Baro Altitude Necessary For IRS Alignment?

[QAVION]

From previous readings on the subject, I seem to recall that IRS's need to know how far away from the centre of the earth the aircraft is both during alignment and subsequent navigation. As I recall, a "distance from the centre of the earth to an average sea level height" is added to airplane baro altitude to estimate the airplane distance from the centre of the earth. In the absence of ADC altitude data during alignment, will the IRS's use some kind of default altitude or can they roughly compute altitude themselves during alignment from the earth's movement?

If they can't align themselves without ADC data, can the IRS's (with FMC's) still be considered a "stand-alone" navigation system?

Thanks.
Rgds.
Q.

[comxtek]

Consider this, the IRU along with the ADC computes the following:
Position (lat long)
True & mag heading
Winds and direction
Attitude
Velocity
Accelerations
Angular Rate Data
Altitude

The IRU only uses ADC data for winds, altitude, and velocity.
The main purpose of the IRU is to sense and compute linear accelerations and angular turning rates about the ariplane's roll, pitch and yaw axis.

*Data taken from B737 Classic AMM

So yes, the IRS is standalone. But for alignment, valid ADC data is needed...or else alignment will not complete. Once aligned, ADC data is not needed for the IRS to do it's job.

[QAVION]

Thanks, Comxtek.

Might give this a try on a few other aircraft when I get back to work, to see how they compare with the 73' (I'd be interested to see what FMC or EICAS msgs are generated).

Cheers.
Q.

[QAVION]

"But for alignment, valid ADC data is needed...or else alignment will not complete."

Hmmm..... Tried to align the IRS's on a 747-400, yesterday, with all the ADC CB's pulled. After 10 minutes, apart from the expected missing displays such as V/S, altitude and speed, the IRS's appeared to align without any problems: ND Map displays with compass headings and PFD attitude displays seemed normal, there were no FMC scratchpad messages... and, as far as I could see, no additional EICAS msgs.

Are our manuals lying to us? :confused:

Rgds.
Q