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This might have been obvious at the time, but I can't find any articles that discuss the FAA's official standpoint on requiring all GA planes to equip. There was and still is a hefty cost to ADSB compliance, which AFAIK means the FAA would have to justify in a NPRM why the mandate produced more value than it cost.

I'm curious what that rationale was.

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The FAA puts the rationale for rule changes in the Federal Register as part of the Notice of Proposed Rulemaking (NPRM). The NPRM for ADS-B Out was published on 10/05/2007. Link: Here

It's 26 pages long. I'll include some of the key points here:

From the FAA's NEXGEN vision:

The third element—performance-based surveillance—relies on technology that permits knowing the exact location of other aircraft in the air and of other aircraft and ground vehicles on the airport surface. The aviation community's experience with ADS-B, which periodically broadcasts an aircraft's location—both horizontal and vertical position and horizontal and vertical velocity—will lead directly to the performance requirements. When displayed in the cockpit, information obtained through ADS-B greatly improves situational awareness in the en route segment, in the terminal area during approaches, and on the airport surface. For additional information on ADS-B activities, see Section VI, FAA Experience with ADS-B later in the preamble.

This rulemaking is important because ADS-B is an essential NextGen building block. Improving surveillance requires advanced onboard equipment with backup capability. Most, if not all, of the surveillance capability as well as the navigation and communications capabilities should be onboard the aircraft so the required capabilities will go wherever the aircraft goes. As part of the rulemaking effort, the FAA established an Aviation Rulemaking Committee under Order 1110.147. This committee has been chartered to deliver a report on how to optimize operational benefits of the ADS-B system and to provide recommendations to the FAA on the rulemaking after the NPRM is published. The scope of the ARC membership is designed to provide the widest range of inputs into the development of the NextGen strategy. The FAA will put the ARC recommendations in the docket established for this rulemaking.

It is this combination of onboard capability and performance expectations that will enable aircraft in the future to fly safely and efficiently despite ever-increasing demands on the airspace.

About the limitations of radar:

To accommodate the projected level of traffic without increasing delay, more comprehensive surveillance in the NAS, including more radar sites in certain areas, would be necessary. Even if more radar sites were commissioned, however, there are many areas in which radar coverage is not feasible, either geographically (e.g., mountainous areas) or in a cost-effective manner (e.g., remote areas). Furthermore, simply increasing the number of radars in the NAS does not solve the inherent limitation of radar technology, and would not allow the FAA to reduce current separation standards.[5]

Consequently, the future of air traffic surveillance cannot be based solely on the use of radar. Radar technology also lacks the capability to provide services on the flight deck. However, the FAA is planning to maintain its current network of primary radars, and expects to be able to reduce a percentage of its secondary radars. This NPRM does not propose to reduce primary radar sites.[6]

Instead, this NPRM would transfer future aircraft surveillance to newer and more advanced onboard avionics that provide more accurate and timely aircraft information. ADS-B has been identified as the technology to facilitate that goal.

ADS-B System:

Implementation of an ADS-B system would not completely replace the primary radar or SSR at this time. In addition, ADS-B does not replace the requirement for transponders. Transponders are still necessary for SSR, which is the FAA's backup strategy in case of ADS-B failure. For more information on the backup strategy, see section IV.C.4, Backup Surveillance Strategy.

The performance requirements for ADS-B avionics proposed in this NPRM would ensure that the aircraft is broadcasting the requisite information with the degree of accuracy and integrity necessary for ATC to use that information for surveillance.[7]

This enhanced surveillance would provide ATC with the enhanced ability to surveil and separate aircraft so that efficiency and capacity could increase beyond current levels to meet the predicted demand for ATC services while continually maintaining safety. Incremental developments in capacity, efficiency, and air traffic control procedures based on radar technology cannot accommodate the anticipated increase in demand for surveillance and separation services, which could result in delays that would far exceed those experienced today. Without ADS-B, the increase in demand could result in increased congestion and the denial of ATC service to some users of the NAS.

ADS-B technology already has been demonstrated successfully in Alaska via the Capstone program.[8]

In Alaska, radar coverage is either very limited or non-existent. ADS-B provides a level of surveillance performance that previously did not exist and has resulted in increases in both efficiency and capacity.

“ADS-B Out” refers to an appropriately equipped aircraft's broadcasting of various aircraft information. “ADS-B In” refers to an appropriately equipped aircraft's ability to receive another aircraft's ADS-B Out information. This proposal only seeks to require ADS-B Out; the FAA is not proposing to require ADS-B In at this time.[9]

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  • $\begingroup$ Very nice. As a way to enhance my searching skills, could you let me know how'd managed to find that so quickly? $\endgroup$ Jul 29, 2023 at 3:37
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    $\begingroup$ It would be good to summarize the answer here quoting from the official source, otherwise this answer is your answer is in another castle $\endgroup$
    – Jamiec
    Jul 29, 2023 at 9:14
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    $\begingroup$ @Jamiec Point made. Done. $\endgroup$
    – Gerry
    Jul 29, 2023 at 13:36
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    $\begingroup$ Regarding the search. The first point is understanding the rulemaking process. That came from working in avionics for 25 years. That process normally starts with a Notice of Proposed Rule Making (NPRM) being published in the Federal Register, which is an official government publication. In it, they propose the rule, why it's needed, and solicit input. At a later point they publish a Final Rule including a discussion of the comments received on the NPRM and changes made. So I just Googled "ADS-B Notice of Proposed Rule Making". I then looked thru the Federal Register links for the first one. $\endgroup$
    – Gerry
    Jul 29, 2023 at 13:48

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