1. General
A. The Autothrottle is a computer-controlled electro-mechanical system which controls engine thrust within engine design parameters. Throttle position for each engine is controlled to maintain a specific engine thrust (N1), or a target airspeed, for all flight regimes from takeoff to touchdown. It is designed to operate primarily in conjunction with the Digital Flight Control System (DFCS) and the Flight Management Computer (FMC); interfacing also takes place with the IRS and several other systems and components.
B. The desired mode of operation is selected on the DFCS mode control panel, and the engine thrust limit is selected through the FMCS control display unit.
C. The Autothrottle system consists of a digital computer, two Authothrottle servo mechanisms, an Autothrottle arm (engage) switch, Autothrottle disengage and takeoff/go-around switches and two Power Lever Angle (PLA) synchros. Airplanes without EFIS have an A/T (AutoThrottle) annunicator display and an A/T warning light on each DFCS Flight Mode Annunciator (FMA) and a fast/slow pointer on each ADI. Airplanes with EFIS have a limit annunciation on the Thrust Mode Annunciator, one A/T warning light on each Autoflight Status Annunciator, and mode annunciation on each EADI. Together with those units from other systems which play a vital role in Autothrottle system operation.Airplanes with Engine Instrument System (EIS); The A/T limit annunciation is displayed on the A/T limit annunciator.
D. Autothrottle (A/T) components are located in the flight compartment, in the electronics equipment area, on the left side of the fuselage, and on both engines.
(1) The DFCS mode control panel is located on the glareshield.
(2) The A/T disengage switches and takeoff/go-around switches are located on the thrust levers.
(3) Airplanes without EFIS; The Flight Mode Annunciators are located on captain's panel and first officer's panel.
(4) Airplanes with EFIS; The Autoflight Status Annunciators are located on captain's panel and first officer's panel . The Thrust Mode Annunciator or A/T limit annunciator is located on the center instrument panel.
(5) The A/T circuit breakers are located on the P18-1 circuit breaker panel. Airplanes with Automatic Thrust Restoration (ATR) feature; The A/T circuit breakers are also located on the circuit breaker panel.
(6) The angle of airflow (alpha) sensor is located on the forward left side of the fuselage.
(9) The Power Lever Angle sensors are located on the left side of each engine.
E. The Autothrottle engage switch (A/T ARM-OFF) is mounted on the DFCS mode control panel, which is located on the pilot's light shield. Several other controls used with (or indirectly associated with) the Autothrottle system are also grouped together on the panel. These include: IAS/MACH readout, N1 mode select, SPEED mode select, and C/O (IAS/MACH changeover switch). The IAS/MACH speed control is not directly linked with the Autothrottle system but IAS and MACH select signals generated in the DFCS play a part in Autothrottle computations. The C/O (changeover) control is directly associated with the IAS/MACH display.
F. The FMCS provides flight guidance data to the Autothrottle system in the form of optimum airspeed and thrust settings to achieve the most efficient and economical mode of operation for the airplane. Controls for this system are mounted on the CDU, which is located on the forward electronic panel. The CDU is used extensively by the Autothrottle system to display self-test (BITE) information, including test pass/fail results.
G. Various Autothrottle interlocks must be satisfied before the A/T ARM-OFF switch will latch in the engaged position. The Autothrottle system will disengage with the loss of internal valid signal, loss of interfacing system valid, operation of either disengage switch on the forward thrust levers, or operation of the A/T ARM-OFF switch to OFF. A disengage warning light is located on the captain's and first officer's Flight Mode Annunciators (airplanes without EFIS) or on the Autoflight Status Annunciators (airplanes with EFIS).
H. A number of externally generated signals are used for system computation and control. These signals, together with signal sources, are given in the following list:
(1) Inertial reference units (IRU) No. 1 and 2 provide pitch and roll attitude, normal, longitudinal and flight path vertical acceleration plus IRU valid logic via an ARINC 429 digital data bus.
(2) Digital Air Data computers No. 1 and 2 provide ARINC 429 digital data consisting of altitude, MACH, total air temperature (TAT) and airspeed. Digital Air Data computer No. 1 provides DADC valid logic and three-wire computed airspeed data.
(3) Radio Altimeter No. 1 provides radio altitude and radio altitude valid logic.
(4) Left ALPHA vane provides angle of attack.
(5) Logic in the landing gear accessory box provides an in-air/on-ground discrete.
(6) Left flap position transmitter provides flap position information.
(7) Flight Management Computer provides ARINC 429 data consisting of flight mode select and system discretes, target N1, continuous N1 limit, climb N1 limit, cruise N1 limit, go-around N1 limit, FMC minimum speed, SAT, and gross weight signals.
(8) N1 transmitters provide N1 signals.
(9) Power Lever Angle (PLA) synchros provide engine power lever position feedback.
(10) Autothrottle servomechanisms provide rate feedback.
