[NorthMountain]

Notice anything peculiar about these aircraft? Points to anybody (else) who knows what's going on here.

[Protect the Bill of Rights]

Veered off the runway?

[Pamlico]

Yes. The engine closest to the pilot is much larger than the other three? What did I win?

[RayChuang88]

Easy answer: they are used as flying engine test platforms. The upper picture is a plane that belongs to GE Aero Engines, and the bottom one is a plane belonging to Rolls-Royce.

However, it should be noted that Airbus has "loaned" one of their A380 prototypes to Rolls-Royce for use as an engine testbed. The Rolls-Royce Trent XWB engine used on the Airbus A350XWB airliner had its test flights mounted on one of the engine pylons of an A380 test plane.

[Yo-Yo]

Engine testing.

[spel_grammer_an_punct_polise]

2nd photo did not post.

[BradyLS]

Inboard port-side engine looks huge in comparison to the other three.

Some sort of live engine prototype testing?

[ltc8k6]

GE’s engine testing 747.

http://images.flugrevue.de/sixcms/media.php/11/thumbnails/ge-747testbed_zoom.jpg.2281987.jpg

747 can also ferry a spare engine.

http://i.stack.imgur.com/SaE8r.jpg

[Robert A Cook PE]

Engine testing for the 1 of 4 engines that is different.

If it runs, fine - they can test at altitude and in clouds/snow/rain freezing sleet.
If it fails, the other three can get the plane back.

For the bigger engines, you have to have a big plane to allow the engine to be tested at full power.

[TalonDJ]

In both the pilot was blond and born on a Tuesday.

[SunTzuWu]

Test bed for new engine.