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gravity components of the airplane. The three signals are manipulated
within the computer to give us local verticle. Three laser gyros are used
to sense the earth's angular rate components on the airplan's lateral,
longitudinal and vertical axes. The IRU computer uses the direction of
angular rate to determine the direction of true north .The present position
( latitude/longitude) must be entered during the 10 minutes alignment
period. When the present position has been entered into the IRU, it is
compared to the last known stored present position. If the entered present
position is within 1 degree of the stored present position, the IRU will
enter the navigation mode.
Fig. 32-3 Alignment
[4] In the navigation mode the IRUs provide outputs for attitude, heading,
accelerations, ground speed, position and drift angle. These outputs are
all computed using inputs from the gyros and accelerometers. The initial
navigation outputs are continuously modified during operation by the
computer using inputs from the accelerometer and gyros, to maintain real
time present parameters. Inputs from the ADCs are used to compute
inertial altitude, inertial vertical speed, and wind speed/direction.
Altitude referenced to 29.92 inHg is used for computing altitude. The
altitude data along with altitude rate is used for computing vertical speed.
True airspeed is used to compute wind data.
Fig. 32-4 Navigation mode
[5] The mode select unit (MSU) provides mode selection for two IRUs
through two mode select switches and monitors the operation of the IRUs
through two sets of alert lights mounted on the unit. The MSU contains
four position mode select switches; one for the left and one for the right
IRU. The position are OFF, ALIGN, NAV and ATT. To minimize
inadvertent mode selection, the switches gave barriers when switching
from “ALIGN”to “OFF”and when switching between the “NAV”
and “ATT”modes. To switch between these modes you must pull the
mode select switch and turn.
ALIGN
When ALIGN is selected, power is supplied to the IRUs and the
IRUs progress through a alignment period of approximately ten minutes.
NAV
The NAV position of the switches enables the navigation mode,
provided that the alignment has been completed.
Fig. 32-5 IRS mode select unit
ATT
When ATT position of the switches places the IRUs into the attitude
mode, which is a degraded mode of operation. The attitude mode
provides only attitude information.
OFF
When the mode selector switches are in the OFF position, power is
removed from the IRUs. When turned OFF, the IRUs go into a shutdown
procedure and shutdown approximately thirty seconds later. In addition,
Two set of four annunciators are installed on the unit, one annunciator
(ALIGN) for an alert purpose and three (ON DC, FAULT, DC FAIL )for
warning purposes.
READING MATERIAL
QUESTIONS TO LESSON 32 READING MATERIAL
1. What is the laser gyro?
2. How does the production of the light beam (lasing ) occur in the gas
discharge region?
3. Why is a high voltage discharge used between the cathods and anodes?
4. Where is light produced from the lasing reflected ?
5. Why is light produced from the lasing reflected at each corner of the
triangle?
6. What does one of the 3 corner contain?
7. How to form the fringe pattern on the readout detector?
8. As long as the gyro is stationary, why is the fringe pattern also
stationary?
9. What axis does the gyro rotate about?
10. Why are the two light beam frequency different?
11. Does the difference in frequencies cause the fringe pattern to move?
12. What is the use of photo diodes?
13. What is called “laser lock-in”?
14. What is the use of dither motor ?
15. Can the dither motor vibrations be felt on the IRU case?
EXERCISE
A. Find out the explanations of the following words in your
“English-chinese technical dictionary”
1. inertial reference system (IRS) [ P1-80 ]
2. flight management computer (FMC) [ P1-63 ]
3. Digital-to- Analog [ P1-43 ]
4. Laser gyro [ P1-90 ]
5. ground speed (GS) [ P1-72 ]
6. drift angle [ P1-49 ]
B. Translate the 1st and 2nd paragraphs of lesson 32 into chinese.
C. Explain the following abbreviations
1. IRS
2. IRU
3. MSU
4. DAA
5. ISDU
D. Answer the question 1～15 that are given for lesson 32 reading
material.
LESSON 33
GROUND PROXIMITY WARNING SYSTEM
VOCABULARY
1. FAA = Federal Aviation Administration [美]联邦航空局
['fedrl][eivi'ein][d'minis'trein]
中国航空网 www.aero.cn
航空翻译 www.aviation.cn
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航空工程英语基础（中英文对照）(53)
