Almost four years after the Air Force said a B-2 sustained “minor damage,” the aircraft returned to the fleet at Whiteman Air Force Base, Mo., on Dec. 16, according to the service.

An engine fire grounded the “Spirit of Washington” in 2010. Since then, Air Force maintainers and engineers have worked to get one of the 20 B-2 stealth bombers remaining in the Air Force fleet back in the air.

The fact there are only 20 B-2s in the Air Force’s fleet increased the urgency of the repair and helped Air Force officials to make the decision not to scrap the project even though it took three years and nine months to complete.

The small number of B-2’s and the lack of spares meant the maintainers couldn’t just call on a part from an aircraft sitting in the boneyard. Replacements had to be remade or the maintainers had to get creative, according to the service.

Maintaining the B-2’s stealth qualities added to the challenge, according to officials. A particular challenge was removing the charcoal created by the fire from the aircraft’s skin.

Maintainers created a process in which they used dry ice pellets to remove charcoal from the skin.

“The team would spray pelletized dry ice on the aircraft, after which the ice would melt, leaving no additional residue or material for the maintainers to clean up,” according to a release. This was the first time this had been done.