PRM Comes to Philly

PRM approaches, used at MSP since 1997, will expand to PHL in early June; ALPA has obtained significant improvements in PRM that will apply to all PRM operations.

Air Line Pilot, May/June 2002, p.12
By Capts. Larry Newman (Delta) and Terry McVenes (US Airways)

This is the way it should happen: Government regulatory agencies and the "stakeholders" in the aviation industry—including ALPA—get together to hammer out a new technological system and associated procedures to improve the efficiency of the air transportation system and aviation safety.

That’s what did happen in working out procedures for Precision Runway Monitor (PRM) simultaneous independent approaches to closely spaced parallel runways at Philadelphia International Airport (PHL). On June 3, PHL will begin PRM approaches to Runways 27L/26—with ALPA’s support and encouragement.

Minneapolis–St. Paul International Airport (MSP) was the first operational site for PRM, starting in 1997, and your ALPA aviation safety representatives have gone through the painstaking but necessary process of resolving operational and safety issues as they were identified.

ALPA has been part of an FAA–airline industry coalition known as the Closely Spaced Parallel Runway Operations Steering Committee since Nov. 6, 2000. Your safety advocates, along with government and industry representatives, have met many times to get PRM approaches where we think they need to be.

PRM made simple

Airport surveillance radar used at most U.S. airports today updates every 4.8 seconds. This design specification, plus display resolution and functionality, limits the minimum lateral distance at which two aircraft can fly independent parallel approaches to 4,300 feet between parallel runway centerlines.

We believe that flying PRM approaches is safe, even though some training questions remain.

Flying approaches to runways less than 4,300 feet apart requires air traffic controllers to make the arrival streams dependent—in other words, it requires controllers to maintain a minimum along-track spacing, or stagger, between aircraft on the adjacent final approach courses. The position of the first aircraft affects what ATC can do with the second. These restrictions reduce airport capacity and increase air traffic controllers’ workload.

PRM uses a high-update-rate radar that updates once per second—almost five times as fast as conventional airport surveillance radar. PRM includes aural and visual warnings, magnified displays, and future-position predictive software, which together significantly improve air traffic controllers’ ability to monitor aircraft on final approach. Because the radar updates so quickly, the controller also sees a set of target trails that provide extremely accurate trend information and also provide an aural alarm to the controller if required separation is jeopardized.

Moreover, during PRM operations, a separate controller monitors each runway, and a coordinator manages the overall situation. Thus three controllers are focused on ensuring required separation between aircraft on final approach.

These features allow controllers to conduct independent parallel approaches with as little as 3,400 feet lateral spacing, and 3,000 feet if one of the ILS courses is offset.

Because a number of U.S. airports have parallel runways less than 4,300 feet but more than 3,000 feet apart, a need for PRM exists. Also, having the option of building new runways closer to existing ones gives airport planners additional flexibility in providing badly needed additional concrete.

PRM can even be used for runways less than 3,000 feet apart; that’s known as simultaneous offset instrument approaches (SOIA) and will be the subject of another feature article in Air Line Pilot soon. For now, just remember that PRM is designed to safely increase capacity at airports with closely spaced parallel runways in IMC to Category I minimums.

Bottom line

PRM is a safe and effective capacity-enhancement tool. The most advanced risk modeling available today has shown that you are at less risk of being involved in a midair collision when flying a PRM approach than when you are flying a standard parallel independent ILS approach.

You’re probably thinking, "No way can it be safer to fly 3,000 feet apart (minimum for PRM) than it is to fly 4,300 feet apart (minimum for non-PRM independent parallel approaches)."

On the surface, that statement would seem true. However, we don’t look at just the surface. We look at the risk using the most advanced modeling techniques available.

Risk modeling

Calculating the risk associated with performing a certain task or procedure is a process pilots often perform. We do it every time we step into the cockpit. We know that managing risk is part of how we do business. We can never eliminate risk. Our goal is to first understand what the risk actually is, and then how to minimize it.

Historically, risk management by the FAA and the aviation industry has been more an art than a science. People who understood airport and aircraft operations sat down and tried to come up with a list of the hazards associated with a particular operation or procedure. They would rank these hazards from most to least hazardous.

While this was useful, it didn’t mathematically identify what level of risk actually existed within various operations. It depended on historical perspective and was subject to the biases that all humans possess. It was, in short, a flawed process.

The FAA Flight Procedures Standards Branch (AFS-420) has developed a data-driven, mathematical modeling process known as Airspace Simulation and Analysis for TERPS (ASAT). It runs various scenarios millions of times, generating a true mathematical picture of the risk involved in a particular procedure, such as flying PRM approaches.

