Predictability of Top of Descent Location for Operational Idle-Thrust Descents

Abstract

To enable arriving aircraft to fly optimized descents computed by the flight management system (FMS) in congested airspace, ground automation must accurately predict descent trajectories. To support development of the trajectory predictor and its uncertainty models, commercial flights executed idle-thrust descents at a specified descent speed, and the recorded data included the specified descent speed profile, aircraft weight, and the winds entered into the FMS as well as the radar data. The FMS computed the intended descent path assuming idle thrust after top of descent (TOD), and the controllers and pilots then endeavored to allow the FMS to fly the descent to the meter fix with minimal human intervention. The horizontal flight path, cruise and meter fix altitudes, and actual TOD location were extracted from the radar data. Using approximately 70 descents each in Boeing 757 and Airbus 319/320 aircraft, multiple regression estimated TOD location as a linear function of the available predictive factors. The cruise and meter fix altitudes, descent speed, and wind clearly improve goodness of fit. The aircraft weight improves fit for the Airbus descents but not for the B757. Except for a few statistical outliers, the residuals have absolute value less than 5 nmi. Thus, these predictive factors adequately explain the TOD location, which indicates the data do not include excessive noise.

Analysis of flight management system predictions of idle-thrust descents

“…The polynomial approximation approach was very similar to a previous paper by the same author [9]. Analysis of actual TOD locations extracted from operational data [10] indicated that the TOD path distance can be approximated well by a linear function in cruise and descent altitudes, descent CAS, wind, and aircraft weight. This prompted a second look at the FMS predictions of TOD location, focusing this time on models linear in descent speed and aircraft weight.…”

“…Stell [10] analyzed operational data for the Airbus 320 family and the B757. To keep absolute error in TOD location less than 5 nmi, the ground predictor must use accurate values of descent speed and weight for all four of these aircraft types, although nominal weight might be acceptable for the B757.…”

Robust Conflict Detection and Resolution around Top of Descent

“…As an example, there can be an error on the order of a few nautical miles between the Flight Management System's (FMS) predicted and actual TOD point. 15 Additionally, depending on the airspace class, aircraft in level flight can pass over another level aircraft 1,000 feet below and be perfectly legal and safe. It is only when aircraft are transitioning altitudes that detections in the vertical plane become a concern.…”

Flight management system execution of idle-thrust descents in operations

“…This paper shows results of analyzing over 50 of these descents, which is a superset of the 11 analyzed in Coppenbarger, Mead, and Sweet [1]. This set of descents has two advantages over the operational data from Denver analyzed by Stell [2]:…”