Last things first:
The ATT mode essentially turns that big (okay actually fairly small now) expensive ADIRU into an AHRS. If the computer fails you can still use the accelerometers for keeping upright and pointed in the right direction.
It provides attitude and heading data based on vertical and heading at the time of initialization of the mode. If you're in flight it is most advantageous to have the autopilot engaged and coupled to the still working system so the heading and attitude will have some basis in reality.
TAS within the ADIRU is used as a comparison valid signal to aid in detecting faults. However, it is used by the aircraft for a number of functions and provides valid bits to allow or inhibit numerous modes of DFCS operation:
Below are extracts from the Systems Description Manual, Sections 22 & 34
Air Data Inertial Reference Unit (ADIRU)
True airspeed is used for calculating the TAS/CAS conversion.
Roll Mode disengage Conditions
The true airspeed (TAS) must be valid in the CWS heading hold mode.
FD Bias out of View (BOV)
The true airspeed (TAS) is invalid
ND Indications
True Airspeed and Wind Speed and Direction
True airspeed does not show on the ND until TAS is more than 100 kts. The normal display when TAS is 100 kts or less is three dashes. This is the NCD indication.
Wind speed and direction does not show until TAS is more than 100 kts. When TAS is 100 kts or less, wind speed and direction is blank.
WXR System Fail, Messages and Display
Loss of CAS and TAS.
These statements will hold true pretty much for any model of 737 whether you have a CADC/IRS or ADIRU setup
Last edited by avioniker; 4th Feb 2004 at 02:28.