F-35 Lightning II Flight Testing At Edwards AFB

The first thing members of the F-35 Integrated Test Force see when they walk through the main entrance to the hangar at Edwards AFB, California, is a large flat screen display with a list of flight test priorities. The items on that list can change from one day to the next.

“Stability is crucial to successful test execution, but we can turn on a dime if priorities shift,” noted Lt. Col. George Schwartz, US government director for the F-35 ITF at Edwards. “The helmet mounted display test we are flying tonight is an example. The program asked us two days ago to fly an additional night flight for HMD testing. We are conducting that mission tonight.”

Edwards normally operates a daylight flying schedule, so a short-notice night mission requires a significant adjustment in schedules and resources across Edwards. “The night mission exemplifies the incredible support the F-35 ITF gets from the base,” Schwartz added.

The F-35 ITF at Edwards consists of more than 900 military, contractors, and civilian personnel from a variety of services, countries, and industries. In 2012, the ITF operated six F-35As assigned to Edwards—three for flight sciences testing and three for mission systems testing—as well as one F-35B temporarily deployed to Edwards from NAS Patuxent River, Maryland, for air start testing.

By the end of 2013, Edwards F-35 ITF will be operating three additional F-35s—two F-35Bs and one F-35C, for a total of nine F-35s. The test pilot population will expand from nine pilots to twelve pilots as well. The additional aircraft and pilots will be involved primarily with mission system testing.

Expanding The Envelope
Since receiving their first two F-35As (called AF-1 and AF-2) in May 2010, Edwards F-35 ITF personnel have been busy expanding the flight envelope.
   
“We spent the first two years turning the F-35 into a flying machine, but the focus has quietly shifted to weaponizing the aircraft in both flight sciences and mission systems,” Schwartz said. “Flight sciences work began with a small envelope. Today we’re flying at the edge of the envelope—at 100 percent loads—out to 1.6 Mach. Thanks to all the incredible work on envelope expansion done by this team, we are flying at seven g’s with no loads monitoring on our mission systems aircraft, and we have proven the aircraft can operate anywhere throughout the full envelope.”

The majority of the envelope expansion has been accomplished on AF-1, AF-2, and AF-4—the three F-35As devoted to flight sciences testing. F-35A AF-1 is flown in flutter tests. AF-2 is flown for most of the loads testing. And AF-4, recognizable by its spin recovery chute, is flown in high angle of attack test missions. These three aircraft alone accumulated about 600 hours of flying time in about 300 flights in 2012—approximately one-fourth of the total 1,167 System Design and Development missions for the entire fleet, which includes the test aircraft at Pax River.

Mike Glass, F-35 ITF site director at Edwards for Lockheed Martin, doesn’t see that level of activity diminishing for the flight sciences aircraft. “Envelope expansion testing remains significant in 2013,” Glass said. “We’ve completed the clean wing flutter flight sciences testing. Now we are installing pylons on the aircraft and doing the same type of flutter and loads testing we did with the clean wing. We will be conducting these tests for the next couple of years but with different load configurations on the aircraft.”

High angle of attack testing with the F-35 began in late October 2012. This testing involves taking the aircraft to its production angle of attack limit, which is fifty degrees. It also involves taking the aircraft beyond this limit to evaluate its characteristics in recovering from out-of-control conditions.

“High AOA testing produces some of the most challenging environments for the engine because the intake gets bad air,” explained David Nelson, lead F-35 test pilot for Lockheed Martin at Edwards. “The bad air creates a potential for producing a flameout, which is basically an engine shutdown. For that reason, air start testing preceded high AOA testing.”

Air start testing involves shutting down the engine and restarting it in flight. All four test pilots involved in high-AOA flight tests have flown air start missions. “The graduation exercise involved turning off the engine at 45,000 feet and then restarting it,” Nelson said. “Everything worked as planned.”

Besides producing conditions that can cause the engine to flame out, flying at high angles of attack can also lead to out-of-control flight. The spin recovery chute mounted at the apex of a four-legged structure on the back of AF-4 is designed to deal with that possibility. The test pilot can deploy this twenty-eight foot diameter parachute in case the airplane gets into an out-of-control condition from which the pilot cannot recover with the standard flight control inputs. The chute has not been needed to date.

“The airplane does quite well at high AOA,” Nelson added, “and the tests have been proceeding smoothly. We went from twenty degrees angle of attack to fifty degrees in only four days of testing.” Nelson and other pilots have also evaluated flying qualities at minus ten degrees AOA, which is the maximum design limit for negative AOA for the airplane. High AOA testing for 2013 will involve a variety of loadings mounted externally.

Loads Testing
Loads testing involves putting the aircraft in highly dynamic conditions to measure the stresses on the airframe and on other components. The tests verify the structural integrity of the F-35 in all flight regimes. Most of the loads testing has been flown on AF-2. US Air Force test pilot Lt. Col. Brent Reinhardt, who has been at the ITF since June 2012, has flown many of these missions.

