ILS and the Landing-Sensor Chain

In CAT III weather, what leads a 200-tonne aircraft down to a 50-ft decision height is the half of the MMR that is an ILS receiver — the two "deviation angles" it outputs feed the AP, the PFD, and the EGPWS mode 5. This article covers the sensor side: what the beams are, how the receiver keeps itself honest (dual channels), why the display is "crossed," how tuning authority flows, who blows the whistle when the two MMRs disagree (LS TUNING DISAGREE), and the ATA-34 rows of the QRH CAT 2/3 equipment list. Guidance laws, capability downgrade, and autoland belong to the auto-flight chapter — this article supplies the goods, it does not drive.

1. ILS — two beams, one pair of deviation angles

Per AMM 34-36-00:

The function of the ILS is to provide the crew and airborne system users with lateral (LOC) and vertical (G/S) deviation signals, with respect to the approach ILS radio beam transmitted by a ground station. The localizer operates in a frequency band which ranges from 108.1 MHz to 111.95 MHz and the glide uses the band from 329.15 MHz to 335 MHz as defined by ARINC specification 710.

The crew tunes a single VHF frequency; the G/S UHF frequency is paired to it and follows automatically. The deviation is in DDM (difference in depth of the 90/150 Hz modulations): LOC ±0.155 DDM, G/S ±0.175 DDM (labels 173/174), refreshed every 50 ms — one PFD "dot," the AP capture criterion, and the "½ dot" of a callout are all dressed up from these two numbers.

The aircraft has two ILS/GLS receivers.

They live inside the MMRs (ILS 1 ∈ MMR 1, ILS 2 ∈ MMR 2), but the antennas are shared. Per AMM 34-36-00:

The A/C comprises two independent MMRs, 40RT1 and 40RT2, linked to: - a common localizer antenna 3RT - a common glide/slope antenna 4RT - a GPS antenna 43RT1 (linked to MMR1) - a GPS antenna 43RT2 (linked to MMR2)

A shared antenna sounds like a single point, but the antenna is a passive piece of metal with a poor repertoire of failure modes; the real redundancy is in reception and computation — that is where errors happen, hence the dual channels of §2.

2. Inside the receiver — the primary channel works, the monitor channel nit-picks

In this MSN group's MMR (the three-subsystem version) the ILS receiver splits into four blocks: VHF reception, UHF reception, a primary instrumentation processor, and a monitor instrumentation processor. The key sentence. Per AMM 34-36-00:

The monitor instrumentation processor performs a validity check of the primary instrumentation processor deviation output, and disables the outputs if the check fails.

The primary and monitor channels demodulate the same RF independently, each computes its own deviation, then they compare — and if they disagree, the receiver would rather fall silent (output disabled) than output a suspect deviation. This is the first link in the CAT III chain of trust: autoland dares to believe the ILS because the ILS distrusts itself first. The second link is outside the box (FMGES compares the two MMRs, §5); the third is in the auto-flight chapter (dual AP, LAND-mode monitoring). The box's housekeeper is the system processor. Per AMM 34-36-00:

The system processor controls the flow of data to the ILS and GPS receivers including mode and frequency selections, GPS initialization, and maintenance data requests.

It also handles one small thing — glide-antenna switching (It is in charge of glide antenna switching.): the relay of the shared antenna between the two MMRs.

3. Crossed display — the PFD reads its own, the ND reads the other

Per FCOM DSC-34-10-30-20:

‐ PFD1 and ND2 display ILS1/GLS 1 information. ‐ PFD2 and ND1 display ILS2/GLS 2 information.

Why cross them? To let each pilot see both receivers at once: the captain's PFD shows ILS 1, the ND (ROSE LS mode) shows ILS 2 — if the two disagree, that one person's two screens argue, with no need to compare across the cockpit. How to bring it up. Per FCOM DSC-34-10-30-20:

‐ The PFD displays the ILS/GLS information if the flight crew presses the LS pb or ILS pb (depending on the aircraft configuration) on the EFIS control panel

The ND side selects ROSE LS. Do not forget the third screen: the ISIS LS relays ILS 1 only — so an "ILS 1 failure" wounds PFD 1 + ND 2 + ISIS, three screens. The ILS/DME collocated range also reaches the PFD. Per AMM 34-51-00:

With ILS/DME collocated stations, the ILS/DME distance is shown in magenta on the L lower corner of the PFD (Item 1).

Magenta = the approach colour; the "DME 4.2" called on final reads this. Audio ident routes as usual through the AMU/ACP. Per AMM 34-36-00:

The Morse-coded audio identification signals are sent to the Audio Management Unit (AMU).

So the SOP's "RADIO NAV check: if the displayed ident is unsatisfactory, do the audio ident" applies to ILS too.

4. Tuning and the APPR key — three legitimate paths, one reminder

Two levels of tuning (the three-layer structure of the radio-nav article, projected onto ILS). Per AMM 34-36-00:

The equipment given below can control the ILS operation: - the Multipurpose Control and Display Units (MCDU) and the Flight Management Guidance and Envelope Computers (FMGEC) for frequency/course selection in normal operating mode. - the Radio Management Panels (RMP) for frequency/course selection in back-up mode.

(RMP backup control of ILS requires both RMPs in STBY NAV.) When does the FCU APPR key give LOC/G-S? This configuration's text gives three legitimate paths. Per FCOM PRO-NOR-SRP-01-70:

The FCU APPR pb arms or engages LOC and G/S modes, if : • An ILS/GLS approach is entered in the flight plan, or • No approach, or only a runway, is entered in the flight plan, and an ILS/GLS is manually-tuned on the RAD NAV page, or • Both RMPs are set to NAV, and an ILS/GLS is selected.

