Operations I: Navigation SOP and Altimetry

The previous articles cover "what the system is"; from here on, "how a flight uses it." Take a sector from cockpit preparation to power-down, and the ATA-34 action lines are five threads: align the IRS once, check three altimeters, sweep the navaids, set the three surveillance items (XPDR / radar / TERR) by phase, and settle the IRS before power-down. Each thread carries counter-intuitive disciplines (why can't you align after engine start? why does TCAS wait until lined up? why turn the TERR image off when NAV ACCURACY is LOW?). Note this article covers only the ATA-34 equipment's SOP action line; GPS PRIMARY monitoring and PBN/RNP approach procedures are FM position management (ATA-22).

1. Cockpit preparation — one alignment, three altimeters and a compass

Alignment type (decided when the CM2 sets the three IR MODE rotaries to NAV). Per FCOM PRO-NOR-SOP-04:

A complete IRS alignment must be performed in the following cases: ‐ Before the first flight of the day, or ‐ When there is a crew change, or ‐ When the departure airport is located between latitudes 2 ° north and 2 ° south, or ‐ When the GPS is not available and the NAVAID coverage is poor on the expected route, or ‐ When the GPS is not available and the expected flight time is more than 3 h.

A fast IRS alignment must be performed if a complete IRS alignment is not necessary.

The five cases split into two groups: the first two are "the account is due" (a fast alignment clears only position/velocity error; only a full alignment clears the accumulated gyro attitude drift — fly four sectors without re-aligning and the drift snowballs); the last three are "this sector cannot afford it" (poor gyro-compass accuracy in the equatorial band; long no-GPS legs living on inertial). The execution discipline. Per FCOM PRO-NOR-SOP-04:

To perform an alignment, the aircraft must be stopped on ground. Do not align IRS during engine start, or while the engines are running. In the case of aircraft movement during the IRS alignment, the flight crew must perform a new full alignment.

(Engine vibration corrupts the gyro-compass search for north — the engineering root of EXCESS MOTION, set as three SOP prohibitions.) The alignment acceptance at the end of cockpit preparation. Per FCOM PRO-NOR-SOP-06:

On the POSITION MONITOR page, check that the IRS are in NAV mode, and check that the distance between each IRS and the FMS position is lower than 5 NM. Select ND in ROSE-NAV or

ARC mode, and confirm that the aircraft position is consistent with the position of the airport, the SID and the surrounding NAVAIDs.

A numeric check (5 NM) plus a graphical check ("I am not on the airport on the map") — the latter catches the "wrong-but-self-consistent coordinates" that 5 NM cannot. The three altimeters. Per FCOM PRO-NOR-SOP-06:

The maximum difference is: ±20 ft between both PFDs And depending on the aircraft configuration: ±60 ft between ISIS and PFDs, or ±300 ft between mechanical standby altimeter and PFDs.

The ±20/±60 gap is an identity gap: behind the PFD is the full ADR SSEC correction, the ISIS only a simplified one — it may be coarser, but not absent. The RADIO NAV check. Per FCOM PRO-NOR-SOP-06:

‐ Check the VOR, ILS / GLS and ADF tuned by the FMGEC. ‐ Modify them, if required, and check that the correct identifier is displayed on the ND and PFD (VOR, ILS / GLS , ADF ). If unsatisfactory, go through the audio check.

The displayed ident comes from the receiver decoding the station's Morse — if it will not display or does not match, use the ear on the raw audio; if that too is wrong, the station is unusable for approach today. The three ATC-preparation rules. Per FCOM PRO-NOR-SOP-06:

To prevent possible interference to radar surveillance systems, TCAS should not be selected before the holding point/lining up.

(The transponder itself still answers Mode S by name on the ground; this restrains TCAS's active interrogation, which would harass others on the surface.) ALT RPTG ON; the RVSM pairing set early. Per FCOM PRO-NOR-SOP-06:

For RVSM operations (Refer to PRO-SPO-50 Required Equipments/Functions for RVSM), select SYS 1 if AP 1 is used, and SYS 2 if AP 2 is used. Only system 1 is available in the Emergency Electrical Configuration.

2. Taxi and take-off — the three surveillance items take their places

Radar (this group's SOP text). Per FCOM PRO-NOR-SOP-10:

To check the departure path, set the MULTISCAN sw to MAN. The flight crew can then set the radar to the AUTO position.

Why MAN first? The Multiscan automatic image is "stored and combined"; stationary on the ground at low tilt, a manual straight sweep of the departure sector shows the raw echo untrimmed by the algorithm — confirm the departure side hides nothing, then hand back to AUTO. Confirm PWS AUTO (a memo watches for you: PRED W/S OFF turns amber from phase 3). TERR division. Per FCOM PRO-NOR-SOP-10:

Consider selecting the radar display on the PF side, and TERR ON ND on the PM side only.

