The U.S. Air Force is disputing claims that its Thunderbirds executed a supersonic flight during a practice session for the Chicago Air and Water Show on Friday. The denial follows reports of two loud blasts heard across the city’s North Side, with some residents suggesting the booms may have cracked windows in the Lake View neighborhood.

Though, contrary to the Air Force’s statement, video evidence appears to indicate a sonic boom, according to an expert in aeronautics.

“Those were clearly sonic booms,” said Matthew Clarke,an assistant professor at the University of Illinois Urbana-Champaign’s Grainger College of Engineering. Clarke told reporters that it’s plausible that only a portion of the aircraft surpassed the sound barrier, generating shock waves that manifested as booms.

Clarke explained that this phenomenon can occur when air moving over the wing generates lift. “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,” he stated. “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.”

The U.S. Air Force released a statement late Friday asserting that “a review of the practice determined the Thunderbird jets did not go supersonic at any point during the demonstration.” When questioned on Monday whether it was possible that only part of the aircraft had exceeded the sound barrier, an Air Force spokeswoman offered no immediate comment.

On Friday,residents of Lake view recounted their experiences to local media,describing the booms as akin to an “earthquake,” “a massive jolt” and “kinda scary.”

This isn’t the first time such an incident has occurred.A similar event involved the Thunderbirds’ Navy counterpart, the Blue Angels.

In 2021, a Blue Angel pilot caused over $180,000 in damage during a practice run after veering off course and passing within 100 feet of buildings while nearing the speed of sound at the Naval Air Facility el Centro in California, according to The Seattle Times.

A Navy investigative report found the F-18 super Hornet unleashed a “noticeably larger localized sonic airflow signature,” in part because of the placement of a rear-facing camera that increased the planes sonic pressure wave beyond Mach 1, even when the pilot’s controls said the plane was flying slower. The blast injured a dozen people.

The Blue Angels, which sometimes perform at the Chicago Air and Water Show, even have a warning on its online FAQ page: “from time to time, spectators may have mistaken the sound of engines at a high-power setting approaching the speed of sound for a sonic boom.”

Ald. Bennett Lawson (44th) reported that residents described the boom as louder than any they had previously experienced at an air show. He noted that the window damage appeared to be concentrated on the first floors of four residential high-rises along the lakefront: 3180, 3600 and 3950 North Lake Shore Drive and 4200 N. Marine Drive.

Lawson has advised residents and building managers to file claims with the city, characterizing the Air and Water Show as a “city-sponsored” event.

“My understanding is the Air Force released a statement like they didn’t do anything differently than before, but those buildings pre-date the air show and never had this problem ever before. So I don’t think the buildings should be required to pay for it. And they shouldn’t have to go through insurance first when it was something that the city sponsored and the performers created,” Lawson said.

Given that the shattered windows were limited to lobbies in the four buildings, damage estimates are not expected to be substantial.

The city’s Office of Emergency Management and Dialog did not instantly respond to a request for comment. OEMC has told Lawson’s office that it was “looking into” the window-shattering incident.

The department of Public Health has not responded to Lawson’s calls.

Lawson also mentioned that inspectors responsible for enforcing the city’s noise regulations still “haven’t migrated over” from the Health Department to the newly resurrected Department of Environment.

He stated that the city lacks effective noise restriction enforcement. “I don’t know that anyone does noise very well in this city,” he said.

The Federal Aviation management on Monday referred questions to the Air Force.

Ari Barjesteh, a Lake View resident, captured video of the two blasts and expressed his belief that the jets had broken the sound barrier. Barjesteh posted the videos to TikTok, where they have since amassed thousands of views.

“In the comments, it’s like a 50-50 split between people who say it was or wasn’t a sonic boom. I truly believe it. Both of them were sonic booms. Like, they’re very distinct, they sounded like bombs going off,” he said. “Jets are loud, but they don’t normally sound like a bomb went off. It shook windows and shook the building, and you heard the car alarms go off up and down Lake Shore Drive.”

In 2012, the Thunderbirds accidentally went supersonic during practice at Davis-Monthan Air Force Base, southeast of downtown Tucson, arizona, according to Tucson News Now. In that instance, the Air Force acknowledged responsibility for the sonic boom, established a claim center, and ultimately disbursed over $22,000 in damage compensation. The Air Force reported that the pilot exceeded the speed of sound for nine seconds while executing a turn.

Back in 1988, two F-4 Phantom fighter jets with the Indiana National Guard flew at supersonic speeds over Kankakee, a city south of Chicago, generating a boom that resonated as far north as Chicago’s South Side.

“Those were clearly sonic booms…they sounded like bombs going off…It shook windows and shook the building, and you heard the car alarms go off.”