NextGen Surface Trajectory-Based Operations: Contingency-hold clearances

Abstract

The purpose of this pilot-in-the-loop taxi simulation was to investigate a NextGen Surface Trajectory-Based Operations (STBO) concept called "contingency holds." The contingency-hold concept parses a taxi route into segments, allowing an air traffic control (ATC) surface traffic management (STM) system to hold an aircraft when necessary for safety. Under nominal conditions, if the intersection or active runway crossing is clear, the hold is removed, allowing the aircraft to continue taxiing without slowing, thus improving taxi efficiency, while minimizing the excessive brake use, fuel burn, and emissions associated with stop-and-go taxi. However, when a potential traffic conflict exists, the hold remains in place as a fail-safe mechanism.In this departure operations simulation, the taxi clearance included a required time of arrival (RTA) to a specified intersection. The flight deck was equipped with speed-guidance avionics to aid the pilot in safely meeting the RTA. On two trials, the contingency hold was not released, and pilots were required to stop. On two trials the contingency hold was released 15 sec prior to the RTA, and on two trials the contingency hold was released 30 sec prior to the RTA. When the hold remained in place, all pilots complied with the hold. Results also showed that when the hold was released at 15-sec or 30-sec prior to the RTA, the 30-sec release allowed pilots to maintain nominal taxi speed, thus supporting continuous traffic flow; whereas, the 15-sec release did not. The contingency-hold concept, with at least a 30-sec release, allows pilots to improve taxiing efficiency by reducing braking, slowing, and stopping, but still maintains safety in that no pilots "busted" the clearance holds. Overall, the evidence suggests that the contingency-hold concept is a viable concept for optimizing efficiency while maintaining safety. Surface Trajectory-Based OperationsThe next generation (NextGen) of the National Airspace System aims to improve the throughput and efficiency of aircraft operations [1]. Realizing these goals will require increased timing precision and reduced aircraft spacing in all phases of flight, including surface operations. In one far-term vision of the Surface Trajectory-Based Operations (STBO) concept, an air traffic control (ATC) surface traffic management (STM) system will issue conflict-free taxi clearances that contain a required time of arrival (RTA) to points on the surface, such as the departure runway, an active runway crossing, or a traffic-merge intersection [2]. Aircraft will be required to reach these specified locations on the airport surface with relatively precise timing.Following these time-based taxi clearances issued by an ATC/STM system will allow aircraft to: 1) arrive at the departure runway in time for immediate take-off; and, 2) arrive at active runways in time to cross the runway (after receiving crossing clearance) during planned or expected landing/takeoff "gaps." These departure STBO capabilities also support future NextGen en route and arrival ti...