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OCR for page 93
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
OCR for page 94
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
OCR for page 96
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
Representative terms from entire chapter:
flight director
