The XB-1's 13th test flight was also its last. Credit: Boom SupersonicShareBoom Supersonic passed a major milestone last month when its XB-1 prototype became the first civil aircraft to break the sound barrier over the continental US. Less than a month later, Chief Test Pilot Tristan Geppetto Brandenburg has once again surpassed Mach 1 in the experimental plane while flying as high as 36,514 feet above the Mojave Desert. This time, however, the XB-1 demonstrated another important achievement during its 13th test flight: passing the Mach cutoff. While the planes engines were still audible on the ground, surpassing the Mach cutoff ensured no one heard a disruptive sonic boom.According to the companys February 10th announcement, Mondays test flight continued efforts to assess aircraft performance and data collection above Mach 1. This included measuring acoustic sonic boom data using strategically positioned sound pressure recording equipment and microphones. While Boom Supersonic engineers used similar methods during XB-1s last flight on January 28th, Mondays test also included capturing what are known as Schlieren images. These visualizations, taken by photographers at ground level under precise conditions, detail the air density changes around a plane, including any supersonic shock waves.Boom Supersonics mission crew was particularly focused on demonstrating that the XB-1 is capable of passing the Mach cutoff, a physics event that only occurs at sufficiently high altitudes. During a Mach cutoff, a sonic boom is refracted inside the atmosphere and prevented from ever reaching ground level. In theory, this would ensure nobody beneath a supersonic plane would hear the extremely disruptive (and sometimes dangerous) concussive burst.Mach cutoff flights ensure any sonic booms dont reach the ground. Credit: Pennsylvania State University / Boom Supersonic But aside from being a nuisance, audible sonic booms in the continental US are illegal thanks to longstanding FAA regulations. For Boom Supersonic to realize its end goal of reintroducing commercial flights faster than Mach 1, it needs to have aircraft engineered to achieve Mach cutoffs.The XB-1 is a one-third-sized proof-of-concept meant to demonstrate the aerodynamics involved in Overture, the companys proposed commercial supersonic jet. Overture is ultimately intended to ferry 64-80 passengers on both cross-continental and international trips at speeds as fast as Mach 1.7about twice as fast as modern subsonic jets. Because XB-1s design created a Mach cutoff on Monday, then it stands to reason Overture will be able to do the same during its Boomless Cruise flights. Get the Popular Science newsletter Breakthroughs, discoveries, and DIY tips sent every weekday. By signing up you agree to our Terms of Service and Privacy Policy.Boom Supersonic also noted this weeks work officially concludes its multiyear flight test program. XB-1 will soon travel from the Mojave Air & Space Port near Barstow, California to its original home in Denver, Colorado. Once there, the company intends to focus its full efforts on scaling XB-1 learnings and technology to build the Overture supersonic airliner.But given its history of past delays, theres still a solid chance Overture wont make its currently scheduled 2029 debut. Thats not to say supersonic travel isnt ever coming back, but compared to an international Mach 1.7 flight, its probably going to take a little bit.