Aircraft

F-35 Flight Testing At Pax

By Eric Hehs Posted 1 December 2012

NAS Patuxent River, located on the Maryland coast where the Patuxent River empties into the Chesapeake Bay, is home to Naval Air Systems Command and the Naval Air Warfare Center Aircraft Division. Long known as Pax River, this air station bills itself as “the place where the future of Naval Aviation begins.” That future is being defined these days at Pax by the frequent takeoffs and landings of two variants of the F-35 Lightning II.

The F-35 Integrated Test Force, or ITF, at Pax, one of multiple test sites for the F-35 program, is responsible for flight testing the seagoing variants of the F-35, which include the short takeoff/vertical landing F-35B variant and the F-35C carrier variant. As of fall 2012, the ITF at Pax operated eight F-35s — five B-models and three C-models with two more F-35Bs and a fourth F-35C slated to arrive before the end of the year.

Integrating The Test Force

The 900 personnel working at the Pax F-35 ITF represent a wide cross section of the aeronautical world. Test pilots, for example, represent the US Navy, US Marine Corps, Royal Air Force, Lockheed Martin, and British Aerospace. Similarly, aircraft are maintained by both military and civilian personnel. The US government as well as Lockheed Martin, British Aerospace, and Northrop Grumman provide flight test engineers.

“We are integrated across the board,” said Navy Capt. Erik Etz, the US government director of test for F-35 naval variants. “While previous test programs I’ve worked on have had integrated teams, I’d say this team is truly integrated,” Etz said. He pointed out that military maintainers work right next to civilian maintainers. The ITF at Pax is just one component of the overall F-35 ITF. “We feed information to the F-35 Joint Program Office, to Lockheed Martin in Fort Worth, to Edwards AFB, to Eglin AFB, and to all the subcontractors.”

“I’ve never worked anywhere in my career in an environment that is so interdependent,” added Joe Iorio, Lockheed Martin deputy director for operations at Pax. Iorio works with employees of Lockheed Martin, British Aerospace, and Northrop Grumman as well as with military and government personnel. “Nothing gets generated here without touching several of those organizations,” he continued. Iorio stressed the interdependence of the ITF as one team. “Just like the name says, we are an integrated test force.”

High-Level Attention

The proximity of NAS Patuxent River to Washington DC, about sixty-five miles to the northwest of Pax, generates a high level of high-level attention. Since receiving the first F-35 test aircraft at Pax in November 2009, the ITF has hosted more than 120 protocol-level visits and events including US and foreign military and civilian dignitaries, as well as numerous visits by industry officials, non-protocol Government, media, and civic groups.

“We do get a lot of visitors,” said Iorio. “It’s easy for them to come down here from DC. But that’s actually a good thing.”

Secretary of Defense Leon Panetta chose to come to Pax River to relieve the two-year probation placed on the F-35B by former US Secretary of Defense Robert Gates January 2011. The probation addressed technical issues on the B-model. “Having someone of such stature deliver a positive message in person meant a lot to our workforce,” Iorio said.

Panetta lifted the probation early during his visit to Patuxent River on 20 January 2012, lauding the workforce and saying, “The STOVL variant is demonstrating the kind of performance and maturity that is in line with the other two variants of the Joint Strike Fighter.” The fighter program is in the System Development and Demonstration, or SDD, phase.

Etz, who briefed Panetta when he visited, labeled the current status of the F-35 flight test program green in most areas. “We are making significant progress in most technical areas in 2012 as we did in 2011.” While progress in some technical areas has been delayed by some hardware challenges, continued Etz, “our team is confident that we will meet the needs of the SDD phase of the program.”

Measuring Progress

The F-35 program measures and reports progress in many ways for the SDD phase. The most common measure consists of test points, which refer to specific points in the flight envelope as defined by speed, altitude, and g forces. Test points and the flight envelope itself are also defined by other factors such as flying at night, carrying and releasing different weapon loads, refueling from various tanker aircraft, and flying with weapon bay doors open.

“Not all test points are created equal,” explained Etz. “Some are more difficult to capture than others. Some of the lower speed test points can be achieved fairly rapidly. Points associated with handling qualities and flying qualities at low speeds can be knocked off in a matter of minutes on some flights. Achieving test points becomes more difficult at higher speeds. Test points at Mach 1.4 and 1.5 and beyond may take twenty minutes or more of flight time based on the specific fuel and airspace requirements needed to reach that point.”

