58 CT-142 IRS

Question 1

IRS components

Answer 1

1. IRU
   The IRU contains an inertial sensor assembly, microprocessors, power supplies, and aircraft electronic interfaces
   Located under the centre floor of the crew compartment
   Housed on a rack that also serves as the electrical and mechanical interface between the IRU and the aircraft
2. MSU
   The MSU provides inertial mode selection, status indication, and remote test initiation
   Located in the flight deck centre console

Question 2

IRS modes of operations

Answer 2

Modes of Operation

Align mode
The IRU aligns its reference axes to the local vertical and computes heading and latitude by measuring horizontal earth rate components
Completes alignment in a minimum of 2.5 minutes at the equator and a maximum of 10 minutes at 60º to 70º latitude
At latitudes above 70º additional alignment time may be required

Nav mode
In the Nav mode, the IRU supplies inertial position reference for the aircraft and provides outputs via the FMS:
Aircraft attitude
Body rates
Body accelerations
True heading
Velocity vectors
Wind data (requires TAS from DADC)
Latitude and Longitude
Inertial altitude
The IRS uses the latitude and longitude entered during the Align mode as the starting point for positional computations.
Provided this information, the navigation components calculate present position, velocity, heading and attitude
The IRU does not permit external updates of latitude and longitude while in the Nav mode
Some inertial systems do allow this

Attitude mode
once rapid levelling has been completed provides valid outputs of:
body axis accelerations
attitude rates and angles
vertical velocity
inertial attitude
No navigation outputs are provided
Should be selected in the air under two conditions
When the MSU fault enunciator illuminates (indicates critical fault & all outputs invalid)
After the IRS has temporarily lost all power

Question 3

IRS Alignment requirements

Answer 3

Operator must enter present position via the FMS
IRU conducts a reasonability test of new position: compares with previous position at last shutdown.
If new position > 1º of the stored value, test is failed
A correct position may fail test if
New IRU has been installed
Aircraft has been moved since last shutdown
Entering identical co-ordinates twice will override the test

Question 4

IRS System performance test

Answer 4

Entered latitude must pass system performance test at end of alignment phase
Latitude entered must be within 30 min of arc of the latitude computed by the IRU during alignment
If the entered latitude passes the system performance test, alignment is complete and it allows entry into the Nav mode

Question 5

IRS integration with the ATS

Answer 5

- Information is fed from the IRU directly to the ATS. The ATS maintains a data base of inertial information, including:
True heading
True track angle (TMG)
Present latitude and longitude
Inertial groundspeed
Wind velocity and direction
System status

Question 6

Two main categories of information provided by IRS?

Answer 6

Navigation and Attitude