Armstrong retired as a master sergeant from Army Special Forces, then worked at the Secret Service's training academy before joining the Federal Air Marshal Service in 2002. With his beefy build and close-cropped hair, Armstrong looks the part of someone who could do serious harm with just his bare hands.
But his students on this day, not so much.
"I've never been a fighter," confessed Mark Gangler, a United Airlines flight attendant dressed casually in a T-shirt and workout pants. "But I think it's important to know the principles of how to protect yourself."
The four-hour class is offered without charge by the Transportation Security Administration at 20 sites around the United States. It's voluntary, so participants must take it on their own time and pay their own travel expenses. Since its inception in 2004, more than 11,000 crew members have taken part.
Inside the training room, more than a dozen pairs of protective goggles were arranged carefully on a table. Next to them were several rubber daggers. A half-dozen large rubber dummies mounted on stands were positioned around the room, ready to be used for practice.
There was a bit of nervous laughter among the group as Armstrong began the lesson on close encounters. He emphasized three points: Move, block, strike. Using "Bob" the dummy, he pointed to vulnerable spots above the shoulders: the ears, the throat, the nose, the eyes.
"You can rupture someone's eardrums this way," Armstrong said as he delivered a hard smack to Bob's ear. "You don't have to do both [ears]. Just one good slap on the side of the ear is going to cause some really good pain in that ear."
The group watched, eyes wide.
And on it went: wind chokes - moves that compress the trachea; blood chokes - moves that restrict blood flow, causing an opponent to pass out; how to deliver a blow to an attacker's head. The best way to avoid breaking a knuckle? Hit with the bottom of the palm instead of a fist.
Move, block, strike.
For flight attendants more accustomed to caring for passengers rather than cold-cocking them, the lessons required a shift in thinking.
"Don't be nice with that arm," Armstrong shouted as the group broke into pairs to practice dodging hits. "You want to put this person on the defensive, not the opposite."
Like others in the class, Gangler, never thought he'd need to know self-defense to do his job. But these days, he and others said, flights are more crowded, space on planes more confined and passengers sometimes more short-tempered and less patient. And even though more than a decade has passed since Sept. 11, terrorist attacks are always on their minds, they added.
Even so, Gangler said it took a push from Hernlem to persuade him and the others to set aside time to take the course.
Hernlem, who flies for United, recalled a recent flight on which a male passenger became belligerent, moving farther and farther into her space. The man eventually backed down, but the incident stuck in her mind.
Federal statistics show that the number of passengers cited for "unruly" behavior has declined from a peak of 310 in 2004. In the first seven months of 2016, 31 such incidents were recorded.
But internationally, reports of unruly airline passengers are on the rise. The International Air Transport Association said airlines reported a 14 percent increase in 2015, compared with 2014 figures. About 11 percent of the 10,854 incidents reported worldwide involved physical aggression.
Sara Nelson, president of the Association of Flight Attendants, who also took part in the recent training near Dulles, said the U.S. numbers may not reflect all the incidents that take place, since many are not reported.
"We know that even if we aren't going to have a life-threatening situation, in many cases our de-escalation skills are going to keep it from rising to that," she said. "The danger has increased for many reasons. [Sept. 11] changed everything."
While much of the day's instruction focused on the physical, Armstrong emphasized that most situations don't have to reach that point. Often, crew members can defuse tension by appearing confident and in control.
"These are last resorts when you have no other options," he said.
Finally it was time for the section dubbed "Unarmed Defense of Handheld Weapons."
The group watched as Armstrong demonstrated how to dodge a knife aimed at his chest. As another instructor moved toward him, Armstrong hollowed out his midsection and angled his body backward as he swung his arms forward, grabbed his attacker's arm and wrested the knife out of his hand. The entire encounter took only a few seconds.
"You will get cut," Armstrong cautioned the group. "But you want to minimize the damage."
And then it was their turn.
Gina Wong grabbed a knife and faced Hernlem. The two eyed each other carefully. Wong thrust the knife forward and Hernlem dodged, hollowing out her stomach and using her arms to block the knife and grab Wong's arm.
"Just throw those hands out," Armstrong shouted as he watched. "Don't try to grab anything. Just initiate that block."
Hernlem's reactions grew quicker with each of Wong's thrusts, her arm blocks steadier and stronger. When they finished, the two colleagues wiped their foreheads and broke into wide grins.
Even with her newfound skills, Hernlem isn't exactly eager to put them to use. But now, she said, she knows she's prepared. If a situation arises, "I feel like I'll have more confidence," she said.
