Flight management system prediction and execution of 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 predictor and its uncertainty models, descents from cruise to the meter fix were executed in a B737-700 simulator with a commercial FMS using vertical navigation. The FMS computed the intended descent path for a specified speed profile assuming idle thrust after top of descent (TOD), and then it controlled the avionics without human intervention. The test matrix varied aircraft weight, descent speed, and wind conditions. The first analysis in this paper determined the effect of the test matrix parameters on the FMS computation of TOD. Increasing weight by 10,000 lb moved TOD about 4.5 nmi farther from the meter fix, increasing along-track wind by 25 kt moved it about 4.6 nmi farther away, and varying the descent speed from 250 KCAS to 320 KCAS moved the TOD about 25 nmi. The execution of the descents was analyzed by comparing simulator state data to the specified speed profile and to the FMS predictions. The FMS generally flew its predicted three-dimensional trajectory accurately, with altitude error less than 200 ft. It engaged the throttle if the speed dropped 15 KCAS below the target speed but allowed the speed to increase arbitrarily above the target unless it reached a performance limit. In the runs with descent speed too slow but correct wind conditions, the FMS meter fix arrival time prediction error was as large as 37 sec. Along-track wind error of 25 kt resulted in a meter fix arrival time error of roughly 30 sec if the target descent speed was met. The data from this analysis are used to estimate accuracy requirements for the ground automation system.