Aircraft Application Program Interfaces. Aircraft Computer Hardware

An application programming interface (API) is conventionally defined as an interface used by one program to make use of the services of another program. The human interface to a system is usually referred to as the user interface, or, less commonly, the human-computer interface. Application programs are software written to solve specific problems.

For example, the embedded computer software that paints the artificial horizon on a heads-up display is an application program. A switch that turns the artificial horizon on or off is an API. Gal-Oz and Isaacs (12) discuss APIs and how to relieve bottlenecks of software debugging.

Aircraft Control. Landau (1) defines a control as an instrument or apparatus used to regulate a mechanism or a device used to adjust or control a system. There are two concepts with control. One is the act of control. The other is the type of device used to enact control. An example of an act of control is when a pilot initiates changes to throttle and stick settings to alter flight path. The devices of control, in this case, are the throttle and stick.

Control can be active or passive. Active control is forcesensitive. Passive control is displacement-sensitive.

Mechanical control is the use of mechanical devices, such as levers or cams, to regulate a system. The earliest form of mechanical flight control was wires or cables, used to activate ailerons and stabilizers through pilot stick and foot pedal movements. Today, hydraulic control, the use of fluids for activation, is typical. Aircraft control surfaces are connected to stick and foot pedals through hydraulic lines. Pistons in the control surfaces are pushed or pulled by associated similar pistons in the stick or foot pedal. The control surfaces move accordingly.

Electronic control is the use ofelectronic devices, such as motors or relays, to regulate a system. A motor is turned on by a switch, and it quickly changes control surfaces by pulling or pushing a lever on the surface. Automatic control is a system-initiated control, which is a system-initiated response to a known set of environmental conditions. Automatic control was used for early versions of automatic pilot systems, which tied flight control feedback systems to altitude and direction indicators. The pilot sets his desired course and altitude, which is maintained through the flight control’s automatic feedback system.

To understand the need for computers in these control techniques, it is important to note the progression of the complexity of the techniques. The earliest techniques connected the pilot directly to his control surfaces. As the aircraft functionality increased, the pilot’s workload also increased, requiring his (or his aircrew’s) being free to perform other duties.

Additionally, flight characteristics became more complex, requiring more frequent and instantaneous control adjustments. The use of computers helped offset and balance the increased workload in aircraft. The application of computers to flight control provides a means for processing and responding to multiple complex flight control requirements.

Aircraft Computer Hardware. For aircraft computers, hardware includes the processors, buses, and peripheral devices inputting to and outputting from the computers. Landau (1) defines hardware as apparatus used for controlling a spacecraft; the mechanical, magnetic, and electronic design, structure, and devices of a computer; and the electronic or mechanical equipment that uses cassettes, disks, and so on. The computers used on an aircraft are called processors. The processor takes inputs from peripheral devices and provides specific computational services for the aircraft.

There are many types and functions of processors on an aircraft. The most obvious processor is the central computer, also called the mission computer. The central computer provides direct control and display to the aircrew. The federated architecture (discussed in more detail later) is based on the central computer directing the scheduling and tasking of all the aircraft subsystems.

Other noteworthy computers are the data processing and signal processing computers of the radar subsystem and the computer of the inertial navigation system. Processors are in almost every component of the aircraft. Through the use of an embedded processor, isolated components can perform independent functions as well as self-diagnostics.

Distributed processors offer improved aircraft performance and, in some cases, redundant processing capability. Parallel processors are two or more processors configured to increase processing power by sharing tasks. The workload of the shared processing activity is distributed among the pooled processors to decrease the time it takes to form solutions. Usually, one of the processors acts as the lead processor, or master, while the other processor(s) act as slave(s).

The master processor schedules the tasking and integrates the final results, which is particularly useful on aircraft in that processors are distributed throughout the aircraft. Some of these computers can be configured to be parallel processors, offering improved performance and redundancy. Aircraft system redundancy is important because it allows distributed parallel processors to be reconfigured when there is a system failure.

Reconfigurable computers are processors that can be reprogrammed to perform different functions and activities. Before computers, it was very difficult to modify systems to adapt to their changing requirements. A reconfigurable computer can be dynamically reprogrammed to handle a critical situation, and then it can be returned to its original configuration.

Aircraft Buses. Buses are links between computers (processors), sensors, and related subsystems for transferring data inputs and outputs. Fink and Christiansen (8) describe two primary buses as data buses and address buses. To complete the function of an MPU, a microprocessor must access memory and peripheral devices, which is accomplished by placing data on a bus, either an address bus or a data bus, depending on the function of the operation.

The standard 16-bit microprocessor requires a 16-line parallel bus for each function. An alternative is to multiplex the address or data bus to reduce the number of pin connections. Common buses in aircraft are the Military Standard 1553 Bus (Mil- Std-1553) and the General-Purpose Interface Bus (GPIB), which is the IEEE Standard 488 Bus.