1. Fiber Optic Gyro System (FOGS) (AHRU)
New Equipment
CN-1716/A
Fiber Optic Gyro System
(FOGS)
Attitude Heading Reference Unit
(AHRU)

2. CN-1716 (FOG) New Equipment will replace the three existing gyros
ASN-43(Direction Gyro)
CN-1314 (Vertical Gyro)
and TRU-2A/A (Rate Gyro)

3. What is AHRU?
The Fiber Optic Gyro System (FOGS) is a Attitude Heading Reference System (AHRS) that provides accurate aircraft pitch, roll, magnetic heading, and turn rate information for integration within an avionics subsystem.

4. How FOGS Work
Fiber Optics for Axial Reference replacing the mechanical gimbaled gyros.
Accelerometers
electromechanically compensate & stabilize in flight
AHRU is a self latitude look up system.
Flux Valve provides initial reference in heading
FOGS filters fluctuations of flux valve which
smoothes out the pitch, roll, and heading output.

5. “FOGS” AHRU CN-1716/UH-60 Gyros Replacement
New Digital Control
Improved display and control capability
Improved maintenance and diagnostics functions
Spec Mean Time Between Failure: 6000 hrs. Manufacturer Predicted MTBF: 15,000 hrs)
Significant Simplification of Compass Alignment (you don’t do anything!)
Improved Navigational Accuracy

6. NAVIGATION SET, AN/ASN-128 HSI VSI AFCS
Interconnections
FLUX VALVE
NAVIGATION SET, AN/ASN-128
HSI
VSI
AFCS

7. Supplies information to Doppler in the absence of GPS.
AHRU and ASN-128
AHRU is not a navigator.
Supplies information to Doppler in the absence of GPS.
Improves accuracy of ASN 128

8. Before

9. After
CN 1716
CN 1716

10. AHRU (FOGS) AHRU AKA: FOG, SUE, MDU, Lisa.... OUTPUTS
Digital Arinc-429, RS-422: (Heading, Pitch, Roll, Body Rates & Accelerations, Vertical Velocity)
Synchro Arinc-407: (Heading, Pitch, Roll)
Analog: (Yaw Rate, Validities, 2 Wires Attitude, Interlock for Autopilot) ACCURACY
Heading: 0.8 deg. Attitude: 0.5 deg

11. SENSOR ELECTRONIC UNIT (SEU) GENERAL VIEW

12. “SEU” aka “FOG” AHRU CN-1716/A

13. Before
Compass Controller

14. After

15. DIGITAL CONTROL PANEL (DCP)

16. CCAL = Compass Calibration
Digital Control Panel
Display
Window
DG MODE
(FREE)
Indicator
CCAL MODE
Indicator
Alignment Mode
Indicator
Failure
Indicator
SLAVED MODE
Indicator
MODE
Selector
Function
Selector
Display Cursor
Movement
Enter Selection
Button
CCAL = Compass Calibration

17. !! The LOWER CONSOLE lights HAVE to be ON !!
SEE the LIGHTS
!! The LOWER CONSOLE lights HAVE to be ON !!
When are we in the 1-212th going to turn the lights on?
Anytime you need info off the DCP and you can’t see it.
It will be acceptable to turn them on during Circuit Breakers and Switches – Set.

18. !! LOWER CONSOLE lights have to be ON !!
AC Power Applied
Initial power on: All lamps illuminate for 2.5 seconds.
!! LOWER CONSOLE lights have to be ON !!
- DCP performs a lamp self-test / system MOC.

19. Turn Lower Console lighting rheostat on to illuminate DCP.
NO DISPLAY?
Turn Lower Console lighting rheostat on to illuminate DCP.

20. AC Power Applied ALN lamp will be on
Initial power on: DCP is performing lamp self-test for 2.5 seconds, THEN:
ALN lamp will be on
ATT fail flag on the VSI will be in view
AHRU will align and self-test within
45 seconds

21. AC Power Applied After 45 seconds:
the SYSTEM will automatically default to the SLAVE mode
and the VSI ATT warning flag disappears

22. BIT STATUS:FOG FAIL ! If the FAIL lamp appears –
The Display Window will show two BIT words:
a. LRU/SRU information and b. the failure code.
(Write up “AHRU Failed x amount of times”. It is not necessary to write up the failure BIT info/code.)
The CDU stores the last 16 faults.