The test program also tracks progress in terms of overall flight hours, flights per month, test points per month, test points per flight, and abort rates for each aircraft. “We track reliability and maintainability metrics as well,” Etz added. “Our aircraft are unique test assets with unique needs for support, so a one-to-one maintenance correlation with the fleet may not be accurate. But I would view the relationship as parallel.”

Envelope Expansion

As test points accumulate, the envelope of the F-35 expands and the expanded envelope eventually filters out to the operational fleet. “Just because we go out and hit a point in the sky doesn’t mean the point is released the next day,” noted Etz. “Achieving a particular test point begins a train of analysis that must be completed before that point is released for fleet use.”

US Marine Corps Lt. Col. Matt Taylor, a test pilot at Pax who has been flying the F-35 since July 2010, explained the methodical nature of expanding the flight envelope: “We start at the center of the flight envelope and move out from there.” When the airplanes first arrived at Pax, they were flown at subsonic speeds, medium altitudes, and low g-forces. “This year we are exploring the high-speed, high-altitude, and high-g edges of the envelope.”

Those edges are defined as Mach 1.6, 50,000 feet, and up to 7.0 g’s for conventional flight for the F-35B; and Mach 1.6, 50,000 feet, and 7.5 g’s for the F-35C.

While many test points are shared across all three variants of the F-35, others are variant-specific. The vertical lift capability of the F-35B, for example, creates a unique flight envelope that goes all the way down to zero airspeed at zero feet altitude. “The F-35B can fly backwards,” noted Eric Faidley, a Lockheed Martin flight test engineer assigned to BF-1. “In fact, its maximum backwards groundspeed is thirty knots.”

The only time an F-35B might hover at thirty knots in reverse in an operational setting would involve an overshot landing, Faidley explained. “In such instances, pilots would typically not back up and, instead, go back around in the pattern and attempt another landing,” he said.

The test team at Pax is also exploring the maximum speed end of the STOVL portion of the flight envelope, which is 250 knots. “The buffet and noise is significant when we have the upper lift fan door all the way open, which is an angle of sixty-five degrees, at that speed,” Faidley said. “That’s a flight condition that we can’t evaluate accurately in a simulator. It’s another reason why we do flight testing.”

The team is also flying the B-model in conventional mode but configured with various STOVL doors open. “The flight conditions mimic failure modes,” Faidley explained. “For example, we intentionally open the upper lift fan door after the engine nozzle has converted from STOVL to conventional flight mode.”

Some of the flight test aircraft have special software that allows the pilot to override the standard control laws that actuate the various doors and nozzle angles. The flight control laws for the STOVL variant have six modes that are associated with specific actuations. Mode 1 defines conventional flight. Mode 4 defines STOVL. The other four modes define transitional states between the two primary modes. “If a pilot loses a hydraulic system in Mode 2, we know that the doors associated with STOVL flight will be positioned a certain way,” Faidley explained. “We are seeing how well the airplane flies in those conditions.”

Ship Suitability

Ship suitability testing creates another set of unique test points for both the F-35B and F-35C. Many of the initial suitability tests are conducted on land instead of at sea on a ship. Teams from Pax have taken the F-35C to JB McGuire-Dix-Lakehurst, New Jersey, several times since June 2011 for a series of carrier-suitability tests. Loads and handling qualities have been evaluated in catapult launches from three types of launch systems, including from two standard steam-powered systems.

Loads and handling qualities have also been evaluated from the US Navy’s first Electromagnetic Aircraft Launch System, or EMALS, which will eventually replace the existing steam catapults on current and future aircraft carriers.

The C-model has gone through jet blast deflector testing at Lakehurst as well. The deflector, located behind the catapults aboard aircraft carriers, diverts high-energy exhaust from the engine to prevent damage and injury to other aircraft and personnel located in close proximity. Microphones, thermometers, and other sensors have been placed around the aircraft at various engine power settings as part of environmental tests. The F-35C has also gone through some initial tests of the arresting gear at Lakehurst. The tests uncovered an issue with the tailhook, which is being addressed with some redesign and additional testing.

