Automatic Flight Control Systems Engineering Essay

We live in a world where technology is, if not being improved, developed by the second. Everyday new improvements, inventions and discoveries are made. One industry that is always on the lead when it comes to new inventions and innovations is the Aviation Industry.

Over the years, aircrafts have been facing major improvements on the structure, fuel efficiency, life-span, range of flight. But one of the best improvements that have been done on every aircraft (commercial) that had the biggest impact in the Aviation Industry and most probably the main reason why the industry has been booming up is the improvements done in the Avionics section, specifically the Automatic Flight Controls.

In the beginning, Pilots were trained to fly the aircrafts alone. But after several years, it is now the pilots programming the computer, telling it where to fly, at what altitude, etc. This computer is the AFCS (Automatic Flight Control System). In today’s modern world of flying, it is the AFCS who is technically flying the aircraft, from cruising to landing, and for some until parking. The AFCS has a lot of advantages when compared to human pilots when it comes to flying. Here are some of them:

The AFCS has the ability to overcome deficiencies when it comes to stability and control.

The AFCS improved the handling qualities. Such as, when the airspeed or the altitude of the aircraft needs to be constant.

The AFCS is more accurate and hence is able to carry out several tasks that the pilot is not able to do.

* Source: Emirates Aviation College’s Automatic Flight Control Systems Book (Chapter 3.1.3)

To get a better understanding of the AFCS, the different parts of it will be discussed, such as the Autopilot System, Flight Director System, Auto Throttle System and etc.

The information about the AFCS will be based on one of Boeing’s classic aircrafts, the 737-500.

FLIGHT MANAGEMENT SYSTEM (FMS)

The Flight Management System is navigation, combined flight control, a Built-In Test Equipment (BITE) and a guidance system. The FMS provides control and operation of five independent subsystems to provide lateral navigation (LNAV) and vertical navigation (VNAV) for performance management and optimum flight profiles. The Flight Management System is not labeled to any control panel or any single component as it is an integration of five independent subsystems. These subsystems are:

Digital Flight Control System (DFCS)

Inertial Reference System (IRS)

Autothrottle

Electronic Flight Instrument System (EFIS)

Flight Management Computer System (FMCS)

* Source: United Airlines’ Boeing 737-322/522 (page 6, Chapter 22-2, Oct ’99) from Emirates Aviation College Library

This system was designed to increase fuel efficiency, safety and decrease workload. For both pilots, this means that they can select full FMS operation or Autopilot Flight Director System (AFDS) for a complete automatic flight. They can even use the Control Display Units (CDU) to provide, for manual flight, reference information. Management and operation is totally under the control of the flight crew. There are only certain operations that can only be implemented by the flight crew. They are: landing rollout steering, thrust reversal, speed brake operation, altitude selection, landing gear and flap operation, instrument landing system (ILS) tuning, thrust initiation, brake release, airplane rotation and steering during takeoff roll.

FMS BUILT-IN TEST EQUIPMENT (BITE)

Ground test capabilities and self-contained in-flight monitoring are provided for the FMS subsystems. The flight management computer (FMC) coordinates the BITE testing of the five subsystems and must be functional for access to any subsystems’ built-in test equipment. BITE for the FMS subsystems is accessed from both CDUs located in the cockpit. Each FMS subsystems run its own system’s test. Included in the tests are for its computers, sensor inputs and several interfaces. Any fault findings during flight are automatically stored for analysis on the ground which is accessed through the bite system.

* Source: United Airlines’ Boeing 737-322/522 (page 8, Chapter 22-4, Oct ’99) from Emirates Aviation College Library

THE BOEING 737-500 CATEGORY

General AFCS Category Capability Chart

Boeing 737-500 AFCS Category Capability Chart

The Boeing 737-500, according to the diagram is in Category 3B. In order for this aircraft to operate at Cat3B weather minimums, the equipment that must be available in Cat2 should be functional. These equipment are:

Both Flight Directors

One Autopilot in CMD

Both Air Data Systems (ADS)

Both windshield wipers

Two generators

Both EFIS systems displaying:

Radio Altitude

Glideslope and Localizer Deviation

Attitude

Autopilot Status Engaged Pitch and Roll Modes

Decision Height

Both Radio Altimeter Systems

Both ILS

Marker Beacon

Both IRS

Now for the aircraft to operate in Cat3B all the equipment listed for Cat2 must be fully operational, provided that the equipment as follows is included:

The Autothrottle

Both Autopilots

Both Hydraulic Systems

* Source: United Airlines’ Boeing 737-322/522 (page 12, Chapter 22-8, Oct ’99) from Emirates Aviation College Library

AUTOMATIC FLIGHT CONTROL SYSTEM (AFCS)

The AFCS or also known as Auto Flight System (AFS) comprises of three independent systems. They are:

A Yaw Damper System

A dual Digital Flight Control System (DFCS)

An Autothrottle (AT) System

These 3 systems provide automatic aircraft stabilization about the roll, pitch and yaw axis. The AT and the DFCS systems control the aircraft with the selected Mode Control Panel (MCP) guidance from N1, heading, radio, IRS, FMC and ADC inputs.

