What must be installed before connecting a tower to an aircraft?

Installing a bypass pin is a critical step before connecting a towbar to an aircraft because it ensures that the aircraft's braking system cannot be accidentally deployed while the aircraft is on the ground and being moved. The bypass pin disables the braking system, allowing the aircraft to be towed safely without the risk of the brakes engaging unexpectedly. This is particularly important for maintaining safety during ground operations, as an unexpected stop could lead to serious complications or accidents.

The other equipment mentioned, such as signal lights, safety wands, and ground power units, serve different operational purposes in aircraft handling, such as directing traffic or providing power, but they do not serve the specific function of ensuring safe towing operations. The bypass pin's role in deactivating brakes makes it an essential safety measure prior to any aircraft movement.