Accurate Hold Pattern Guidance for Fixed Hold Time

Maurice F. Hutton and Brenda D. Carpenter

Abstract:	The guidance function in a digital avionics computer determines the desired reference path and velocity to be flown. This function also computes the guidance errors by comparing the actual aircraft position and velocity estimated by the navigation system to the desired position and velocity. For hold pattern guidance, the desired reference path is a racetrack consisting of two straight line segments connected by turns at each end. A racetrack flown at constant true airspeed and turn rate will have parallel straight legs and semicircular turns when no wind is present. These two properties of the closed loop ground track become invalid in the presence of wind. The closed loop hold pattern must pass over a specified point on the ground (holding fix) with a specified direction of the straight line segment (inbound leg) which ends at the holding fix. The racetrack must be flown an integer number of times which meets the specified hold time. Meeting the hold time requirement while compensating for wind makes this particular guidance design difficult. The problem is further complicated by the need to account for the entry path which captures the racetrack guidance path after flying over the holding fix for the first time from a direction other than the specified inbound leg direction. This hold pattern guidance will be implemented on the Army Special Operations Aircraft (SOA) which include the MI-IdOK and MI-I-47E helicopters as part of a software upgrade to the digital avionics. The capability to make adjustments to the flight plan during the mission while still accurately managing time is an important SOA requirement. A SOA design requirement for hold pattern guidance is to meet the specified hold time within thirty seconds. This paper indicates how the hold pattern guidance algorithms are designed to meet the hold time accuracy requirement while satisfying all of the hold pattern constraints and compensating for wind. The design approach and assumptions used in the development of the SOA hold guidance algorithms arc described in this paper, including the method used to achieve the hold time. Closed loop simulation results based on the helicopter dynamics. pilot model. flight director control laws. and guidance algorithms are also presented. These results include a plot comparing the ‘actual helicopter ground track to the computed reference guidance path.
Published in:	Proceedings of the 53rd Annual Meeting of The Institute of Navigation (1997) June 30 - 2, 1997 Albuquerque, NM
Pages:	743 - 752
Cite this article:	Hutton, Maurice F., Carpenter, Brenda D., "Accurate Hold Pattern Guidance for Fixed Hold Time," Proceedings of the 53rd Annual Meeting of The Institute of Navigation (1997), Albuquerque, NM, June 1997, pp. 743-752.
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