Interphone and the Ground-Crew Call

Interphone is the "person-to-person" part of internal communication — no external radio, just linking crew, cabin and ground personnel within the aircraft and around the ramp. This chapter's interphone comes in three parts: the flight-crew interphone (crew among themselves, plus the ground engineer via the external power panel); the service interphone (flight crew, cabin crew and ground service over the aircraft's jacks on the ground); and the cockpit-to-ground-crew call (the pushbutton-and-horn call between cockpit and ramp engineer).

The third part hides this article's most memorable, cross-chapter gem: the ground-crew call system's external horn doubles, on battery power, as the aural warning for an APU fire, an equipment-ventilation fault and the ADIRS running on batteries — because it hangs on the hot battery bus and keeps sounding when main power is gone.

1. Architecture

① Flight-crew interphone (part of CIDS)
   cockpit crew ←→ AMU ←→ CIDS ←→ external power panel jack ←→ ground engineer

② Service interphone: ground telephone
   13 jacks (whole aircraft) ──audio──▶ amplifiers in both CIDS directors ◀── cabin handset
        ▲ 5 isolation units (a shorted jack is isolated, not dragging the net down)
   condition: nose gear compressed (on ground) → automatic; SERV INT OVRD forces it

③ Cockpit-to-ground-crew call: pushbutton + horn, on hot bus 702PP
   cockpit→ramp:  CALLS/MECH pb → external-panel COCKPIT CALL blue + external horn
   ramp→cockpit:  COCKPIT CALL pb → FWC buzzer + amber MECH flash on ACPs
   ★ on battery power the external horn doubles as the aural warning for:
     APU FIRE · BLOWERS LO FLOW (equipment ventilation) · ADIRS ON BAT · BAT FEED WARN (all on ground)

2. Flight-crew interphone

The most basic part. Per FCOM DSC-23-20-10:

This system allows the flight crew to communicate among themselves and, through a jack on the external power panel, with the ground mechanic.

Operationally it is the ACP of article 3: select INT and set INT/RAD to INT (or use "direct mode"). Mechanically it is part of the CIDS — the AMM states the cabin-and-flight-crew interphone system is part of the CIDS (23-43 → 23-73), so its signal routes through the CIDS directors (article 7). For now: flight interphone = via the AMU and CIDS.

3. Service interphone — the ground "extensions"

The service interphone is the ground maintenance/servicing "telephone network" across the aircraft. Per FCOM DSC-23-20-30:

The system provides communication between: The flight crew and the service interphone jacks. The flight attendant stations and the service interphone jacks. The different service interphone jacks. The service interphone system comprises: Thirteen interphone jacks. An OVRD switch located on the overhead panel. The audio lines from the interphone jacks are connected to both CIDS directors.

By default it works only on the ground (AMM §3: nose gear compressed, LGCIU giving no "ground signal" makes it operate automatically, with or without external power). The OVRD switch handles the "gear not compressed but communication needed" case (e.g. aircraft on jacks). Per FCOM DSC-23-20-30:

Auto: Ground personnel can communicate with the flight crew by means of the service interphone jacks after the aircraft has landed. The landing gear must be compressed. ON : Communication is possible when the landing gear is not compressed. The ON light is white.

One engineering detail worth remembering — the isolation units. Per AMM 23-44-00:

The isolation units isolate the connected service interphone jacks, if there is a short circuit.

Ramps are cold and wet with many jacks; when one jack floods and shorts, an isolation unit drops just that jack rather than letting one bad jack pull down the whole service-interphone network — the "zonal isolation against a single point" idea, in communications.

4. Cockpit-to-ground-crew call — a two-way pushbutton and horn

This is the most direct call between cockpit and ramp engineer, two-way, one path each. Per FCOM DSC-23-20-30, cockpit calling the engineer (MECH pb, overhead):

Pressed (and held): COCKPIT CALL lights up blue on the external power panel. An external horn sounds. Released: COCKPIT CALL remains lighted. The ground mechanic can extinguish it by pressing the HORN RESET pb on the external power panel.

