Which item should not be left behind in the aircraft according to ATP procedures?

The Blue Book, which typically contains critical operational information and procedures for flight crews, should always be with the pilot or crew and not left behind in the aircraft. This ensures that all pertinent information is accessible during flights, contributing to safe and effective operations. It often includes important data related to aircraft performance, emergency procedures, and operational guidelines, making it essential for decision-making and compliance with regulations.

In contrast, while items like ground handling equipment, flaps control levers, and flight logs are important, they are often specific to the aircraft's operation or maintenance and may not be required to be kept within the cockpit or with the crew during flights. Therefore, the Blue Book's role as a comprehensive reference tool makes it paramount to always have it readily available, reinforcing its importance compared to the other items listed.