The nation observes the 15th anniversary of 9/11 today, and for former combat pilot Heather Penney, the anniversary is a reminder of the moment she was willing to sacrifice her life to take down one of the highjacked commercial airliners that posed a threat that day.

Lt. Penney, 26, went by the call sign "Lucky," piloted an F-16 and was among the first generation of female U.S. combat pilots when she got the call on Sept. 11, 2001: Stop United Airlines Flight 93, the fourth highjacked jet, which was headed to Washington.

She knew it would likely be a suicide mission.

When Penney took off, she didn't have missiles or munitions to stop the aircraft.

"We wouldn't be shooting it down," she told The Washington Post in 2011, on the 10th anniversary of 9/11. "We'd be ramming the aircraft. I would essentially be a kamikaze pilot."

From the Post:

"A third plane hit the Pentagon, and almost at once came word that a fourth plane could be on the way, maybe more. The jets would be armed within an hour, but somebody had to fly now, weapons or no weapons.

"'Lucky, you’re coming with me,' barked Col. Marc Sasseville.

"They were gearing up in the pre-flight life-support area when Sasseville, struggling into his flight suit, met her eye.

"'I'm going to go for the cockpit,' Sasseville said.

"She replied without hesitating.

"'I'll take the tail.'

"It was a plan. And a pact."

Sasseville said he hoped to be able to eject from his aircraft in time. Penney wasn't so sure.

"If you eject, and your jet soars through without impact, ..." Penney told the Post, implying that she might have to remain aboard the aircraft to ensure the mission was successful.

Penney had a personal connection that made her brave decision even more poignant: Her father was a United Airlines captain who flew routes up and down the East Coast.

"Yes, John [Penney] was a captain for United Airlines at that time," her mother, Stephanie Penney told the Post in a subsequent interview. "He flew 757s and had been flying trips into and out of the East Coast the month before. Heather would not have known for sure that her dad wasn't the captain on United 93."

"This sounds cold-hearted; I mean, that was my daddy," Penney told the Post. "But, frankly, there was no way for me to know, and it would not have changed what I needed to do at all."

As it turned out, Penney and Sasseville didn't have to complete their mission.

Hours later, they were notified that United 93 went down in a field in Pennsylvania: As we now know, a group of passengers rose up against the highjackers, sacrificing their lives to prevent the plane from reaching Washington.

"The real heroes are the passengers on Flight 93 who were willing to sacrifice themselves," Penney told the Post. "I was just an accidental witness to history."

You can hear more about her story in an ABC News segment shared on Pop Sugar and below.