Behind the Scenes: Testing The Big Engines

Although unsuitable weather conditions cut short my first flight in January, I was offered a second chance to make a longer flight during a mission in late April to evaluate the CFM Leap-1B for Boeing’s 737 MAX. CFM is the joint venture between GE and Snecma, so the test team included engineering personnel from the French engine manufacturer.

Though the test was focused on the Leap engine, my main target was to witness the GE test team in operation and see how it had completely transformed itself to handle the recent surge in new engine programs. We flew for more than 7 hr. through large swaths of the giant block of Edwards AFB-controlled R-2508 restricted airspace. I also wanted to find out more about plans to test the GE9X, the world’s largest engine in development for Boeing’s 777X.

Each flight is the carefully choreographed culmination of months of careful planning by the engine maker’s Flight Test Operation unit working in cooperation with GE Aviation’s Evendale, Ohio, headquarters as well as the appropriate airframe maker and, if the test involves a CFM engine, GE’s partner Snecma. In response to a mounting workload including flight testing three versions of the Leap-1, GE’s Passport business jet engine and doing other work, the test unit devised new procedures to enable a busy flight schedule of airline-like intensity. The operation’s two Boeing 747 flying testbeds conducted 110 Leap-1A/B flights in the past year. In addition, the 747-100 made 20 flights of the Passport engine. The tally of 130 flights for 2015 was more than had been flown over the previous five years combined, with some of the most intense weeks seeing both aircraft sometimes flying twice within a 24-hr. period.

With the 747-100 due to be retired later this year, the burden of future testing will fall on the ample shoulders of the 747-400, which has received several flight deck modifications for its role. One of the most obvious is a specially made engine throttle lever for the test engine. Gold-colored and fitted with two handles, the throttle lever was fashioned by “a real talented machinist here at GE,” says Steven Crane, chief test pilot of GE’s flight test operation. “The offset handle allows us to fine tune throttle operations without interfering with the pilot flying.”

A tailor-made flip-down display for each pilot (above the co-pilot's navigation display) shows engine test parameters, live exterior camera images and other key data. When not in use, the display stows away beneath the glareshield.  Designed by former GE chief test pilot Tom Drechsler and Crane, the display has “all the things we want as one source for test pilots, such as N1K and N2K [corrected low- and high-pressure spool speed]. It’s a high-definition screen and is a repeater of the data and video systems downstairs [on the main deck]. It doesn’t interfere with flight controls, and it also allows us to see the navigation display for good situational awareness,” says Crane.

Although my flight involved testing the relatively small Leap-1B, future work will include flight tests of the GE9X, the world’s largest engine. Having worked on the GEnx-1 program Crane says the higher-thrust powerplants represent a different challenge. “Testing a larger engine is a lot more physical because you are doing a lot more changes with the rudder, and in pitch and roll. We are creating asymmetric conditions with high-power fuel cuts and tests of that nature, so you have to anticipate with the non-test engines coming up on thrust.

“I find ways to stay within the test beta by using pitch and, much as you’d pitch down for airspeed as that thrust comes back up, you may be pitching up to make sure you don’t go beyond your airspeed parameters. So all those things are happening at once—it’s a lot of fun.”

“One of the bigger challenges is managing four different engines,” says GE senior engineering test pilot Jon Ohman (right).  “If we flew this on the line, we’d have them matched up and have them in auto-throttle almost for the full flight, but here we are working different power settings all the time, and doing power chops and bursts. Also, I was surprised at how much this aircraft reacts to asymmetric thrust. Even when you are bursting the power on the inboard engines, the airplane reacts significantly,” says Ohman who, like Crane (left), is a former military test pilot.

Then there is the change in scale. “We could take 20,000 lb. of total fuel on the F-35C, the biggest variant. On this, 20,000 lb. is our minimum fuel.  That was as much gas as I could take off with the F-35!”