What happened
On 17 May 2025, a Boeing 737-86N, registration G-NPTA, was conducting a commercial cargo flight from East Midlands Airport to Edinburgh Airport. Because the cargo included a significant amount of dry ice, the operator's specific procedures necessitated that the aircraft's air conditioning pack control switches be set to the high position throughout the flight.
As the aircraft approached the top of its descent, the crew received a cabin altitude warning. In response to the loss of pressure, the flight crew donned oxygen masks and initiated an emergency descent. Once the aircraft had levelled off, the commander inspected the overhead panel and discovered that both air conditioning pack switches had been moved to the off position. After the crew switched the packs back on, pressurisation was restored, allowing the flight to proceed to its destination without further incident.
The investigation
The investigation examined the circumstances surrounding the switch configuration and the crew's actions. The commander noted that the required setting for the dry ice was an unusual configuration for standard operations. While the commander believed the switches were correctly positioned at the start of the flight and during the top of climb scan, he could not account for how the switches moved to the off position.
One possibility considered was that the crew may have inadvertently selected the wrong setting—the off position instead of the high position—while attempting to rectify a perceived error in the initial setup. The crew reported that neither the pilot nor the co-pilot recalled manually moving the switches during the flight.