F-35 Flight Test Perspectives

By Eric Hehs Posted 15 September 2008

Eighteen test vehicles, more than 5,000 sorties, and 10,000 flight test hours are just a few ways to sum up the testing planned for the System Development and Demonstration, or SDD, phase of the F-35 program. Thirteen flyable aircraft and five ground test airframes will be flown, pushed, poked, and prodded during this phase. The flyable aircraft fall into two basic categories: flight sciences and mission systems.

Flight sciences aircraft are used to expand the flight envelope. The nine flight sciences aircraft are composed of four F-35A conventional takeoff and landing, or CTOL, aircraft (including the first aircraft, called AA-1, which was manufactured before the results of a weight reduction program were applied to the F-35 design); three F-35B short takeoff/vertical landing, or STOVL, variants; and two F-35C carrier variant aircraft. These aircraft will be used to evaluate flying qualities, stability and control, high angle of attack, environmental systems, propulsion, flutter, loads, dynamic response, and store separation.

Mission systems aircraft are used to test systems not associated with expanding the flight envelope. The four mission systems test aircraft, divided into one F-35A, two F-35Bs, and one F-35C, will focus on interoperability, stores integration, and avionics integration. Systems associated with mission systems testing include communications (datalinks and satellite communications), distributed apertures, and electro-optical targeting.

Testing of the F-35’s active electronically scanned array radar, helmet-mounted displays, avionics associated with navigation and identification, and multifunction displays will also be done with the mission systems aircraft.
The Cooperative Avionics Test Bed (referred to as CATB, or the CATbird) will support mission systems testing.

This Boeing 737-300, originally built in 1986 and purchased by Lockheed Martin from Lufthansa in 2003, will be used to integrate and validate the performance of all F-35 sensor systems before they are flown on the first Lightning II fighter. The CATB will be key to developing the F-35’s ability to collect data from multiple sensors in a dynamic environment and fuse it into a coherent picture for the pilot.

The delineations between flight sciences and mission systems testing are not as strict as they may sound. Weapon integration, for example, involves both flight sciences and mission systems aircraft. Test drops for clearing a particular weapon for a specific variant of the F-35 involve flight sciences aircraft. Testing that evaluates how well systems on the aircraft help the pilot identify targets or how accurately a weapon is delivered to the target involves mission systems aircraft.

The ground test aircraft also fall into two categories: static testing and durability testing. Static tests involve applying forces to the airframe to determine the strength of the structure. Durability tests involve applying repetitive forces to the airframe to simulate stresses and strains the aircraft will experience during its lifetime. Both the F-35A and the F-35B require a static and durability ground test airframe. Only a static test airframe is needed for the F-35C.

Standing behind these eighteen test aircraft, 5,000-plus sorties, and 10,000 flight test hours associated with the SDD phase of the F-35 is a large group of dedicated professionals from the Flight Test department. Code One editor Eric Hehs interviewed several of these professionals to get their perspectives on F-35 flight testing. 

Starr Hughes   joined the F-35 program in 2003 as a flight test engineer and test conductor for the first F-35, or AA-1, which is a CTOL version of the Lightning II. She brings flight test experience from Raytheon where she worked on the flight test team for the Hawker Horizon business jet. She first heard about the Joint Strike Fighter program when she was at the University of Kansas. Lockheed Martin’s winning the contract was a topic of conversation in the aeronautical engineering department.

My main activities involve planning and conducting flight tests for AA-1. As a flight test engineer, I prepare the test plans and conduct the tests. The ultimate goal for my team is to expand the flight test envelope for AA-1. The aircraft is flying with many limitations right now. But the more we fly, the more limitations we remove. For example, we are expanding the flight envelope for aerial refueling. With our initial test, we were cleared to refuel at only one altitude, 20,000 feet (plus or minus 2,000 feet), and at a limited airspeed.

