The Boeing 777-9 test fleet: N779XW, XX, XY and XZ

Boeing launched the 777X in November 2013 with a plan to deliver in 2020. The program slipped first because of the GE9X engine, whose high-pressure compressor needed redesign in 2019, then because the 737 MAX crashes changed how the FAA certifies Boeing airplanes: the agency withdrew the authority it had delegated to Boeing engineers and insisted on reviewing the 777X's flight controls and systems itself. Type Inspection Authorization, the formal start of FAA certification flying, did not come until July 2024, and a month later a cracked thrust link, the strut that transmits engine thrust to the wing, grounded the fleet again. Testing resumed in early 2025, and Boeing now expects to deliver the first 777-9 to Lufthansa in 2027, seven years late.

The four aircraft divide the work. WH001 does flutter, stability and handling; WH002 the engines and systems; WH003 avionics and autoland; WH004 the cabin, environmental systems and, in 2026, the first flights in an airline interior for Lufthansa. Boeing has also built two 777-8 freighter test aircraft that will join later.

The 777-9 is a 777-300ER stretched by four frames to 76.7 meters, with a new carbon-fiber wing whose tips fold up on the ground so the 72-meter span fits the gates built for the old 777. Its GE9X engines each produce about 110,000 pounds of thrust from a fan 134 inches across, larger than a 737's fuselage. The cabin is wider than the 777's because Boeing thinned the sidewalls, and the windows and cabin altitude come from the 787. Boeing has orders for more than 600 777Xs from Emirates, Qatar, Lufthansa, Singapore, Cathay Pacific, British Airways and others, which is why the airlines have tolerated the delay.

Test flying takes the aircraft to places airliners avoid: Boeing's high-altitude site at Colorado Springs, hot-weather testing in Arizona and the Middle East, cold soak in Alaska or Canada, and crosswind work at Lubbock, Texas. The tracker catches them departing Boeing Field, climbing to 43,000 feet over the Cascades or spending hours in holding patterns over Moses Lake.

A 777 that is too long, with engines that are too big and wingtips that fold: on the ground the outer 11 feet of each wing stands vertical. The test aircraft wear Boeing's blue-and-white 777X livery with the WH0 numbers on the nose gear doors and large orange trailing cones or test booms on some flights. On the tracker they broadcast N779XW, N779XX, N779XY and N779XZ with Boeing test callsigns beginning BOE, flying out of Boeing Field (BFI) or Paine Field (PAE).