At ten to nine on a clear Tuesday morning, Lieutenant Colonel Tom Froling lifted off from Selfridge Air National Guard Base in Harrison Township, Michigan, and turned north. The trees had not started to change yet. It was, by every measure available to him at that moment, a completely ordinary day at work.

He was flying an F-16C. His flight lead, Major Doug Champagne, was ahead of him. The plan was a low-level run up Military Training Route VR1624 over the little town of Peck, out across Lake Huron, and then bombing and strafing passes on the targets at Grayling Range. Twenty-millimetre target practice rounds in the gun. Home by lunchtime.

Thirty-one minutes before he released the brakes, a flight attendant named Betty Ong had picked up a seatback airphone on American Airlines Flight 11 and told a reservations agent in North Carolina that the cockpit was not answering.

The question that made no sense

The range work went fine. Dive bombing from altitude, working down, then strafing passes with the cannon. Heading home via Saginaw, a hundred miles out from Selfridge, Froling called in to his operations desk as required.

His squadron commander came on the radio immediately, which was not normal, and asked him a question that made no sense at all: had he fired the gun? Then, had he fired it out.

Froling assumed he was in trouble. He thought he had missed a line on the maintenance forms. He had not; he had flown the mission exactly as briefed. What he did not know was that somewhere above him in the chain of command, someone was counting bullets.

The 20 mm rounds in his gun were inert practice slugs with no explosive filler. But they are made to match the ballistics of the real thing, which means they are solid metal travelling very fast, and they would do catastrophic damage to an airliner.

Froling only learned much later why he had been asked. NORAD had contacted his wing about a possible intercept of United Airlines Flight 93, the fourth hijacked aircraft. Someone needed to know what, if anything, the airborne Michigan jets could actually shoot with.

It never came to that. Flight 93 went into a field in Pennsylvania after the passengers tried to retake the cockpit.

Turning the wrong way with a hot gun

Small procedural things gave it away before anyone told him anything. He had asked to practise a couple of simulated flameout approaches, the drill a single-engine fighter pilot runs in case the motor quits. Denied. Make it a full stop.

Aircraft landing south on Selfridge’s runway 19 normally turn right into the de-arm area so the gun can be made safe. Froling and Champagne were sent left, guns still hot, and marshalled to parking spots they never used.

He shut down. His crew chief put a ladder against the side of the jet, climbed up, and said: you are not going to believe this, the Pentagon and the World Trade Center have just been hit.

By then American 11 had been gone for well over an hour. Ninety-two people had been aboard it — eighty-one passengers, five of them hijackers, and eleven crew, Betty Ong among them. In the squadron operations room the televisions were on and nobody was saying much. Go home, they told him. Come back in eight hours. We will probably be on alert.

Coming back to a different air force

When he came back, the jets he had flown practice rounds in that morning were being loaded with live AIM-120 AMRAAM and AIM-9 Sidewinder missiles and, by his count, 505 rounds of live high-explosive incendiary 20 mm. Photographs from those days show Selfridge F-16s carrying four AMRAAM, two Sidewinders and three external tanks — a long-endurance combat load.

Maintenance crews pitched tents on the ramp and slept next to the aircraft. That was the loadout you would carry over Iraq. They were carrying it over Michigan.

The following morning, 12 September, the day Operation Noble Eagle formally began, someone woke him at four. Airborne at six over southeastern Michigan, he found something no American fighter pilot of his generation had ever seen.

At around 8,000 feet over Detroit Metropolitan Airport he looked down at airliners parked nose to tail across every available foot of concrete, a continent’s worth of diverted traffic stacked up like a jigsaw.

Then the AWACS called a contact at 82 miles, no transponder, no radio. Two armed F-16s went supersonic to run it down. At thirty miles it was identified as a military helicopter out of Grand Ledge on an unannounced VIP flight. Minutes, he reckons, from an intercept with live missiles on the rails.

Stadium lights over Manhattan

A later call came at three in the morning: the F-15s on the East Coast were weathered in, and New York needed covering. He took off into pouring rain, climbed above the weather, ran east, and let down through cloud to 12,000 feet over the city while it was still dark.

Ground zero was lit up with stadium lights as the crews worked through the rubble looking for survivors. He says he will never forget it. Above it there was nothing but his two jets, a tanker, and a NATO AWACS with an accent.

Ask him whether it could happen again in the same shape and he says no, that the hijackers had one shot at it and used it. Ask him where the exposure is now and the answer is less comforting: a thousand drones at once, electronic warfare, the power grid. Twenty-five years on, the man who flew an empty American sky is worried about a sky that is far too full.

An F-16 pilot describes being scrambled on the morning of 11 September 2001. Froling flew with the Michigan Air National Guard; other Air National Guard units launched from bases along the East Coast that morning.

Sources: interview with Lt Col (Ret.) Tom Froling by Tom Demerly for The Aviationist; the 9/11 Commission Report; National September 11 Memorial and Museum; NORAD; U.S. Air Force and 127th Wing historical records; Britannica; Garrett M. Graff, The Only Plane in the Sky.