F-35 Lightning II Flight Tests

By Eric Hehs Posted 15 June 2007

The F-35 Lightning II took to the air for the first time on 15 December 2006 with chief test pilot Jon Beesley at the controls. During this maiden flight, Beesley performed a military power takeoff and executed a series of maneuvers to evaluate the handling qualities of the aircraft. The airplane flew to 15,000 feet and a maximum speed of 225 knots. The F-35 test program has since expanded the flight envelope of this first Lightning II and will continue to expand the envelope in the coming months. More importantly, this first aircraft is being used to evaluate the performance of highly sophisticated subsystems that form a baseline for subsequent F-35s.

Beesley has an extensive flight test résumé that begins with graduation from the US Air Force Test Pilot School in 1979. After working on several classified programs, he became one of the first USAF pilots to fly the F-117. When he left the Air Force in 1986 to join General Dynamics in Fort Worth, Texas, he initially flew developmental flight tests for an innovative night attack system for the F-16 called Falcon Eye. This program was one of the first to use helmet-mounted displays, or HMDs, and head-steered infrared devices on a tactical aircraft.

In 1990, Beesley became a project test pilot on the YF-22 during the Advanced Tactical Fighter competition. He was principally involved with evaluating and demonstrating the flying qualities of the YF-22. Many of these flights demonstrated the tremendous high angle of attack capabilities of the aircraft. Longtime Code One readers may recall his article on flight testing the YF-22, "Report From the Future," in 1991.

After the US Air Force selected the F-22 as the winner of the Advanced Tactical Fighter competition, Beesley became the Fort Worth project pilot for the F-22 program. He was the second pilot to fly the Raptor and one of the lead pilots in envelope expansion flights. Over his career, he has accumulated more than 5,000 hours of flight time in more than forty-five different types of aircraft.

Beesley became chief test pilot for the F-35 program in 2002. He will be in charge of flight testing all three variants to be produced: the F-35A conventional takeoff and landing, or CTOL, variant; the F-35B short takeoff/vertical landing, or STOVL, variant; and the F-35C carrier variant, or CV. Code One editor Eric Hehs interviewed him for his impressions of flying the first F-35 and for his perspective on flight testing this and subsequent Lightning II fighters.

What is your strongest memory from the first flight of the F-35?

The thrust impressed me most. The first flight profile called for the F-35 to immediately go to 15,000 feet. I had to keep the speed at 225 knots during the climb since I had to keep the gear down, which limited the maximum speed.

I used nose attitude instead of modulating engine thrust to control airspeed during the climb to 15,000 feet. In other words, I had to raise the nose to slow down the airplane. I took off and started pulling back on the stick. I had to keep pulling back to stop from accelerating over the 225-knot limit. So I reached a rather steep angle, about twenty-five degrees of pitch. The steep angle, witnessed by the crowds on the ground, highlighted the raw power I was experiencing in the cockpit. The thrust surprised me. Not in the sense of "Gee, how am I going to handle all of this power?" But more like, "Wow, this is more than I expected."

What was your overall impression of the airplane after that flight?

Overall, I was impressed by how well the entire first flight came together. I started the airplane, ran through all of our ground checks, taxied out to the end of the runway, and took off. The test team told me I taxied out to the end of the runway much faster than I did for any of the taxi tests. But I was ready to go and so was the airplane.

I was also pleased with how smoothly the airplane went through all the ground checks and how smoothly the airplane flew. As an example, the flap schedules on the original F-22 shook the Raptor at speeds above 200 knots. This objectionable buffet was addressed right away through a software change. Paul Metz [first pilot to fly the F-22] and I are the only two pilots who ever experienced that buffeting. I thought that I might experience some sort of buffeting with the first F-35, but I didn't.

We learned a lot from the F-22. Our engineers deserve a lot of credit. In fact, many of those who completed the checkout and testing of similar systems on the F-22 Raptor are performing the same work on the F-35. To name a few prominent examples: Kevin McTeague works on electrical systems; John Magbuhat works on flight controls; Paul Thoennes works on hydraulics; and Roy Schoberle from Pratt & Whitney works on the F135 engine. Many others with similar experience did the design integration work over the last several years. We also have some seasoned veterans involved in flight testing the new airplanes, which includes Mary Beth O'Loughlin as the test conductor for the first flight. We have a great team.

