Pope Francis has explained why he took a tumble last week during a public Mass outdoors in public while on a pilgrimage.

Reporters aboard the papal plane flying him back to Rome after the five-day trip to Poland asked him Sunday night how come he fell while sprinkling incense around the outdoor altar at the Jasna Gora monastery, in Czestochowa, before a huge crowd at Poland's most popular Catholic shrine.

Francis says the July 28 tumble happened this way: "I was watching (an image of) the Madonna, and I forgot the step."

He credited his reflect action with having him escape any injury.

"I let myself fall, and this saved me. Because if I tried to resist it, I would have gotten hurt."

Vatican officials said Francis, 79, was uninjured.