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The I/O computer also uses status information from the inertial computer to control the output discretes. The FAULT discrete can also be set, along with the fault ball (if installed), by BITE hardware.
(8) IRU internal BITE
(a)
When the mode select switch is moved from OFF to ALIGN or NAV, a self-test BITE signal disables the 115v ac input and inserts the 28v dc battery as the power source for 5 sec. This tests the battery and associated power switching circuits. IRU BITE continuously monitors unit performance. It also initiates self tests during the power-up sequence. BITE detected faults are classified as critical or non-critical. Critical faults are faults which could cause erroneous outputs of heading, attitude, angular rates or accelerations. If BITE detects a critical fault, BCD data is removed from the bus, BNR data is transmitted with a sign/status matrix of failure warning , the fault ball (if installed) will be set and the IRU 28 Vdc FAULT output discrete is set. If BITE detects a non-critical fault (for example, the ADC input bus wraparound test) no action is taken during flight. When ground speed goes below 40 knots, the fault ball and the FAULT output discrete is set but the output busses remain valid.
(b)
BITE monitors and tests the following: 1) Power supply 2) Discrete Input/Output 3) Sensor/electronics 4) Computer clock/interfaces 5) Input receiver and output transmitters 6) Alignment, air data input, and computed data reasonableness 7) Derived latitude compared to entered latitude 8) Keyed in longitude compared to stored longitude 9) Memory operation 10) Instruction execution 11) Internal temperature
(c)
A watch dog timer (WDT) monitors and detects failures that prevent the CPU from executing its program. The CPU is programmed to reset the WDT every 20 ms (50 Hz rate). If the CPU is unable to reset the WDT because of a computer failure, hangup, or memory failure, the WDT will time out. This will set the fault ball (if installed) and display the IRS LEFT (CENTER, RIGHT) message on the main EICAS display. In addition, a fault word will be written into BITE memory.
(d)
Non-volatile BITE memory stores the latest calculated latitude and longitude and accumulated BITE status record of the last two power-on periods. It also stores a summary of IRS discrete message words for the last 6 power-on periods.
(e)
When BITE detects a failure, the fault ball is set (if installed).
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3. Mode Select Unit (MSU)
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A. The MSU has three rotary switches for selecting the operational modes of each IRU. Each switch has four positions: OFF, ALIGN, NAV and ATT. The MSU is located on the pilot's overhead panel, P5.
(1)
OFF - IRU is off. Power is removed from the IRU internal circuitry.
(2)
ALIGN (alignment) - During the ALIGN mode, message(s) appear on the main EICAS display, local vertical and true heading are determined with respect to true north, latitude is estimated, and velocities are set to zero. The position information given to the IRS is checked for accuracy against the estimated value for latitude and the stored value for longitude. The alignment, when properly completed, allows the IRU to advance to the NAV mode. Normal alignment takes 10 minutes to complete.
(3)
NAV (navigation) - The NAV mode is the normal operating mode for the IRS. In this mode, the IRS performs inertial navigation functions and outputs normal IRS data to be displayed or used by other systems.
(4)
ATT (attitude) - The ATT mode is used as a back-up mode when a BITE failure is detected or when both AC and DC temporary power loss occurs in the NAV mode causing loss of alignment. In this mode, only attitude, attitude rates, accelerations and magnetic heading are output to the user systems. Most other words are transmitted as failure warn. Heading will not be valid until a heading value is entered from the POS INIT page on the CDU. The IRUs must be cycled through OFF to re-enter ALIGN or NAV. The implication is that if ATT mode is entered, NAV mode can not be re-entered until after accomplishing a full alignment. There is a two second delay before the IRUs will recognize that ATT mode has been selected. This is to prevent inadvertent selection of ATT mode.
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