Boeing 747-500X, -600X and -700X (never produced)

Boeing announced the 747-500X and 747-600X at the 1996 Farnborough Airshow. Neither was built. They were the most ambitious 747 derivatives ever seriously proposed, and their failure shaped everything that came afterwards.

What was proposed

The concepts combined the 747 fuselage with an entirely new 251 ft (77 m) span wing derived from the 777, more powerful engines, and much heavier landing gear — nose gear going from two tyres to four, main gear from 16 to 20.

Concept	Length	Passengers	Range	Gross weight
747-500X	250 ft (76.2 m) — an 18 ft stretch	462	Up to 8,700 nmi (16,100 km)	Over 1.0 million lb
747-600X	279 ft (85 m)	548	Up to 7,700 nmi (14,300 km)	1.2 million lb
747-700X	The -600X wing on a widened fuselage	650	Same as a 747-400	—

The -700X is the one worth pausing on. Widening the 747 fuselage would have meant abandoning the cross-section the aircraft had carried since 1969 — effectively a new aeroplane wearing a familiar name.

Why none were built

The cost. Boeing estimated the changes — above all the new wing for the -500X and -600X — at more than $5 billion, and could not attract enough airline interest to launch.

That figure is the crux. A new wing is not an incremental change; it is most of the expense of a new aircraft, requiring fresh certification of the structure, the aerodynamics and the systems that run through it. Boeing was being asked to spend new-aircraft money on a derivative, and the airlines would not commit.

What happened instead

The 747X programme was terminated, and Boeing spent the following years studying what could be improved without a new wing. That produced the 747-400XQLR — better range and much lower noise, on the existing airframe.

The QLR did not launch either, but its work fed directly into the 747 Advanced, which Boeing launched in November 2005 as the 747-8.

The irony is that the 747-8 eventually got its new wing after all — a complete redesign, with raked wingtips much like those studied here. It reached 250 ft 2 in in length, almost exactly the -500X’s proposed dimension, nine years later and after Airbus had committed to the A380.

By then the market for very large aircraft was already closing. Only 155 747-8s were built, and Airbus fared no better with the A380. Boeing’s inability to launch the 747X in 1996 looks in hindsight less like a failure of nerve than a fairly accurate reading of demand.

Related

• The 747 trijet, studied in the early 1970s
• The 747-400XQLR, the QLR that followed the 747X
• The Yellowstone Project, Boeing’s plan to replace its whole product line