(3)
Each air data computer processes pneumatic inputs of pitot and static pressures and total air temperature inputs to provide shaft rotation for 21 electromechanical output devices such as potentiometers, synchros and rate generators. Shaft position information is transferred to the airplane instruments and systems requiring air data inputs by standard three-wire synchro outputs, two-wire ac voltage analogs and dc voltage ratios.
(4)
Abnormal computing performance or power failure triggers a self-monitoring failure warning system within each computer. The computer also provides failure warning signals to interfacing systems when a related computation occur. These signals can appear singly or in combination. A failure warning is programmed to appear if the failure condition persists for a time period of more than two seconds. The delay is intended to minimize nuisance type warnings. The failure warning system activates a magnetic fault indicator on the computer front panel and warns each affected instrument and system. When a fault has occured the fault indicator must be reset manually after the computer has been repaired.
C. Digital Air Data Computer (P/N HG480C1)
EFFECTIVITY
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(1)
The digital air data computer (DADC) is a solid-state device that uses pitot-static pressure and air temperature inputs to compute air data information required by airplane displays and system. In addition to the above inputs, 115 volts ac and 26 volts ac are provided to multipin connectors at the rear of the computer.
(2)
The computer consists of two pressure transducers, 11 plug-in printed circuit cards, a power supply and chassis assembly. The front panel (Fig. 1) provides two switches for self-test actuation, a self-test indicator light, a fault isolation test connector for off-aircraft trouble shooting and pitot static pressure input connectors. Two electrical connectors mounted on the back of the chassis provide interconnection with the airplane wiring.
(3)
The TEST SELECT switch manually selects built-in test to be performed as follows:
SLF-TEST - transmits fixed values on all the computer signal output lines.
DELTA - transmits change values for altitude, computed airspeed and mach.
SLEW _ transmits slew rate values for altitude, computed airspeed, mach and true airspeed.
The PUSH TO TEST pushbutton switch initiates the built-in test and the TEST VALID WHEN LIT indicator light comes on within 2 seconds after manually activated test is successfully completed and remains on if there is no failure.
4. Operation_________
A. Functional Description
(1) System Operation (Fig. 2)
(a)
Central air data system outputs are all derived from three types of input data taken from the air environment external to the aircraft. Pitot and static air pressures are sensed from pneumatic hoses of the pitot-static system and total air temperature is sensed directly from the atmosphere by total air temperature probes.
(b)
Sensing true static air pressure external to a moving aircraft is difficult over the entire operating limits of an airplane. The central air data computers each contain universal static source error correction circuits which compensate for this error as a function of mach number. The universal correction circuit is adjusted for a particular airplane by jumpers on the airplane side of the computer connector. When a standard air data computer is plugged in, it receives a correction factor peculiar to that particular airplane. This correction factor is included in the determination of computed airspeed, pressure altitude and mach number.
EFFECTIVITY
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(c)
Hold synchro outputs from pressure altitude, mach and computed airspeed modules are clutched synchros whose clutches are activated only on command from the autopilot/flight director. The issue of a hold command engages a clutch in the hold synchro and the hold synchros then provide high resolution error information to the autopilot/flight director.
(d)
Each central air data computer monitors itself for abnormal servo loop performance and/or power failure through a failure warning logic assembly. There is no master warning of computer failure on the flight deck. Detection of a failure causes one or more of four failure warning flag signals to be issued to one or more of the interfacing systems. Some of these systems then show flags and/or warning lights on the flight deck and some react to the warning without any external display.
(e)
Environmental data inputs are provided continuously to the central air data system. Since the computers are on continuously as long as the system circuit breakers are closed, valid outputs are provided to interfacing systems after an initial 30-minute warmup period.
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