Precision Runway Monitor (PRM) approaches are independent, simultaneous operations to runways spaced between 2500 and less than 4300 feet apart. The approach courses are normally parallel but may be offset by between 2.5 and 3.0 degrees depending on the runway separation.
The PRM system provides air traffic controllers monitoring the No Transgression Zone (NTZ) during PRM approaches with precision, high update rate, secondary surveillance data.
PRM approaches require specific pilot training. Company policy will detail the requirements or, for general aviation, consult the Aeronautical Information Manual (AIM).
Communications and control procedures
Pilots are required to concurrently monitor a PRM monitor frequency as well as the Tower frequency for the approach, therefore dual VHF comm is required. This protects against any blocked "breakout" instruction. The tower transmits on both frequencies The aircraft listens to both frequencies but only transmits on the Tower frequency.
One Tower Controller and one Monitor controller are assigned to each PRM runway. The monitors watch for off course deviations, especially those toward the NTZ, a safety buffer between the two parallel runways. If an aircraft strays off course, the monitor will attempt to return the aircraft to the final approach course. If a conflict is developing, for example if an aircraft is entering the NTZ, then the monitor may issue a "breakout" instruction involving a turn away from the NTZ and either a climb (normally) or a descent (rarely). Pilots must execute this instruction without delay and therefore all "breakouts" are hand flown. "Breakouts" are rare but crews should brief their aircraft specific procedures for responding to a "breakout" call, covering such topics as autopilot, flight director, pitch, roll and power, before commencing the PRM approach.
If, during a "breakout", TCAS issues a climb or descend Resolution Advisory (RA) opposite to the controller's instruction, pilots should follow the TCAS RA while executing the turn portion of the controller's instruction.
For further information see the separate article on Simultaneous Close Parallel PRM Approach