Analysis of flight management system predictions 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 using vertical navigation in a B737-700 simulator and a B777-200 simulator, both with commercial FMSs. For both aircraft types, 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 location, and it compared the results to those for the current ground predictor in the Efficient Descent Advisor (EDA). The second analysis was similar but considered the time to fly a specified distance to the meter fix. The effects of the test matrix variables together with the accuracy requirements for the predictor will determine the allowable error for the predictor inputs. For the B737, the EDA prediction of meter fix crossing time agreed well with the FMS; but its prediction of TOD location probably was not sufficiently accurate to enable idle-thrust descents in congested airspace, even though the FMS and EDA gave similar shapes for TOD location as a function of the test matrix variables. For the B777, the FMS and EDA gave different shapes for the TOD location function, and the EDA prediction of the TOD location is not accurate enough to fully enable the concept. Furthermore, the differences between the FMS and EDA predictions of meter fix crossing time for the B777 indicated that at least one of them was not sufficiently accurate.

Flight management system execution of idle-thrust descents in operations

“…The other major source of this error is likely error in the descent wind forecast used by the FMS, which Bronsvoort et al [7] investigated. Stell [8] test bench simulator, which could increase prediction error. Further research is needed to see whether ground predictors might be able to reduce these errors.…”

Strategies for Choosing Descent Flight-Path Angles for Small Jets

“…26 The Global 5000 analysis results suggested that fixed-FPA decents were much more predictable than idle-thrust descents typically performed by large commercial jets, of which the ground predictions had top-of-descent errors around 5 to 10 nmi. 18,27 While a simple, fixed-FPA descent procedure using prescribed clearances was introduced for the purposes of the flight tests, EDA itself lacks a defined descent procedure and corresponding algorithm for small jets. This hampers EDA's capability to deliver small jet arrivals to the metering fix on time and impacts EDA's ability to maintain separation involving the arrival aircraft.…”

New altitude optimisation algorithm for the flight management system CMA-9000 improvement on the A310 and L-1011 aircraft

“…Reynolds, Ren and Clarke (6) concluded after different tests in the Nottingham East Midlands Airport that CDA effectively reduced fuel burn and noise near the terminals simply by keeping the aircraft at the higher altitude possible before creating the descent. Stell (7) used an efficient descent advisor, which is a method to predict the latest descent point (equivalent to the TOD) in order to apply the 3D PAM technique, but it still needs an improved ATC in order to operate at its maximal efficiency.…”