One watches the weather, one watches the ground (the two images are mutually exclusive) — the standard division at an airport with both mountains and convection. Set the XPDR code per the airport; TCAS goes TA/RA only when lined up (or TA per the situation: closely-spaced parallel/converging runways, single-engine pre-set). If TA ONLY was used for take-off. Per FCOM PRO-NOR-SOP-13:

If the takeoff was performed with TA only:

— TA ONLY is a scenario-specific temporary downgrade; once the scenario passes it must be reset (forget, and you give up RA protection for the rest of the flight).

3. Climb and cruise — accuracy monitoring and the RVSM table discipline

With GPS PRIMARY present, navigation accuracy is guaranteed automatically by the integrity system; only its loss brings in the human. Per FCOM PRO-NOR-SOP-15:

Check on FMS PROGRESS page that the required navigation accuracy is appropriate according to the flown airspace. For more information, Refer to PRO SPO 51 PBN.

RVSM is the great exam for ATA-34's sensors in the cruise. The bill of materials. Per FCOM PRO-SPO-50:

RVSM regulations require the following equipment/functions to be operative : ‐ 2 ADRs + 2 DMCs ‐ 1 transponder ‐ 1 autopilot function ‐ 1 FCU channel (for altitude target selection and OP CLB/OP DES mode engagement) ‐ 2 PFD functions (for altitude indication) ‐ 1 FWC (for altitude alert function).

The ground check (flight preparation). Per FCOM PRO-SPO-50:

Check that the difference between each altitude indications (in the QNH reference) displayed on PFDs and the airport elevation is less than 75 ft.

Before entering RVSM airspace, and about once per hour within it, cross-check on the standard barometric reference (the table widens with altitude/speed — the higher, the larger the static error). Per FCOM PRO-SPO-50:

FL 300 M 0.82 120 (37 m) 130 (40 m) 320 (98 m)

(Full table: FL50 = 55/55/90, FL100 = 60/60/130, FL200 = 95/100/220, FL410 = 145/150/380.) With only two ADRs, one more action. Per FCOM PRO-SPO-50:

If only two ADRs are operative, the altimeter indications on PFD and standby altimeter should be recorded. This information may be useful in case of subsequent PFD altitude discrepancy or loss of both remaining ADRs.

(Copy a "health baseline" first, so there is evidence if they argue later.) Flying discipline: keep the AP engaged, do not overshoot/undershoot a level change by more than 150 ft; and the five situations that must be reported to ATC. Per FCOM PRO-SPO-50:

When the aircraft is in RVSM airspace, the flight crew must notify the ATC of the following situations, because they may affect the aircraft's ability to maintain the flight level: ‐ The failure of both autopilots ‐ The loss of altimeter system redundancy (only one PFD indication remaining) ‐ An excessive discrepancy in altitude indications, and no way to identify the valid indication ‐ The failure of any other equipment, that affects the aircraft's ability to maintain the flight level ‐ The encounter of greater than moderate turbulence.

The essence of RVSM is "every aircraft can prove its altitude is trustworthy"; fail to prove it and leave the airspace.

4. Descent and approach — the ritual of changing QNH

Near the transition level, the baro change is "three places in sync plus one moment of doubt." Per FCOM PRO-NOR-SOP-17:

The flight crew must take into account any QNH change on the EFIS control panels, standby altimeter and on the FMS PERF APPR page.

(The third place is the most missed — the PERF APPR page QNH feeds the approach energy computation and the BARO minima; set the EFIS but not the page, and the MDA logic still uses the old value.) The "one doubt." Per FCOM PRO-NOR-SOP-17:

The flight crew should pay attention to a barometric reference that significantly differs from the one used for approach preparation. That could be the symptom of a barometric reference error. In this case the flight crew should consider the confirmation of the barometric reference from all available sources.

(A 1 hPa mishear = 28 ft, a 10 hPa mishear = 280 ft; ATIS, METAR, and the controller read-back are three independent sources. The EGPWS geometric altitude is the last net — but the net should not be tested.) One quick item. Per FCOM PRO-NOR-SOP-16:

Preset the QNH on the ISIS.

(On the day the main instruments are lost, the standby already has the right QNH — the value of the word "preset.") The three TERR rules (descent prep). Per FCOM PRO-NOR-SOP-17:

‐ In mountainous areas, consider displaying terrain on ND. ‐ If use of radar is required, consider selecting the radar display on the PF side, and TERR ON ND on the PM side only. ‐ If NAV ACCURACY is LOW, do not use the TERR on ND.

