View Full Version : A320 ISIS. What are the ADIRU inputs for?

21st Feb 2010, 09:33
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?


21st Feb 2010, 11:25
Hi MB2002,

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

21st Feb 2010, 12:07
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.

21st Feb 2010, 12:21
Thanks spannersatcx. That would make sense.

21st Feb 2010, 13:59

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.)


Fly safe,


21st Feb 2010, 14:57
Thank you guys


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.

21st Feb 2010, 16:37
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?

Down...2 greens
19th Nov 2011, 12:59
Just a thought: the ISIS displays Mach Number - wouldn't a temperature input (from ADRs) be necessary to compute this?

19th Nov 2011, 16:21
No. Mach number is independent of temperature. It is determined only by IAS and pressure altitude.

19th Nov 2011, 16:32
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.

Max Angle
19th Nov 2011, 16:41
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.

Neptunus Rex
19th Nov 2011, 17:36
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.)

19th Nov 2011, 18:11
Sorry, Max Mach number and the speed of sound are two different things. The speed of sound is dependent - Mach number is not.

Max Angle
20th Nov 2011, 00:20
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?

20th Nov 2011, 07:07
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.

20th Nov 2011, 08:26
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.

20th Nov 2011, 11:19
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 ēK) over ISA sea level temperature


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


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.

20th Nov 2011, 16:48
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.

20th Nov 2011, 18:21
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?).


20th Nov 2011, 19:39
Yes it is independent for the required functions it performs, it is enhanced for the desired functions.

20th Nov 2011, 20:22
I thought ISIS stood for Independent Standby Instrument System...Nope, it's Integrated Standby Instrument System, but I agree, it should be able to function independent of the main system.

20th Nov 2011, 20:24



The back of ISIS is equipped with two pressure connectors:
- one is connected to the standby pitot probe for total pressure
- the other one is connected to the combined standby static probes for
static pressure acquisition.
The ISIS indicator contains the following subassemblies:
- one pressure module,
- one inertial module,
- one computation module,
- one display module and,
- one interface module.
The pressure module is connected to the total and static pressure
connectors. Each pressure line is connected to a pressure sensor in
the pressure module. The inertial module is composed of three
gyrometers (gyrolaser) measuring angular speeds and two specific
linear accelerometers (pitch + roll). The computation module includes
a Central Processing Unit (CPU), which calculate the operational
parameters (attitude, altitude and airspeed), and a graphic card, which
receives the data from the CPU and sent it to the display module. The
display module is fitted on the front face of ISIS. It is of the Liquid
Crystal Display (LCD) type. The front face is also fitted with several
knobs for operation purposes. The interface module is composed of:
- a filtering board linked to an electrical connector at the back of ISIS,
- the 28V DC power supply unit,
- the interface board which links the pressure, inertial, computation
and display modules.


ISIS is supplied with 28V DC from the ESSential BUS bar. In case
of loss of this bus bar, the HOT BUS bar automatically takes over,
provided the Computed Air Speed (CAS) is greater than 50 kts. In
case of a power supply cut-off less than 50 ms, there is no effect on
the display.


When ISIS is energized, a delay of 90 seconds is necessary for
initialization. ISIS computes and displays own air and inertial
parameters. When the airspeed data is not valid, the CAS information
is provided as a back-up by Air Data/Inertial Reference Unit (ADIRU)
1 or ADIRU 3 regarding the ATT/HDG selector switch position. The
ADIRU also provides a Ground Speed (GS) information for
flight/ground condition, as a back-up. A BAROmetric selector knob
enables the display and adjustment of the standard barometric pressure
in hPa. When the Landing System P/BSW, located on the upper right
part of the indicator, is pushed, the G/S and LOC scales come into
view. The BUGS P/BSW allows to display the BUGS page. This page
is used to program characteristic speeds and altitudes displayed on
the related speed and altitude scales. The (-) P/B is used to get access
to the next bug and the (+) P/B to return to the previous bug. Air and
inertial parameters are processed by the computation module using
data from the pressure and inertial modules. The pressure module
sends the total and static pressure data while the inertial module sends
accelerations and angular rates data. Once computed, the parameters
are sent to the display module for indication to the crew. The LS data
is sent by the Multi-Mode Receiver (MMR) 1. Air Data/Inertial
Reference System (ADIRS) parameters computed by ISIS are sent to
the Flight Data Interface and Management Unit (FDIMU). The ISIS
can operate from 0 to 600 kts without deterioration of airspeed data.
It can operate from -2.500 to 55.000 ft and up to 40.000 ft/min without
deterioration of altimeter data. For the horizon data, the ISIS can
operate from -180 to +180 deg without deterioration.

20th Nov 2011, 21:13
Nope, it's Integrated Standby Instrument System, but I agree, it should be able to function independent of the main system.

So you would prefer conventional stby horizon and altimeter over ISIS? It replaces those, under most circumstances gives heading, ils et cetera.. Doubt the old whisky compass will ever go away, hell I have one in my truck!

23rd Nov 2011, 00:41
I believe te ADIRU also supplies the lateral acceleration indicator

23rd Nov 2011, 03:47
Has anyone ever tried flying the ISIS ILS? Those two tiny GS
and LOC indicators are about as big as a gnat's tit - I almost
need a pair of binoculars just to see the bloody things.

23rd Nov 2011, 05:36
ISIS or steam guages, what would you prefer? And why if you wish to comment.