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Optimal Top of Descent Analysis in Respect to Wind Prediction Errors and Guidance Strategies*

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Optimal Top of Descent Analysis in Respect to Wind Prediction Errors and Guidance Strategies* Trans. JSASS Aerospace Tech. Japan Vol. 15, No. APISAT-2016, pp. a45-a51, 2017 Optimal Top of Descent Analysis in Respect to Wind Prediction Errors and Guidance Strategies* By Adriana ANDREEVA-MORI and Tsuneharu UEMURA (5mm) Aeronautical Technology Directorate, Japan Aerospace Exploration Agency, Tokyo, Japan (Received January 31st, 2017) Modern airliners use the profiles calculated by the onboard flight management system (FMS) to execute safe and efficient descents. Since the wind often varies greatly between the cruising altitude and the end-of-descent altitude, the FMS uses both predicted and measured wind to determine the descent profile. Even so, the actual wind encountered along the descent changes the profile. When a constant descent speed is maintained at idling thrust, the aircraft deviates from its path and needs to either fly an additional steady level flight segment to the metering fix or deflect speed brakes to ensure the speed constraints at the metering fix are met. This research analyses the optimal top of descent in respect to such wind prediction error and fuel burn. Numerical simulations for the Boeing 767-300 are done and it is shown that an early descent of 0.5 nm would save, on average, 0.9 lb of fuel for an idling descent from 30,000 ft to 10,000 ft and at a constant speed of 280 kt, and decrease the number of cases where the necessary deceleration could not be achieved due to lack of enough lateral distance by 77%, thus improving safety and easing operations. Key Words: Top of Descent, Flight Management System, VNAV-SPEED, Wind Disturbances, Fuel Optimal 1. Introduction newly developed time and energy managed operations (TEMO) concept with a typical flight management system and Current flight management systems (FMSs), with which argue that TEMO’s performance in terms of arrival time most modern airliners are equipped, can compute a relatively control is superior to existing systems. optimal descent profile based on information available Here, the authors approach the problem from a different onboard the aircraft, such as aircraft weight and performance perspective. The goal of this research is to evaluate how wind model, and information available from the ground, such as prediction errors influence the pre-calculated descent wind predictions. However, despite all technological advances, flightpath using current FMS functions, especially the Top of the prediction errors cannot be fully nullified, so the wind that Descent (ToD) from the perspective of fuel burn. We attempt an aircraft encounters along its descent is always slightly to adjust the ToD to compensate for the inefficiency, thus different from the one predicted. As a result, even though the proposing a more robust descent path calculation. This FMS plans an idle-thrust descent, wind disturbances cause the research models real descent control operations based on the aircraft to either add more power, fly an additional level piloting experience of one of the authors. We consider only segment or deflect speed brakes in order to comply with the the vertical path disturbances, assuming that the aircraft flies a operational constraints along the way. This results in a less predefined lateral path. efficient descent than the one originally calculated by the FMS, that is some extra fuel is burnt to compensate for external 2. Typical Descent Path Calculation disturbances. The descent time from cruising altitude to metering fix also changes due to the difference between Modern aircraft are equipped with an FMS that provides predicted and actual wind. Most researchers consider such valuable guidance for safe and efficient flights. During the wind prediction errors to set accuracy limits and constraints on descent phase, the role of the FMS is to calculate an efficient descent paths calculated by the FMS. Therefore, much profile at idling or close to idling thrust, which complies with research is being done on eliminating or minimizing the all performance and operational constraints imposed by the effects of wind prediction errors by introducing new hardware airspace, for example. In other words, the FMS calculates the and software solutions that enable timely accurate continuous descent profiles based on airspeed and altitude restrictions, as descents. Lenz et al. 1) presented a concept involving an iPad well as on the end-of-descent (here, the metering fix) device that allows the aircraft to meet its predicted time of waypoint. Furthermore, the system requirement of future arrival with an accuracy of 10 sec. Dalmau et al. 2) compared FMSs is to enable the aircraft to fly a 4D trajectory accurately, where the aircraft should be able to compute and execute the © 2017 The Japan Society for Aeronautical and Space Sciences. *Presented at the 2016 Asia-Pacific International Symposium on most fuel-optimal trajectory with an error of less than 10 s Aerospace Technology (APISAT-2016), Oct. 25-27, 2016, Toyama, Japan from