Incident Overview
An investigation into an incident involving a commercial aircraft found that a propeller strap was not removed before engine start. The strap had been placed on the left engine propeller after the preceding flight. During the final external walkaround, the first officer (FO) removed the strap extension from the propeller strap and the aircraft stairs, but left the propeller strap attached to the left engine cowling and propeller. The FO did not rotate the propeller to the required dressed orientation. The FO later could not explain why they removed the extension but not the strap, and had difficulty recalling parts of the walkaround. The FO reflected that the walkaround is a straightforward, highly practiced task that does not require much attentional effort, suggesting the removal of the extension was an unintentional action slip.
Walkaround and Cue Limitations
After removing the strap extension, the FO returned to the flight deck carrying it and placed it in its normal location. The stairs were retracted and the door closed, which would not have been possible if the extension were still attached. Returning to the flight deck with the extension in hand provided a perceptual cue that the walkaround was complete and both straps (extension and propeller) had been removed. These circumstances limited opportunities for the FO to detect the omission.
Engine Start Checklist Error
Following the walkaround, the flight crew began the engine start checklist. The checklist required the captain to verify the left engine was clear, but the captain did not identify the propeller strap and proceeded to start the engine. The captain reported having a very high expectancy that the strap had been removed, having observed the FO enter the cabin holding a strap. The captain assumed this was both the propeller strap and extension. The captain had no reason to inspect the recovered strap closely, as that was not normal practice. From the left seat, the propeller strap appeared as a thin fabric against the blade in the shadow of the engine, requiring the pilot to lean forward to see it. The captain acknowledged not checking the left propeller thoroughly, attributing this to their expectancy that the strap was already removed.
Contributing Factors
The investigation identified two contributing factors: the first officer inadvertently disconnected and removed the strap extension from the propeller strap, leaving the propeller strap on the blade, and did not rotate the propeller to the required orientation; and the captain did not detect the propeller strap during the engine start checklist.