The collision risk, in the case of PRM, is defined as the center of gravity of two aircraft coming within 500 feet of each other. Penetration of this 500-foot bubble is known as a "test criterion violation," or TCV. Each TCV that occurs during the modeling process is analyzed to determine why it occurred and whether it is valid.

Valid TCVs are used to form the basis of the calculation of risk involved in a particular procedure. In the case of PRM, the FAA determined that the risk of a TCV occurring—i.e., two aircraft CGs passing within 500 feet of each other—is 4 × 10-8. This exceeds the current target level of safety for final approaches of 1 × 10-7. In other words, flying PRM approaches is safer than our current operations.

No risk analysis can deal with the probability of human error occurring, such as the risk of a pilot’s hearing "turn left immediately" and then turning right. Neither is the probability of a pilot’s ignoring a controller’s instructions, nor of a controller’s saying "left" and meaning "right." No one has yet been able to calculate the probability of any of these scenarios occurring—which brings us to the safety issues ALPA identified, and the solutions to those safety issues.

TCAS—the way it was supposed to be

Until recently, the Attention All Users Page for MSP directed the pilot to set TCAS in the TA ONLY mode during PRM approaches. The reason was that the FAA had not included TCAS in the initial PRM risk modeling and thus could not allow it to be operated in the TA/RA mode because any effects that TCAS might have on the risk of conducting PRM were unknown. This was a reasonable position for the FAA to take.

What was not reasonable, in ALPA’s opinion, was to expand PRM approaches without doing the modeling work necessary to validate our ALPA policy that TCAS should be used in the TA/RA mode from takeoff to touchdown.

We believe that TCAS must be used in the TA/RA mode as it was intended to be used—to account for those rare human errors that no modeling process can predict. How could we possibly agree to increase capacity without allowing for the inevitable mistakes? We can’t, and we won’t.

Dealing with this issue was a painstaking process, but the perseverance of your ALPA safety representatives paid off. The FAA conducted an analysis using TCAS in the TA/RA mode; I am pleased to report that use of TCAS will now be in accordance with your current operational directives. Unless approved otherwise, that means leave the TCAS in the TA/RA mode from takeoff to touchdown, including during PRM approaches.

ALPA will continue to oppose any capacity-enhancement procedure that requires TCAS to be switched out of the TA/RA mode at any time. Not only does this prevent the loss of the non-human backup—TCAS—for the all-too-human pilot and controller, it also prevents the classic human-factors trap set up by the requirement to reach down and switch the transponder out of the TA/RA mode, and then remember to turn it back on again during a go-around.

Primary separation responsibility

The primary responsibility for ensuring required separation between aircraft during simultaneous closely spaced parallel approaches falls on the controller. The odds are that if a controller ever instructs you to perform a breakout, he or she will do so before you get a TCAS RA.

Even though the odds of getting a RA are remote, you must respect the RA if you get one. Don’t wait to find out why the RA has occurred. The only safe thing to do is to follow the RA’s vertical guidance. It will still separate you from the threat aircraft, although not by as much distance as will a timely breakout instruction, which contains both a climb or descent and a turn. Remember that the controller is primary, but TCAS is there if the PRM procedures fail for any reason.


The very top of the Attention All Users page for each PRM approach advises that you must have special training on PRM approaches before you can fly them. As in all training requirements for airline pilots, the FAA Principal Operations Inspector (POI) reviews and approves the PRM training program for that particular airline.

Most of the major air carriers, because of a recommendation of the Air Transport Association, have decided to train pilots in flight simulators for PRM approaches. This was one of ALPA’s goals, and our insistence at the table paid off to some degree.

Some regional airlines, which have less access to simulators, have elected to use non-simulator-based training to prepare their pilots for PRM approaches. The Regional Airline Association has also agreed to work with its members to help them develop simulator or aircraft-based training. This obviously is less than what we wanted, but the FAA would not agree to mandate simulator training. The POIs are still responsible for ensuring that the training that all pilots receive meets the requirements set forth in the FAA regulations.

A few airlines have elected not to participate in PRM at all and are therefore not conducting any PRM training. Other carriers are training only some of their pilots, as not all aircraft types will be flying into PRM-equipped airports. This is an economic decision, and when the economics say it’s time to participate, these airlines will have to conduct PRM training.

