OBBI 231000Z 31009KT 290V360 9999 SCT030 31/25 Q1015 NOSIG
You would think this is your brain playing tricks on you if seen in a photo, it looks ridiculously photo-shopped. An oddity at best, but this oddity is a reality, and is referred to a Podded or Mounted Engine.
Typically the heavier four engine transporters are used to transport the engine, but this can also be done on dual engine aircraft. During the flight the added engine is inoperative, and special fuel, weight, and drag consideration have to be made.
An additional mounted engine can also be used for testing and experimentation. Honeywell utilizes a Boeing 757 with an extra mounted engine to conduct power, aerodynamic, and efficiency tests.
Aircraft Engine Manufacturer Pratt & Whitney also uses this method in testing. The company tested the PW1200G engine using a Boeing 747SP. Additionally, P&W previously operated testbed aircraft based on a Boeing 720-023B.
Boeing itself runs a testbed aircraft dubbed B757 F-22 this aircraft was actually used to develop the F-22 Raptor and host a number of test equipment and adjustable surfaces for a wide range of experimentation.
A similar testbed based off the Boeing 737 was used to develop the JSF F-35 the test aircraft was dubbed JSF CATB “Joint Strike Fighter Corporate Avionics Test Bed”. similar in function to the B757 F22 the nose and surfaces were modified to mimic the F-35 Lightning II, testing flight dynamics, and proof of concept.
Engine manufacturers tend to use this method in testing engine performance or surface dynamics in an actual operation environment. This means they can "Live" test an engine during actual flight at actual altitude. Due to the fact that the engine is live, these aircraft are registered as EXPERIMENTAL and follow extremely rigid operating and safety procedures.