Jennifer Chisler, a civilian employee for NAVAIR, is a member of team assigned to evaluate ship suitability for the F-35. The team looks at all the fixed-wing aircraft that go aboard ships. “We check the aircraft structurally to see if they will survive short takeoffs and vertical landings or catapult takeoffs and arrested landings,” she explained. “Catapult takeoffs generate a big acceleration impulse and transmit a lot of vibrations to the structure through the launch bar,” she continued. “Equipment inside the aircraft sometimes doesn’t respond well to that impulse. It’s our job to make sure it does.”

Chisler and Faidley were part of the team sent to the amphibious assault ship USS Wasp (LHD-1) in October 2011 for the initial sea trials of the F-35B. The tests were used to collect data on the aircraft’s ability to perform short takeoffs and vertical landings on a ship at sea. The team also determined how well the F-35B integrates with the ship’s landing systems and deck and hangar operations. While underway, the F-35B pilots logged more than twenty-eight hours of flight time and completed seventy-two short takeoffs and seventy-two vertical landings.

The experience was one of the highlights of my career,” said Faidley, the test conductor for the first vertical landing of the F-35 on a ship. “Not only was it the first time an F-35 landed on and operated from a ship, it was also the first time we set up a flight test control room at sea.”

Faidley, Chisler, and the rest of the team lived on the Wasp for three weeks as it operated in the Atlantic fifty to sixty miles from the coast. “We can test how the F-35B deals with crosswinds more easily at sea because we can position the ship at whatever angle we want,” said Faidley. “We also evaluated how the F-35 affects ship operations, for example, by measuring temperatures of the flight deck and structural loads on rooms directly below the flight deck during takeoff and landings.”

Weapon Separations

More recently, the test team at Pax has been focusing on dropping weapons from the internal bays of the F-35. The F-35B dropped its first 1,000-pound GBU-32 Joint Direct Attack Munition on 8 August 2012, shortly after Code One visited Pax. Duriel Holley, lead flight test engineer for CF-1, is involved in the planning of dropping the same weapon from the F-35C. “While CF-1 is used primarily for envelope expansion and flutter testing,” he explained, “it is also being used for weapons environment tests.”

Just as all the other testing, weapon separation tests build up from basic to complex. On the ground, engineers check clearances of the weapons after they are loaded into the bay. Clearances are checked again dynamically in pit testing, which involves ejecting a weapon from a parked aircraft into a cushioned pit.

The flying portion of the testing begins with installing the weapons into the aircraft weapons bay and flying it with the doors closed. The final step before actual weapon separation test involves the aircraft flying with weapons and opened bay doors. “We place a variety of sensors in the bay to collect vibration, temperature, and acoustical data,” Holley said. “The weapon itself will have instrumentation on it as well. We do weapon testing for every weapon and weapon combination the airplanes can carry. Some of these loadings are worst case, maximum loads. Our test plans covers JDAMs, GBU-31, GBU-32, and AIM-120 AMRAAM.”

Mission Systems

All of the testing described so far deals with how the aircraft behaves under certain prescribed conditions, or flight sciences testing. The other category of flight testing, called mission systems testing, deals with how the aircraft detects what is going on around it and how well it conveys that information to the pilot.

Mission systems tests are used to evaluate the functionality of the various electronic systems and sensors on the aircraft, including communications (datalinks and satellite communications), radar, countermeasures, distributed apertures, and electro-optical targeting. Before these systems are tested in an F-35, they are checked out on the ground in the mission systems integration laboratory in Fort Worth, Texas, and in the air in the Cooperative Avionics Test Bed (referred to as CATB, or the CATbird), which is also based in Fort Worth.

Because they lack a radar and other sensors, BF-1, BF-2, BF-3, CF-1, and CF-2 are used exclusively for flight sciences testing. BF-4, BF-5, and CF-3 have the hardware and software needed for mission systems testing, though they are often used for certain flight sciences tests as well.

“The capabilities of legacy fighters evolved as sensor technology evolved,” explained Taylor. “But the sensors usually evolved independently. So the cockpit devotes individual displays for a given new technology.” The pilot may be running the radar from one set of displays and a datalink from a different set of displays. The two different displays don’t interact.

“Sensors can evolve cooperatively in the F-35,” he continued. “The display shows the tactical environment to the pilot. We may be unaware of which sensor or sensors were used to provide that information, though we can find out if we want to. But we usually don’t care. We want to know the location of the bad guys and the target—and when we can shoot them.”

