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SYSTEM FIFTY FIVE X

What's New?

Mike Keirnan
Manager, Installations & Certification


The System Fifty Five X is an evolutionary step from the original System 55 introduced in 1994. As the System 55 approaches 1,000,000 hours of fleet total flight time, S-TEC has used that experience to design and implement enhancements that make this autopilot easier to use, more precise to fly, and compatible with the latest navigation developments available in general aviation.

The System Fifty Five X comes with the same features and functions as the original 55 but the enhancements make using the Fifty Five X a substantial improvement.

One of the most obvious changes incorporated in the Fifty Five X is a new black on silver LCD display.

Several improvements have been made to the NAV and Glideslope Couplers to allow intercept and track with additional authority. The new algorithm will be able to adjust to turbulence or wind conditions that have caused the aircraft to deviate slightly from the intended flight path in the past.

Another navigation enhancement is a change in the control software that automatically switches the NAV Coupler to the intercept sequence when the course pointer is changed more than 20 degrees. This enables the autopilot to make large course changes without additional commands to the autopilot. For example, if you are approaching a VOR in NAV mode for a non-precision approach and want to turn to the outbound course, all you need to do is move the course pointer to the new heading. This will have the same effect as pressing the NAV button again to reset the normal gain sequence to intercept and capture the new course.

The heading mode also received attention. Two levels of heading gain are now used when tracking the heading bug. While flying a heading, the autopilot will track using a higher gain than was used in the original System 55. When the bug is slewed to a new heading, the heading gain is briefly reduced for a smooth roll into the turn and to minimize overshoot of the newly selected heading. After reaching the new heading, gain will again be increased to reduce minor excursions while tracking the bug.

In the pitch section, the altitude hold circuit was enhanced to reduce altitude loss in turns. The accelerometer, which is the core of the pitch control section of all S-TEC autopilots, is now used to compensate for the lost lift vector in turning flight. When a turn rate is sensed by the roll circuit, the autopilot commands additional elevator input to increase the "g"appropriate for the turn rate sensed. The result is a 50% improvement in altitude excursions experienced during a standard rate heading bug turn. A Beech A-36 that might have lost 50 feet during a turn in cruise with a System 55 will deviate less than 25 feet with a Fifty Five X controlling the aircraft through the same maneuver.

The flight director algorithm has also been improved to utilize pitch acceleration to improve short term response in pitch. If you are trying to fly an approach in rough air with the flight director, the autopilot computer will provide more accurate guidance to reduce deviations from the desired altitude, vertical speed or glideslope reference.

One of the most obvious changes incorporated in the Fifty Five X is a new black on silver LCD display. The new display improves contrast and has improved backlighting. The S-TEC Altitude Selector/Alerter and Remote Annunciator (options for the Fifty Five X) have also been upgraded with the black on silver displays. An additional mode annunciation, GPSS, has been added to both the Fifty Five X display and the remote annunciator.

Now that I've brought it up, GPSS has been integrated into the circuitry of the Fifty Five X and is the biggest enhancement to the original System 55 design. The Fifty Five X is capable of receiving ARINC-429 serial roll steering information (see Approach Volume 5, Issue 4) and following the bank angle commands (converted to rate commands in the autopilot). Roll steering is a navigation source available from the more recent GPS navigators offered by Garmin and other GPS manufacturers. By pressing the NAV button a second time, the Fifty Five X will annunciate GPSS while using roll steering guidance to track multi-leg flightplans enroute and execute transitions and procedures available in the GPS navigator's database. Roll steering is a capability that was, until recently reserved for transport category and business jet aircraft with sophisticated flight management systems and autopilots that cost as much as a complete GA airplane. GPSS changes the way you will use your autopilot.