We toured a specially modified Boeing 747 used as a 'flying laboratory' to test new jet engines. See inside.

Leading American manufacturer GE Aerospace is building the world’s largest jet engine for Boeing’s coming 777X — and it needs a massive plane to test it.

The enginemaker bought a now-32-year-old Boeing 747-400 from Japan Airlines in 2010 to serve as its modern Flying Test Bed, or FTB.

It replaced the aging Boeing 747-100 that GE had had since 1992.

The latest FTB, which Business Insider toured at the UK’s Farnborough International Airshow in July, has one main purpose: to test and prove the industry’s newest commercial engines.

Thousands of feet of cables line its cabin, all connecting to test sensors, computer stations, and giant metal boxes that occupy most of the first floor of the specially modified 747 and collect a behemoth amount of data.

So far, the GE 747 has been instrumental in certifying engines that now power planes like the Airbus A320, the Boeing 737, and China’s homegrown Comac narrowbody.

The dual-level plane’s latest project is the GE9X, an engine so large it can fit an entire Boeing 737 fuselage.

It’s rated for 110,000 pounds of thrust, but holds the world record for the highest thrust recorded by a commercial jet engine, reaching 134,300 pounds during testing.

The ultra-powerful GE9X engine, which received FAA certification in 2020 but has continued to undergo additional testing, is being developed for Boeing’s yet-to-be-certified 777X.

While the 777X is seven years behind schedule, in part due to quality, production, and supply chain issues at Boeing, it’s expected to enter service in 2027 with German flag carrier Lufthansa.

Here’s a closer look at the 747 testbed that’s been critical to the 777X’s certification.