Capabilities associated with mission systems are being developed in a series of software blocks. Block 1 covers basic functions of the navigation system, communication systems, and sensors. With Block 1, the aircraft are limited to subsonic airspeeds, an altitude of 40,000 feet, maximum g force of 4.5, and a maximum angle of attack of eighteen degrees. Block 2A, which as of the summer of 2012 was being flown at Pax on BF-5, covers Multifunction Advanced Data Link, the current Link-16, maintenance data link, and a mission debriefing system.

Block 2B, which is the initial warfighting version of the software, adds capabilities associated with air-to-air and air-to-ground missions. It also has the complete set of maintenance functions. With Block 2B, the aircraft can be flown at supersonic speeds (up to Mach 1.2 for B- and C-models); a maximum g force of 5.5 and 7.5 for B- and C-models respectively; and a maximum angle of attack of fifty degrees. The software also covers various loadings of the AIM-120 air-to-air missile, 2,000-pound JDAM GPS-guided bombs, and 500-pound GBU-12 laser-guided bombs.

Block 3 is the full warfighting version of the software, which is scheduled to be installed on production F-35s beginning with the ninth production lot called Low-Rate Initial Production 9, or LRIP 9. Mission System testing will pick up speed when BF-17, BF-18, and CF-8, all mission systems aircraft, join the test fleet in late 2012 and early 2013.

Pilot Perspectives

All the F-35 test pilots at Pax are qualified to fly both variants. A subset has the qualifications necessary for executing STOVL test missions, that is short takeoffs and vertical landings. The ease of operating the aircraft in STOVL mode allows that test capability to be distributed broadly among the pilots. “A number of our pilots came here with no STOVL experience, but now they are flying STOVL test missions,” noted Etz.

“The ease of landing the B-model in STOVL mode is unprecedented,” explained Taylor, who had no STOVL experience before joining the F-35 ITF at Pax. “In the Harrier world, learning to operate in STOVL mode takes months of training. For us it is a couple of flights in the simulator and one, maybe two, flights in the airplane, because it is so intuitive. It is easy to land the F-35B in STOVL mode. We will never hear a Harrier pilot say the AV-8 is easy to land. The F-35B will hold whatever condition you command it to hold. It is like driving a perfectly aligned car down a perfectly straight highway with no wind. The F-35B will go straight until you tell it to do something else.”

“One of the beauties of this airplane is that it is so simple to land,” added Dan Levin, a Lockheed Martin test pilot and lead test pilot for the ITF at Pax. “Harrier airframes burn up about half their life in training pilots to land vertically. Landing vertically in a Harrier is a complex task. I’m a fixed-wing fast-mover pilot, and I was ready to perform STOVL operations after ten minutes in the simulator. STOVL operations are simple and intuitive. The flight control system is automated in the right ways. The pilot doesn’t even notice the transition between conventional flight and STOVL mode.”

Levin is one of a handful of pilots who have flown all three variants of the F-35. “All three variants are remarkably similar in terms of pilot-vehicle interface,” he said. “That is one of the beauties of this program. A pilot can go from one aircraft to another almost seamlessly. As for flying qualities, the A- and B-model are very similar. The C is a little different because of the larger wing. But none of the differences jump out. Landings in the A and B are similar to the F-16 in terms of speed and angle of attack. The C-model is as solid as a rock and pilots land at a much slower speed—high 120s to low 130s [knots]. The angle of attack for landing is much lower in the C-model so the pilot can see over the nose. The angle of attack control is very precise.”

Ease of vertical and carrier landings promises to significantly reduce the training time needed for these operations with the F-35B and F-35C, when compared to the aircraft the two variants are replacing. “The training required to keep a pilot comfortable in the STOVL environment is going to go to near zero,” Levin said. “The slow speed handling qualities of the C-model will decimate the training requirements needed to get pilots to land safely on the aircraft carrier deck. Eliminating those training requirements will skyrocket the value of the F-35C to the US Navy. Operating these airplanes will be cheaper and safer.”

“The ease of taking off and landing these aircraft is impressive,” added Taylor. “However, the ability to execute the mission is more important. That is, can the aircraft get pilots to the target, help them destroy it, and get them out safely? That is where the F-35 will really be impressive.”

Eric Hehs is the editor of Code One.
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