The FAA has commissioned the first implementation of a new generation of precision runway monitor (PRM) equipment at Detroit/Wayne Airport (DTW), to permit simultaneous close parallel approaches to adjacent runways having less than 4,300 feet between their centerlines. At DTW these are Runways 21L and 22R, separated by 3,000 feet, the current lower limit. The Detroit procedures also cover simultaneous approaches to the airport’s 21L and 22L, although these are 6,000 feet apart.

Currently, PRM procedures are in place at ATL, CLE, SFO and STL, but these use an earlier radar-based system that will slowly be superseded. The new system, called PRM-A, for alternative, uses Sensis’s ASDE-X airport surveillance radar, supplemented by a network of the company’s unmanned multilateration (MLat) stations far beyond the airport boundaries, and similar to those recently commissioned to enhance winter traffic operations into and out of
Rifle and Hayden, Colo.

The ASDE-X operates as a conventional secondary surveillance radar, interrogating transponders as it rotates through the surrounding airspace, while the MLat stations triangulate once per second on every transponder response out to beyond 30 miles. The fused result of both techniques produces the extremely accurate track required for radar controllers to monitor simultaneous approaches on closely spaced parallel runways. At DTW, a dedicated controller using a specially modified screen of the Raytheon Stars radar display suite at the local Tracon monitors each individual PRM-A runway approach.

The PRM-A screens show long, rectangular no-transgression zones (NTZ), clearly marked in white outlines between the inbound localizers of each runway. A controller watching the operation on the PRM-A display screen would typically see the target aircraft blips, tagged with their individual IDs, track down the localizer centerlines in string-of-beads fashion to the runway. Occasionally an aircraft will drift off the centerline, to be corrected quickly by the crew.  

If the drift continues toward the outer edge of the NTZ, the system computer will detect a potential transgression and alert the controller by turning the edges of the NTZ box from white to yellow, whereupon the controller issues a warning to the crew. Should the aircraft continue and cross into the NTZ, the outline changes from yellow to red, and the controller issues an immediate breakout command to the aircraft on the adjacent approach, since it is the one now threatened. The logic here is that the aircraft that has “blundered” into the NTZ may be having ILS, VHF com or other problems, and is assumed to be unable to respond to the controller’s instructions.

The breakout phraseology is simple and unambiguous: “Traffic alert, [aircraft call sign] turn [as instructed] immediately, heading [degrees], climb/descend and maintain [altitude].” Further, the procedure stipulates that the breakout be hand flown, recognizing that an autopilot response would be slower. And while a descending breakout command on an approach would certainly sound startling, the FAA states it would be used only in the absence of other reasonable options, and in no case would it be given below minimum vectoring altitude, thereby ensuring at least 1,000 feet of obstruction clearance.

Sensis, which has provided a similar system to Australia’s Sydney International airport, states that PRM-A enables more flights to land per hour for increased throughput at DTW. Industry observers believe that the system will likely become essential when increasing zoning and environmental restrictions limit the construction of new runways to inside an airport’s boundaries, which will mean more closely spaced parallels in the future. As well, some specialists believe that as controllers and operators accumulate experience with PRM-A, the 3,000-foot separation limit will be gradually reduced. Here, the interesting argument is that simultaneous approaches on closely spaced runways are virtually unaffected by wake vortices, compared with those that can affect the staggered spacing on independent parallel approaches.

There are specific training requirements to fly PRM approaches, but they are not onerous. The required avionics equipment–ILS/DME, conventional transponder and dual VHF com–is already installed in virtually every corporate aircraft. (In PRM, each runway has a dedicated controller with a unique VHF frequency, to which both VHFs must be tuned.) Importantly, and contrary to some interpretations of PRM-A operations, an ADS-B transponder is not necessary.