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Appendix B Example of a Modern Win~l-Shear Penetration System This appendix describes the basic elements and operation provided by the flight control computer (autopilot ~ (FCC ~ and thrus t management computer (autothrottle) (TMC) of an existing modern jet transport. Control in wind shears has been an important consideration during the design and development of the FCC and TMC automatic functions, particularly during takeof f, approach and landing, and go-around. The FCC and TMC use air data from the air data computer (ADC) and inertial data from the inertial reference unit (IRU) to estimate three- component wind velocities. Once the wind components are isolated, they are processed to remove noise and turbulence. Knowledge of the longitudinal ~ fore and at t) component of wind is used to improve airspeed control through throttle and elevator commands and as a predictive term to enhance lateral automatic landing (autoland) and rollout performance. The vertical component is used to enhance the angle-of-attack referenced minimum-speed control through either elevator or throttle commands and to enhance path control to shears. Control system modes related to the terminal area are subdivided into those applying to approach and landing, go-around, and takeof f . Approach and Landing To certify for automatic landing, the TMC must demonstrate airspeed hold within 5 knots of target during the approach, for all environmental conditions, including wind shear encounters, and not violate VREF (1.3 Vat ); further, it must provide a TMC retard function during the flare. Thus, the target speed set ire the mode control panel (MCP) is VREF + ~ knots. Some of the features employed to achieve these ob ject Ives are as follows: o Tight airspeed control using true airspeed and inertial acceleration along the f light path as the primary feedback variables . The second derivative of wind speed between wind shears and turbulence. 93 is used to discriminate
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Aft throttle limiting to a position above minimum thrust is invoked to prevent the engine from retarding to the low power settings from which engine acceleration is very poor. This is very significant for preventing large airspeed losses in a wind-shear situation involving an increasing headwind followed by a decrees ing one . Minimu~speed protection, very similar to that used for the takeof f flight director, is used with an angle-of-attack target set for VREF. o Power-lever angle feedback is used to speed the TMC motor through regions of mechanical deadband to minimize the reaction t ime of the engine to a shear. Glideslope and flare control in wind shears are greatly enhanced with predictive terms driven from filtered vertical and longitudinal wind-speed estimates. A vertical wind will eventually result in an equal inert ial vertical speed if airspeed is constant and no pitch correction is made. If pitch control responds only to inertial vertical speed and position, control will lag the disturbance. The predictive term provides an attitude command change proportional to the low-frequency vertical wind change to counteract the vertical acceleration change as it occurs and thereby minimizes the subsequent change in vertical speed. This term ef fectively provides for weathervaning into the vertical wind. The predictive term for longitudinal winds operates s imilarly to that for the vertical wind. Airspeed change, with the high-frequency wind component removed, commands pitch-attitude change such that the aerodynamic lift is held roughly constant despite a loss or gain in airspeed. Go-Aro and When a pilot selects automatic go-around, the throttles advance to maintain speed and capture a predetermined (e.g., 2000 feet/minute) climb rate. The thrust required is deduced from inertial vertical speed and inertial acceleration along the flight path (energy rate) plus the airspeed error and is therefore responsive to wind shear conditions. The FCC controls airspeed with elevator much the same as the takeoff flight director, except that the initial rotation (until 100-feet altitude and sufficiently positive vertical speed are attained) is performed using inertial data only. This inertial submode ensures that the initial rotation is performed even though air data signals may have failed. The initial rotation is controlled by the profile of the vertical speed command compared with the inertial speed plus change in ground speed and inertial accelerat ion along the flight path, both of which cause positive pitch rates when they increase in reaction to the advancing throttles. 94
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Upon attaining 100 feet of altitude above ground level and suf ficiently positive vertical speed, the ground-speed error is replaced by an airspeed error s ignal ~ from a suitably selected target speed) and minspeed protection is enabled. The minspeed reference is 1.2 Vs until flap retraction from the go-around setting, whereupon it transitions to 1.3 Vs. The commanded speed, when not limited by minspeed, is adjusted to distribute energy 60 percent to vertical and 40 percent to acceleration along the flight path for speed increases and 100 percent to vertical speed when there is insufficient energy to maintain level flight. Takeof f When the TMC is engaged during takeoff, the power is advanced to maximum or to a pilot-selected aerated value. At 80 knots the thro ttles are fixed to prevent a TMC servo failure from caus ing a thrust reduction. FCC outputs during takeoff are fed only to the flight director. The roll flight director at liftoff controls the track occurring at that time, to minimize obstacle clearance problems if a wind shear or engine failure should occur. The pitch flight director is fundamentally speed-through-the-elevator control with special processing for takeoff, which allows the flight director to accommodate any pilot rotation rate. The airplane acceleration is restricted so that 60 percent of the available excess energy goes into climbing. If insufficient energy is available to maintain level flight at constant speed, as in a severe shear condition, a deceleration, through increased angle of attack, is commanded so as to trade kinetic energy for potential energy to maintain level flight. The energy status is deduced from the potential flight path or energy-rate function, the combination of vertical speed and inertial acceleration along the flight path--there is no reliance on engine data. The control law uses filtered airspeed, inertial acceleration along the flight path, and vertical speed as active feedbacks, plus, when under minspeed control, angle of attack derived from a vane with the high-frequency vertical wind component filtered. Additionally, minspeed control employs inertial data and control/ configuration information to remove angle-of-attack variations not associated with speed changes. The minspeed control forces vertical speed to reduce to near level flight before giving up additional speed. Di spl ays Driven from the FCC and TlIC Two displays driven from the FCC and TMC, the flight director and the fast/slow indicator, provide guidance for manual operation, monitoring for automatic operation, and warning for speed-limit violation. FCC mode commands are also provided on the electronic attitude d irection indicator (EA1)I ~ for manual control by use of the flight director. The control laws are largely similar for automatic and flight director operations, except that the pilot closes attitude loop errors rather than the pitch and roll inner loops. 95
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When the FCC is engaged, the fl ight director ' s errors are proper t i one l to the d i f ferenc e b e tween the c ommand and pr imary feedback s ignals and thus provide a means of monitoring FCC performance. For example, wi th automat ic go-around engaged, the error between the fl ight director ' s bar and pitch attitude is proportional to speed error . The fast/slow display is driven from the TMC and performs functions similar to those of the flight director. When no automatic speed mode is engaged, the fast/slow display provides guidance for controlling throttle to attain and track airspeed to the greater of that selected on the MCP or minspeed. The display is driven by the same control law that drives the TMC during approach, except that integral control is not used. With the removal of high-frequency wind variations from the signal and the use of inertial acceleration along the flight path, the fast/slow director provides a responsive, smoothly varying signal that enhances manual speed control through the throttle. When any automatic speed control mode is engaged, the fast/slow indicator reflects the difference between airspeed ~ less high- frequency wind component ~ and the greater of the MCP speed or minspeed, and therefore acts as a clean signal for monitoring airspeed control. If the MCP selected speed is less than minspeed and airspeed is less than 3 knots above minspeed, the mode annunciated changes to ALPHA to advise the pilot why the automatic system is no longer clos ing on the selected speed. I f airspeed should fat 1 more than 3 knots below minspeed, the fast/slow pointer changes to an amber color and flashes to warn the pilot of excessive speed deviation and that pilot actions may be required. Direct horizontal wind information is displayed on the electronic horizontal situation indicator (EHSI ), which shows wind direction by means of an arrow and shows wind magnitude in knots on a digital readout. The EHSI also provides an onboard weather radar display. 96