How has your impression of the F-35 changed in subsequent flights?

I continue to be impressed with the performance of the aircraft. The F-16s flying chase don't have near the fuel capacity or payload capability as the F-35. The Lightning II does very well in comparison. For example, the F-35 often forces the chase aircraft into afterburner when it is in military power.

The airplane's handling qualities continue to be very good throughout the flight envelope. When I raise the landing gear, the airplane flies very smoothly. The landing gear is sequenced, which is unique for a fighter. The nose gear comes up first, then the main gear follows. The gears drop down in reverse order. Another strong impression is that the airplane wants to fly a lot faster than we are allowed to fly at this point in the flight test program. Most of the time we fly at about thirty to forty percent of available thrust. This airplane can go out to high subsonic speeds very easily without using afterburner.

Describe the basic progression of the first flight tests.

On the first flight takeoff, we received an air data degrade caution message. It indicated a mismatch in the lower-level comparison in the air data system, specifically with angle of attack. However, we had no loss of capability. Simply put, readings from the right and left air data probes need to agree within a certain tolerance, and they didn't on the first flight.

Because the air data system is redundant, we were able to fly on the left probe after the right one was turned off. The caution message cut the flight short, but we still managed to perform some of the planned maneuver blocks, which included throttle transients and one-half stick and pedal inputs. The handling qualities in these maneuvers were excellent with a notably smoother response and a better roll rate than I expected.

The greatest accomplishment of the first flight was the performance of the subsystems. The integrated power package, electrical, electro-hydrostatic actuators, flight control computers, and other subsystems worked without a problem for the entire flight. The performance of these systems is a great testimony to the team that brought the F-35 to first flight. After the faulty probe was replaced, we performed an additional 110-knot taxi test on 4 January to calibrate the new probe. We gathered additional air data on subsequent flights during January to further calibrate the air data system.

On Flight 2, we cycled the landing gear and then flew formation for the first time with the gear up. On Flight 3, we performed the first military power takeoff. On Flight 4, we performed the first low-altitude maneuvering. On Flight 5, we performed the first afterburner engine transient as well as performing other engine transient testing. On Flight 6, we conducted a fuel dump test. This test was conducted early in the flight test program to gather real-world data to inform design decisions on the fuel dump mechanization for the carrier variant, or F-35C. We performed higher angle of attack maneuvers on Flight 6 as well.

On Flight 7, we evaluated the speed brake operation. The F-35, like the F-22, doesn't have a dedicated speed brake like most previous fighters. Instead, it decelerates through the flight control software by deflecting control surfaces in the same manner as the Raptor. We use the leading-edge flaps as well as the trailing-edge flaps and the rudders to slow the airplane. Unlike the F-22, the F-35A and F-35B have no ailerons. That explains why it uses a combination of leading- and trailing-edge flaps and rudders to slow down. I found that the buffet levels were very low, essentially the same as buffet levels of the F-16 with the speed brake in operation. Deceleration rates in the F-35 are similar to the
F-16 as well, which is a design goal.

On Flight 8, we flew the software fix for the air data system issues we saw on the first flight. The new software allowed me to use full lateral stick rolling maneuvers. Handling qualities during these rolls were outstanding with roll rates matching predictions. We had to cut this flight short because our chase aircraft had a mechanical problem.

On Flight 9, we performed the first afterburner takeoff. Flight 9 was also our longest flight to that point, 1.5 hours. We took off with 3,500 pounds short of a full fuel load and landed with about 4,000 pounds of fuel remaining. So we shorted ourselves more fuel than the entire internal fuel capacity of an F-16 and still flew for 1.5 hours without aerial refueling. During Flight 9, we also flew close formations, power approaches, and maneuver blocks to sixteen-degrees angle of attack at 20,000 feet.

