3. Planning and performance
3.1 Cost index
Cost index expresses how time is valued against fuel, on a scale of 0 to 999. It is recalculated daily for each type and each destination, so there is no single correct number — in practice it sits between 3 and 30.
It is usually higher outbound from the home base than at an outstation, because fuel is cheaper where the airline buys it.
Whether to change it depends on which way you are flying.
| Out of the hub | Normally no adjustment. Exceptions: a crew duty-time problem, or a destination that must be reached before a night curfew |
| Into the hub | Depends on the delay and on the connections — the more valuable the passenger and the tighter the connection, the faster |
Why you cannot simply crank it up. A cost index of 999 on the A320 gives an optimum flight level around FL250, and the fuel burn at that level is enormous. So speed is increased only as far as the optimum level does not have to change. The cost index at which the optimum level starts to fall differs by type and sits somewhere between 150 and 250; going past it is very rare.
That is the whole trick: a cost index change is a speed decision until it becomes an altitude decision, and the moment it becomes an altitude decision it has stopped saving anything.
3.2 Fuel
Normal conditions. Plan to have at least one hour of remaining fuel in the tanks after standard flight fuel.
Where threats are expected — weather, air traffic control, anything that can reasonably be foreseen to cost time — plan so that after a diversion to the alternate there would still be 45 minutes in the tanks, plus the delay those threats are expected to cause.
The second rule is not the first rule with a bigger number. It is a different calculation, made from the alternate backwards.
3.3 Landing flap
There is no recommended setting. Airbus withdrew the recommendation, and the crew is expected to think about it before every landing and ask what actually makes sense today.
| Long roll-out available, landing distance no problem | Flaps 3 |
| Short runway, or a long taxi back | Flaps Full |
| (M) Contaminated runway | Flaps Full — this one is not a judgment |
The test. Take Flaps 3 whenever you could vacate where you sensibly intend to without reverse and without autobrake medium. If you would need either of those, take Flaps Full first.
The combination to avoid is Flaps 3 with full reverse and autobrake medium. Arriving at it means the earlier decision was wrong.
Steeper glide paths and tailwinds count too. If you cannot hold the final approach speed without speedbrakes, the decision was wrong — and the answer is Flaps Full, not more speedbrake.