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Interval management – the next step toward domestic CPDLC

By David Bjellos
Contributing Writer

While European airspace may appear to have more CPDLC than the US, it is only operational for aircraft equipped with ATN-B1 (and is exclusively ground-based). When domestic CPDLC begins in the US, it will be an improved version that will incorporate ATN-B1 and FANS-1/A+. This will be known as ATN-B2. Transition from domestic to oceanic airspace will be seamless once all the coastal ARTCC facilities are operational in 2025.
March 2022 marked the 20th anniversary of controller–pilot data link communications (CPDLC). For flightcrews operating internationally, this benefit has already been well established.

The long-anticipated enroute automation and modernization (ERAM) efforts envisioned by FAA, and, more broadly, NextGen, will begin incorporating domestic CPDLC later this year.

Currently, only 3 ATC centers in the National Airspace (NAS) apply CPDLC actively – Kansas City, Indianapolis, and Washington.

Albuquerque center will be the 4th to incorporate CPDLC once trials with American Airlines begin.

ACSS, a joint venture between L3Harris Communications and Thales, is partnering with FAA and American Airlines, which will have its entire Airbus A321 fleet equipped with ADS-B In technology.

This will enable interval management (IM) operations for departure, enroute, and arrival spacing in participating airspace.

Data will be gathered for one year to be evaluated and shared with the wider aviation community.

Components of IM and CPDLC integration

FANS-1/A+. The US future air navigation system (FANS) comprises the foundational trinity of communications, navigation, and surveillance (CNS). The 1/A refers to authority/approvals by Boeing and Airbus respectively. The + symbol references the latency issue, or time delay incurred during electronic messaging. FANS is primarily a satellite-based form of CNS, but can also use VHF datalink, Mode 2, or VDL-2.

cpdlcATN-B1. Europe enlisted ICAO to help establish its protocols that would cover the busy airspace that serves the continent, UK, Scandinavia, and eastern European countries. These protocols don’t need the complex satellite (Inmarsat or Iridium) interface. Instead, they were designed according to the Aeronautical Telecommunications Network, Baseline 1, or ATN-B1 protocols – based exclusively on VHF. This was formerly known as LINK 2000+.

ATN-B2. NAS domestic CPDLC will be a combination of ground-based ATN-B1 and satellite-based FANS 1/A+. It will be known as ATN-B2 (Baseline 2). This will allow for innovative features such as 4D trajectories, dynamic required navigation and performance (DRNP), and, ultimately, more robust features, including IM. This has also been referred to as FANS-3 or FANS-C.

IM. Mitre Corp, along with FAA and industry, have developed the protocol of terminal sequencing and spacing (TSAS) with the goal of enhancing arrival throughput. Interval management is administered by ATC and applied by flightcrews in suitably-equipped aircraft.

ATC will identify IM opportunities and then authorize flightcrews to use ADS-B In to govern speed relative to an electronically-linked “lead aircraft” with the goal of separation (being either distance or time). This sub-component of IM is called in-trail procedures (ITP), and would replace the existing system of controllers issuing verbal speed and vector instructions to maintain separation. IM trials have shown the system improves enroute and arrival delays significantly.

ITP. This technology has been used for some time on oceanic routings. The benefits of ITP are improved fuel economy, reduced emissions, and avoidance of turbulent altitudes.

While this article focuses on domestic CPDLC and IM/ITP, the operational experience gained from oceanic aircraft trials is clear – ITP-equipped airliners saved an average of 573 lb of fuel per flight, even when no ITP maneuvers were performed. Lessons learned from long-haul aircraft on the busiest airways and airspace on Earth (NAT-HLA) have provided valuable data for domestic ITP integration.

ITP uses precise ADS-B location data to allow for altitude changes that would otherwise be blocked by non-radar separation procedures. Flightcrews operating ITP-equipped aircraft have cockpit displays and special algorithms that calculate ITP distance for the flightcrew to determine if a safe climb or descent can be performed. Instead of being trapped by the proximity of other aircraft, ITP-equipped and trained flightcrews can request ITP maneuvers to their preferred flight level. Air traffic control, which has the total traffic picture, can authorize the requests. Because of ADS-B precision, less separation is required for ITP maneuvers so they can happen more often.

Regulatory approvals – OEM installations

In 2020, the Radio Technical Commission for Aeronautics (RTCA) and the European Organisation for Civil Aviation Equipment (EUROCAE) completed the complex regulatory minimum operational performance standards (MOPS) for IM avionics.

Supporting the MOPS initiative, FAA sponsored demonstrations in 2019 at SFO (Intl, San Francisco CA), showing extremely precise spacing for arriving aircraft (within 5–10 seconds of the spacing goal).

Pilots and controllers who participated said that the system worked well and predictably. The only critiques by both parties were the automated speed reductions during the final 25–30 miles to the runway, which were not intuitive for the trailing aircraft (or ATC).

Proposed solutions to this were recommendations to add speed limiting points (SLPs) during the STAR phase (similar to European operations), which became more predictable for both flightcrews and ATC.

Not surprisingly, RTCA also found that the best efficiencies occurred when aircraft were routed via RNAV SIDs and STARs with existing speed constraints at intermediate fixes.

The continued introduction of more RNAV-specific arrivals and departures to our busiest airports is the result of these trials, along with the cost benefits associated with more efficient routing and indicating the way to a more automated method of controlling aircraft in the 4D environment. Airbus and Boeing are now offering new aircraft to be equipped with ADS-B In technology.

Business aviation OEMs are participating in trials with new equipment as well, and approval for domestic CPDLC is expected toward the end of 2022 for new and late-model airframes.

ADS and CPDLC are robust, integral components for aircraft, but, given the CVR requirements, will be relegated to Part 25 transport aircraft (at least for the time being). As digitization improves and the concept becomes more widely accepted, we may see CPDLC filter down to smaller Part 23 aircraft operating in the NAS under IFR.


Covid-19 showed the world how quickly a robust global air transport system can be reduced to a whisper, but the pandemic’s receding effects have also shown the system’s resilience and a still vigorous demand for travel.

The limited resources that are airports, runways, terminals, and gates can be better utilized through optimized spacing management.

Additional ancillary benefits will be reduced flight time, fuel burn, and airframe/engine cycles. Aviation has unfairly taken the brunt of accusations of being excess carbon emitters – claims that have been shown to be demonstrably false, or at least unethically exaggerated.

No credit has been given for the timely transport of goods and services – even the heroic efforts expended by all segments of the industry during the pandemic have been forgotten quickly.

Domestic IM and in-trail procedures for suitable aircraft will become commonplace, and we will all benefit, including – especially – the environment.

We live and work in an exciting time for technological advances in our industry – perhaps it was always so – and embracing this latest iteration of maximizing efficiencies will continue to show the world that aviation is crucial to our lives both now and in
the future.

DavidDavid Bjellos is a senior contributor to PP and has written for the magazine since 2004. A director of aviation for over 25 years, he is an active airman flying a G550, and is a past member of the Flight Safety Foundation Executive Committee for Business Aviation, Chief Pilots Roundtable, HAI Board of Directors.