A.C. In 't Veld

Static and dynamic angles of repose in loose granular materials under reduced gravity

Granular materials avalanche when a static angle of repose is exceeded and freeze at a dynamic angle of repose. Such avalanches occur subaerially on steep hillslopes and wind dunes and subaqueously at the lee side of deltas. Until now it has been assumed that the angles of repose are independent of gravitational acceleration. The objective of this work is to experimentally determine whether the angles of repose depend on gravity. In 33 parabolic flights in a well-controlled research aircraft we recorded avalanching granular materials in rotating drums at effective gravitational accelerations of 0.1, 0.38 and 1.0 times the terrestrial value. The granular materials varied in particle size and rounding and had air or water as interstitial fluid. Materials with angular grains had time-averaged angles of about 40° and with rounded grains about 25° for all effective gravitational accelerations, except the finest glass beads in air, which was explained by static electricity. For all materials, the static angle of repose increases about 5° with reduced gravity, whereas the dynamic angle decreases with about 10°. Consequently, the avalanche size increases with reduced gravity. The experimental results suggest that relatively low slopes of granular material on Mars may have formed by dry flows without a lubricating fluid. On asteroids even lower slopes are expected. The dependence on gravity of angle of repose may require reanalysis of models for many phenomena involving sediment, also at much lower slope angles.

Enhanced Self-Spacing Algorithm for Three-Degree Decelerating Approaches

A current trend in aircraft noise abatement around airports is exploiting the benefits of revised arrival and approach procedures with computational aids, such as onboard and ground-based trajectory prediction algorithms and displays. The challenge for these upcoming advanced noise abatement procedures is to mitigate the noise impact without sacrificing runway capacity. A proposed solution, implemented in the three-degree decelerating approach, is to delegate the task of spacing the aircraft to the cockpit during the approach. To assist the pilots, a flap scheduling algorithm with complementary interface has been developed that takes noise nuisance and in-trail spacing into account The design and functionality of this support system is presented and evaluated with three experiments. Monte Carlo simulations indicated adequate and consistent performance and robustness of the self-spacing algorithm for various wind and traffic scenarios. A pilot-in-the-loop simulator experiment verified that, with the aid of the algorithm, pilots were able to execute the noise abatement procedure consistently while maintaining safe spacing. The support system reduced pilot workload up to an effort level comparable to current standard approaches. The concept was demonstrated in flight, which confirmed the conflict-free performance benefits and the feasibility of self-spacing during continuous decelerating/descent approaches under actual flight conditions.

Three-Degree Decelerating Approaches in High-Density Arrival Streams

The positive effects of continuous descent approac h (CDA) on aircraft noise, fuel consumption and emissions are evident. Nevertheless, the negative effect on capacity restrains continuous descent approaches from being used in high density traffic. A solution to this problem is the transfer of the separation t ask from the air traffic controller to the pilot. This paper discusses the feasibility of impl ementing the Three-Degree Decelerating Approach (TDDA) at high density traffic airports in a distance- or time-based self-spacing environment. In other research, time-based separation was implemented in the TDDA to prevent transient motions in the arrival stream res ulting in possible loss of separation caused by trajectory predictions of the leading aircraft b ased on previous states. For the same reason aircraft intent-based trajectory prediction of the lead aircraft in a distance-based scenario was introduced in this research. A fast-ti me simulation tool was developed to simulate arrival streams of different aircraft type s and weights that execute the TDDA in both self-spacing scenarios under actual wind condi tions. The simulation tool was used to quantify the performance differences between distance- and time-based self-spacing in terms of capacity, noise reduction, and loss of separatio n. For the time-based scenario no effects of leading aircraft on trailing aircraft could be iden tified. However, an increase in separation with a negative effect on the airport capacity to a ssure safe separation was required. In the distance-based self-spacing scenario a slow-down ef fect was observed, leading to a decrease in the noise reduction towards the end of the arriv al stream. The deteriorating noise reduction was solved by altering the initial separa tion between aircraft in the arrival stream. A sensitivity study showed that the TDDA performance in a distance-based scenario is not degraded dramatically by realistic deviations from the desired situation.