The third is the counter-intuitive exam point: with an inaccurate position, the TERR image draws "the mountain looked up at the wrong position" — a map that looks safe is more dangerous than none (the EGPWS itself surrenders TERR STBY at gross inaccuracy, but LOW may not reach that threshold; the human retires before the machine). Changing BARO on approach automatically pops up ECAM STATUS if not empty — the moment to complete the "capability review."

5. Parking and power-down — settling the inertial system

Parking (after engine shutdown). Per FCOM PRO-NOR-SOP-22:

On the FMS POSITION/MONITOR page, check that the deviation does not exceed the following:

This grades the inertial system's exam: over-tolerance drift is logged in MAINTENANCE STATUS and reported. Power-down (securing). Per FCOM PRO-NOR-SOP-23:

The flight crew should not turn off the ADIRS during transits at latitudes above 82 ° N or 82 ° S, in order to avoid excessive alignment time.

(The operational landing of the alignment-time formula |5 min/cos lat|: 82° is already in the 17-minute band, higher still and the gyro-compass can barely find north — better to leave it running than make it re-find north.) The final ten seconds. Per FCOM PRO-NOR-SOP-23:

After turning off the ADIRS, wait at least 10 s before turning off the electrical supply, in order to ensure that the ADIRS memorize the latest data.

In those ten seconds it is "writing its diary" — the quality of the next fast alignment and the maintenance trend depend on this diary. Cut power early and a segment of data is lost.

6. QFE and high-pressure QNH — two altimetry easter eggs

The QFE option is fitted, so the FCU can set QNH or QFE. Egg one — using QNH on a QFE-option aircraft has a specific note. Per FCOM PRO-NOR-SOP-18:

If the QNH altimeter setting is used for an aircraft with the QFE Option , Refer to DSC-22_20-30-20 QNH Use for Aircraft Equipped with QFE Option.

(The QFE configuration changes the FM vertical-profile reference frame — the detail is an ATA-22 matter; on the ATA-34 side, remember the EGPWS geometric altitude in QFE needs a manual MCDU QNH, or TERR STBY.) Egg two — QNH > 1050 hPa: the transponder cannot report altitude (a coding floor), so ALT RPTG OFF + notify ATC, restoring it above 1000 ft — mechanism and full procedure in the transponder article. Signpost only: next time an ATIS reads 1052 at a polar anticyclone airport, think of the transponder first.

Key numbers

Self-test

[!note]- Q1. A 40-minute turnaround, same crew, at 43°N — full or fast? And at 0.1°S? Fast at 43°N (same crew, not first flight, not within ±2°). At 0.1°S it is within ±2° latitude → full alignment required.

[!note]- Q2. Alignment at 7 minutes, a tug pushes half a metre — next step? EXCESS MOTION → the system reverts to a full alignment (9 min 30 s); position need not be re-entered. Wait it out.

[!note]- Q3. Cockpit prep, ISIS reads 45 ft above the PFD — dispatchable? On an RVSM flight, PFD 80 ft above field elevation? ISIS-PFD 45 ft is within ±60 ft — acceptable. RVSM ground check requires PFD vs field elevation < 75 ft; 80 ft fails.

[!note]- Q4. Why is TCAS off at the gate but the XPDR on, and where is the difference? The XPDR still answers Mode S selectively (surface surveillance); TCAS's active interrogation would harass others, so it waits until lined up.

[!note]- Q5. RVSM cruise FL350, ADR1-ADR2 differ 135 ft — from the table, normal? (interpolate FL300/FL410) FL300 limit 120, FL410 limit 145; FL350 interpolates to ≈ 132 ft. 135 ft slightly exceeds — investigate.

[!note]- Q6. ATC gives QNH 998 but ATIS at prep was 1021 — the right reaction, and the suspicion? A large difference is a possible baro-reference error; confirm from all available sources (ATIS/METAR/read-back) before setting.

Key takeaways

References

• FCOM PRO-NOR-SOP-04 — alignment five cases, fast alignment, three prohibitions.
• FCOM PRO-NOR-SOP-06 — ISIS check, three altimeters, RADIO NAV, ATC three rules, IRS ALIGN acceptance.
• FCOM PRO-NOR-SOP-10/13 — taxi radar MAN→AUTO, TERR division, TCAS timing.
• FCOM PRO-NOR-SOP-15/16/17/18 — NAV ACCURACY, ISIS preset, QNH three places, TERR rules, QFE note.
• FCOM PRO-NOR-SOP-22/23 — IRS performance check, 82° latitude, 10 s data save.
• FCOM PRO-SPO-50 — RVSM equipment, cross-check table, discipline, five reportable situations.

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.