We are in the process of expanding the altitude to 30,000 feet and increasing the refuel-ing airspeed. Opening this portion of the envelope to aerial refueling makes the testing more efficient as we conduct other types of testing.

This first Lightning II was manufactured before the results of a weight-reduction program were applied to the F-35 design, so envelope expansion tests we’ve done with it apply mostly to AA-1. Still, we have used AA-1 to test critical subsystems common with subsequent aircraft, such as the integrated power package, cockpit displays, electro-hydrostatic actuators, the electrical system, and many components of the propulsion system.

I followed AA-1 to Edwards AFB in California where it was used in a series of tests for engine airstarts as well as for aerial refueling and acoustics. The testing gave us a look at engine/aircraft integration and airstart capabilities. The airstart characteristics for all the systems were as expected. We also verified inlet recovery predictions for the power and thermal management system, or PTMS.*

The best part of my job is conducting test flights. I enjoy talking to the test pilots and observing missions from the control room. The work requires a lot of coordination. I also enjoy the detail associated with writing joint test plans that are used by the aircraft flight test lead to plan actual missions. Working with a variety of disciplines rather than just one or two keeps my job interesting.

Day-to-day activities preparing for a mission include putting together a set of test cards to meet the day’s objectives, leading an engineering brief, and then conducting the test from the control room. Of course to get that far, our team has to coordinate with engineering, pilots, and flight operations, to name a few.
The biggest challenge the flight test group is facing often seems to be just keeping the airplanes in the air. Hardware breaks, and we fix it. But we learn from those experiences. AA-1 and subsequent aircraft will benefit from these early tests.

After AA-1 flight testing is complete, I plan to work across all three variants of the F-35. I will support the aircraft through airworthiness testing as a test conductor. I will also train other flight test engineers who will be deploying to the various test sites.

*The PTMS performs aircraft functions traditionally performed by an auxiliary power unit, environmental control system, and emergency power unit. The PTMS includes the equipment necessary to provide aircraft main engine start, auxiliary power, cockpit cooling and pressurization, avionics cooling, mechanical equipment thermal management, and pressurized air for the onboard oxygen generation system and the onboard inert gas generation system.


Graham Tomlinson came to the F-35 program in 2002 as a test pilot for BAE Systems. He piloted the first flight of the F-35B in June 2008. He was an advisor for BAE during X-35 flight testing. Tomlinson went to work for BAE Systems as a test pilot in 1983 and spent most of his career in Harrier flight testing programs. He began his flying career with the Harrier in the Royal Air Force. He was the British military test pilot for the AV-8B at NAS Patuxent River, Maryland, when that aircraft was introduced to the US Marine Corps. He is a graduate of the Empire Test Pilot School in the United Kingdom.

I was asked to come to the F-35 program to test the STOVL variant because of my experience at BAE Systems as a flight test pilot in the Harrier; BAE has been doing Harrier work since 1960, so we know STOVL. When I arrived six years ago, we were discussing specifications. We wanted to make sure the specifications were sensible for the needs of the fleet pilots.

Those specification discussions have now turned into design discussions. The bulk of our work has been getting the design right. In particular, we have been refining the control laws. Basically, these are the instructions programmed into the flight control computer that tell the airplane to fly the way the pilot commands it to fly. Now that we have started flight testing, my job has changed to a more classic flight test pilot job. I fly the airplane to help flight test engineers test their ideas. We test pilots think of ourselves as the conduit between design engineers and fleet pilots. All the test pilots on the F-35 were fleet pilots once in their careers. I also help write the flight test plans that will get the answers and results needed to eventually turn this aircraft into an operational fighter.

The biggest challenge to the program is financial and political—that is, keeping the program sold. That’s always the biggest challenge. Does the world stay in a current state of disarray long enough before someone decides what we really need is a three-inch square robotic minicopter? The big picture is always the most important issue on any aircraft program. Once the program is securely funded, our biggest technical issues are things like sensor fusion. Providing pilots with the information they most need to complete their mission is a big challenge.

