I'm curious about how these engines are flight-tested. Especially since this one is so close to the ground.
I'm assuming all four engines are started up normally prior to commencing the take-off roll. The three 744 engines are run up to full T/O thrust while the engine being tested is left at idle, or do they balance the thrust and use the in-testing engine as a fourth 744 engine?
Once the testbed has taken off, the engine being tested is run through the various tests scheduled for that flight.
What happens if the single 744 engine on the same wing as the test engine fails past V1? Do they run up the test engine to equivalent thrust to complete a 3-engine takeoff or do they risk aborting? Or is the aircraft sufficiently light that they can accomplish a takeoff with just two engines? In the greater trade-offs in life, damaging a test engine with FOD is lower cost than trying to abort and running the aircraft off the end of the runway and potentially costing the lives of all on board.
It isn't trivial testing an engine so low to the ground. However, an engine must be tested at all appropriate Mach numbers.
Early takeoffs might takeoff lightly loaded with three engines.
Later on a fleet of Street street sweepers will make sure that runway is clean. Teams will ensure the runway is clean. A team will calculate exactly where each thrust lever should be (on the test engine, the controls will be hacked and the pilots only command full throttle, flight idle, cruise and maybe thrust reverse (usually not this low, but I've seen crazier). The precise thrust is controlled by the engineer in the back.
There is a reason experimental aircraft must be stenciled above every door, including the entry into the 747 avionics Bay behind the front gear.
I love 747 test beds. Deactivate the center fuel tank and you easily take off on 3 engines. No muss, no fuss. Recall how light the plane is loaded. Instead of 50 or so tons of payload, it is maybe 15 tons of engine and engineering stuff and crew. They've sold most of the seats, galleys, and lavs off for spares long ago, so there is several more tons of saved weight. The flights without the test engine providing thrust rarely have more than 8 hours of fuel, so a 747 is a rocket with plenty of spare thrust.
Heck, the thing is too light, so those weights they use to proof load elevators or proof cranes are fixed onto the cargo floor to help with weight balance.
And everybody on board signs a waiver and goes through a bunch of training. To say the least, those 8 evacuation spool reels in the cockpit are verified in cert. Another 8 evacuation reels are bolted in a standard position by L1 and R1. And everyone knows how to dive down through the avionics compartment to get out. And the floor plates to the cargo hold have the carpet removed for quick access. Let's not forget the L5 door escape rope or top deck slide... Every seat has an oxygen bottle and first flight is by only parachute trained crew. OK, part of why I like 747 test beds is the flight test crew can get out!
So this allows doing tests that would be insane with passengers. Such as the required test of the low altitude, low Mach number, 1.5 G turn to verify no compressor stall at maximum takeoff thrust.
. Yea... Lots of 1.5G turns at different Mach numbers and thrust levels... Lots of thrust transients... Everything needed to be certain a new plane can fly with two of the new engine must be tested before a new airframe enters flight test.
Yes, the Trent 1000 is an upgrade. But high altitude relight, in flight shutdown (cold soaked) and other tests must be done.
Even crosswind flying at different thrust levels to try and induce stalls.
But for the low ground clearance, much more is spent keeping the runway clean.