Controller Initiation and Monitoring of a Relative Spacing Task during Closely Spaced Parallel Runway Operations: Interval Management Paired Approach Human In The Loop Experiment

March 2019
Topics: Air Traffic Management, Avionics, Human Factors Engineering
William J. Penhallegon, The MITRE Corporation
Hans Stassen, The MITRE Corporation
Kara J. MacWilliams, The MITRE Corporation
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The Federal Aviation Administration’s (FAA) Next Generation Air Transportation System leverages new technology and concepts to improve efficiency and enhance safety in the National Airspace System. One such concept is Interval Management (IM) which refers to an application suite, enabled by Automatic Dependent Surveillance – Broadcast (ADS-B), providing greater air traffic system throughput and efficiency by improving inter-aircraft spacing precision. IM avionics (termed flight deck-based IM [FIM] Equipment) provide speed commands to the flight crew enabling them to manage a desired spacing relative to another aircraft. IM is a tactical capability used to manage spacing based on a separation standard, miles-in-trail restriction, or any air traffic control spacing objective. The closed-loop nature of IM allows for more accurate speed adjustments to achieve the desired spacing for a given flight segment than can be provided by a ground system.

IM Paired Approach (PA) is intended for dependent approaches to Closely-Spaced Parallel Runways (CSPRs) (i.e., separated by less than 2,500 feet). This application provides a means for CSPR approach and landing operations to continue in weather conditions where visual separation cannot be maintained. IM PA maximizes arrival throughput in Instrument Meteorological Conditions (IMC) by minimizing the in-trail spacing between successive arrivals without compromising safety.

An IM PA aircraft pair consists of a Lead Aircraft, equipped with ADS-B Out avionics, and a Trail Aircraft equipped with ADS-B In FIM Equipment on the CSPR. A Collision Safety Limit (CSL) is defined between the two aircraft such that the risk of collision is less than 10-9 in the case that either aircraft deviates laterally from its approach path into the adjacent path. A Wake Safety Limit (WSL) defines the risk the Trail Aircraft encounters the wake vortices from the Lead Aircraft at a rate no worse than current operations. All IM PA operations require a CSL; however, the need for a WSL is based on aircraft weight category and crosswind conditions. The FIM Equipment provides speeds to the Trail Aircraft flight crew to manage spacing relative to the Lead Aircraft and keep the Trail Aircraft inside the safety limits for the entire approach to its runway threshold. A new separation standard is envisioned for IM PA operations and it will likely be based on allowable safety limit values as implemented at each airport.

The FAA, National Aeronautics and Space Administration (NASA), The MITRE Corporation’s Center for Advanced Aviation System Development (CAASD), and others have been conducting research on various aspects of PA for over two decades. In 2016, RTCA Special Committee 186 (SC-186) Working Group 4 decided to make PA an IM application and include it in the updated FIM Equipment requirements. Through this process, stakeholders determined controllers must be able to monitor the operation relative to a separation standard. This requires controllers to be able to assess separation within an IM PA pair and take effective action before separation is lost. Therefore, the IM PA concept was changed to move the safety limit monitoring function from being a flight crew responsibility and FIM Equipment capability to a ground capability, likely to be hosted in the Standard Terminal Automation Replacement System.

The FAA’s Surveillance Broadcast Services (SBS) Program Office tasked MITRE CAASD to develop and execute a Human-In-The-Loop (HITL) simulation experiment to validate IM PA concept changes, address open controller acceptability and feasibility questions, provide input to air and ground system requirements, and mitigate technical risks associated with the revised PA concept. The primary goal of this HITL simulation was to examine new controller monitoring functions. A secondary goal was to examine acceptability and information requirements for IM PA initiation.

Addressing the primary goal included an evaluation of the terminal controller monitoring task for IM PA aircraft pairs established on final approach, especially with respect to both minimum and maximum separation values that change over the course of the approach, and for lateral deviations. Related to this goal was the acceptability of a single Monitor controller to manage Lead and Trail Aircraft, or whether separate monitors should be required. The experiment also included an evaluation of prototype display features to facilitate this monitoring task. These features were developed by MITRE CAASD for the FAA’s Closely Spaced Parallel Operations Program Office in coordination with SBS and the FAA’s Operational Concepts, Validation, and Requirements office in 2017.

Data collection occurred in April 2018 and twelve currently certified Air Traffic Controllers participated in the simulation. Four controllers were from the Northern California Terminal Radar Approach Control facility and had experience in the areas being simulated. Eight controllers were from other terminal facilities with parallel runway approach operations. The controller participants managed scenario traffic that included IM PA operations in both Final Approach and Final Monitor positions. Two pseudopilots controlled and responded as all aircraft. Two confederate controllers served as Local controllers.