Note that path (2) requires that the plan has no approach. If the plan holds a non-precision approach and you also manually tune an ILS, the system gives a reminder, not a reversal. Per FCOM PRO-NOR-SRP-01-70:

This message is a reminder to the pilot that, although an ILS/GLS is tuned on RAD NAV page, the available approach guidance modes are APP NAV - FINAL when the APPR pb is pressed in on the FCU.

Guidance follows the plan, not the frequency — to fly ILS, change the approach in the flight plan first. (This is a notable configuration difference: in another group the same scenario gives LOC/G-S when APPR is pressed.)

5. LS TUNING DISAGREE — the FMGES as referee

Per FCOM PRO-ABN-NAV:

This alert triggers when the FMGES detects a discrepancy between MMR1 and MMR2 on the landing system type, frequency, channel or course.

The two receivers must sing the same score — type (ILS), frequency, channel, course; one mismatch and the auto-flight does an immediate three-part response. Per FCOM PRO-ABN-NAV:

When the alert is triggered, the AP/FD: ‐ Inhibits APPR mode arming, or ‐ Disarms the APPR mode if already armed, or ‐ Reverts to basic AP/FD modes if APPR mode already engaged.

The consequence line INOP SYS logs CAT 2 — two receivers off the same page lose even the CAT 2 qualification. This alert is also the best footnote for "why the RMP backup must tune ILS on both panels": tune only one and the two MMRs split instantly.

6. The CAT 2/3 equipment list — ATA-34's contribution

The QRH operational-data "Required Equipment for CAT2 and CAT3" is the bill of materials for capability, and the ATA-34 rows are: ILS receivers ×2 (dual at every level); RA: 1 for CAT 2 (displayed both sides), 2 for CAT 3; ADR/IR: 2/2 for CAT 2 and CAT 3 SINGLE, 3/3 for CAT 3 DUAL; auto callout: 1 for autoland; PFD 1 + PFD 2 + standby attitude at every level; 1 DH indication for the PM (not FMGS-monitored). Two usage philosophies. Per QRH OPS.10A:

‐ Flight crews are not expected to check the equipment list before approach. When an ECAM or local caution occurs, the crew should use the list to confirm the landing capability.

‐ On ground, the equipment list determines which approach category the aircraft will be able to perform at the next landing.

Airborne, read the FMA capability annunciation (the FMGS monitors it for you), and open the table only when an ECAM sounds; on the ground, dispatch reads the table. One frequently asked clause. Per QRH OPS.10A:

‐ In the case of an engine failure during a CAT III approach, revert to CAT 3 SINGLE minima.

(The guidance-level CAT II/III limits, DH/AH figures, and wind limits are taught in the auto-flight limitations; this table governs "are the sensors present.")

7. GLS — a provisioned room, no key issued

The GLS function block exists in the AMM for this MSN group. Per AMM 34-36-00:

The GLS function uses the ground stations and the GPS to compute the approach data (LOC and G/S) for the landing system. The GLS ground station data are transmitted to the localizer antenna, and the GPS data are transmitted to the GPS antennas.

The principle in one line: a differentially-corrected "electronic ILS" — the ground station transmits not a beam but correction data (over VHF, borrowing the LOC antenna's ear), and the deviation is computed on board relative to the defined approach path. But the general options list and the QRH configuration table both judge GLS = No: the hardware family is provisioned, the function is not activated — every "GLS" in the FCOM is an optional marker for this configuration. Teaching line: "not activated, do not fly a GLS approach; but seeing 'GLS' text is not cause for alarm — it is the room of a same-family aircraft." (FLS/SLS likewise: not fitted, so the LS family here is pure ILS.)

Key numbers

Self-test

[!note]- Q1. The LOC and G/S bands, and why does the crew tune only one frequency? LOC 108.1–111.95 MHz; G/S 329.15–335 MHz. The G/S UHF frequency is paired to the tuned VHF and follows automatically.

[!note]- Q2. Why is the shared antenna not a fatal single point, and where are the three links of the ILS chain of trust? The antenna is passive with few failure modes; errors happen in reception/computation. Trust chain: primary/monitor within the receiver → FMGES between the two MMRs → dual-AP LAND monitoring.

[!note]- Q3. The captain's PFD glideslope comes from which receiver? The ND's? To what end? PFD 1 = ILS 1, ND 1 = ILS 2 — so one pilot sees both receivers; a disagreement makes their own two screens argue.

[!note]- Q4. The plan holds a VOR approach, you manually tune ILS and press APPR — what guidance, what message, and how to actually fly ILS? Guidance = APP NAV–FINAL (per the plan); message CHECK APPR SELECTION; to fly ILS, change the approach in the flight plan.

[!note]- Q5. The three-part response to LS TUNING DISAGREE, and what capability survives? Inhibit APPR arming / disarm APPR / revert to basic modes. CAT 2 is lost (INOP), so no CAT capability survives beyond CAT 1.

[!note]- Q6. ILS 1 fails — which three screens are wounded, and who else is caught on the ground (hint: mode 5)? PFD 1, ND 2, ISIS. On the ground, ILS 1 failure inhibits GPWS mode 5 (the glideslope mode).

Key takeaways

References

• FCOM DSC-34-10-30-20 — dual receivers, MMR ownership, crossed display, LS pb.
• FCOM PRO-NOR-SRP-01-70 — APPR three paths, CHECK APPR SELECTION.
• FCOM PRO-ABN-NAV — LS TUNING DISAGREE trigger and response.
• QRH OPS.10A — CAT 2/3 equipment list, usage philosophy, single-engine downgrade.
• AMM 34-36-00 / 34-51-00 — ILS function and bands, shared antennas, primary/monitor channels, system processor, GLS function, ILS/DME magenta range.

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.