The US National Transportation Safety Board (NTSB) has determined that water leaking from lavatories formed ice that jammed the controls of a Delta Air Lines Boeing 767 last July.
“The water likely froze on one or more of the components which led to the pilots’ limited control of the ailerons,” says the NTSB in a final report published on 20 April.
The incident involved a Delta 767-300 (registration N181DN) operating flight 211 from Prague to New York’s John F Kennedy International airport on 7 July 2022.
While airborne, flight attendants told the pilots that water was leaking from the jet’s two mid-cabin lavatories. Pilots traced the leaks to a lavatory toilet and a “filter canister under the sink”. The crew stopped the leaks by shutting off the water supply.
Shortly later, the pilots received caution messages in the cockpit. In response, the first officer disengaged the jet’s autopilot and tried to turn the aircraft manually.
“However, he was unable to move his control wheel to the left more than a couple of degrees of deflection and was thus unable to input the needed correction,” says the NTSB’s report.
The pilots responded by completing a “jammed or restricted flight controls” procedure and communicated with Delta’s maintenance controllers. “It was determined that frozen water was a suspected cause,” says the report.
The pilots therefore began descending the jet to 9,000ft, where the air temperature was predicted to be warmer.
“As the airplane descended through 12,000ft, a minor ‘jolt’ was felt by the flight crew. After this, the control wheel operated normally, and the flight continued to and landed safely at JFK,” the NTSB says.
Delta later told the NTSB that prior to the flight the aircraft had been “out of service for maintenance which required pulling the forward and aft lavatory drain mast heater” circuit breakers.
“It is possible that these [circuit breakers] were inadvertently left open,” which could have allowed water inside the lavatory drain to freeze, causing water to backup into the lavatories.
“The water likely travelled into and drained through the canted pressure bulkhead in the vicinity of aileron control system components located within the main landing gear wheel well,” says the NTSB. “The water likely froze on one or more of the components, which led to the pilots’ limited control of the ailerons.”