A320 ISIS. What are the ADIRU inputs for?
 

Hi, folks

I have been looking at the schematics, even in the AMM, and I can't find what information is received from the ADIRU 1 (3 in case we do the switching).
The ISIS gives air data information parting from the stby probes, and it has its own rate gyros and accelerometers. So, why does it need an ADIRU?

thanks

Hi MB2002,

Good question! I think it's an error on the schematic diagram. The ISIS should be completely independent of ADIRU information.

The ISIS indicator receives digital data inputs from the following components:
(a) Inertial Reference

Two ARINC 429 high-speed buses provide the ISIS indicator with the optional magnetic heading indication (label 320).
In normal configuration, the parameters displayed on the ISIS indicator come from the ADIRU 1.
When a failure is detected, switching on the ADIRU 3 has to be performed manually through the ATT/HDG selector switch on the SWITCHING panel.

Thanks spannersatcx. That would make sense.

Good question...
 

Microburst2002,

Good question. I don't have the answer. Obviously, you've had a good look at FCOM 1.34.25 P1 which shows the schematic. Hence, the question.

For those reading this without convenient access to the FCOMs, the inputs are: ILS1 or MMR1, ADIRU1, ADIRU3, Standby Pitot, Standby Static, DC ESS, HOT BUS 1. All these things are shot into the ISIS Accel/Gyrometer. (Whatever that is?)

Based on the description and operation, the display can 'precess' and may need to be 'set right'. This poses the valid question: What do the ADIRUs 1 and 3 have to do with it....if it has to be 'reset'?

Again, I don't know.

Further, if you'll have a look at FCOM 3.02.34 P6, which is the abnormal for loss of ADRs..(further, loss of IRs), the list of inopertive components does not include the ISIS. So, obviously, there is not a required input from the ADIRUs 1 and 3 for the thing to function.

(This makes sense....I can't image a standby attitude/airspeed system that is dependent on primary systems for operation.)

Interesting.....


Fly safe,


PantLoad

Thank you guys

Spannersatx

I did read that paragraph in the AMM, too, but I found it ambiguous. It says "the parameters", as if it meant several of them, like ALT, SPD, ATT...

However I think it is as you suggest: ADIRUs only provide HDG to the ISIS that include that option and all the other parameters are calculated by the ISIS by its own means.

Right now I don't recall if the ISIS is working when the ADIRUs are still OFF when entering the cockpit first flight in the morning. I think it is. They shoud be, I guess. Otherwise they wouldn't meet their STBY function. Or would they? With bat only, one ADIRU will be supplied from the HOT BUS 2, so why not?

Anyway. As usual, almost no info in the FCOMs, and even the AMM lacks it.

optional functions
 

AMM says that the IR inputs are required for "optional functions".
wondered what that could be?..
then, noticed the ISIS in our SIM also provides HDG ! (but not the ones onboard)
I wonder if anyone else has such a configuration?

Just a thought: the ISIS displays Mach Number - wouldn't a temperature input (from ADRs) be necessary to compute this?

No. Mach number is independent of temperature. It is determined only by IAS and pressure altitude.

I will back up Spanner's answer with confidence on ALL ISIS systems. ADIRU/IRU's etc supply heading info only, yes the ISIS will operate with all other functions W/O IRU's aligned. The only question I forgot the answer to was that the ADIRU provides a reference to North, I believe if you align the responsible IRU and ISIS is up then the IRU is lost heading will still be operational on the ISIS as it had allready been given true north info.

Actually Mach No. or the local speed of sound is almost solely dependent on air temperature, it is directly proportional to the square root of the absolute temperature.

The inputs to a Machmeter are pitot pressure and static pressure, no temperature inputs are required.

For a given Mach Number, TAS will increase with temperature, which is why the Supermarine Swift, being Mach limited, went to Libya to capture the world speed record. (Mike Lithgow, September 1953, 737.7 mph.)

Sorry, Max Mach number and the speed of sound are two different things. The speed of sound is dependent - Mach number is not.

Really, Mach 1 is the local speed of sound, Mach .5 is half the local speed of sound and Mach 2 is twice the local speed of sound. How are they different?

Mach number is a dimensionless ratio, the speed of sound is is a speed. Two different things. MAch number is stated to be indeendant of temperature because temperature does not change its relationship to pressure altitude or indicated airspeed.

Max:
 

M = v/a where M = Mach no, v=TAS, and a=the local speed of sound.

Consider both v and a are dependent on temperature, correct?

Since the antecedent (v) and consequence (a) are acted upon by the same number, namely temp, it has no effect on the overall ratio.

TAS = EAS / square root of relative density

where relative density is actual density over ISA sea level density

MN = TAS / speed of sound at ISA sea level times square root of relative temperature

where relative temperature is actual temperature (in ºK) over ISA sea level temperature

Therefore:

MN = EAS / square root of relative density / standard speed of sound times square root of relative temperature

From the gas law equation:

relative pressure = relative density * relative temperature

so:

MN = EAS / standard speed of sound times square root of relative pressure

where relative pressure is actual pressure over ISA sea level pressure.

According to this, you only need pressure altitude and EAS (from undisturbed airstream dynamic pressure) to calculate MN. No need for temperature.

Talk about thread drift!

Back to the original question

Later ISIS indicators have what is called a "boresighting" function. This uses ADIRU attitude inputs to accurately match the ISIS attitude to that of the ADIRU's to minimise errors.

When installing a new "boresighting" ISIS the boresighting function needs to be used to, in effect, calibrate the display. It's a sort of once only caging of the gyro.

Remember that a data bus input to another device contains all the info on that bus. The receiving unit extracts the info it requires in order to function. Earlier units extracted only heading. These later units also take attitude data.

Hope this helps.

Thanks all, I'm now totally baffled.....
I thought ISIS stood for Independent Standby Instrument System...
If it still relies on ADIRU inputs, it's NOT "independent"......

As an ancient, I come from the era of the SFENA standby horizon, which was a little 3" electromechanical gyro-driven horizon (most pilots here will be familiar with the "SFENA", or its American license-manufactured equivalent). No ARINC 429, nothing... just an electrically-powered self-contained high-inertia gyro, that ran off the DC emergency bus (via an inverter), and that would keep running (inertia) for several minutes, even after the DC went....

Without access to the AMM, can somebody point me to the relevant ISIS documentation (if it exists on the 'net, or has been referenced earlier on one of the AF447 threads?).

CJ

Yes it is independent for the required functions it performs, it is enhanced for the desired functions.