* Source: United Airlines’ Boeing 737-322/522 (page 10, Chapter 22-6, Oct ’99) from Emirates Aviation College Library

YAW-DAMPER SYSTEM

The Yaw Damper System is composed of various components, such as:

Yaw Damper Engage switch

Warning Annunciator

Yaw Damper Coupler

Integrated Flight System Accessory Unit (IFSAU)

Yaw Damper Engage solenoid

Position Transducer and Transfer Valve on the rudder Power Control Unit (PCU)

Yaw Damper position Indicator

* Source: United Airlines’ Boeing 737-322/522 (page 12, Chapter 22-8, Oct ’99) from Emirates Aviation College Library

The Yaw Damper System smoothens the flow of air that causes the aircraft to yaw and thus causes it to Dutch Roll. The Dutch Roll is a phenomenon that occurs when a sidewind hits the aircraft which causes it to yaw. This yawing motion exposes one side of the wing to the wind more than the other side. The exposed side of the wing, gains more speed and more speed generates more lift on that particular side. The lift that was generated on the exposed side of the wing causes the aircraft to roll. Hence, the term Dutch Roll.

The Yaw Damper System is a stability augmentation system that works full time, providing yaw axis damping for the complete flight, including both takeoff and landing. It is connected in series so that none of the rudder feedback is applied to the pedals. This now allows the Yaw Damper System to operate in an independent manner without interfering the initiated rudder commands by the flight crew. Both the yaw rate and the yaw direction are detected using a rate sensor located in a Yaw Damper Coupler. The rate sensor of the Yaw Damper is sensitive that the rudder is displaced at a proper time to dampen out any sidewinds hitting the aircraft before it can obviously affect the flight path of the aircraft.

* Source: United Airlines’ Boeing 737-322/522 (page 12, Chapter 22-8, Oct ’99) from Emirates Aviation College Library

The Yaw Damper authority is only ±3° deflection of the rudder. Its actuator gets no turning commands from the longitudinal axis AP. The Boeing 737-500 does not have a 3-axis AP. It only has 2 which is the pitch and roll axes only. In this aircraft, turn coordination is not available. A roll command which sends a cross feed signal in proportion to the bank angle due to aircraft banking, is sent to the Yaw Damper to prevent undesirable opposition to roll attitude.

* Source: United Airlines’ Boeing 737-322/522 (page 12, Chapter 22-8, Oct ’99) from Emirates Aviation College Library

Yaw Damper System Diagram

Yaw Damper Operation

The Yaw Damper is confined to 3 modes of operation. These modes are: synchronization mode, engaged mode interlocks and logic and engaged mode operation. Before the yaw axis engagement, the Yaw Damper coupler is in the synchronization mode for 2 seconds. The synchronization mode provides a null output to the electro-hydraulic transfer valve to prevent a sudden change in the Yaw Damper coupler. The mode is accomplished by returning the controlled signal back to the output amplifier. The output of the valve amplifier is reduced to null as the output of the integrator increases, causing the signal to be cancelled. Prior to the engagement of the Yaw Damper its actuator remains at the center which nulls the position feedback to the Yaw Damper’s coupler.

* Source: United Airlines’ Boeing 737-322/522 (page 14, Chapter 22-10, Oct ’99) from Emirates Aviation College Library

The system is activated by placing the Yaw Damper switch on the flight control module to “ON”. Both “B”s of the flight control switch and system hydraulic power must be ON and available, respectively, in order to power the Yaw Damper actuator portion of the PCU. 2 seconds after the Yaw Damper is on, the “YAW DAMPER” light extinguishes. The Integrated Flight Systems Accessory Unit (IFSAU) contains the logic to engage the system, monitor the engagement and illuminate the amber-colored Yaw Damper disengaged annunciator light. If, for over 2 seconds, the AC power is lost, the “YAW DAMPER” light illuminates and the Yaw Damper switch returns to “OFF” position. The Yaw rate gyroscope which is located inside the Yaw Damper coupler senses the movements by the yaw axis. The coupler only responds to yaw movements that produce the Dutch Roll. Before the command signals are applied to the transfer valve which is located inside the Yaw Damper actuator, it is first filtered and amplified. The transfer valve then sends hydraulic pressure in order to deflect the rudder and reduce the aircraft’s oscillation by the yaw axis. The output of the Yaw Damper actuator is summed with the rudder pedal manual input in order to move the main rudder Power Control Unit (PCU) that controls the rudder. Now as the actuator moves the linear variable displacement tranducer (LVDT) supplies the signal of the position feedback to nullify the signal coming from the yaw rate gyroscope. When the movement (by the yaw axis) stops, the feedback signal returns the rudder to the original position.

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