Weather Balloon Strikes United Airlines 737 MAX, Shattering Windshield

The National Transportation Safety Board has published its preliminary report on the recent midair collision between a United Airlines Boeing 737 MAX and a weather balloon in the skies over Utah.

United Flight 1093 was en route from Denver to Los Angeles on October 16 when the aircraft collided with an object, while in cruise flight at 36,000 ft near Moab, Utah. The object was later identified as a “global sounding balloon” operated by WindBorne Systems. The balloon’s impact shattered the first officer’s windshield, injuring the captain. The flight diverted to Salt Lake City.

Impact At Cruise Altitude Showered Pilots With Glass

According to the NTSB, the 737-8 was in cruise when the captain “noticed an object distant on the horizon.” Within seconds, the aircraft experienced “a significant impact” to the first officer’s forward windshield, “along with a loud bang,” showering both pilots with glass fragments.

The captain suffered “multiple superficial lacerations” to his right arm, but the first officer was uninjured. Despite the violently shattered outer pane, the cabin pressurization remained stable.

Crew Diverted Immediately As Windshield Overheated Light Illuminated

After the impact, the captain transferred control to the first officer and began administering first aid to himself while coordinating with dispatch and flight attendants. Shortly afterward, the first officer’s window overheat light illuminated, and the crew requested a diversion. The flight diverted to Salt Lake City International Airport, where it landed without further incident. Emergency medical personnel met the aircraft at the gate.

All 111 passengers and crew, except for the captain, escaped injury, but the aircraft damage was classified as substantial.

Both the cockpit voice recorder and flight data recorder were shipped to the NTSB’s Vehicle Recorder Laboratory. Specialists in meteorology, materials engineering, aircraft performance, and air traffic control have been assigned to the investigation.

Lost High-Altitude Research Balloon Matched The Aircraft’s Track

WindBorne Systems reported that one of its unmanned research balloons launched the previous day from Spokane, Washington, had stopped transmitting just minutes before the collision. The balloon had reached a pressure altitude of 35,936 ft — within 60 ft of the aircraft’s altitude — and was drifting eastward across Utah at the time of the strike.

The balloon was operating legally as an unmanned free balloon. A Notice to Airmen (NOTAM) had been issued the previous day for launch operations in Spokane, but it expired hours before the incident. It did not apply to the balloon’s later location in Utah.

The balloon’s last known coordinates placed it directly along the aircraft’s ground track. The NTSB’s preliminary review of the flight data recorder showed the 737 was flying nearly along the balloon’s path, at a speed of 395 knots (455 miles per hour).

A mapped overlay included in the report shows the aircraft and balloon passing within 8.3 nautical miles of one another moments before impact.

Boeing 737 MAX Windshield Structure Performed As Designed

The aircraft’s shattered windshield was transferred to the NTSB Materials Laboratory in Washington, DC, for further analysis.

Boeing 737 windshields are required to withstand a four-pound bird strike without penetration. The windshield installed on the aircraft involved in the collision with the balloon is a multi-layer laminated structure built by PPG Aerospace, which includes two tempered glass layers separated by urethane and vinyl interlayers and is framed by a stainless-steel Z-bar.

The outer pane, which is non-structural, absorbed the balloon impact but broke apart, sending fragments into the cockpit. The inner structural pane remained intact, and prevented depressurization which could have led to a more serious incident.

Lightweight Balloon Still Caused Substantial Damage

Palo Alto-based technology firm WindB​​orne provides weather insights to several U.S. government departments, including the National Oceanic and Atmospheric Association, the National Center for Atmospheric Research, the U.S. Air Force, and the Office of Naval Research via its data-as-a-service offering. The company has launched over 4,000 balloons and files a notice with the Federal Aviation Administration ahead of each launch.

WindBorne’s GSB design uses thin, low-tensile-strength plastic film for its envelope and ballast container, with no metal frame or rigid structure. The design is intended to minimize damage in the event of impact. Despite this, the collision with the aircraft was forceful enough to fracture the outboard windshield layer.

In a company blog post about the NTSB report, WindBorne stated, “Our flight systems are designed both to prevent mid-air impacts and, in the rare event one occurs, to ensure safety through redundant structural and operational safeguards. We are deeply relieved that Flight UA1093 landed safely and with minimal damage.”

WindBorne also said it had “implemented four additional safety measures to further reduce the possibility of any future aircraft-balloon interactions as well as mitigate harm should an impact occur again.”

The company has cut the time its balloons spend at altitudes where they may encounter commercial air traffic by 50%. In addition to the dashboard that air traffic controllers can use to track the company’s balloons, WindBorne is also sending automated reports on all balloons in the airspace every two hours and issuing additional alerts as balloons enter and exit airspace regions. The company has also connected its global ADS-B data, used to track aircraft positions, via an API feed, so aircraft can adjust flight paths to avoid intersecting a balloon’s path.

Further, the company is refining the design of the data-gathering payload housing and testing lower-density ballast materials to reduce the balloon’s overall mass and mitigate its impact energy.

The NTSB will continue its investigation into the factors that led to this incident and will publish a final report.