After initiating and monitoring IM PA operations with varying supporting IM PA (display) Tool configurations, monitor configurations, and off-nominal deviations, results suggest given the appropriate tools and training, the IM PA longitudinal and lateral separation monitoring task should be feasible and acceptable to controllers. IM PA pair-wise separation assurance appeared to be a straightforward task for controllers as only a single violation was observed within an IM Trail Aircraft and IM Lead Aircraft pairing, despite the introduction of far more off-nominal deviations than would be expected in actual operations. This instance involved a controller appearing to attempt to manually manage an IM PA Trail Aircraft’s speed. It was not a result of the controller failing to notice a developing situation.

Separation was also examined between other aircraft combinations (e.g. between the trail aircraft of a leading pair and the lead aircraft of a following pair). Twelve total instances of separation violations were observed between aircraft pairs not performing IM PA. It was unclear if this was due to the presence of IM PA in the operational environment or was related to controllers in the simulation not having current separation tools such as Automated Terminal Proximity Alert available to them. Although these violations occurred in the context of simulation events that were designed to stress test the concept and ground tools, their presence still suggests that tools and procedures need to be fully integrated to ensure that separation between aircraft pairs not performing IM PA can be maintained while IM PA operations are in progress. In addition, IM PA setup spacing requirements should ensure that between-pair separation can be sufficiently maintained during compression on final.

Controller participants on average reported: low and acceptable workload; acceptable tasks in each of the monitoring positions; acceptable levels of traffic awareness for all types of aircraft; comfort in allowing the IM Trail Aircraft to manage their own speeds; and confidence they could assess whether the separation between the IM Trail Aircraft and Lead Aircraft would be maintained. They also agreed IM PA is operationally desirable and compatible with terminal approach operations, though real-world facility and airspace integration may be challenging. Controller participants appeared to be comfortable monitoring IM PA when both safety limits (CSL and WSL) that changed over the course of an approach were active at the same time.

Due to the potential logistical and operational challenges certain facilities may have employing two Local controllers, the HITL experiment examined whether a single, combined Monitor controller can effectively provide separation for two CSPR final approaches involving IM PA operations. Results suggest under nominal conditions, a single, Combined Monitor controller is likely to be able to effectively and acceptably provide separation within and between IM PA pairs. However, further study is recommended to examine whether separate monitors may ultimately be required for safety to manage extreme off-nominal lateral deviation situations.

The simulation also produced several findings and recommendations with respect to the IM PA Tool display elements. Graphic depiction of the safety limits were preferred to numeric distances presented in the Trail Aircraft data block. As the WSL only became active late in the approach, controllers found a preview feature helpful in indicating whether an aircraft would eventually require it. Alerts were introduced to direct controller attention to aircraft that were starting to encroach on either limit and controllers found these useful.

For the lateral monitoring task, a 4:1 aspect ratio Final Monitor Aid display was found to significantly help controller lateral deviation assessment; however, questionnaire responses did not appear to suggest it should be a minimum requirement. A Warning was introduced to inform controllers when a lateral deviation occurred and IM PA needed to be terminated. Controllers found it useful and offered suggestions to increase its saliency.

Most controllers reported a Final Monitor controller is the most appropriate position to monitor IM PA operations, though this may ultimately depend on the facility. They also expressed a strong preference to provide speeds manually to the IM Trail Aircraft when they observed a spacing situation start to develop. However, the practicality of this may depend on whether a Final Approach controller or dedicated Monitor controller is providing separation within and between IM PA pairs. Controller manual PA speed management must be balanced with the potential for undesirable frequency overrides and workload, which may make it highly challenging to implement.

With respect to the experiment’s secondary goal to evaluate the IM PA initiation task, controller participants on average agreed Final controllers: can acceptably initiate the IM PA operation; can acceptably ensure separation during IM PA operations before transferring aircraft to the Local controller given the appropriate training and tools; and were comfortable with the IM Trail Aircraft managing speed to achieve the spacing goal. However, concerns were raised regarding the phraseology, available CSL and WSL information at the time of initiation, and the available time and airspace to perform the initiation task as simulated.

Overall results suggest given the appropriate tools and training, the IM PA longitudinal and lateral separation monitoring and initiation tasks should be acceptable to controllers. In a related IM PA flight deck features evaluation performed in 2018, pilots also found IM PA to be generally acceptable, even after experiencing significant lateral deviations by the Lead Aircraft. The current IM PA concept appears feasible and MITRE CAASD recommends the FAA continue its development.

This study provided initial findings on the initiation and monitoring of an IM PA operation. The results are intended to provide concept validation for the avionics standard development activities and A-IM Concept of Operations, as well as specific recommendations for the FAA IM Initial Program Requirements document with respect to controller information needs and supporting ground tools. These results and recommendations should be considered by RTCA SC-186, FAA SBS Program Office, FAA Aircraft Certification, and FAA Flight Standards as IM PA proceeds through concept maturation.

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