Your aircraft has radio failure at night and you need to land?
(Pilot Controlled Lighting)
There are multiple scenarios where a pilot may be unable to use the Pilot Controlled Lighting (PCL) at a non-towered airport. Use the Chart Supplement for specific information on the use of PCL for any given airport.
Since radios may fail in multiple ways, it is a good practice to continue attempting to communicate, or in this case, continue attempting to activate the PCL, if possible. For instance, if the radio is capable of transmitting an un-modulated carrier frequency, this may be sufficient to trigger the lighting system even though the pilot is unable to communicate with Air Traffic Control (ATC).
FAR 91.3 Responsibility and authority of the pilot in command.
Places the final authority on the safe operation of the airplane on the Pilot in Command (PIC).
The PIC must determine if landing at an unlit airport is safe, or constitutes an emergency. If the PIC determines it is unsafe, and has the resources to divert to a towered airport i.e. (fuel, weather, etc), then he should divert and land with the use of light gun signals from the ATC tower.
Refer to the Aeronautical Information Manual (AIM), Chapter 6, Section 4 and Federal Aeronautical Regulation (FAR) 91.185 and 91.125.