the ToD to the runway 3). Little information is publicly available regarding the control laws and logic used in the FMS. a45 Trans. JSASS Aerospace Tech. Japan Vol. 15, No. APISAT-2016 (2017) The research presented here assumes a standard FMS not updated once the descent is initiated. Therefore, we assume necessarily equipped with high-accuracy 4D functions. The that the wind prediction error at the ToD is zero since the main reason is that the authors wanted to analyze descents and current wind is accurately measured; however, further along develop robust ToD proposals applicable to present-day the descent path, a wind prediction error exists. Only the airliners, thus focusing on short-term rather than long-term vertical plane motion is considered here. The vertical motion guidance strategies. Additionally, considering the lifecycle of is controlled by the so-called VNAV-mode. The main modes an aircraft is usually longer than 20 years, it is safe to say that are VNAV-PATH (path-managed descent) and some of the aircraft in production now will still be flying in VNAV-SPEED (speed-managed descent). In a VNAV-PATH 2035,. Therefore air traffic management should be able to mode, the aircraft follows its calculated path at idling thrust accommodate them along with more modern aircraft. Here, and disturbances such as wind prediction errors are we assume that once the descent trajectory is planned, it is not compensated by speed adjustments. Thrust is only added when recalculated. Assuming idling thrust, the ideal descent profile the speed deviation allowed exceeds a certain margin. In a from cruising altitude to the metering fix consists of several VNAV-SPEED mode, on the other hand, the aircraft segments, as shown in Fig. 1. The aircraft starts its descent at maintains its target speed at idling thrust, but might deflect the ToD, following a constant Mach number profile until the from its calculated path due to external disturbances. This crossover altitude, where it switches to a constant calibrated research considers VNAV-SPEED guidance only, that is the airspeed descent (constant CAS descent). This descent target CAS is maintained regardless of the wind encountered continues until a certain altitude, in most airspaces around along the way. 10,000 ft, where speed constraints are enforced. In reality, the Details on the dynamics assumptions and constraints are aircraft decelerates gradually following a shallow glide slope shown in the next section. path. However, for simplicity and clarity of the simulation, we consider a level segment idle-thrust deceleration (shown in 3. Aircraft Flight Dynamics Assumptions and Profile purple). The end of this deceleration segment is the metering Constraints fix, which is the end of the numerical simulation considered in this research. Some researchers refer to this part of the descent The flight dynamics model used in most FMSs is a as a “performance path” 4). Later, the descent continues at a simplified point mass performance model 5). The same model lower CAS where the aircraft profile is determined based on in also used in EUROCONTROL’s BADA ver. 3.11 aircraft published procedures and specific airspace constraints. Further performance model 6), applied in the numerical simulations down, lift devices are deployed for the final drag developed during this research. In this section, we first configuration, allowing the aircraft to approach and land provide a brief description of the point mass model simplified safely. for movement in the vertical plane only then present the performance assumptions that need to be added in order to Top of Descent describe the control strategy simulated here. Next, we show Const. Mach Top of Descent No. Descent the constraints on trajectory and flight profile under which the Crossover Altitude simulations have been performed, and finally discuss the wind Const. CAS Const. CAS Descent Descent prediction error model used. Speed, Altitude Speed, Altitude Altitude Altitude Constraint（10,000 ft） Constraint（10,000 ft） 3.1. Aircraft dynamics model Idle Deceleration Idle Deceleration Level Segment Level Segment The numerical simulations presented here are, in general, based on the BADA ver. 3.11 model. We consider the Distance Distance movement of a Boeing 767-300 aircraft (assumed weight Fig. 1. Standard descent profile calculated by the FMS (left). For lower 240,000 lb), defined in body-centered coordinates (x, y, h)， cruising altitude, the crossover altitude exceeds the cruising altitude, so true airspeed VTAS, heading ψTAS, flight path angle in respect the constant CAS segment immediately follows the ToD (right). to the air mass γTAS, bank angle ϕTAS, thrust T, drag D, mass m and wind wx，wy andwz. This research considers only the Since this research considers only the part of the descent vertical movement of the aircraft, so the point mass equations from cruising altitude to the end of the deceleration segment at can be simplified as follows: ẋ = VTAS cos γTAS + wx （1） 10,000 ft (metering fix), the flight profile can be broken down ̇ into two segments: idle-thrust descent from ToD to 10,000 ft h = VTAS sin γTAS （2） and idle-thrust level flight deceleration at 230 ft.
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