 On Flight 10, we flew with the HMD for the first time. The mission included full-stick 360-degree rolls, snap engine transients in afterburner, and close formation flying. We also landed in fifteen-knot crosswinds for the first time. Flight 11 involved several lower altitude maneuver blocks as well as maneuvering with the speed brake. Jeff Knowles, the second pilot to fly the F-35, completed his first flight on Flight 12. I took the aircraft to 30,000 feet on Flight 13, performed a touch-and-go landing, completed maneuvers to seventeen-degrees angle of attack, and cycled the aerial refueling door.

As far as envelope expansion goes, we have conducted engine transients up to maximum afterburner from takeoff to 30,000 feet. We have been to 345 knots, 3.5 g's, and sixteen-degrees angle of attack and seventeen degrees with the landing gear down. We have three engines available for AA-1 but have flown only one. We want to fly as many hours as we can on it.

Summing up the flying characteristics: the F-35 flies a lot like the F-22 and has the size and feel of an F-16. The F-35 is a solid and very responsive airplane.

How does this test progression compare to previous fighter flight test programs you have worked?

The F-35 envelope expansion and flying qualities work is similar to previous fighter programs. That similarity may give the impression that we're conducting the same tests in the same ways. But that impression is false. A superficial comparison between the development of this fighter and the development of legacy fighters neglects mission capability.

Our customers are getting a whole lot more in the F-35 program. They are getting a baseline configuration with capabilities that required twenty or thirty years to develop for the F-16: infrared sensors, targeting pods, night vision systems, head-mounted cueing systems, and agile beam radars to name a few. During those years of development, the Air Force and Lockheed Martin conducted separate test programs to validate those capabilities. Those capabilities are all incorporated in this phase of the F-35 program. A truer comparison between legacy programs and the F-35 program would include the development time and cost for these additional capabilities.

Are any of these capabilities and systems unique to the F-35?

The F-35 has many unique capabilities. The helmet-mounted display and the integrated power package, or IPP, are two good examples. We began flying the HMD on Flight 10 and have flown with it on all succeeding flights. The HMD is much more than a helmet-mounted sight, which is flying in operational F-16s today as the joint helmet-mounted cueing system, better known as JHMCS. Our HMD also functions as a head-up display. That is, it shows all the information normally placed on the HUD, including speed, altitude, heading, and flight path information.

The system is working very well, and pilots quickly forget that the flight symbology is being displayed on the helmet rather than on a conventional head-up display. We don't have a HUD on the first F-35. And we have no plans to put one in any other F-35. Putting an HMD in the first airplane is a gutsy call. We are on track with its development. The initial results of incorporating an HMD in the test program have been better than we expected. The HMD is a significant jump in technology. This system has been performing very well.

The IPP, my second example, is a sophisticated turbine that acts as the auxiliary power unit on engine starts. When the engine is running, the IPP functions as an environmental control system, or ECS. When required, it also functions as an emergency power unit during emergency mode transitions. The IPP, then, performs the functions of three subsystems found on legacy fighters.

The first F-35 represents a configuration of the aircraft before the company undertook a significant weight-reduction effort. Why is the program testing an aircraft that is not completely representative of subsequent production models?

While the internal structure may be different, the shape of this first F-35 is almost identical to subsequent production versions. So gathering aerodynamic data on this configuration gives us an opportunity to evaluate performance characteristics on a real aircraft as opposed to making predictions using models or simulations. Additionally, testing and integrating all of the new systems in the F-35, as I described previously, gives us more than a year's head start on problems that we may encounter in testing and integrating these same systems in subsequent aircraft. Along with the HMD and IPP, other systems and features incorporated on subsequent F-35s include the F135 engine, electrical system, fuel system, electro-hydrostatic actuators, cockpit, weapon bay doors, and bay ventilation. So this first version of the Lightning II gives us an outstanding opportunity to reduce risk as we move forward with the program.

