ISIS and the Standby Instruments

When all three ADIRUs and all six displays fall silent, two "old-school," independent instruments remain in the cockpit: a 3-ATI screen on the centre panel (the ISIS) and a fluid-damped magnetic compass on top of the windshield centre post. Their reason to exist is written in the AMM's first sentence — to let the crew check the main systems; their ultimate role is written in the FCTM — in the "unlikely event" of a total IRS loss, the standby instruments are the only remaining reference for attitude, altitude, speed, and heading.

Small as it is, the ISIS packs a miniature ADIRU: its own gyros, its own accelerometers, its own pressure sensors, its own alignment ritual, and its own quirks.

1. The role — the main system's inspector, the last survivor at the end

Per AMM 34-20-00:

The standby navigation system enables checks by the flight crew of the navigation data provided by the main system: the Air Data/Inertial Reference System (ADIRS) (ATA 34-10-00).

Its everyday job is umpire: whenever a NAV IAS/ALT/ATT DISCREPANCY sounds, the procedure's first line is "cross-check with the standby instrument." In a 3-into-2 vote, the ISIS is the fourth ballot.

The Integrated Standby Instrument System (ISIS) indicator replaces the three conventional standby instruments: - The standby altimeter - The standby horizon indicator - The standby airspeed indicator.

Beyond the three-in-one it also carries Mach, baro reference, LS, and BUGS. The casing is a standard 3-ATI unit (ARINC 408A), but its internals are digital. Per AMM 34-22-00:

This unit includes three single-axis rate sensors (with analog outputs) and their temperature probes, two acceleration sensors and their temperature probes, and an electronic board.

Three gyros, two accelerometers — one accelerometer fewer than an ADIRU: enough for attitude, but no inertial navigation. Speed and altitude come from its own pneumatics. Per AMM 34-22-00:

The standby airspeed-indicator function measures the pitot/static pressure differential from the standby air-data system and gives the airspeed indication in knots.

The "standby air-data system" is the STBY probes (pitot 3 + static 3 pair) — the same probe family as ADR 3. That bloodline is double-edged: on cross-check, "ADR 3 and ISIS agree" cannot rule out a common-mode blockage (the NAV ADR 1+2+3 FAULT procedure deliberately notes STBY INST MAY BE UNREL); but conversely, if they disagree, something is wrong in that probe family or a computer.

2. The 90-second start-up ritual, and three kinds of "reset"

Per AMM 34-22-00:

Immediately after the ISIS indicator is energized, the display shows these four yellow digital boxes for approximately 90 seconds: - ATT for the standby horizon function - SPD for the airspeed indicator function - ALT for the altimeter function - INIT 90 S for the initialization function.

Those 90 seconds are the ISIS's miniature alignment — the gyros and accelerometers finding the horizontal. It, too, hates motion: an excessive-motion test runs throughout; if the aircraft was moved, attitude still displays at the end but an ATT:RST flag asks you to redo it. The redo is the same as an in-flight reset. Per FCOM DSC-34-10-20:

The attitude indication can be reset by pressing this pushbutton for at least 2 s. The aircraft must be level during this procedure.

The reset takes about 10 s, with an "ATT 10s" countdown. When is it needed? The ATT:RST flag has three triggers: excessive motion during initialisation; after 350 hours of continuous power (a periodic settling of accumulated gyro error — inevitable on an aircraft that stays powered through turnarounds); and WAIT ATT flagged for more than 10 s. The relationship between WAIT ATT and ATT:RST is "dizziness" versus "injury": a brief loss of attitude data (say, a manoeuvre exceeding the sensor range) shows WAIT ATT first and blanks the ball; recover within 10 s and it restores automatically; only if it cannot does it escalate to ATT:RST. The sensor "dizziness threshold" is in the AMM. Per AMM 34-22-00:

a monitoring starts when the angular speed in the x, y or z axis becomes more than 95 deg./s.

On the accelerometer side, a sustained x/y acceleration above 3.25 g pauses monitoring and raises WAIT ATT — never reached in normal transport flight, a backstop for "abnormal manoeuvre and the standby instrument must still live." Maintenance also has a boresighting procedure. Per AMM 34-22-00:

This function gives a procedure to make sure that the ISIS is correctly installed (ISIS attitude data aligned with the attitude data on ADIRU1 or ADIRU3).

So "ISIS and PFD attitude differ by half a degree" is not necessarily a failure — it may be a mounting misalignment; do not rush to remove the unit (reducing spurious removals is exactly why the function exists).

