What safety feature must all carabiners have when used in tree trimming?

A self-locking mechanism is essential for carabiners used in tree trimming because it enhances user safety by preventing accidental unclipping or disengagement during work at heights. This mechanism reduces the risk of a fall, which can occur if a carabiner were to open unintentionally due to movement, pressure, or friction.

In the context of tree trimming, where workers often rely on their equipment for stability and security, having a carabiner that locks automatically ensures that the connection between the worker and their harness or lanyard remains secure. The self-locking feature can be engaged through various mechanisms, such as a screw gate, twist lock, or automatic lock, all designed to ensure that the gate remains closed except when intentionally opened.

While quick releases, reversible designs, and flexible hinges may provide certain functionalities in specific applications, they do not offer the same level of security and reliability needed for fall protection in tree trimming activities. Therefore, a self-locking mechanism is not just a preferred feature but an essential safety requirement for carabiners in this field.