What happened
During the approach to landing at ARN, the crew of an Embraer 175 operated by PLL LOT moved the flap lever to position "1" while the aircraft was traveling at a speed of 260 knots. Upon receiving a system warning indicating that the speed exceeded the allowable limit for that configuration, the pilot flying (PF) immediately retracted the lever to position "0".
The captain, acting as the pilot flying, believed that the brief movement to position 1 and its subsequent retraction would not result in the deployment of the flaps and slats. Consequently, the crew did not immediately notify the Maintenance Control Center (MCC) regarding the incident. The technical services were only notified after the aircraft had landed at WAW.
The investigation
Investigators reviewed flight data recorder readings while the aircraft was on the apron. An initial review failed to confirm the overspeed condition due to an error in the analysis rule. Following a correction to the rule, a secondary analysis revealed that the allowable speed limit for the flaps had been exceeded by 36.5 knots.
Based on this finding, maintenance personnel performed a first-phase inspection of the flaps. At the request of an MCC engineer, a flight safety inspector conducted a detailed flight analysis. Following the updated data, the maintenance engineer proceeded with a second-phase inspection, specifically targeting the slats, in accordance with the Aircraft Maintenance Manual (AMM 05-50-07).
Findings
- The movement of the flap lever to position 1 at excessive speed was caused by the pilot flying issuing the "Flaps" command without verifying the current airspeed.
- The pilot monitoring (PM) also failed to verify the airspeed before moving the lever.
- The crew's assumption that a momentary lever movement would not trigger the extension process contributed to the delay in reporting the event.