One of the FAA’s less publicized programs to increase airport capacity is the gradual introduction of the precision runway monitor (PRM). Currently commissioned at Minneapolis, St. Louis and Philadelphia, and soon to come online at San Francisco and New York JFK, the PRM allows aircraft to make simultaneous independent approaches to closely spaced dual or triple parallel, or dual converging, runways.

Currently, without a PRM, simultaneous ILS approaches are not permitted in IMC when the centerlines of adjacent parallel runways are less than 4,300 ft apart. This rule came about after analysis showed that there was a statistical possibility, even though remote, that two aircraft approaching simultaneously could inadvertently deviate toward each other and, having no accurate knowledge of the other’s position, create a potentially hazardous situation. In VMC, of course, when the other traffic is clearly in sight, it’s a quite different situation, as operators into San Francisco’s 28L and 28R–separated by just 750 ft–are well aware.

But the result of the IMC rule has been that many airports in the U.S. and overseas have to close down perfectly good runways as soon as the ceiling or visibility falls below limits well above those applicable to normal IMC approaches. At that point, the airport usually reverts to single-runway operations, with resulting reductions in capacity and increasing delays.

A solution to this is the precision runway monitor, which allows independent IMC approaches down to normal decision heights on parallel runways that are as close as 3,000 ft apart. Here, the PRM covers the approach and missed-approach areas with a dedicated 32-nm-range rapid scan radar–a very high accuracy system that uses an advanced nonrotating electronic antenna to sweep through 360 deg in one second, compared to the 4.7-sec sweep of a conventional rotating surveillance radar. It is this update speed and high accuracy that qualify the PRM for the task, while tests of ADS-B, LAAS, transponder multilateration and other techniques have been found wanting.

The positions of the landing aircraft are continuously presented on two large high-definition displays, each with a separate controller monitoring progress on one of the two runways. The permanent background to the controllers’ screens shows the two runways and their ILS approach paths running vertically up the screen, and the target aircraft stand out clearly as they move down the screen to the threshold. The display also shows each aircraft’s predicted position with a 10-sec lead time.

Displayed between the two ILS approach paths on the screen is a clearly delineated 2,000-ft-wide rectangular no transgression zone (NTZ), which extends 10 to 15 mi from the runway threshold and a half-nautical-mile beyond its end.

Controller/pilot communications are transmitted over a dedicated VHF frequency for each of the two runways as well as over a secondary control tower frequency. The aircraft’s radios must be separately tuned to the frequency assigned to its runway and to the tower. In normal operations, there is little communication between the controller and the pilot, except when one of the aircraft moves off the centerline toward the NTZ. The controller will then immediately advise the pilot to return to the centerline. If the aircraft continues toward and crosses the NTZ boundary, the other aircraft will make a breakout maneuver, normally a turn away from the other aircraft and a climb to an assigned missed-approach altitude.