And the engineer calling the cockpit (COCKPIT CALL pb on the external panel):

Pressed: This calls the cockpit. The MECH lights flash amber on the ACPs and a buzzer sounds. Released: The MECH lights go out after 60 s if they are not reset on the ACPs. The buzzer stops.

This is exactly the "MECH amber flash, self-cancel at 60 s" of article 3 — the engineer pressed the external-panel button. The mechanic call horn is in the nose-gear well; the system works only on the ground with the nose gear compressed.

5. The battery-power aural warning — the horn's second identity

This is the point to keep. The ground-crew call system hangs on the hot bus 702PP. Per AMM 23-42-00:

The ground crew call system is supplied with 28VDC power from the hot bus 702PP.

The hot bus is on the battery directly, through no busbar relay, so it stays powered when all main power is gone and only the battery remains. This gives the system's external horn a second identity — a battery-power aural warning. Per AMM 23-42-00:

Moreover, when the aircraft is battery supplied, the system comprises an aural warning function for the following status: APU fire; ADIRS supplied by batteries; equipment ventilation faulty.

The operating section gives the fuller trigger list (all on the ground). Per AMM 23-42-00:

In addition this system provides warnings through the external horn for the following circuits: APU FIRE on ground; BLOWERS LO FLOW on ground with engines shut down; ADIRS ON BAT on ground; BAT FEED WARN on ground.

[!warning]- A horn sounding "for no reason" on the ramp is the battery warning you Scene: the aircraft is parked, main power off, only the battery live, and a horn near the nose gear sounds insistently. This is not someone pressing the mechanic call — the aircraft is using this system's horn to shout, on behalf of one of four things: an APU fire (ATA-26); equipment-ventilation blowers at low flow (BLOWERS LO FLOW — the avionics bay is overheating); the ADIRS running on the battery (ADIRS ON BAT, ATA-34); or BAT FEED WARN (a battery-feed anomaly, ATA-24). Why this system? Because it is on the hot bus and alive when main power is not — the "last horn that can still make a sound". This also explains why the MEL treatment of the CALLS/MECH items has to weigh this warning role.

Self-test

[!note]- Q1. What do the three interphones each connect, and which one is actually part of the CIDS? Flight-crew interphone (crew + ground engineer via the external power jack) — part of the CIDS; service interphone (crew/cabin/ground service over 13 jacks); cockpit-to-ground-crew call (pushbutton + horn).

[!note]- Q2. Under what condition does the service interphone work automatically? What do the SERV INT OVRD Auto and ON positions do? What do the isolation units protect against? Nose gear compressed on the ground → automatic. Auto = works after landing with gear compressed; ON = works with gear not compressed. Isolation units drop a shorted jack so it does not pull down the whole network.

[!note]- Q3. Cockpit presses MECH, engineer presses COCKPIT CALL — where does each light/horn/buzzer appear, and how is it cleared? MECH pb → external-panel COCKPIT CALL blue + external horn (engineer clears with HORN RESET). COCKPIT CALL pb → amber MECH flash + buzzer on ACPs (self-cancel at 60 s or RESET).

[!note]- Q4. With only the battery live and a horn sounding on the ramp, what four things might it be warning of, and why is this system still powered? APU fire, BLOWERS LO FLOW (equipment ventilation), ADIRS ON BAT, BAT FEED WARN. It is on the hot bus 702PP, alive when main power is gone.

Key takeaways

References

• FCOM DSC-23-20-10 — flight-crew interphone (crew + ground engineer via external power jack).
• FCOM DSC-23-20-30 — service interphone (13 jacks / CIDS directors / OVRD), two-way ground-crew call.
• AMM 23-40-00 / 23-42-00 — interphone families; ground-crew call system + battery aural warning (APU fire / ADIRS on battery / equipment ventilation), hot bus 702PP, ground-only with nose gear compressed, four external-horn triggers.
• AMM 23-44-00 — 13 service jacks, both CIDS directors, automatic with nose gear compressed, 5 isolation units against short circuit.

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.