23. Starting Procedure
Step 1. Mode Selector – As desired (you always desire SLAVE while you are at FT Rucker)
a. The Lower Console Lights have to be on for the lights on the DCP to be seen.
b. The default mode is SLAVE – Therefore, you should not have to move, touch or otherwise fondle anything for it to be in the SLAVE mode.

24. Step 1. Mode Selector – As desired. (Continued)
Starting Procedure
Step 1. Mode Selector – As desired. (Continued)
Do Not select anything other than SLAVE!
Do Not select DG unless you are above 70° Latitude (Be sure you have CAO approval to be out of the LFA)
THOU SHALL NOT select CCAL!!

25. Call Maintenance Starting Procedure
Step 2. HSI – Check to see that HSI heading agrees with a known magnetic heading [STANDBY (Magnetic) COMPASS].
If the HSI does not line up after the 45 second alignment time + your patience -
Call Maintenance

26. Failures
If a failure is detected, the FAIL lamp will illuminate, and the display window will show a message indicating which AHRU has failed.
Press Enter to clear the fail light
If the fail light clears – continue mission
If fail light does NOT clear – CALL MAINTENANCE

27. ELECTRICAL POWER CHARACTERISTICS
A Primary power interruption of less than 200 milliseconds does not cause a system realignment.
Power interruptions more than 200 Milliseconds
CAUSE SYSTEM IN-AIR RE-ALIGNMENT
The ATT and HDG warning flags will be in view to indicate the system is doing a re-alignment.
DURING THE RE-ALIGNMENT PHASE, MAINTAIN STRAIGHT and LEVEL UNACCELERATED FLIGHT (2.5 to 45 seconds)

28. “Hold the Controls” NOTE During aircraft shutdown,
when engine power control levers are moved from the Fly to Idle position, or
when the generator switch position is changed to off, and
electrical power assignment is changed from Main Generator to APU Generator,
an un-commanded right or left pedal drive of up to ¼ inch may occur, returning to neutral position within 3 seconds.

29. If the Collective Position Sensors are out of null
CLTV NULL = YAW DRIFT
If the Collective Position Sensors are out of null
the aircraft will have a
greater tendency to YAW at a HOVER
than with the older system.

30. especially if you have seen the fail light on the DCP.
YAW DRIFT
If the YAW DRIFT is more than you are accustomed to or otherwise seems abnormal –
Write it up,
especially if you have seen the fail light on the DCP.

31. GYRO ERECT IN SLAVE MODE
FAST ERECT SYNC
may be activated in air to perform a fast Heading error resetting in a magnetic sterile area.

32. This is a MAINTENANCE FUNCTION
CCAL
This is a MAINTENANCE FUNCTION
Don’t go there!!

33. COMPASS CALIBRATION MODE Lest ye be tempted
Once CCAL begins it MUST be completed!!! (You don’t know how)
CCAL is ONLY used on ground.
You CAN NOT get out of CCAL using the MODE select button!!!
HOLD THE CONTROLS!!!!!!
(you don’t know why but it can be ugly!)
FOGS CAN NOT BE ZEROIZED after initial calibration.

34. COMPASS CALIBRATION (CCAL) MODE
IN CCAL MODE THESE FUNCTIONS ARE ENABLED:
COMPASS CAL START
COMPASS CAL STORE
COMPASS CAL COEFFICIENTS READ
COMPASS CAL COEFFICIENTS WRITE
BIT STATUS REQUEST
BIT HISTORY REQUEST
INDEX SET REQUEST (NEVER EVER CHANGE!!!)
ETI GET REQUEST
If you don’t know what this means –
why are you in CCAL?

35. GO FLY!!! Summary Turn on Lower Console Lights Be sure MODE is SLAVE
Be sure HSI heading agrees with a known magnetic heading
GO FLY!!!