Probabilistic Alerting Threshold
Development for Parallel Approach Operations
Research Overview
1. A probabilistic model of parallel approach scenarios has been developed, incorporating parameters such as blunder characteristics, pilot response delays, and type and accuracy of state information available to the alerting system. A typical parallel approach scenario is diagrammed in the figure below:
(click on picture for full size)
2. This model has been used to develop an alerting logic.
3. Evaluations of the alerting thresholds have been carried out using simulator trajectories.
Significance
Independent approaches to parallel runways increase the number of operations per hour that an airport can support. Ground-based monitoring systems allow independent parallel approaches to runways spaced less than 5,000 feet apart. These systems are limited by sensor accuracy and human response delays. The current minimum runway separation for which the FAA will currently allow independent parallel approaches is 3,400 ft. An airborne alerting system will reduce the overall time required for an avoidance maneuver to be initiated, thus allowing independent approaches at reduced runway spacings. Current goals hope to reduce this runway spacing to 2,500 ft or less.
Approach
A numerical model has been developed to perform Monte Carlo simulations of parallel approaches. Monte Carlo simulations attempt to estimate the probability that an event will occur given that there are/will be uncertainties in current and/or future state measurements. For the current purposes, there are uncertainties only in the future trajectory of the intruding aircraft.
Pilot response delays and avoidance maneuver trajectories have been assigned deterministic values within the model. Reasonable errors in intruder position, climb rate, heading, and bank angle measurements were intoduced to a set of nominal initial intruder states at the beginning of each simulation. Monte Carlo simulations were performed to determine the probability of a collision given this set of nominal states.
These same calculations were carried out for several sets of nominal states. Points in the state-space where the probability of collision was above some value comprise the alerting thresholds. Some simplifications were made to the calculation of these thresholds, as well as the storage of this information for retrieval during testing.
Accomplishments
A set of alerting thresholds for an airborne parallel approach monitoring system has been defined. Based on intruder heading, bank angle and airspeed, a range limit is obtained from a lookup table. The alerting logic determines whether the intruder is within that range. If so, the logic calculates a longitudinal point based on the lateral distance between the own aircraft and the intruder, as well as other state measurements. These lateral-longitudinal points will fall on a curve similar to the dark curve indicated in the figure. An alert is issued if the intruder is less that 800 feet longitudinally from the calculated point.
(click on picture for full size)
The logic has been tested against pilot-flown simulator trajectories of both blunders and normal approaches, with good results. Missed detections are absent and the number of false alarms is low (around five percent over all blunder trajectories with no false alarms occurring during normal approaches).
Future
The alerting logic is currently based on a series of lookup tables for heading, bank angle and airspeed. Preliminary work to incorporate intruder altitude and climb rate into the logic has shown some problems using an algorithm similar to the current logic, especially when only one avoidance maneuver may be issued. Work is needed to determine an algorithm to issue alerts which take intruder's relative altitude and climb rate into account.
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