London (CNN) Mary Ellis, a "pioneering aviator" who was one of the last surviving female World War II pilots, has died at the age of 101.

Ellis helped deliver Spitfires and bombers to the front line as part of the UK's Air Transport Auxiliary (ATA) during the war.

UK Air Chief Marshal Stephen Hillier mourned her on Twitter.

Another terrible loss. Mary Ellis, pioneering female aviator, Air Transport Auxiliary veteran, an inspiration to generations. I'll always remember her proudly reminding us at RAF100 events that she was older than the RAF itself! RIP Mary. pic.twitter.com/xktUU6R7Qn — ACM Stephen Hillier (@ChiefofAirStaff) July 26, 2018

"Another terrible loss," Hillier wrote. "Mary Ellis, pioneering female aviator, Air Transport Auxiliary veteran, an inspiration to generations. I'll always remember her proudly reminding us at RAF100 events that she was older than the RAF itself! RIP Mary."

Ellis, then Mary Wilkins, joined the ATA in 1941 after hearing a radio commercial. She flew around 1,000 planes over four years including 400 Spitfires and 47 Wellington bombers.

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