What type of signal is used when radio communication between ATC and an aircraft is unreliable?

When radio communication between ATC and an aircraft becomes unreliable, ATC light gun signals are utilized as an alternative method of communication. These signals consist of visual indications produced by a light gun operated by ATC personnel. The light gun can emit different colors: steady green indicates that an aircraft is clear to land or take off, steady red means to stop, flashing green signifies that the aircraft should taxi, and flashing red indicates that the aircraft should not land.

The use of light gun signals is particularly valuable in situations where radio communication may be compromised due to interference, equipment failure, or other issues, allowing for safe and effective communication with pilots who may not be able to receive or transmit voice communications. This method ensures that aircraft can still operate safely even when typical radio procedures are unavailable.