Let's take the cockpit as one example of the similarities between this and subsequent aircraft. With the exception of two switches, the AA-1 cockpit is the same as the next F-35, which will be a STOVL variant. And that F-35B STOVL cockpit will be the same across all three variants. On the STOVL airplane, one switch will read "conversion" instead of "hook." All of the other switches are the same. While the engine page on the F-35B has a display that deals with STOVL, most every other display on this variant is the same as the displays on the other variants. The missions systems are the same on all three variants. This commonality reduces the total scope—and expense—of the program. We are combining into one program what would have involved three separate and independent development programs in the past.

The electro-hydrostatic actuators, or EHAs, are another excellent example of risk reduction we're accomplishing on AA-1. This is the first real electric jet. The flight control actuators, while they have internal closed-loop hydraulic systems, are controlled and driven by electricity—not hydraulics. The F-35 is the only military aircraft flying with such a system. We proved that the approach works on six flights of the AFTI F-16 during the concept demonstration phase of the JSF program. We already have many more flights on EHAs on this test program. Because we are flying production versions of the EHAs on AA-1, we won't have to prove the EHA design on subsequent F-35s.

What are the immediate production plans for subsequent F-35s, and how will those aircraft be used in the flight test program?

Current plans call for fifteen flight test aircraft, including AA-1. The next four aircraft produced will be F-35B short takeoff/vertical landing, or STOVL, variants. These will be followed by three conventional takeoff and landing, or CTOL, aircraft. Then the first three carrier variant, or CV, aircraft will be produced followed by another STOVL aircraft and one more CV. Two more CTOL aircraft complete the production run of test aircraft. AF-1 and AF-2, the next CTOL variants to be produced, will be used for flight sciences; that is, they will be used to test aero-dynamics and flight controls and to expand the flight envelope. AF-3, 4, and 5 will be used to develop and test mission systems.

We will have three F-35B, or STOVL, variants for flight sciences and two F-35Bs for testing mission systems. The first flight sciences B-model will be dedicated to STOVL operations. The other two B-models will be used to expand the flight envelope.

We will have four F-35Cs dedicated to the flight test program. The first two carrier variants will be used for flight sciences. The third aircraft will be used for carrier suitability testing. The fourth aircraft will be used to test mission systems.

We had as many as six aircraft devoted to testing mission systems for the F-22. We have seven aircraft in this program. Fortunately, everything we do on the F-35A for mission systems applies to the F-35B and F-35C. The variants have only minor differences in terms of antenna sizes and shapes.

But the real virtue of this flight test program is that we have seven flight sciences aircraft. While the F-22 had only one true flight sciences aircraft, we need more because we have three variants as well as many external payload configurations that require testing as well. The potential external loadings on the internal weapon stations and six external hard points create a very large test matrix, which will eventually include most of the weapons carried by the F-16, F/A-18, Harrier, and A-10.

What will be the biggest challenge for the flight test program?

For AA-1, our biggest challenge is to be aggressive enough to find out all the things we don't yet know about the aircraft's performance. We have some real opportunities to learn how EHAs work at high speeds. Proving the HMD is another challenge. Testing the first aircraft gives our predictions for subsequent aircraft credibility. We want to knock off all the big risks with this first airplane and reduce all the other risks for future airplanes. After that, a big challenge is managing fourteen flight test aircraft in three test sites. Testing short takeoffs and vertical landings is always a challenge. First, we have to make STOVL work. We have to make short takeoffs and vertical landings as straightforward and as easy as possible. Pilots should not have to spend most of their training time on the first and last five minutes of the flight. How we mechanize transitions from horizontal to vertical flight will free up time for training skills more pertinent to the mission.

Developing mission systems will be a huge challenge, and testing those systems is one of the more critical parts of the program. The CATBird, a 737 modified to carry the F-35 sensor suite and associated systems, will help us reduce risk associated with mission systems. The number of weapons and configurations to clear also represents a challenge. If pilots can't employ weapons, the airplane is of no value. And we are testing these weapons in a large envelope. The F-35 can maneuver post-stall like an F/A-18. So we have a lot ahead of us. But we are certainly up to these challenges.

Eric Hehs is the editor of Code One.