What happened
On June 17, 2017, a Boeing 737-800 was performing a climb following departure from Girona Airport (LEGE). During the initial phase of the climb, the flight crew received clearance from air traffic control (ATC) on frequency 133.025 MHz to climb to flight level (FL) 240. Shortly thereafter, the same ATC unit issued a subsequent clearance allowing the aircraft to continue climbing to FL 300. The monitoring pilot performed a readback of this instruction to confirm the new altitude.
While the aircraft was passing through FL 250, the ATC controller notified the crew that their clearance was only valid up to FL 240. The crew responded by clarifying that they had received and read back an instruction for FL 300 several minutes prior, noting that ATC had not corrected the readback at that time. Following this exchange, ATC issued a new clearance for the aircraft to climb to FL 260. The crew reported that the controller acknowledged the discrepancy by stating "NO PROBLEM."
The investigation
The investigation focused on the communication exchange between the flight crew and the ATC unit operating within the Barcelona TMA (LEBL). The inquiry examined the sequence of altitude clearances issued and the crew's adherence to readback procedures. The investigation established that a discrepancy existed between the altitude the crew believed was authorized and the altitude the controller intended to authorize.
Findings
- The investigation determined that the issuance of an incorrect clearance by ATC was the probable cause of the incident.
- The crew followed standard procedures by confirming the altitude change via readback.
- There was a lack of immediate correction from the controller following the crew's confirmation of the higher altitude.