Boeing, Honeywell, NASA Flight-test Interval Management

During flight-tests involving up to three jets converging on the same point in space but separated by specific time intervals, engineers from NASA, Boeing and Honeywell observed as pilots managed an intricate air traffic management choreography enabled by sophisticated software, all with the goal of improving efficiency for the busy airspace surrounding popular airports.

The flight deck-based interval management (FIM) trials spanned six weeks and involved 18 days of flying, with five to seven different scenarios tested, and 130 test runs. Testing was done at Grant County International Airport in Moses Lake, Wash. The aircraft involved were Honeywell’s Boeing 757 and Falcon 900 and a United Airlines 737. The project is funded by NASA and is part of communications, navigation, surveillance/air traffic management (CNS-ATM) modernization research; this particular phase was the culmination of the ATM Technology Demonstration-1 (ATD-1) program, and Honeywell was a subprime contractor to Boeing, the prime contractor for the trials. 

Honeywell built the FIM equipment, according to Rick Berckefeldt, Honeywell senior program manager, surveillance programs, advanced technology, and Boeing coordinated the flight-testing and provided the flight-test director to oversee the trials. 

Proper Arrival Spacing

For the test pilots flying the scenarios, the main interface is a tablet computer into which crew had to input various parameters to set up the proper spacing for the arrival. The idea is to give the crew a clearance that adds another parameter to typical arrival and approach procedures, this being not only an aircraft to follow but a spacing goal, defined as time-in-trail. An example might be, “Cleared for the SUDBY transition to Rnav Z 32R approach, achieve and maintain 120 seconds time-in-trail of N889H by [such-and-such intersection].” In these tests, the Falcon 900 was the target aircraft, so wasn’t FIM-equipped, while the 737 and 757 flew with the FIM equipment.

Because this was a trial using prototype software, pilots had to enter winds at various altitudes, aircraft routing, spacing goals and the termination point to make the spacing and timing work. “All that was hand-entered for the experiment,” Berckefeldt said. “In a mature [version], it would ingested via datalink. Likely FIM would be hosted in the flight management computer, then flown coupled to the autopilot.” 

The key technology that enables the FIM experiment is ADS-B IN, which allows the computers to see exactly how far apart the target and following aircraft are. “All that information is fed into the FIM logic,” he said, “then acted on. The output is speed commands to the pilot.” 

What this means for the pilots flying the following aircraft is that once the clearance is entered into the FIM computer, their job is first to achieve the commanded time spacing before reaching the specified fix. Once the spacing is nailed, then it’s just a matter of maintaining that spacing, in this case to within 10 seconds, all the way to the final approach fix or perhaps the end of the runway. “Most of our runs were well within plus or minus three seconds,” he said.

If the aircraft falls outside the required spacing, the FIM computer will issue commands either to speed or slow. At the same time, there may be other aircraft merging into the arrivals stream, so keeping the spacing within parameters is critical. “We had many runs where we had three aircraft,” he explained, “each coming on a different transition, some to different Stars, all of which merged and all to the 32R Rnav Z approach. That’s what makes FIM far more realistic; it’s a real-world tool.”

Long-term Goal: Automation

While the pilots in these tests were flying the airplane, although using the autopilot, Berckefeldt sees this technology maturing in about 10 years and being flown by the autopilot and directed by flight management computer (FMC) commands. RTCA Special Committee 186 is working on developing interval management minimum operational performance specifications, he said. “Once that’s available and the FAA has the ground automation that supports it, we will likely see this forward-fit into aircraft with FMCs. The nav display will have awareness, and the PFD will have a modified speed tape [showing] the commanded speed for situational awareness. But it will all be flown coupled to the autopilot. Aircraft will manage the spacing that the FIM algorithm determined it needs to fly to achieve the spacing goal.”

ADS-B IN allows pilots to see other aircraft, and in this case the FIM display included Honeywell’s cockpit display of traffic information ADS-B IN software. The 757 and 737 pilots could therefore see each other on the FIM display. However, “This is not a separation tool,” Berckefeldt explained. “ATC owns separation and is responsible for keeping IFR traffic separated. Spacing allows ATC to provide a clearance that provides the spacing goal. In a mature world, controllers can issue clearances, and this reduces their workload. They’re not constantly trying to provide target speeds and vectoring traffic; they know this arrival stream is relatively well ordered, and they can spend more time on separation and safety.”

In practical terms, the FIM will change the way controllers work the inbound flow of aircraft. “ATC will be supported by new ground automation,” he added, “which will assign landing runways and sequence airplanes and suggest which aircraft should be paired and what spacing intervals will be. This will start in Center airspace then approach then the tower. There will be a lot better integration between ATC organizations.”

Ultimately, FIM could help reduce spacing for arrivals into busy airports. “Today a lot of the additional spacing that ATC adds is to account for human error, the associated congestion and all the reasons why airplanes could be closer than they should be,” Berckefeldt said. “They have to add spacing so they don’t violate the minimum [spacing standards]. With tools that eliminate a lot of that error, ATC will get comfortable and confident in the way the system operates and can then lower spacing intervals at busy airports where at push times there’s metering and it’s congested and runway utilization is a primary resource. If we can have a well ordered arrival stream with reduced spacing, that opens up landing slots. The capacity of the airport goes up without any compromise in safety.”

The controllers at Seattle Center, Moses Lake Tracon and tower and Boeing Field, he said, “did an outstanding job of allowing the experiment to run smoothly and setting up the procedures. We recorded terabytes of aircraft performance data, video and crew feedback.”