3. Reading conventions — same source as the PFD, different clothes

Speed tape. Per FCOM DSC-34-10-20:

A white scale moves in front of a yellow triangle indicating the airspeed. The scale ranges from 30 to 250 kt, with a mark every 5 kt, and from 250 to 520 kt, with a mark every 10 kt.

Mach. Per FCOM DSC-34-10-20:

The Mach number is displayed in green when it goes above 0.5. And it disappears when it goes below 0.45.

The 0.5/0.45 hysteresis prevents flicker near the threshold, and is the origin of the difference-table footnote (ISIS Mach not displayed below M0.50 in climb). Altitude window. Per FCOM DSC-34-10-20:

The altitude scale ranges from -2 000 ft to 50 000 ft, every 100 ft, with digital altitude indications every 500 ft.

The hundreds are large digits, the tens and units roll drum-style, and a white NEG appears beside negative altitude. The baro knob is "press for STD, one turn = 10 hPa." Per FCOM DSC-34-10-20:

This knob enables the selection of a barometric pressure, setting a variation of 10 hPa per knob rotation.

The standard barometric pressure can be selected by pressing the barometric knob.

Range 745–1100 hPa — the altimetry discipline later (±20/±60 ft tolerance, the two-way transition-level reset, the descent-prep preset) all include "set the ISIS too." The lateral acceleration (electronic reincarnation of the slip ball) has a cool parameter — full deflection at 0.2 g, hidden above 1 g. Per AMM 34-22-00:

When the lateral acceleration is 0.2g left and right sides, there will be a maximum deviation of the symbol. The symbol is not shown when the lateral acceleration is more than 1g left and right sides.

The attitude ball uses the PFD vocabulary (a line every 2.5° in pitch, a red chevron pointing back at extremes), with one quirk to warn students about. Per FCOM DSC-34-10-20:

When leveling the wings, after performing a small turn of a small bank angle, the displayed roll attitude may temporarily be incorrect by a few degrees.

The two-accelerometer IMU corrects attitude slowly against "gravity = down," and the centripetal acceleration of a turn fools it for a few seconds (the same physics as the ATT-mode erection cut-out — the deluxe version has a guard, the stripped-down version relies on you knowing).

4. The LS function — a borrowed ear that goes wrong in reverse

Per FCOM DSC-34-10-20:

When the LS pb is pressed, the deviation scales are displayed.

The LS deviations that are displayed correspond to the LS approach: ‐ Selected on the MCDU, or ‐ Tuned on the RMP.

The physical source is a single receiver. Per AMM 34-22-00:

One ARINC 429 low-speed bus gives the ISIS indicator with the LS function (label 173 for the localizer and label 174 for the G/S).

The ISIS LS is an exclusive relay of ILS 1: if MMR 1 fails or ILS 1 is dispatched inoperative under the MEL, the ISIS LS is caught in the same net (the MEL explicitly writes "ISIS ILS indication considered inoperative"). On NCD it shows an empty scale without the diamond. The critical caution is worth memorising in full. Per FCOM DSC-34-10-20:

Do not use the ISIS LS for takeoff using the localizer of the opposite runway, or for a back-course localizer approach. The LOC deviations are given in the wrong sense.

Mechanism: the LOC beam's 90/150 Hz modulation maps to "left/right" only on the front-course side; used from behind the antenna, left and right are reversed. The PFD has display logic to protect this; the small ISIS screen does not — the low-visibility-take-off technique of holding runway centreline on the LOC must read the PFD yaw bar, not the ISIS.

5. BUGS — available, but the manual advises against it

The BUGS page offers 4 speed bugs and 2 altitude bugs, set by the knob. The FCOM attaches a warning-off note. Per FCOM DSC-34-10-20:

Use of the ISIS bugs function is not recommended because, in the event that both PFDs are lost in flight, when the ISIS bugs were previously set for takeoff, then for the approach, the bugs would remain at the takeoff characteristic speed settings.

Cold logic: a bug is dead, configuration and weight are alive. On the day both PFDs go dark, a row of "take-off speeds" becomes a source of misinformation on approach — better none at all. For a speed reference, the pitch/thrust tables and the QRH/OPS speed cards are the living water.

6. Power supply — dual 28 V DC and the "50 kt rule"

Per AMM 34-22-00:

The ISIS indicator is supplied with 28VDC from: - The ESS bus (403PP) first (when the aircraft electrical network is energized), and - The HOT bus (701PP), when the speed detected by ADIRU1 and/or ADIRU3 is more than 50Kts.

