What must be installed before connecting the tow bar to the nose gear of an aircraft?

Before connecting the tow bar to the nose gear of an aircraft, it is essential to install the bypass pin. This pin is crucial because it ensures that the nose gear can be maneuvered safely without the risk of accidental retraction while the tow bar is being attached. The bypass pin disables the nose gear's retraction mechanism, providing a secure connection point for towing operations.

Using a bypass pin helps prevent damage to the aircraft's systems and enhances safety by ensuring that the gear remains extended during the tow process. Proper procedures like these are vital in fleet service, where safety and operational efficiency are paramount.