Recently, a United Airlines Boeing 737 flying from Seattle-Taipei International Airport to Denver International Airport experienced a mishap during flight when the main cabin door handle was not fully locked.
Shortly after taking off from Seattle around 4 p.m., flight attendants reported hearing a loud "bang" at an altitude of approximately 10,000 feet.
The unusual noise occurred during the aircraft's climb over Washington, Idaho, Wyoming, and Colorado. Despite the anomaly, the pilots assessed the situation and did not declare an emergency, continuing their flight to Denver.
The Boeing 737 uses a plug-in cabin door design introduced in the 1960s, a design that has been continued in subsequent models (including the 737-900). This type of door relies on pressure differentials to automatically lock as altitude increases.
Pilots reported that operating the door, due to its manual control mechanism, requires significant physical strength, sometimes necessitating substantial upper-body power.
Records indicate that the cabin door handle of this aircraft model occasionally shifts during flight. While uncommon, this shift does not pose a safety risk.
Even if the handle disengages from its aerodynamic recess, the door lock function remains intact. To open the door, the handle must be pulled further out and fully rotated-impossible at cruising altitude, where the pressure differential would keep the door tightly sealed against the fuselage.
This design limits any unintentional opening, unaffected by slight handle displacement.