“Loads missions can be physically demanding,” he said. “Some test points are hard to hit. I am diving at the ground at sixty degrees, doing Mach-one-point-whatever, and pulling 5.6 g’s while doing a roll—all this maneuvering just so we can hit a loads point at given speed and altitude conditions. Depending on the point, a lot of the runs start at Mach 1.3 and at altitudes nearing 50,000 feet. During the rolls, I increase the g’s so the flight test engineers on the ground can determine if we are overstressing any part of the airplane.”

Jennifer Schleifer is one of the flight test engineers who monitors and measures the loads on the aircraft during these test missions. Assigned to AF-2, she arrived at Edwards with the aircraft in May 2010. “We are flying on the edges of the structural envelope,” she explained, “and we have to make sure the airplane does not cross an edge. We spend a lot of time in the control room making sure that we won’t exceed structural limits.”

“We’re flying at Mach 1.6 and at more than seven g’s,” added Reinhardt. In a lot of the loads tests, pilots perform rolling maneuvers at a particular g. “Once we clear out a portion of the envelope at that g, we move to a higher g and repeat the testing process. We are shooting for a continuous g roll for 360 degrees through a certain block of altitude.”

In these maneuvers, the F-35 is often pushed to a very high roll rate, which is around 200 degrees per second.

“Operational pilots will never execute some of the maneuvers we’re performing in the airplane,” said Reinhardt. “But the maneuvers are part of building a flight envelope. We are verifying that the airframe will be fine structurally if it stays within the limits we are testing here.”
 
When not flying or conducting an actual mission, test pilots and flight test engineers practice the missions in a simulator. “We go to the simulator with a pilot to see if the more challenging loads points are achievable,” added Schleifer. “In the simulator, we can determine what Mach and what altitude the pilot needs to set up a particular run. We easily spend four hours in the simulator for every flight. We often return to the simulator to rehearse the points the morning of the flight. More practice in the simulator translates to greater mission efficiency in the air.”

Mission System Testing
“Flight sciences testing is fun,” Nelson said, “but it has its limits. Once an aircraft is good to nine g’s, it’s good to nine g’s. There’s no updating the flight envelope thereafter. Mission systems, on the other hand, will evolve for the life of the F-35, just as capabilities continue to evolve for the F-22 and F-16.”

Mission system testing deals with how the aircraft detects what is going on around it and how well it conveys that information to the pilot. Mission system 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.

Mission systems, combined with stealth, define the F-35. They separate fifth generation fighters from previous generation fighters.

“The F-35 was designed as a stealthy sensor platform,” added Reinhardt. “The aircraft can carry two 2,000-pound bombs and two AIM-120s internally. A similarly configured F-16 must carry those bombs and missiles externally, in the wind stream. Plus the F-16 has to add external fuel tanks as well as external targeting and countermeasure pods. These external loads reduce performance. And they increase radar cross section. We have to look at the whole picture when comparing fighters.”

Before mission systems are tested in the F-35s at Edwards, 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 CATbird), which is also based in Fort Worth.

The mission systems fleet at Edwards originally consisted of F-35A AF-3, AF-6, and AF-7. Unlike the flight sciences test aircraft, these three F-35s fly with a full complement of electronic systems and sensors found on operational F-35s. This current fleet will be increased with the three additional F-35s scheduled for delivery in 2013, which will also be used for mission system testing. F-35B BF-17 arrived in March. It was joined by BF-18 in April. CF-8 is expected to arrive later in the year.

“The additional aircraft coming in will help with multi-ship missions,” explained Glass. “As you can imagine, launching four aircraft for a mission at one time with only four aircraft available can be a real challenge even for an operational unit. Having six aircraft should improve our success.”

These multi-ship missions represent the increasing complexity and continuing evolution of mission system testing. Most of the mission system testing performed with the F-35 prior to 2013 involved single aircraft and even single sensors with limited sensor fusion, that is, the process for taking inputs from two or more sensors, combining them, distilling them, and then conveying them in an intuitive form to the pilot.

“At the system level, we are moving from testing individual systems or testing small federated groups of systems to testing fusion, where all of the sensors work together,” explained Capt. Nathan Yerrick, a US Air Force flight test engineer at the Edwards F-35 ITF.

“Eventually we will have all systems on,” Yerrick continued. “In terms of mission profiles, we had single F-35 operations early on. That is, one F-35 would go out with a chase aircraft. Now we are adding another F-35 as wingman, and the two F-35s are flying against multiple, maneuvering targets. In the next year or so, we will have our first four-ship F-35 mission with multiple maneuvering targets.”

Software
Because mission systems are common for the most part across all F-35 variants, the mission system testing done on an F-35A applies to the F-35B and F-35C. Similarly, the software that underlies the evolution is shared.

