Frozen Drain Leads to Flight Control Issues
 

N181DN, a Boeing B767-332 aircraft operated by Delta Air Lines, was conducting flight DAL211 from Prague-Václav Havel Intl (LKPR), Czech Republic to New York-John F. Kennedy Intl (KJFK), NY. When in cruise flight at about 56°35' N, 038°48' W, the flight crew was alerted to a flooding issue in the mid-cabin lavatories, followed shortly after with information that the forward left lavatory was also flooding. A short time after the flooding issue was discovered, the autopilot disconnected. The flight crew attempted to connect all 3 autopilot systems, but none would remain connected. The aircraft began to drift direction to the right, and correction to the left could not be made. The flight crew contacted company dispatch and maintenance control and completed the quick reference handbook (QRH) for a jammed flight controls state. The flight crew declared an emergency with Canadian air traffic services (ATS), began a descent and coordinated with ATS for a diversion to Goose Bay (CYYR), NL. At about 100 nm east of CYYR, while at an altitude of 12 000 feet, the aircraft flight control function returned to normal operation. The flight crew terminated the emergency and coordinated a diversion to Boston-Logan Intl (KBOS), MA. While enroute to KBOS at 11 000 feet a recalculation of fuel determined that the flight could continue to KJFK, where a landing was made without further incident.

Maintenance personnel replaced both grey water drain masts, and replaced a seal in the left wheel well canted pressure deck above the aileron feel centering and trim mechanism. The entire roll control and related autopilot systems were inspected and repairs effected as required.

Happened on 7th July this year.

How did flooding freeze the controls so bad.... I do understand that it might have formed ice in the unheated/unpressurized areas... but still, this seems very concerning.

I do get that the electronics certainly wouldn't like it, but then again, the 767 is (IIRC) completely non FBW.

AVHerald reports:

On Apr 21st 2023 the NTSB released their final report and the docket concluding the probable cause of the incident was:

The failure of maintenance personnel to close the drain mast heater circuit breakers which resulted in the formation of ice in the forward drain mast, an improper flow of wastewater into the main landing gear wheel well, and the formation of ice on one or more aileron system components.

Qantas had a similar issue several years back, though there was no flight control restriction as regards hand flying it. The ATSB report.

They flooded the galley, water dripped onto 3 of the 4 Generator Control Units, AC busses 1,2+3, AP, AT, wx radar & the Effo's screens were lost. The cause was found to be a failed drain line heater that allowed ice formation and subsequent backing up of waste water which then flowed through a split in the drain line, onto inadequate drip shields over the E&E bay and finally into the avionics.