The biggest challenge to the program is financial and political—that is, keeping the program sold. That’s always the biggest challenge. Does the world stay in a current state of disarray long enough before someone decides what we really need is a three-inch square robotic minicopter? The big picture is always the most important issue on any aircraft program. Once the program is securely funded, our biggest technical issues are things like sensor fusion. Providing pilots with the information they most need to complete their mission is a big challenge.

The flying we have done recently is reducing that unknown. We opened the STOVL doors in flight. That flight provided a lot of aerodynamic information that helps to confirm our models. Next year when the new engine is installed, we will tie down the F-35B over a hover pit and run it in STOVL mode. The pit simulates free air testing so that we will acquire a lot of data in those tests. But actual flights are always different. There is no such thing as a perfect simulator or a perfect ground rig. I’m sure we will encounter some unexpected events in the air. That’s why we do flight testing. I’m confident those unexpected events will be more interesting than exciting.


Paul Dotson joined the F-35 program in 2002 to work on the integration of the instrumentation system for the nine F-35s associated with flight sciences testing. He has worked flight test integration for twenty-four years on such flight test aircraft as the F-16 Block 25, the Indigenous Defense Fighter for Taiwan, EC-130V, C-130J, and X-35. He is a graduate of Texas A&M University.

Flight test instrumentation refers to the recording and monitoring equipment fitted to the aircraft to monitor aircraft behavior. Instrumentation touches almost every system on the airplane—from an accelerometer on the vertical tail that measures flutter to strain gauges on a landing gear that measure landing forces. Those of us who work with instrumentation get a big picture of the aircraft and its subsystems.

Each F-35 aircraft in this phase of the program has an instrumentation lead engineer. The leads negotiate and gather measurement requirements from various technical teams and create a master measurement list—a list of parameters that engineers measure during the flight tests. From that master list, they design a data system for each aircraft to accommodate those measurements. They then create an equipment installation and transducer installation requirements list for each section of the aircraft.

Each of the three primary partners—BAE Systems, Lockheed Martin, and Northrop Grumman—is responsible for instrumentation in its section of the aircraft. All three companies follow common design practices. Flight test instrumentation in Fort Worth tries to make the installations common across the aircraft to reduce complexity and cost. The instrumentation systems are more common between AF-1 and AF-2; CF-1 and CF-2; BF-2 and BF-3 [designations for the A, B, and C-model test aircraft] since flight test tasking and roles for these aircraft are very similar. The similarities also allow them to function as backups.

Traditionally, we build the airplane first and then put it in a modification hangar for six months where we install the instrumentation. The schedule on the F-35 demands that we install the instrumentation as the aircraft are being manufactured. Airplanes coming off the assembly line now are almost completely instrumented and ready to perform flight tests.

We monitor about twice as many parameters on the SDD F-35s as we monitored on the X-35. We are in the 1,000-parameter range. We are monitoring more capability on F-35. For example, we have digital weapon separation cameras on the F-35. These cameras are used to track the weapon as it releases to make sure it clears the aircraft. We didn’t drop weapons from the X-35.

We have taken advantage of miniaturization in digital electronics. The instrumentation data system is integrated into the aircraft on the flight sciences aircraft instead of filling a weapon bay.

The biggest challenge we face is getting all the various entities involved to work well together. We are dealing with three aircraft variants and two types of flight test aircraft within each variant—mission systems and flight sciences. We have three companies building major portions of each variant. We also have two engine manufacturers supplying different engines. Instrumentation has to account for all these differences.


Stefano Filoni joined the F-35 program in 2004 to develop test plans for F-35 climatic testing, which is scheduled for the climatic test laboratory at Eglin AFB, Florida, in 2010. He also writes test plans for utility and subsystem testing for the F-35B. Before joining the F-35 program, Filoni worked for Alenia Aeronautica in Italy as a flight test engineer on the C-27J Spartan program. He was responsible for performance and handling quali-ties for the Spartan. He was also involved in hot- and cold-soak testing for that two-engine airlifter that makes use of the engines and avionics developed for the C-130J Super Hercules. Filoni graduated from the University of Naples Federico II in 1999. He came to the United States in 2004 to work on the F-35 program.

