Performance Optimisation and Fuel Management

The prediction engine (article 24) can count; this article is about what it saves you and how to read its books: where the ECON speeds come from, the two ceilings (OPT and REC MAX), the economics of the cost index, the engine-out re-computation, and the fuel-policy defaults — ending with the credit boundary: when the FMS's fuel arithmetic may no longer be believed.

1. The optimisation four-pack

Per FCOM DSC-22_20-10-40:

The FMGEC minimizes cost by optimizing the following items: ‐ Takeoff, approach, and go-around speeds (F, S, Green Dot, VAPP) ‐ Target speed for CLB, CRZ and DES phases (ECON SPD/MACH) ‐ Flight Level (for flight crew's information) ‐ Descent profile from CRZ FL down to the destination airport.

Four things: the manoeuvring speeds (note the division with article 18 — the VLS family is computed by FE; the FM here selects operating values by weight); the three phase ECON targets; the optimum flight level (explicitly for information — flying it is your decision); and the descent profile. ECON eats five inputs — cost index, cruise level, gross weight, the wind/temperature model, and the performance factors (article 24's reading glasses). And the freeze rule:

The computer processes the ECON SPDs for the climb and descent phases before the initiation of the flight phase, and freezes the values once the flight phase becomes active. … The computer continually updates ECON CRUISE MACH (SPD), taking into account current weather conditions and modifications to the flight plan.

Climb and descent ECON are baked before their phase and frozen at activation; only cruise ECON stays live. So "wind the CI down mid-descent to slow us" does not work — the descent speed is already frozen; change it on the PERF DES page's MANAGED field (still a managed profile — article 22) or go selected. (Below FL250 ECON is a speed, above it a Mach.)

2. OPT FL — advice that jumps

Per FCOM DSC-22_20-10-40:

The optimum flight level (OPT FL) indicates the most economic flight level for a given cost index, weight, weather data. It requires a 5 min minimum cruise time, at a minimum cruise flight level of FL 100. It is continuously updated in flight until the aircraft is predicted to reach the top of descent within 5 min.

The OPT FL is a compromise between fuel and time saving. As a result, the flight crew may observe jumps in OPT FL due to slight GW, ISA or wind changes.

Two preconditions (five cruise minutes available at it, and at least FL100), continuous updating until five minutes before T/D — and it jumps, legitimately, on small weight/ISA/wind changes, because it optimises the fuel-and-time compromise rather than fuel alone. (Which is also why the FCOM/QRH tables' fixed-Mach OPT never equals the FMS's CI-weighted OPT FL.) It shows dashes in two cases — engine-out detected, or the descent phase active — and it feeds on propagated winds, closing the loop with article 24's stale-wind advice: after a level change, feed the actual winds into the lower levels or the OPT is polluted.

3. REC MAX — five beams, the lowest one is the ceiling

Per FCOM DSC-22_20-10-40:

The recommended maximum altitude is the lowest of the maximum altitude that: ‐ The aircraft can reach with a 0.3 g buffet margin ‐ The aircraft can fly in level flight at MAX CRZ rating ‐ The aircraft can maintain a V/S of 300 ft/min at MAX CLB thrust ‐ The aircraft can fly at a speed above the Green Dot and below the VMO/MMO ‐ The aircraft is certified at.

Buffet margin, level-flight thrust, residual climb, a usable speed corridor, and the certificate — the lowest beam is the roof. With two footnotes that matter:

Anti-ice is not taken into account for this computation. If icing conditions are expected, use the performance application of the EFB. A maximum altitude using a 0.2 g buffet margin is also computed. It is not displayed, but the system uses it to limit CRZ ALT entry.

Anti-ice is not in REC MAX — icing forecasts send you to the EFB performance application. And a hidden sixth number exists: a 0.2 g-margin ceiling, never displayed, used to reject your CRZ ALT entries — the source of ENTRY OUT OF RANGE at absurd levels and kin to the STEP family's ABOVE MAX (article 22).

4. The cost index — one number's economics

Per FCOM DSC-22_20-10-40:

CI is the ratio of flight time cost (CT) to fuel cost (CF). CI = CT/CF (kg/min or 100 lb/h). CI = 0 corresponds to minimum fuel consumption (MAX Range). CI = 999 corresponds to minimum time.

