Last Friday at around 10:20 am, when the US Air Force’s Thunderbirds were carrying out their practice for Saturday’s air show, residents claimed to have heard two loud blasts.
Attendees at air shows can see daring aerial performances, but as NBC Chicago explained, this past weekend’s Chicago Air and Water Show was about the “sights and sounds.”
Of course, air shows are loud events, but it was the practice day that some residents in the Windy City noted.
Last Friday, the United States Air Force Air Demonstration Squadron, more famously known as the Thunderbirds, brought some thunder with them. The team was practicing for their demonstration over the city’s North Side near Lake Michigan when several windows shattered after a loud boom. Multiple media reports claim one of the Thunderbird’s F-16 Fighting Falcons went supersonic, seemingly resulting in the shattering of the windows.
An Air Force spokesperson denies the allegations, telling the Chicago Sun-Times that the aircraft in question “did not go supersonic at any point during the demonstration.”s
The Federal Aviation Administration (FAA) also reported no unusual activity.
Did the US Air Force’s F-16s Create a Sonic Boom?
A loud, thunder-like sound is caused by an object that travels faster than the speed of sound, as the object pushes air molecules aside. It creates a shock wave that compresses and then rapidly decompresses. For a sonic boom to occur, the object would need to travel at roughly 767 miles per hour, the speed of sound at or near ground level.
The US Air Force’s F-16 Falcons can reach a maximum speed of 1,500 mph (Mach 2) at altitude, but at air shows, the team stays below Mach 1, or the speed of sound. During some maneuvers, notably flat passes, a solo Fighting Falcon can reach speeds around 700 mph, just enough to avoid creating the sonic boom.
The Air Force certainly understands sonic booms, as it has conducted faster-than-sound flights since October 14, 1947. That year, a Bell X-1 piloted by US Air Force Captain Charles E. “Chuck” Yeager reached the speed of 1,127 kilometers or 700 miles per hour, Mach 1.06, at an altitude of 43,000 feet. (Air is less dense at greater height—meaning that a demonstration aircraft flying at 700 miles per hour near ground level would not break the sound barrier, while an aircraft flying at the same speed several miles in the air would.)
The Air Force soon found that the speed creates the sonic boom.
“There is a probability that some damage—shattered glass, for example—will result from a sonic boom. Buildings in good repair should suffer no damage by pressures of less than 16 pounds per square foot. And, typically, community exposure to sonic boom is below two pounds per square foot,” the service explains on its website.
However, last Friday at around 10:20 am, when the Thunderbirds were carrying out their practice for Saturday’s air show, residents claimed to have heard two loud blasts, and several windows were shattered in the lobby of three buildings on the city’s North Lake Shore Drive.
“We just came back, and my husband and son were there, they say even the tables were shaking. It was so loud the feeling was that the building is moving,” Miriam Altman, a resident in one of the affected buildings, told Fox 32 Chicago.
The Experts Weigh In—Against the Air Force
Even as the Air Force denies that the Thunderbirds created any thunder, experts disagree.
“There’s air pressure in front of the moving aircraft and air pressure behind it. When it exceeds that speed of sound, it’s going so fast that those two bodies of air, if you will, or areas of air, will collide. And so that happens really quickly. And then you get this very loud explosive sound,” Patricia Ward, head scientist at the Griffin Museum of Science and Industry in Chicago, also told the Fox affiliate.
She added, “When a sheet of glass is really large, it can be more brittle. And so even a small crack, if you will, it’s that vibration, and when that vibration creates some damage, it then can shatter.”
The pilots may have thought they were flying below Mach 1, but combined, they still created the same effect.
“Those were clearly sonic booms,” Matthew Clarke, assistant professor at the University of Illinois Urbana-Champaign’s Grainger College of Engineering, told the Chicago Sun-Times after viewing video of the F-16s during the practice.
“Even though the global flow may not be faster than the speed of sound, there are places locally faster than the speed of sound, creating shock waves,” Clarke added. “While I can’t say that the whole plane went supersonic, I can say, from the video, shock waves [were created] from parts of the aircraft.”
This would not be the first time that a US military aircraft caused damage during practice for an air show. A United States Navy Blue Angel F-18 Super Hornet caused more than $180,000 in damage in 2021 when the aircraft veered off course and passed within 100 feet of a building at Naval Air Facility El Centro, California. In that incident, a rear-facing camera confirmed that the aircraft’s sonic pressure exceeded the speed of sound, despite the pilot’s belief that the Super Hornet was flying slower. In addition to the damage to property, a dozen people were injured.
In 2012, the Air Force’s Thunderbirds “accidentally went supersonic,” the Chicago paper of record added. That occurred during a practice at Davis-Monthan Air Force Base, near Tucson, Arizona. The service acknowledged the accident, which eventually cost $22,000 in damage on the ground.
It was also in 1988 that a pair of F-4 Phantom jet fighters operated by the Indiana National Guard created a sonic boom that was heard on Chicago’s South Side.
About the Author: Peter Suciu
Peter Suciu has contributed over 3,200 published pieces to more than four dozen magazines and websites over a 30-year career in journalism. He regularly writes about military hardware, firearms history, cybersecurity, politics, and international affairs. Peter is also a contributing writer for Forbes and Clearance Jobs. He is based in Michigan. You can follow him on Twitter: @PeterSuciu. You can email the author: [email protected].
Image: Wikimedia Commons.