Capabilities associated with mission systems are being developed in a series of software blocks. Block 1 covers basic functions of the navigation system, the communication systems, and the sensors. With Block 1, the aircraft is limited to subsonic airspeeds, 40,000-foot altitude, 4.5 maximum g force, and eighteen degrees maximum angle of attack. Block 2A covers the Multifunction Advanced Data Link, the current Link-16, the 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); at maximum g force of 5.5 and 7.5 for B- and C-models, respectively; and at 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, the full warfighting version of the software, is scheduled to be installed on production F-35s beginning with the ninth production lot, called Low-Rate Initial Production 9, or LRIP 9.

“We will wrap up the last of the mission system testing for Block 2A in summer 2013 and have already started testing 2B in the spring,” explained Eric Schutte, US government mission systems lead engineer at Edwards. “We corrected a lot of issues during our tests with 2A. The electro optical targeting system, for example, is working a lot better now. Link 16 is working well, too. We performed some interoperability tests with Link 16 last December. We will be doing a lot more interoperability testing with Block 2B.

“The software has come a long way,” Schutte added. “This is an incredibly complex airplane. Getting all the systems talking to each other can be a real challenge.”

Weapon Testing
Software updates are also delivering more weapon capability to the F-35. The test aircraft at Edwards began flying with weapons in 2012. The first bomb separation test occurred from F-35A AF-1 on 16 October 2012. The first AMRAAM separation test came three days later. The Edwards F-35 ITF is gearing up for about another twenty weapon drops in a series of weapon delivery accuracy tests for the spring and summer of 2013.

“We’ve done separation tests with the AMRAAM and a GBU-31,” said Bobby Rocha, a weapons integration engineer at the F-35 ITF. “These are the first steps toward actual weapon launch.”

Early weapon tests fall into the flight sciences regime. The initial separation tests are used to verify that the weapon separation characteristics conform to predictions. These initial tests are done on flight sciences aircraft—mostly on AF-1.

“We have a defined envelope for weapon releases,” Rocha noted. “We start with benign releases at higher altitudes, at one g, and at Mach 0.8. Then we come down in altitude and release at increased pressures. After that, we do releases at g forces above and below one g. Some of these test profiles are to establish an envelope so they are conducted at the edge of the operational envelope.”

As the envelope is established, the tests transition to the mission systems aircraft. “The weapon delivery accuracy tests are flown on the mission systems aircraft,” Rocha continued. “The delivery tests will be fairly simple at first. They will determine that the aircraft can hit a target with the weapon. That involves making sure the weapon receives the updates it needs from the aircraft, guides properly, and hits its target. The releases from mission systems aircraft will become more operationally representative and more complex as the testing proceeds.”

Maintenance Evaluations
Besides flight testing, the F-35s operating from Edwards are also being tasked to verify technical data used to maintain the aircraft and to evaluate and test the overall system for maintaining the F-35.

“For technical data, we have a list by US Air Force specialty codes for maintenance actions we want to evaluate,” explained Mary Parker, deputy for logistics at the F-35 ITF. “Whenever we have a maintenance task on the airplane that can be used to verify the technical data, representatives from the US government and Lockheed Martin are right behind the maintenance technicians asking if the techs have the information and the right tools they need. We are making sure that the maintenance task instructions can be performed in the field.”

The Edwards ITF has recently completed evaluations for servicing and towing the aircraft in chemical protection gear as well as for maintaining the engine. The chemical protection gear consists of overgarments, boots, and gas masks. “We are also evaluating weapons loading, which covers loading AMRAAMs and JDAMS into the internal weapon bays while wearing chem gear,” Parker continued. “In an upcoming phase, we will evaluate maintenance items related to low observable restoration. The maintenance personnel will be wearing chem. gear in these evaluations as well.”

Maintenance at the ITF is performed by personnel from Lockheed Martin as well as by civilians and military personnel working for the US Government. Four technicians come from international air forces—two from the Netherlands, one from Norway, and one from Canada.

“In many ways, the F-35 is easier to maintain than the F-16,” said Capt. Terje Vik, a maintenance lead from the Royal Norwegian Air Force. Vik has been at the F-35 ITF since the aircraft first arrived May 2010. “The F-35 has fewer LRUs [line replaceable units] and is more software driven. Normal scheduled maintenance is reduced. And the computer interface replaces a lot of test equipment. The aircraft also has more built-in test capability. Overall, fewer people are required to maintain the F-35.”

Delivering Capability
While the priorities on those flat screen panels positioned at the main entrance may change from day to day, the overarching goal for the F-35 ITF at Edwards remains constant: To deliver a highly capable fighter that is safe and meets all of its requirements.

“The testing we are doing now is focused on delivering capability,” concluded Schwartz. “Ultimately, we are delivering that capability to future generations of fighter pilots who will be operating the F-35.”