapons tests at both sites [Edwards and Patuxent River],” says McFarlan.
Stovl F-35Bs will return to the assault carrier USS Wasp in August for a second round of sea trials in stronger winds. “We will also bring along weapons,” he says. The DOT&E, meanwhile, says weapons integration has been limited by deficiencies in the radar, electro-optical targeting system, sensor fusion and helmet-mounted display.
Ground and flights tests have demonstrated that the stealthy F-35's radar signature is meeting design requirements, says McFarlan. “We have shown that if the aircraft passes the test in the [anechoic] chamber, it will pass in flight,” he says. “We are now in the mode of showing we can maintain the [low-observable] characteristics after a year in flight.”
In ground durability testing, only the F-35A airframe has so far completed a full 8,000-hr. lifetime. In December, fatigue tests on the F-35B were halted for a second time after the discovery of multiple cracks in a bulkhead flange during the 7,000-hr. inspection. Previously, a crack in a carry-through bulkhead was found after 1,055 hr. of testing, and McFarlan says Stovl test aircraft at Pax River will have to go through structural upgrades beginning this year.
Recent test highlights include hovering the F-35B for 10 min. “It was record, hovering at max performance with more than 5,000 pounds of fuel before doing a vertical landing,” he says. “We have done a lot of night flying to understand the helmet and DAS [distributed aperture system], and done night hovering as well.” McFarlan says testing of the troubled helmet is making progress and now focusing on latency of the DAS imagery projected on the visor during night flying.
—With Amy Butler and Bill Sweetman in Washington.
|2012 Actual||1,092||18% greater than planned|
|Cumulative Total||2,533||13% greater than planned|
|2012 Baseline||4,711||28% less than planned|
|Brought Forward||2,319||From future years|
|2012 Total||8,750||35% greater than planned|
|Cumulative Total||20,006||5% greater than planned|
|Source: Director of Operational Test & Evaluation, 2012 Annual Report|
|Aircraft||LRIP 1-3||LRIP 4/5||LRIP 6-8||LRIP 9+|
|Max. altitude (ft.)||40,000||40,000||40,000||50,000|
|Max. airspeed (kt.)||450||550||700||700|
|Angle of attack (deg.)||18||18||50||50|
|Weapons||None||None||InternalAIM-120,GBU-31/32,GBU-12||Full SDD,internal andexternal|
|Sources: Joint Program Office and Director of Operational Test & Evaluation 2012, Annual Report|