The C-5 was procured under the “total package procurement” concept, in which a single contract covered development and production. It was meant to control costs; instead, as weight grew and technical problems mounted, it exposed Lockheed to huge losses and the Air Force to political embarrassment. In 1968 Air Force cost analyst A. Ernest Fitzgerald testified to Congress that the programme was running roughly US$2 billion over estimates. He was later removed from his post, and his case became a landmark in the history of US government whistleblowing.

The engineering problem was the wing. Weight-saving measures left it vulnerable to fatigue, and cracks appeared early in service. The Air Force imposed payload limits and, from the 1980s, re-winged the C-5A fleet. At the same time it ordered 50 new C-5Bs with the stronger wing and other improvements, the last delivered in 1989. The financial strain of the C-5 and other programmes contributed to Lockheed’s crisis of the early 1970s, when it needed a US government loan guarantee to survive.

By the 1990s the C-5’s reliability was poor and its original engines and analogue cockpit were dated. The Air Force chose to modernise rather than retire. The Avionics Modernization Program gave the fleet a glass cockpit with digital avionics, and the Reliability Enhancement and Re-engining Program replaced the TF39s with General Electric F138 engines, a military version of the CF6-80C2 airliner engine, and upgraded more than 70 other systems.

Fifty-two aircraft, a mix of C-5Bs and selected C-5As, were rebuilt as C-5M Super Galaxies, with the remaining older aircraft retired to storage. The new engines gave much better take-off performance, climb and fuel efficiency. In September 2009, in its test campaign, a C-5M set 41 records in a single flight, including carrying 176,610 lb to over 41,100 ft in under 24 minutes, according to Lockheed Martin. In 2012 Lockheed projected that the modernised aircraft would serve for at least another 40 years.

• C-5A: original production version, 81 built 1968–1973; TF39-GE-1C engines; later re-winged.
• C-5C: two C-5As modified with an enlarged cargo compartment to carry large satellites and space hardware for NASA.
• C-5B: improved production version with stronger wing and upgraded systems, 50 built 1985–1989.
• C-5 AMP: Avionics Modernization Program glass cockpit fitted across the fleet.
• C-5M Super Galaxy: 52 aircraft rebuilt with GE F138 engines and some 70 system improvements; last delivered in 2018.
• L-500: proposed commercial freighter; never built.

The C-5 has always been expensive to own. Its development overrun became a political cause in the late 1960s, the wing modification of the 1980s added to the bill, and through the 1990s and 2000s the C-5 fleet’s mission-capable rates were notoriously low, with older aircraft spending long periods awaiting parts. Each flying hour consumes large quantities of fuel and requires a big crew and extensive maintenance.

We give a hedged estimate of roughly US$30,000 to 40,000 per flying hour, based on published US Air Force reimbursement rates for the type; fully burdened costs are higher. The C-5M modernisation was justified on the argument that new engines and systems would cut fuel burn, improve reliability and extend the fleet’s life by decades at less cost than buying a new outsize airlifter, of which none was available off the shelf.