Recognise the 50 kt threshold? It is one user of the ADR low-speed discrete LSW 1 (50/54 kt). Intent: at low speed on the ground only the ESS bus feeds it, avoiding a slow drain of the battery while parked; once the roll passes 50 kt (about to fly) the hot bus joins — from then on, even if the whole network collapses, the ISIS feeds directly on the battery. There is also an anti-transient design. Per AMM 34-22-00:

The transparency time for the ISIS indicator is not less than 50 ms (memory of the configuration on power supply transients between 50 and 200 ms). The display stops during the power supply cutoff between 50 and 200 ms.

The picture may "freeze" for an instant during a supply switch — not a hang, but holding its breath while power returns, remembering the baro setting. The ISIS also borrows a ground-speed line from IR 1/IR 3 as an air/ground backup. Per AMM 34-22-00:

Two ARINC 429 high-speed buses supply the ADIRU ground-speed information to the ISIS indicator. This information is used as a backup source for the ISIS ground or flight condition.

Which IR feeds it depends on the ATT HDG rotary — a fourth identity for that selector. A typical air/ground effect: the white MAINT flag appears only on the ground (below 60 kt), never in flight.

7. The standby compass — the aircraft's last "unpowered" instrument

Per AMM 34-22-00:

The standby heading is given by an independent magnetic compass which gives the aircraft magnetic heading to the flight crew.

Per FCOM DSC-34-10-20:

There is a compass located on top of the windshield center post. The deviation card is located above the compass.

The construction is nineteenth-century reliable: a magnetic element floating freely in damping liquid. Per AMM 34-22-00:

The standby compass contains a magnetic element (6) which can freely turn in the compass bowl (3), fully put in a damping liquid.

Maintenance has two adjustments: a scale on the top plate (rotating the housing to correct installation offset) and two small holes B/C (a special key turns E-W and N-S compensator magnets to correct deviation). The crew's interface is a curve. Per AMM 34-22-00:

On the standby compass side, there is a correction curve that the crew can see. This curve lets the pilot correct the value to get the magnetic heading of the aircraft.

When is it truly used? The last line of the ATT-mode IR alignment procedure — "cross-check heading regularly with the standby compass and update as required": ATT-mode heading drifts, and this compass is what feeds it (read the value through the correction curve first). Its lighting runs from a separate STBY COMPASS switch, and it can be dispatched inoperative under the MEL (with three IRs working and the switching check passed as prerequisites).

Key numbers

Self-test

[!note]- Q1. Are the ISIS's attitude, speed/altitude, LS, and air/ground data self-produced or bought in? From whom are the bought-in ones? Attitude: self (own IMU). Speed/altitude: self (own pneumatics from STBY probes). LS: bought in from ILS 1 (MMR 1). Air/ground backup: bought in from IR 1/IR 3 ground speed (ATT HDG selects which).

[!note]- Q2. After seven days powered through turnarounds, why might the ISIS raise ATT:RST? How is it handled, and with what attitude prerequisite? 350 hours of continuous power accumulates gyro error. Press RST ≥ 2 s with the aircraft level; ~10 s reset.

[!note]- Q3. WAIT ATT appears — what first? When does it clear itself, when does it escalate? It is a brief loss of attitude data; wait. Clears automatically within 10 s if attitude recovers; escalates to ATT:RST if not.

[!note]- Q4. Why can the ISIS LS not be used to hold direction on an opposite-runway take-off, and what replaces it on the PFD side? The LOC deviation sense is reversed behind the antenna. Use the PFD yaw bar instead.

[!note]- Q5. ISIS and ADR 3 both read high on speed — can they clear each other? Why not? No. They share the STBY probe family, so a common-mode blockage would fool both together — which is why the all-ADR-off procedure notes STBY INST MAY BE UNREL.

[!note]- Q6. Why the "hot bus only after 50 kt" design, and who sends the 50 kt signal? To avoid draining the battery while parked; the hot bus joins once the roll passes 50 kt (about to fly) so the ISIS survives a network collapse. The signal is the ADR low-speed discrete LSW 1 from ADIRU 1/3.

Key takeaways

References

• FCOM DSC-34-10-20 — full ISIS function, compass location, LS caution, BUGS advisory, flag family.
• AMM 34-20-00 — standby-system role.
• AMM 34-22-00 — three-in-one, IMU internals, 90 s init, monitoring thresholds, power supply, interfaces, compass construction, boresighting.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.