2 Quick Q's

[Cron]

Nothing in my notes for either of the below, so any help appreciated.


1. The purpose of the TAS input, from the air data computer, to the Inertial Navigation System is for:



2. ATT Mode of the Inertial Reference System (IRS) is a back-up mode providing:


Many thanks in advance.

[mono]

1. Wind vector. Display shown on the lower RHS of the EHSI. This shows the wind speed and relative direction and is calculated using TAS, drift, ground speed a few other parameters I can't remember. This is also fed on to the FMS for use in calculating fuel burn, etc.

2. Horizon attitude and heading only (magnetic heading must be manually inserted for the heading to be displayed and should be cross checked with the direct reading compass every 15 mins (ish))

[avioniker]

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

[alexban]

mono:
If I remember well,the wind direction and speed is determined by the IRS. But again,maybe I'm wrong.
In case of iced pitot/probes is there any valid speed information you can use ? from the irs or gps? Tas , or GS?
thks alex