(11) DFCS control panel provides Autothrottle engage logic and ARINC 429 digital data. This data, which is generated in the DFCS computer, consists of N1 select, SPEED select, target airspeed, target mach, and selected DFCS modes.
(12) Landing gear up/down logic is provided by a landing gear position switch.
(13) Torque switch mechanisms provide torque switch logic.
I. Electrical power for the Autothrottle system is provided by No. 1 electronic power bus via 115 volt ac circuit breaker, A/T SYST AC, and 28 volt dc circuit breaker, A/T SYST DC, located on load control center P18.
2. Autothrottle Computer
A. The Autothrottle computer performs the necessary computations for automatic control of engine thrust, and provides throttle positioning commands to the servomechanisms.
B. The A/T computer contains a built-in test program, accessed through the Flight Management System-control display unit. The built-in test is capable of performing a quick A/T verification, for use during turnaround; or an extensive diagnostic routine, for use during overnight maintenance. The alpha-numeric test readout on the front of the computer is intended for use during bench testing of the computer.
3. Autothrottle Servomechanisms
A. The A/T servomechanisms provide the electromechanical interface between the autothrottle computer and the throttle cables.
B. Separate servomechanisms are provided for throttle control of each engine. They are located between the floor beams, in the aft area of the electronic equipment area. The servomechanism for engine-1 is located on the left.
C. Each servomechanism is comprised of an actuator assembly, a torque switch mechanism, a torque switch assembly, and an optional throttle position potentiometer. All except the torque switch mechanism are line replaceable.
D. The torque switch mechanism is installed in-line with the throttle cables. Replacement requires rigging of the throttles. This mechanism consists of input and output quadrants, a torque sensing mechanism, a friction brake to prevent throttle creep, and an override detent mechanism. The torque sensing mechanism activates the torque switch, when override force exceeds 2 pounds nominal on the thrust lever. The override detent mechanism allows the pilot to override the servo, in event of a clutch control failure. Override requires 18 pounds nominal force, at the throttle levers.
E. The torque switch is attached by two captive screws. Electrical connection is through an integral wire bundle, terminated at a terminal block mounted near the servomechanism. The switch provides an input to the clutch control circuit.
F. The throttle position potentiometer is optional on the some Boeing airplane. When installed, it provides throttle position data to a digital flight data recorder (DFDR). It is attached to the servomechanism, by two captive screws and electrical interface is through an integral wire bundle, terminated at a terminal block near the servomechanism.
4. DFCS Mode Control Panel
A. The DFCS mode control panel (MCP) provides the pilots with a central control for the automatic flight control functions of the airplane. The MCP provides control of the autothrottle, autopilot, and flight director.
B. The DFCS mode control panel is located on the glareshield (P7).
C. Three controls and one light are directly related to A/T operation: A/T ARM-OFF , N1 and SPEED mode select illuminated push-type switches, and the A/T arm light. Two controls and one display provide manual speed selection for either the Autothrottle or the Autopilot/Flight Director, depending on the engaged DFCS mode; these are: the rotary IAS/MACH select control, a liquid crystal digital display for IAS/MACH, and the IAS/MACH pushbutton changeover (C/O) switch. Five other illuminated momentary push-type switches, and the altitude and vertical speed controls and displays, are provided to control the DFCS pitch axis, and affect the A/T operation.
D. The momentary push-type switches are used to manually select or deselect the associated mode. They are illuminated to indicate that the engaged mode can be manually deselected. The mode logic is latched by the FCC.
E. The A/T engage (ARM/OFF) switch is two-position, solenoid held. The switch is held in ARM, if the A/T engage interlock circuit is satisfied.
F. With N1 mode selected, the A/T controls the throttles maintaining N1 limit thrust. In SPEED mode, the A/T maintains the airplane's airspeed.
G. The rotary IAS/MACH control provides IAS/MACH
reference adjustment, unless VNAV mode is selected for the DFCS. Either the A/T or DFCS uses the manually
adjusted IAS/MACH reference, depending on which system provides
the airspeed control. The IAS/MACH reference is displayed by the
speed reference marker on the captain's and first officer's IAS/MACH
indicators, and digitally displayed in the IAS/MACH window. The
changeover (C/O) push-button switch manually changes the display
between IAS and MACH. The remaining controls and indicators,
provided to control the DFCS pitch axis modes,
indirectly affect the A/T operation, as the A/T mode must be
compatible with the DFCS mode. When the DFCS is controlling the
airplane's airspeed, the A/T maintains N1 at the target value.
When the DFCS is controlling the
airplane's vertical path, the A/T maintains the airplane's
airspeed at the speed reference value.
5. Autothrottle Disengage and TO/GA Switches
A. The A/T disengage switches are located on the end of each thrust lever handle and provide a means of disengaging the A/T. The TO/GA (Take-Off/Go-Around) switches are located on the aft edge of each thrust lever and provide a means of engaging the A/T in the takeoff or go-around mode. All four switches are momentary contact type switches. Each has a set of normally open contacts and a set of normally closed contacts.