The climatic test plans I am creating will test the F-35B at high and low temperatures. However, before the program actually takes an aircraft to Eglin for these tests, we must complete a lot of work. For example, we have to build a tie-down and platform for the tests because none currently exist for this new airplane. We also have to design the ductwork that removes the exhaust from the facility. Because we will be testing a STOVL aircraft, the downward thrust has to be redirected out of the facility as well.

For the actual climatic testing on the F-35, we will use a mission systems aircraft—that is, a test aircraft outfitted with all the avionics and systems found on an operational aircraft. High-temperature and low-temperature test-ing involves multiple test runs. We will simulate entire missions during these tests. We start the engine, retract the landing gear, fire the gun, release weapons, extend the landing gear, and perform everything in between.

For high-temperature tests, our baseline temperature is 59 degrees Fahrenheit. We perform the same test at 113 degrees F and finally at 120 degrees F. The cold tests are more involved. We perform the first cold test at minus 15 degrees F. We simulate an alert launch at this temperature. A pilot climbs in the jet, starts the engine, and performs a simulated takeoff—all within five minutes from the start of the test. Then we perform a self-start test at minus 25 degrees F. Self-start means starting the aircraft without help from an outside source.

The last cold test is at minus 40 degrees F. We will cold soak the aircraft to minus 65 degrees F for this test.

Other test conditions to be covered during the climatic lab trials include snow, high humidity, rain, freezing rain, and icing. I am looking forward to conducting an actual flight test. The F-35B has three test conductors now, but we are training more to prepare for the additional test aircraft. 


Steve Potton joined the F-35 program in July 2003 as a flight test engineer for BAE Systems. He has worked for BAE Systems for the last twenty years in flight test. He began his career on the Harrier in Dunsfold in the south of England. He later moved to Warton in the northwest of England where BAE performed development trials for the Harrier. He came to the United States specifically to work on the F-35 program.

I lead a group of flight test engineers on the day-to-day planning for BF-1, the first STOVL version of the F-35 for this phase of the program. My STOVL flight testing experience on the Harrier drew me to the F-35 program. We conduct briefings for the aircrew and the flight test control room team before each test flight. Our overall purpose is to thoroughly test the airplane before it goes to the operational fleet.

One of our biggest challenges is the amount of people with whom we have to coordinate before we perform any test. This coordination effort is driven by the complexity and sophistication of the aircraft and where it is in its life cycle. With the Harrier, we were updating aircraft that had already been in service for many years. The magnitude of what was new with the Harrier when I worked on it is tiny in comparison with what we are developing for the F-35.

For example, the control laws for the STOVL mode are one of the most technically impressive aspects of this aircraft. They are an order of magnitude more complex than the Harrier. But they will make the F-35B much easier to operate and, therefore, more effective. Pilots will not have to spend as much time training to operate in STOVL mode. They will be able to devote more training to tactics and operational effectiveness.

We spend a lot of time finding problems with the aircraft. Problems are naturally perceived as a negative. But finding and solving problems at this early stage should be viewed as a real positive. Our job is to make sure the airplane is both safe and effective. That’s what flight testing is all about.

With that said, everyone enjoys seeing the flights come off successfully. I also like testing the complete aircraft rather than testing some small piece of it. Another satisfying aspect of my job is getting to the end of the week and feeling that our testing has been of value to the overall program.

BF-1 is going through a modification period through the end of 2008, which includes installing a new engine, updating software, and making a few structural changes. Early next year we begin envelope expansion flying in STOVL mode. Everyone is looking forward to that.

Eric Hehs is the editor of Code One.