Real-time Wind Profile Estimation using Airborne Sensors

Wind is one of the major contributors to uncertainty in continuous descent approach operations. Especially when aircraft that are flying low or idle thrust approaches are issued a required time of arrival over the runway threshold, as is foreseen in some of the future ATC scenarios, the on-board availability of both dependable and accurate wind estimates becomes a necessity. This paper presents a method for real-time estimation of a wind profile in the terminal maneuvering area, based on data transmissions of nearby aircraft that produces real-time wind profile estimates in a form that is usable for accurate trajectory estimation. The wind estimation algorithm is designed to process data that is of the form currently used in the Aircraft Meteorological Data Relay (AMDAR)-program. The algorithm combines a stochastic estimation based on this data and a traditional logarithmic estimator in order to be able to produce a valid estimation even when there are no data available from other aircraft.

Control Space Analysis of Three-Degree Decelerating Approaches at Amsterdam Airport Schiphol

Amsterdam Schiphol Airport currently uses a Continuous Descent Approach during night time operations only, due to reduced runway capacity caused by unpredictable individual aircraft behavior. The Three-Degree Decelerating Approach (TDDA) has been developed to increase predictability and runway capacity by switching the sepa- ration task from Air Traffic Control to the pilot on board the aircraft. The research described in this paper identifies the factors that influence the control space of aircraft performing a TDDA in a real-life setting. Control space is defined as the difference between the maximum and minimum duration to perform the TDDA. Using different control strategies, a fast approach or slow approach can be flown. A fast-time simulation tool was built to perform simulations with different aircraft types, initial weights, wind speeds and directions. Preliminary simulations indicate that a flap scheduler is needed to optimize control space, and the flap scheduling algorithm was enhanced to find optimal flap schedules for all wind conditions. The results of these simulations show that the influence of wind direction depends on aircraft aerodynamic characteristics, which mainly depend on the drag characteristics of the aircraft and aircraft weight. Furthermore, the results can be used to determine whether a TDDA can be executed using different aircraft and under different wind conditions.

Assisting Air Traffic Controllers in Planning and Monitoring Continuous-Descent Approaches

An interface was developed to assist controllers in metering, sequencing and merging aircraft flying a Continuous Descent Approach. This Time-Space Diagram displays predicted aircraft approach trajectories in a way that allows controllers a better insight into the separation between aircraft flying continuous descents. The paper discusses how a baseline Time-Space Diagram display can be enhanced with ‘whatif’ predictions, providing support for routine controller spacing techniques such as speed instructions and the use of direct-to commands. An experiment with eight professional air traffic controllers compared the Time-Space Diagram to a conventional plan view display in low and high traffic rate scenarios. Use of the Time-Space Diagram display yielded a significant reduction in controller workload. It also reduced the number of instructions to the pilots, suggesting a reduced workload for the flight deck as well. Controllers’ opinions on the usefulness of the ‘what-if’ predictions were mixed, and these additions did not lead to any significant performance improvements.

A Time-Space Diagram as Controller Support Tool for Closed Path Continuous Descent Operations

Tactical control during a closed-path Continuous Descent Operation stops the aircraft from following its optimized descent. To mitigate tactical control, air traffic controllers apply arbitrary large spacing buffers to account for the unpredictability of the aircraft trajectory from the controller’s point of view. A controller support tool is required for early de-confliction, spacing, and sequencing to facilitate these operations without the need to apply large buffers. The Time-Space Diagram controller support tool was developed to make the constraints and complexity of a Continuous Descent Operation perceptually evident and provide tools and information to the controller to be an active problem solver. This paper addresses the further development and validation of the interface. The concept of Visual Momentum was applied to enhance the efficiency of working the multi-display interface that consists of the Plan View Display and Time-Space Diagram. Direct Manipulation Interfaces were added to enable the controller to plan and implement actions, such as speed and altitude control. A controller-in-the-experiment was setup to validate the interface. In the experiment the subjects used either the Time-Space Diagram support tool or a stack list that provided the required spacing and time to lose or gain as a baseline. Both interfaces enabled the subjects to space the aircraft safely and efficiently. Compared to the baseline, the Time-Space Diagram interface freed time to plan traffic ahead using the Direct Manipulation Interfaces, which according to all subjects worked intuitively. The number of instructions per aircraft was decreased by 25%. Early accurate speed control was applied and use of heading vectors was no longer necessary in most scenarios. As a result aircraft commenced their continuous descent at a higher altitude and greater distance from the runway. The controller workload was significantly reduced and the level of Situational Awareness increased.