Left engine command, lb
Tail engine command, lb
Airspeed error
Right engine command, lb
Pitch attitude rate
Pitch attitude
Airspeed error
Flightpath angle
960318a
(a) Center-engine airspeed control mode.

Left engine command, lb
Tail engine command, lb
Airspeed error
Right engine command, lb
Pitch attitude rate
Pitch attitude
Airspeed error
Flightpath angle
960318b
(b) Center-engine dynamic control.
Logic is provided to prevent the wing engines from being driven to very low power settings where the engine response is excessively slow. Integrator windup protection is also provided. A priority logic attempts to minimize saturation of pitch or roll commands. In both axes, the thrust command is converted in the PCA logic into an EPR command that is the normal engine control parameter. This strategy provides more accurate thrust control than a throttle command and allows very small thrust commands to be accurately achieved.
The go-around portion of the standard (MD-11) TOGA mode was implemented. If the TOGA button on the center throttle was pushed, the PCA system leveled the wings and commanded a 3° climb. Coupled approach modes were not implemented initially; their subsequent addition is discussed later in this paper.
The control modes were selectable by a .ight test engineer using the MCDUs. Variable gains, .lters, multipliers, and gain schedules were available at most points within the PCA software, providing much .exibility for testing. References 12 and 13 discussed more detailed design and analysis of the PCA control laws.

PCA IMPLEMENTATION
Reference 14 discussed the PCA system implementation in detail. PCA logic resides in one of the two .ight control computers (FCCs). The FCC also provides a host of other functions including autopilot, autothrottle, navigation, stall warning, and .ight management system. Honeywell developed the code for the PCA control laws and interfaced them to existing sensor signals and software that sent EPR commands to the engine FADEC computers over the existing ARINC 429 data bus. This electronic thrust command did not move the throttle levers in the cockpit. Pratt & Whitney modi.ed the FADECs to accept a full-authority EPR command in place of the normal ± 5 percent EPR commands. For this initial .ight test, a PCA disengage capability was also incorporated in case the throttles were moved more than a preselected amount (default was ± 2° ), thus instantly giving the pilots normal throttle control if needed. The .rst FCC contained the PCA software; the second FCC software was unmodi.ed.
In some instances, to reduce the cost of the .ight test, features were not incorporated that would be desirable in a production system. For example, there was no way to display a PCA-engaged annunciation without relatively expensive changes to a symbol-generating computer. In the bank-angle mode, hardware changes would have been necessary to display negative bank angles for left turns. For a concept demonstration .ight test, these features were not thought to be necessary.
For this .ight test, the standard MD-11 altitude capture and hold feature was not needed and was not implemented. Coupled approach modes were not initially implemented in the PCA logic; their eventual implementation is discussed later.
For the PCA tests, the approach idle thrust feature was implemented for any .ap or slat setting except fully retracted. In this way, for testing with the slats or .aps up, the .ight idle thrust setting would be available; approach idle would be too much thrust for a no-.aps approach.

PCA SYSTEM OPERATION
The PCA system was enabled by selecting FCC 1 (which contained the PCA software) and disabling FCC 2 by lowering the AFS 2 override paddle switch. The PCA system was engaged by pushing the Auto.ight button on the FCP (.g. 12). PCA system engagement was indicated by the AP 1 (autopilot 1) indication in the PFD roll window and the empty box on the PFD speed window (normally, a speed command indication was there). Longitudinal control through the thumbwheel commanded FPA. Lateral control was effected through the HDG/TRK knob. In the track mode, turning the knob preselected the track, and pulling the knob sent the command to the PCA software. In the bank-angle mode, the knob responded as soon as it was turned.
　
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