General aviation pilots are directed to train in accordance with the FAA Aeronautical Information Manual and the Handbook for General Aviation. They can meet the PRM training requirements by watching an updated video, participating in the FAA’s Wings Program, or by completing a biennial flight review. This is in line with most general aviation training programs.

The FAA based its decisions on training upon several studies that it conducted earlier in the PRM program. These studies indicated that pilots of single-pilot general aviation aircraft, which are more maneuverable and less complex than transport-category aircraft, had less difficulty complying with ATC instructions during a blunder in PRM approaches than their counterparts flying the big iron.

The FAA’s decision to not mandate simulator training was also based upon this study. The FAA analysis, along with that done by the Mitre Corporation, showed that pilots were able to adequately perform the breakout maneuvers whether or not they practiced them in the simulator.

This demonstrated, in their opinion, that neither simulator nor aircraft-based training would be necessary from a safety standpoint. Quite frankly, we have some reservations about the accuracy of their studies, and the FAA has agreed to take another look at the results. This is a step in the right direction. ALPA is continuing to press this issue with the FAA’s Flight Standards Division.

We believe that flying PRM approaches is safe, even though some training questions remain. It is a matter of risk management based on the target level of safety discussed earlier, the very low probability of a breakout, and changes we were able to obtain in the FAA’s Air Traffic Controller’s Handbook that say controllers may issue the descending breakout only as a last resort. As long as your carrier meets the minimum standards as set forth by the FAA, through the POI, you should participate in PRM. Again, ALPA will continue to pursue our concerns with the FAA. However, it is safe to fly PRM.

Attention All Users page

We have worked hard on the contents of the Attention All Users page that accompanies every PRM approach. Our goal has been to keep it simple but effective. Before each PRM approach, the flight crew should review the Attention All Users page to ensure that they are complying with the requirements for flying the approach.

We’ve refined the information to be what we believe is necessary for safe PRM operations. However, it does not take the place of adequate PRM training, and it is not intended as a PRM training tool.

Backup comm frequency

The backup communications frequency is a key part of the risk mitigation during closely spaced (PRM) parallel approaches. ALPA insisted that we either get a dedicated backup frequency or that antiblocking devices be installed in our airplanes to ensure the pilots’ ability to hear the controller’s instructions if the controller has to call on the pilots to fly a breakout maneuver.

We got the dedicated backup frequency. You must have two operational VHF communications radios to conduct PRM approaches. The FAA will not permit this requirement to be waived by the minimum equipment list (MEL).


Current NOTAMs direct the flight crew, through the dispatcher, to notify ATC when they are "Unable PRM." This is a planning tool for both ATC and aircraft operators. You can expect to hold for as long as 30 minutes when PRM approaches are being used and you cannot participate—whether because of MEL restrictions, lack of PRM training, or the captain exercising his or her pilot-in-command authority, based on his or her judgment.

Regarding captain’s authority, please use discretion when turning down a PRM approach. ATC arrival acceptance rates will be based on all pilots flying PRM approaches. Controllers have to make a hole in the approach stream to accommodate a non-participant. This creates additional workload for controllers and reduces airport capacity.

Right now, the ATC procedure is to make the non-participant hold until ATC can work him or her in. The occasional non-participant will not be a major problem. However, if too many pilots turn down PRM approaches during a rush, ATC will stop using PRM approaches, and all flights to and from that airport will be delayed.

Again, ATC will set arrival acceptance rates based on what PRM can do. Two separate independent streams, using PRM, will permit more aircraft to land in an hour than two dependent streams. Yours may not be the only aircraft affected by refusing a PRM approach.

Some closing thoughts

We didn’t get everything we wanted when we started our negotiations over PRM, but our goal was to first ensure that the PRM approach was, from a safety viewpoint, the way it had to be, and we accomplished this. Some of our "should be’s," such as simulator training for all pilots, were simply not attainable at this time, but we are continuing to pursue them.

Today we develop capacity-increasing procedures by building consensus. The FAA works hard to try to ensure that everyone can live with the procedures it develops. General aviation—including business aviation—has a big stake in airspace and airport capacity, and these fliers must be heard.

Your ALPA representatives remain engaged with the FAA and the rest of the aviation industry through efforts such as the Closely Spaced Parallel Runway Steering Committee, while insisting that our safety requirements be met. PRM is the latest testament to our success.

Our insistence on using verifiable scientific data to support FAA programs and directives is really beginning to pay off. We believe many within the FAA and the aviation industry have started to understand and accept this as the right way to do business. It is an effective developmental process built on mutual respect and understanding, not on intimidation.