What happened
On June 12, 2017, a Boeing 738 operating a flight from Reina Sofia (GCTS) to Poznań (EPPO) experienced a technical malfunction during transit. The crew received a 'MASTER CAUTION - FLT CONTROLS B LOW PRESSURE' warning, which led to the identification of a gradual leak in hydraulic system B, with fluid levels dropping from 84% to 81% over a 30-minute period. In response, the crew performed the appropriate non-normal checklists regarding hydraulic fluid loss and prepared for approach and landing.
During the descent, the STB RUDDER system activated automatically following flap extension. While the aircraft was configured for landing without further mechanical difficulty, a procedural error occurred during the transition from approach to tower control. Upon instruction from the approach controller to establish contact with the tower, the crew tuned the incorrect frequency (119.775 MHz instead of 119.975 MHz). Consequently, the crew proceeded to land the aircraft under the mistaken belief that they had received landing clearance from Air Traffic Control.
The investigation
The investigation focused on the sequence of events leading to the unauthorized landing and the technical status of the hydraulic system. The inquiry established that the crew had correctly identified and managed the hydraulic leak by following established checklists. The investigation also reviewed the technical maintenance performed following the event, noting that the hydraulic fluid was replenished and system tests were conducted. No further leaks were detected, and the aircraft was returned to service.
Findings
- The primary cause of the incident was crew error, specifically the failure to obtain proper landing clearance from the tower controller.
- The crew utilized an incorrect radio frequency (119.775 MHz) when attempting to contact the tower, which prevented effective communication with the controller.
- The crew's error was compounded by the false assumption that landing authorization had been granted despite the lack of successful communication on the correct frequency.