B. The A/T disengage switches are designed to be operated with the thumb while the hand is on the throttle levers. The two disengage switches are wired in series and the normally closed contacts form a part of A/T engage circuits. Pressing either switch will disengage the A/T. The normally open contacts are in the A/T warning light circuit and are used to reset the warning light.
C. The TO/GA switches are positioned to operate with a finger tip while the hand is on the throttle lever. Pressing either switch activates the takeoff or go-around mode depending on the mode interlock requirements of both A/T and DFCS.
6. Autoflight Status Annunciator
A. An Autoflight Status Annunciator is provided for each pilot for warning indication of disengagement of the Autopilot, the Autothrottle, or the Flight Management Computer system.
B. The Autoflight Status Annunciator comprised of three warning lights with push-to-reset features and a test switch.
C. The A/T warning light flashes whenever the Autothrottle is disengaged. The warning light is reset by pressing the A/T warning light on either Autoflight Status Annunciator, or by pressing an A/T disengage switch. The A/T warning light illuminates steady red when the Autothrottle is in BITE.
D. The test switch is a three position switch, spring loaded to the normal position. The A/T warning light illuminates steady amber in position "1" and steady red in position "2".
7. Angle-of-Airflow (ALPHA) Sensor
A. The angle-of-airflow sensor provides the Autothrottle with a measurement of airflow angle, relative to the mean chord line of the wing. This angle is commonly called the alpha angle.
B. The signal from the sensor is used to provide feedback to control the maximum angle-of-attack.
C. There are two sensors mounted on the airplane. The sensor mounted on the left side of the airplane, near the nose, provides the alpha signal to the Autothrottle computer.
D. The sensor consists of a synchro transmitter, mounted to the fuselage skin from the inside of the airplane. Index pins are provided to assure correct alignment. An airflow vane is mounted on the synchro shaft, external to the airplane. The airflow vane contains an electrical heater, used during flight to prevent icing.
8. Flap Position Sensor
A. The flap position sensor provides the A/T with a measurement of the flap position. The flap position is used to compute the minimum safe speed and the maximum angle-of-attack. Flap position is also used in the generation of control logic.
B. The flap position sensor is mounted aft of the rear spar, and approximately centered on the left outboard flap. Lower the flaps for access.
C. The flap position sensor is a synchro transmitter having dual output windings. The sensor is mounted on the flap gear drive by 4 screws. Electrical interface with the sensor is through an electrical connector mounted on the end of the synchro case.
9. Power Lever Angle (PLA) Synchro
A. The PLA synchros provide a measurement of the throttle input command to the engines. The signal is used by the Autothrottle for throttle position feedback.
B. The PLA synchro is mounted on the engine fuel control box, on the lower left side of each engine. The synchro is attached to the fuel control box by two bolts. Electrical interface is through an integral wire bundle, terminated in an electrical connector.
10. Autothrottle Flight Modes
A. A/T mode selection must be compatible with the DFCS mode engaged. If the F/D is OFF, and the A/P is OFF or PCWS, either N1 or SPEED can be selected for the Autothrottle.
B. As a basic rule, with a DFCS pitch mode selected, and the A/T ON, airspeed is directly controlled by either engine thrust (N1) or pitch attitude, but not both at the same time.
C. TAKEOFF - airspeed is maintained by following the F/D pitch commands, and the A/T sets engine thrust (N1), either full or reduced.
D. CLIMB - when the DFCS is maintaining airspeed (MCP SPD or VNAV SPD), the A/T maintains CLB N1 LIMIT. When the DFCS maintains vertical speed, the A/T maintains MCP SPD.
E. ALT ACQ - the DFCS acquires the selected altitude, and the A/T maintains either MCP SPD, or if the DFCS climb is accomplished in VNAV SPD, the A/T maintains FMC SPD.
F. CRUISE - the DFCS maintains the selected altitude, and the A/T maintains MCP SPD or FMC SPD.
G. DESCENT - with the DFCS controlling vertical speed, the A/T maintains MCP SPD. If the DFCS is in a speed mode (MCP SPD or VNAV PATH), the A/T retards the throttle levers to the aft stop, and then inhibits throttle control (RETARD-ARM).
H. APPROACH - after G/S engage, the DFCS controls the descent rate to maintain the glide slope, and the A/T maintains the selected approach airspeed (MCP SPD). At less than 27 feet radio altitude, the A/T retards the throttles to the aft stop (RETARD-ARM), then disengages 2 seconds after touchdown.
I. GO-AROUND - the DFCS controls airspeed, and the A/T maintains either reduced or full GA N1. Reduced go-around is engaged by pressing the TO/GA switch one time. A second press of the TO/GA switch engages maximum go-around. Other modes can be selected after engines have achieved the selected N1 limit and radio altitude is greater than 100 feet.