NTSB: Three climb/descent cycles resulted in multiple injuries on a Delta Air Lines flight in July

The National Transportation Safety Board (NTSB) has released its preliminary report about a severe turbulence event involving a Delta Air Lines Airbus A330-900 operating flight DL 56 on July 30, 2025.

According to the NTSB, the A330-900, registered as N422DZ, flying from Salt Lake City International Airport (SLC) to Amsterdam Schiphol Airport (AMS), encountered severe turbulence while cruising above Wyoming, the United States, at around 17:23 local time (UTC -6).

The flight crew, which included the captain, first officer, and a relief first officer, recalled that before flight DL 56’s departure, they had discussed the weather conditions en route, including potential weather during the climb and turbulence east of SLC. As such, they anticipated that they would deviate from their flight path after crossing the mountains east of their origin airport.

“The departure was relatively smooth, until they reached the Wasatch Mountains, where they experienced some turbulence.”

The NTSB noted that after passing flight level (FL) 300, the turbulence cleared enough for the cabin crew to begin service. At FL350, where they had stayed for around 5 minutes, the pilots stated that “the turbulence predicted on their electronic flight bag weather application was ‘in the green’ and they had observed only light cirrus clouds,” prompting the captain to turn off the seatbelt sign.

Continuing to climb to FL370, the pilots noticed some weather buildup and requested a deviation to the right. Air Traffic Control (ATC) suggested a turn left, and following an assessment that it would keep them above the cloud deck and upwind of weather buildups and a scan of the horizon and the weather radar, they turned left.

However, after the turn was completed near Creston, Wyoming, the A330-900 encountered an overspeed event, exceeding the maximum operating speed by between 15 and 20 knots (27.7 and 37 kilometers per hour). In addition, an updraft carried the aircraft to FL380, with the aircraft’s autopilot disengaging.

“Shortly thereafter, the airplane descended ‘rapidly’ to FL355, and a climb/descent cycle repeated about three times over a two-to-three-minute period, which resulted in the airplane re-entering a cloud layer.”

The first officer, acting as the pilot flying (PF), initiated the ‘All Attitude Upset Recovery Strategy’ process at least once during the event before eventually recovering and re-engaging the autopilot.

Preliminary review of the flight data recorder (FDR) data indicated that the upset lasted around two-and-a-half minutes, and during this period, “acceleration oscillated, reaching a maximum positive peak of about 1.75 g and a minimum negative peak of about -0.5 g.” The investigators noted that the pitch angle also oscillated, “ranging from a low of about 5° nose down to a high of about 10° nose up,” while roll angle variations had been a maximum of about 40° left wing down and about 5° right wing down.

The computed airspeed fluctuated between 230 kts (425.9 km/h) and 290 kts (537 km/h).

“A convective Significant Meteorological Information (SIGMET) was in effect for the time and area of the accident.”

The SIGMET warned of thunderstorms moving at 10 kts (18.5 km/h), with cloud tops above FL450.

Following the encounter with turbulence, the captain, acting as the pilot monitoring (PM), divided duties inside the cockpit. The PF continued to fly and communicate with ATC, while the captain coordinated with the cabin crew and Delta Air Lines’ dispatch. The relief first officer walked through the cabin to assess the injuries to the passengers and cabin crew.

After the latter returned to the cockpit, the pilots decided to divert to Minneapolis-Saint Paul International Airport (MSP), a Delta Air Lines hub, to avoid the weather. They also assessed overweight landing performance data and possibly suitable runways.

Flightradar24 records showed that the A330-900 landed at MSP at around 19:50 local time (UTC -5), with the airport’s emergency services, as well as maintenance personnel, meeting the aircraft at the gate. 

In addition to the injuries to passengers and cabin crew members, the right main landing gear’s (MLG) brakes were hot.

The NTSB stated that “24 of the 246 passengers were evaluated by emergency medical technicians, with 18 transported to local hospitals for additional assessment; information regarding their conditions is currently unavailable.”

Of the 10 flight attendants, two sustained serious injuries, while five had minor injuries. Three cabin crew members and the trio of pilots had no injuries as a result of the severe turbulence event. The preliminary report pointed out that the widebody’s interior components had minor damage.

The aircraft’s flight activity history on Flightradar24 indicated that the A330-900 returned to commercial service on August 2, 2025, operating flight DL 164 from MSP to AMS. The NTSB will continue investigating the event.