Zero buys maximum range, 999 buys minimum time; long-range cruise has its own equivalent — a representative operator's QRH states it directly: CI for LRC = 40 kg/min (metric; 53 in the pound edition). The manuals endorse exactly two in-flight changes: short of fuel → CI 0; running late → CI 999 — otherwise leave it (the airline optimised it per route). And remember article 21's USING COST INDEX 0 message after ENABLE ALTN: the system quietly zeroes the CI on a diversion, and the prompt exists so you notice.

5. Engine-out — the performance system changes its testimony

Per FCOM DSC-22_20-10-40:

The FMGES computes an engine-out target speed for each flight phase. It computes an engine-out maximum altitude (EO MAX ALT) at long-range cruise speed, and displays it on the PROG page. The new speed target becomes Green Dot in climb phase or EO CRZ SPD in level flight. The system computes the flight plan predictions down to the primary destination assuming that the cruise phase is at the lower of CRZ FL or EO MAX ALT. For the engine-out obstacle strategy, the system computes a drift down descent at Green Dot and down to a level off altitude.

Per-phase EO speeds; EO MAX ALT on PROG, computed at LRC speed; the whole prediction re-run at the lower of the cruise level and EO MAX ALT; and for obstacles, a green-dot drift-down to a level-off altitude. Alongside, every "fine-tuning" feature goes dark — OPT FL dashes, steps cleared, RTA deleted (article 22), preselections wiped (article 19) — the system keeps only the survival arithmetic: how high can we stay, how fast should we fly. Keep the two speeds apart: EO MAX ALT answers "how high can single-engine cruise live" (LRC); drift-down answers "how do we come down" (green dot) — two altitudes, two speeds, don't blend them. The EO triad for revision: FG changes the speeds (article 16/19), FM clears the fictions (article 19), performance lowers the ceiling (here).

6. Fuel policy — the default parameter table

Per FCOM DSC-22_20-10-40:

Up to 8 fuel policy records may be specified in the navigation database. If a specific data is not provided in the database fuel policy, the value is the default value given here after: PARAMETERS DEFAULT VALUES RTE RSV (Percentage of route reserve) 5 % ... FINAL TIME (Time for final holding pattern) 30 min ... FINAL ALT (Altitude for final holding pattern) 1 500 ft AGL TAXI (Fuel for taxi) 600 kg (1 300 lb) FINAL DEST (Final holding pattern is flown at ALTN) ALTN

Fuel policy lives in the navigation database (up to eight records — the AMI's neighbour, article 04), with these defaults filling the gaps: route reserve 5 % (capped, still computed in flight, excluding the alternate segment per the policy flags); final hold 30 min at 1500 ft AGL at the alternate; taxi 600 kg. Three items are crew-changeable on INIT FUEL PRED / FUEL PRED: RTE RSV, FINAL TIME, TAXI. FUEL PLANNING can also run backwards — given plan, level, CI, ZFW/ZFWCG, the FM computes the minimum block by policy. And the alternate's six assumptions (article 21) get their FCOM detail here: default FL220 (< 200 NM) or FL310, CI = 0, initial weight = the landing weight at the primary destination, average track, constant ΔISA and cruise wind, and no steps allowed in the alternate segment — conservative estimation, not precision. Before engine start read INIT FUEL PRED; after start, FUEL PRED.

7. The credit boundary

Per a representative operator QRH (in-flight performance section):

Note: In case of failure impacting the fuel consumption, the fuel predictions provided by the FMS are no longer reliable (except in One Engine Inoperative OEI condition). The flight crew must still compute and monitor the actual fuel consumption.

The FMS only knows the diseases in its model. Engine-out it knows (section 5's full re-computation). A stuck flap, gear that won't retract, a fuel-jettison anomaly — it has no model for, and its EFOB becomes fiction. The correct move for a gear-down return is not "check the FMS EFOB": it is the QRH's fuel-penalty factor table times hand arithmetic (conservative multipliers, decision-grade). The legal frame agrees: the AFS limitations file the FMS's fuel/time predictions as advisory only (article 33) — predictions never substitute for statutory fuel monitoring.

8. Operating the books

The daily three looks (synthesis): PROG — OPT and REC MAX bracketing the present level (before requesting higher: does REC MAX leave the buffet margin, and does OPT say it's worth it); FUEL PRED — EFOB at destination and EXTRA (EXTRA below zero = policy fuel no longer closes; think CI 0 or a re-route); VERT REV — per-waypoint EFOB/EXTRA (article 21).

