F-35 Begins Year With Test Objectives Unmet

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Flight testing of Lockheed Martin’s F-35 Joint Strike Fighter enters 2011 at a stepped-up pace, but with many key 2010 objectives still unmet and significant program changes looming.
While the program exceeded its year-end target of 394 flights, the objectives of clearing the conventional-takeoff-and-landing (CTOL) variant to begin pilot training, and the short-takeoff-and-vertical-landing (Stovl) version for training and initial ship trials, were not accomplished as planned in 2010.
Testing required to obtain “ready for training” (RFT) flight clearance for the CTOL F-35A is expected to be completed in January, says J.D. McFarlan, vice president of F-35 test and verification. RFT is required to fly the first production F-35s, which are not covered by a development flight-test clearance.
The first low-rate initial production (LRIP) batch of F-35As, aircraft AF-6 and -7, are slated to be delivered to Edwards AFB, Calif., by May to help with development testing, while the LRIP 2 batch, beginning with AF-8, will be delivered to the Eglin AFB, Fla., training center.
RFT clearance for LRIP 1 and 2 involves a limited flight envelope of 350 kt., Mach 0.8 and 4g, plus initial mission-system functionality, says McFarlan. Additional testing planned for 2011 will expand the flight envelope for LRIP 3 aircraft to 550 kt., Mach 0.95 and 7g, and add mission-system capability. Aircraft have already been flown to 580 kt., Mach 1.3 and 7g during risk-mitigation tests, he says.
For the Stovl F-35B, RFT clearance involves both conventional-flight and powered-lift modes, including initial ship clearance, and has been pushed into 2011 by delays caused by mechanical-reliability issues with the test aircraft. These are being overcome, says McFarlan, with the monthly flying rate increasing over the second half of 2010.
While the F-35As at Edwards have each been averaging 10 flights a month since June, the F-35Bs at NAS Patuxent River, Md., have struggled to match their productivity. “But the last few months have shown Stovl is capable of that rate,” he says, with three aircraft each flying nine times in November.
“Stovl has not caught up, but the pace is quickening,” says McFarlan. Failure-prone cooling fans have been replaced and a unreliable upper lift-fan door actuator redesigned, and no problems were experienced in the last quarter, he says.
Vertical landings, halted since September after the discovery of wear on auxiliary inlet-door hinges, are set to resume this month. McFarlan says some hinge components have been redesigned and operation of the lift-fan door rescheduled to reduce airloads on the auxiliary doors during semi-jet-borne flight.
The lift-fan door was programmed to open to 65 deg. below 120 kt., and to 35 deg. above that airspeed. But with the large door fully open, loads on the auxiliary-inlet doors behind it are reduced, so the schedule has been changed to keep the lift-fan door open 65 deg. up to 165 kt. during a short takeoff, he says.
The program has logged only 10 vertical landings since March 2010, seven of which count toward the 40 required for initial ship clearance. Originally set for March, ship trials are now slated between August and November. This window is based on when the LHD-class amphibious assault carrier can be modified with instrumentation to measure the ship environment during Stovl operations.
Flight testing so far shows the up-and-away handling qualities for all three variants are good, including during aerial refueling and crosswind landings, McFarlan says. Mission-system software stability has been good “and we are pleased with the sensor performance. There is tremendous capability in the radar and electronic warfare system.”
Nose wander in transonic maneuvers has been tackled with changes to control-surface scheduling and air-data calibration, and updated flight-control software was released at the end of the year. This is expected to also correct discrepancies between predicted and measured sideslip angles and control-surface loads. Engines with design changes to reduce screech and allow use of full augmentor are being installed.
Wing roll off has been experienced during transonic maneuvering in the CTOL F-35A, but was expected and planned solutions have worked so far, McFarlan says, including scheduling of leading- and trailing-edge flap positions and rates. Roll off occurs when the shockwaves on the wing do not move symmetrically as angle of attack changes.
The same solutions will be tested on the Stovl F-35B and F-35C carrier variant (CV), which because of its larger wing has been fitted with pop-up spoilers as a precaution. “We will try the aerodynamic techniques first,” McFarlan says. F-35C test objectives for 2011 include RFT clearance and land-based catapult and arrestor testing at NAS Lakehurst, N.J.
Overall, F-35 flight testing ended 2010 close to its goal of more than 3,700 test points, but while the CTOL F-35A and F-35C CV were well ahead of plan, Stovl and mission-system testing fell short. More than half the test points required for Stovl RFT and ship clearance remain to be accomplished in 2011.
A major replan of the F-35 program is to be announced by early February.