PRM approaches allow you to descend through the clouds just a few hundred feet away from other aircraft landing on parallel runways. Here's what you should know before flying your next, or first, PRM approach.

When Are PRM Approaches Published?

Precision Runway Monitor (PRM) approaches are independent, simultaneous operations to runways spaced less than 4,300 feet apart. The approach courses are normally parallel, but may be offset by 2.5 to 3.0 degrees, depending on the runway separation.

You can find RNAV, LDA, and ILS PRM approaches at some of the busiest airports around the country with close, parallel runways. While they require no additional navigation equipment, there's a lot going on in the background for pilots and air traffic controllers.

Detroit, San Francisco, Chicago, and Atlanta are the four US airports that have PRM approaches. PRMs allow these airports to increase arrival rates for landing traffic in low weather conditions, reducing delays and increasing fuel savings.

Monitor Frequencies

On PRM approach plates, you'll find a "monitor" frequency. As you fly a PRM approach, you'll transmit and receive radio calls from the tower frequency in comm #1. Meanwhile, you'll listen to the monitor frequency in comm #2. At no point during a PRM approach do you transmit over the monitor frequency.

Crews are required to monitor this frequency in their standby radio for any breakout instructions issued by a monitor controller. Aircraft flying PRM approaches are required to have Dual VFH Comm capabilities because of this.

Priority is given to the monitor controller in the event of breakout instructions, so the pilots can react immediately (pictured below).

So why separate tower frequencies and monitor frequencies? If an aircraft is transmitting on tower frequency, the monitor frequency controller can issue breakout instructions at any time, allowing aircraft to begin separating themselves immediately. More on that in a bit...

Briefing The PRM

As you set up your approach, you should brief all PRM approach procedures and frequencies in detail. The FAA has an "Attention All Users Page" (AAUP) published for each PRM approach. On it, you'll find a guide detailing everything you should cover in your briefing, from breakout procedures to communication requirements.

Are You Authorized To Fly A PRM?

If you're a pilot under Part 121, 129, or 135, you must complete all company training as stated in your carrier's Op Specs. Not all air carriers are certified to fly PRM approaches.

If you fly a transport category airplane under Part 91, you must be familiar with PRM operations as contained in section 5-4-44 of the AIM. In addition, "pilots operating transport category aircraft must view the FAA video "ILS PRM And SOIA Approaches: Information For Air Carrier Pilots." Click the following link for additional information and to view the video: http://www.faa.gov/training_testing/training/prm/.

If you're a pilot under Part 91 and NOT flying a transport category aircraft, all that's required is for you to familiarize yourself with AIM requirements. That being said, we, and the FAA, STRONGLY recommend you complete the PRM training provided by the FAA.

No Transgression Zone (NTZ) + ATC Requirements

ATC systems often include a high-update radar, high-resolution radar display, and software that can autonomously track aircraft in near real-time. "Controllers are provided with visual and aural alerts that depict an aircraft's current position, velocity, and a ten-second projected position" (FAA AFH 4-65).

A "No Transgression Zone" (NTZ) is established between the two approach courses to prevent imminent traffic conflicts. An NTZ monitor controller and one tower controller are assigned to each PRM runway. If an aircraft strays off course, the monitor on the righthand runway will instruct the aircraft to return to the final approach course.

Breakout Instructions And Pilot Actions

When an aircraft encroaches on the NTZ and poses an immediate risk to other traffic, a breakout instruction will be issued. Pilots will hear something like this:

As a pilot, it's your job to follow the instruction without delay. All breakouts must be HAND FLOWN on PRM approaches. Disconnect the autopilot immediately and execute the instructed maneuver.

If you receive a TCAS Resolution Advisory (RA) during a breakout opposite to the controller's directions, follow the TCAS RA while adhering to the controllers turn instructions. Remember, RAs will only provide maintain altitude, climb, or descend warnings...never turn instructions.

Have You Flown A PRM?

Where did you fly your last PRM? Tell us about your experiences in the comments below.