Test pilots at Edwards AFB, California, are set to begin high angle-of-attack (AOA) flights and weapons separation testing on the Lockheed Martin F-35 Joint Strike Fighter (JSF).

So far this year, pilots at the isolated desert base have flown about 350 sorties, says Lt Col George Schwartz, director of the F-35 integrated test force and commander of the 461st Flight Test Squadron. Much of the activity has focused on high speed tests which have seen the F-35 being repeatedly pushed out to its maximum speed of Mach 1.6 and 700 knots calibrated airspeed-often fully laden with internal weapons.

Another ongoing theme for the Edwards test pilots is maturity testing for the software required for the F-35 training mission currently underway at Eglin AFB, Florida. The test pilots at the base have also flown night aerial refueling missions and have completed all of the engine air starts required for the F-35A conventional take-off and landing variant (CTOL).


 ©Lockheed Martin

For the engine air start tests, the F-35 needed some modifications, one of which was the addition of a second cockpit pressurization system. The added pressurization system was necessary because the tests involved shutting down the jet's engine at high altitude. "We finished that up with two engine restarts at 40,000ft and 37,500ft," Schwartz says.

Though the air start sorties were challenging in a single-engine fighter, the testing was necessary in order to move onto exploring particularly difficult parts of the jet's flight envelope. "That allowed us to go into high AOA testing where we will start expanding the envelope from 20° AOA all the way up to 50° AOA," Schwartz says. "It's going to start probably in September."

Right now, engineers are in the final stages of attaching an anti-spin parachute to aircraft AF-4. "We've finished almost everything for that," Schwartz says. The next step will be to test deploying the chute during runway taxi trials in order to make sure it works properly.


 ©Lockheed Martin

Initially, test pilots will simply push the F-35 out to 50° AOA. But then the veteran aviators will have to intentionally depart the aircraft from controlled flight in order to gauge how the jet behaves under those conditions. They will also evaluate the F-35's departure recovery procedures and its departure resistance characteristics. "It's the kind of stuff a test pilot dreams of doing," Schwartz says.

Like the transonic region of the flight envelope, high AOA testing flight is particularly tricky. While there have been improvements made, there are still some transonic roll-off problems--where the aircraft begins an uncommanded roll at speeds between Mach 0.9 and Mach 1.2--that have yet to be fully ironed out on the F-35. Those problems are being fixed with tweaks to the F-35's flight control laws. But Schwartz says, similar discoveries are possible in other challenging parts of the envelope like high AOA flight. "We expect to find stuff and we'll get it corrected," Schwartz says. "That's why we're here."


 ©Lockheed Martin

There are also ongoing trials with external weapons loads and pilots at Edwards hope to start testing weapons separations soon. Currently, all weapons pit drop tests required for the system development and demonstration (SDD) phase, save for the 250lbs GBU-39 small diameter bombs, have been completed. "We're just getting ready to do our first weapons separation and that'll be in the October timeframe," Schwartz says. "We're going to do a GBU-31 and an AIM-120."

In addition to physical tests of the airframe and weapons, there are also extensive mission systems trials ongoing at Edwards. The integrated test force has already finished vetting the F-35's Block 1B software, which begins to fuse some of the data from the jet's myriad sensors, Schwartz says. That software is now being deployed with the training fleet at Eglin AFB. So far, the software has become more and more stable as testers wring out the problems and the code is corrected in later releases. But there have been some "minor integration issues," Schwartz says.

Currently, Edwards is testing the "very last part" of software Block 2A. Schwartz says that testing is complete for the low rate initial production Lot 4 jets' software. The test force is hoping to be flying with the Block 2B software load starting in the fall. "That's the one that going to be going into the first big operational test period," Schwartz says. It is also the software block that the US Marine Corps hopes to declare initial operational capability with on the F-35B.

Mission systems testing will accelerate in the next year. There are currently six F-35s at Edwards, but three more are scheduled to arrive by early 2013. Two of those will be USMC short take-off vertical landing (STOVL) F-35B-model jets while one will be a US Navy F-35C carrier variant aircraft. "Those jets will be dedicated to mission systems testing," Schwartz says. "Especially with the mission systems testing, it's in the heart of the development right now."

Source: Flight International