The engine-out diversion's performance side (joining articles 19/21): EO detected → read EO MAX ALT on PROG (LRC basis) → drift down at green dot if needed → CLOSEST AIRPORTS (EO state automatically included) → SEC plan or NEW DEST.

Two cold-knowledge traps: an inserted OPT STEP point is fixed and no longer updated (one text stops updates within 20 NM only — both variants exist, so teach the conservative reading: once inserted, you watch it); and the paper OPT tables never equal the FMS OPT FL (fixed Mach versus CI-weighted) — a difference, not a discrepancy.

[!warning]- Four misconceptions this article corrects (1) Changing the CI in the descent does not slow you down — ECON descent speed froze at phase activation; use PERF DES or selected speed. (2) REC MAX does not include anti-ice — icing goes to the EFB; and an invisible 0.2 g ceiling polices your CRZ ALT entries. (3) EO MAX ALT (LRC) and the drift-down (green dot) are different answers to different questions — don't fly one at the other's speed. (4) After any consumption-affecting failure except engine-out, the FMS fuel predictions are unreliable — fuel-penalty factors and hand arithmetic, not the EFOB column.

Self-test

[!note]- Q1. The optimisation four-pack — and which ECON phases are live versus frozen?

Manoeuvring speeds (F/S/green dot/VAPP), phase ECON targets, optimum flight level (information only), and the descent profile. Climb and descent ECON freeze at phase activation; cruise ECON updates continuously.

[!note]- Q2. OPT FL's two preconditions, its two dashes cases, and why it jumps?

At least 5 min of cruise at the level, minimum FL100; dashes on engine-out or with the descent phase active. It optimises the fuel/time compromise, so small GW/ISA/wind changes legitimately move it.

[!note]- Q3. The five REC MAX beams — is anti-ice included, and what is the 0.2 g version for?

0.3 g buffet margin; level flight at MAX CRZ; 300 ft/min at MAX CLB; a speed between green dot and VMO/MMO; certification. Anti-ice: no — use the EFB. The undisplayed 0.2 g ceiling limits CRZ ALT entries.

[!note]- Q4. The CI formula, its extremes, and the LRC equivalent?

CI = time cost / fuel cost (kg/min). 0 = minimum fuel (max range); 999 = minimum time; a representative operator's QRH puts LRC at CI 40 (metric). Sanctioned in-flight changes: fuel-short → 0, late → 999.

[!note]- Q5. After an engine failure, what altitude does PROG show and at what speed is it computed — and the drift-down speed?

EO MAX ALT, computed at long-range cruise speed; predictions re-run at the lower of CRZ FL and EO MAX ALT. The obstacle-strategy drift-down flies green dot to a level-off altitude.

[!note]- Q6. The RTE RSV / FINAL / TAXI defaults — and which three items may the crew change?

5 % route reserve; final hold 30 min at 1500 ft AGL at the alternate; taxi 600 kg. Crew-changeable: RTE RSV, FINAL TIME, TAXI (on INIT FUEL PRED / FUEL PRED).

[!note]- Q7. What CI do alternate predictions use — and why are they conservative by construction?

CI 0, plus: banded cruise level (FL220/FL310), initial weight = destination landing weight, average track, constant ΔISA/wind, no steps — an estimate designed to overstate rather than flatter.

[!note]- Q8. Returning gear-down: can the FMS EFOB be trusted? What is the correct method?

No — the FMS has no model for configuration anomalies (engine-out is the sole modelled exception). Use the QRH fuel-penalty factors with manual computation, and keep monitoring actual consumption.

Key takeaways

References

The optimisation list, ECON computation and freeze rules, OPT FL clauses, CI definition and usage scenarios, and the OPT STEP fixed-point note per FCOM DSC-22_20-10-40 (optimisation and cost-index sections); REC MAX composition with its anti-ice and 0.2 g footnotes, the engine-out computation set, the fuel-policy default table, alternate assumptions and FUEL PLANNING per its computation section. Fuel-penalty method, the OEI exception and the LRC cost index per a representative operator QRH (values vary by operator and airframe — use your own). The advisory-only status of FMS fuel/time predictions is a limitation collected in article 33. The daily-three-looks routine is an integrative synthesis.

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.