Masters Theses

Date of Award

8-1996

Degree Type

Thesis

Degree Name

Master of Science

Major

Aviation Systems

Major Professor

Ralph D. Kimberlin

Abstract

The S-3 airplane’s Automatic Flight Control System (AFCS) was designed and developed during the early 1970’s. The AFCS was documented as unreliable during developmental flight tests and continues to be considered unreliable without any major improvements since the S-3A Viking was introduced into the Navy in 1974. The AFCS in the S- 3 aircraft is composed of numerous components with several deficiencies related to performance and reliability. The Flight Data Computer (FDC), the major component in AFCS, is the least reliable component and has been implicated in inciting several aircraft mishaps. As a result of one mishap investigation, all roll modes of Autopilot System (APS) were disconnected in all S-3 aircraft. Maintenance has also been identified as a problem. The original estimate for the FDC Mean Flight Hour Between Failure (MFHBF) was 500 hours, but the actual MFHBF has proven to be much less and requires an expensive support and logistics network. Current MFHBF for the FDC is now below 40 hours which has seriously impacted aircraft reliability and its ability to perform missions in support of the Navy Carrier Battle Group. This study reviews each of the major components and functionality within the AFCS of the S-3 aircraft. Although the S-3B and ES-3A are different models of the original S-3, for this study both aircraft are considered the same since their flight control systems and AFCS are almost identical. Several aspects of current AFCS status, failures, deficiencies, and trouble shooting are noted which indicate that the AFCS system has many performance, reliability, and maintenance problems. Current operational status and reliability of the S-3 AFCS has severely affected aircraft readiness prior to flight and has eliminated several APS modes for pilot relief and tactical mission maneuvering. Maintenance and troubleshooting efforts are long and tedious without an effective means to isolate AFCS related failures. The current AFCS Built-In-Test (BIT) is not being used by fleet maintenance personnel because it has minimal credibility and does not isolate failed AFCS components. Besides the FDC, many other AFCS components are considered undependable and contribute to overall system unreliability. There is currently a program to replace the Flight Data Computer (FDC) with a new form, fit, functional replacement called the Digital Flight Data Computer (DFDC). It has been designed to correct many known reliability issues and deficiencies of the current S-3 AFCS. If properly designed, the DFDC could become the most reliable component in the AFCS, be used to isolate other component failures in minimal time, and enhance S-3 AFCS flight qualities and mission performance. However, without improving other components of the AFCS, the enhancements given by the DFDC will not become apparent without a dramatic increase in reliability of each of the other AFCS problem components and the availability of spare AFCS parts in the supply system. Although the MFHBF of the DFDC is forecasted by the manufacturer as being greater than 2000 hours, conservative in-house estimates indicate an MFHBF of approximately 310 hours. This, combined with the fact that the original builder of the DFDC will also be the depot level repair facility, does not guarantee quality performance, reliability. and workmanship. The builder has an inherent interest in producing more units and repairing failed units. Without some kind of production reliability acceptance testing or contractual agreement for DFDC reliability, the Navy will travel down the same familiar path it has taken with the failed FDC.

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