Optimization of Cruise Step Altitudes

For the optimum selection of cruise altitudes, the altitude changes (ΔH) (in feet (e.g. 1000 ft, 2000 ft, 4000 ft) are specified.
The altitude profile is then optimized such that specific air range (SAR)

\[\begin{aligned} SAR = \frac{V}{\dot m_F} = \frac{V}{F \cdot TSFC} = \frac{V \cdot L/D}{m g \cdot TSFC} \end{aligned} \]

is maximized at all cruise mission increments; i.e. if SAR is lower at the actual altitude H than at the altitude (H+ΔH) a step climb is performed. For the climb to the higher flight altitude a constant rate of climb (ROC) of 300 ft/min is used. If the consumed fuel (including the climb fuel) until the top of descent point is higher than that if continuing on the same flight level the step climb is not performed.

Fuel Planning Method

Minimum fuel quantity

The minimum fuel quantity (Q) calculated for flight planning is defined as:

\[\begin{aligned} Q = taxifuel + TF + CF + AF + FR + Add + XF \end{aligned} \]

• TF: Trip Fuel
• CF: Contingency Fuel
• AF: Alternate Fuel
• FR: Final Reserve Fuel
• Add: Additional Fuel
• XF: Extra Fuel

Representation of the Different Fuel Quantities

Taxi Fuel

“AMC OPS 1.255
Taxi fuel, which should not be less than the amount, expected to be used prior to take-off. Local conditions at the departure aerodrome and APU consumption should be taken into account.”

Taxi fuel is usually a fixed quantity for an average taxi duration.
For the A320 for example, it is equal to 140 kg (300 lb). This corresponds to a
12-minute average taxi fuel.
Based on statistics or evaluation, the taxi duration and taxi fuel may need to be adjusted.

Trip Fuel

The required fuel quantity from brake release at the departure airport to the landing touchdown at the destination airport, is referred to as trip fuel. This quantity takes into account the necessary fuel for:

• Takeoff
• Climb to cruise level
• Flight from the end of climb to the beginning of descent, including any step climb/descent
• Flight from the beginning of descent to the beginning of approach,
• Approach
• Landing at the destination airport

Contingency Fuel

Contingency fuel is the greatest of two quantities:

• The fuel necessary to fly for 5 minutes at 1500 feet above the destination airport at holding speed in ISA conditions
• One of the following quantities:
  • 5% of trip fuel,
  • With airworthiness approval, 3% of trip fuel with an available en route alternate airport

Alternate Fuel

Alternate fuel takes into account the necessary fuel for:

• Missed approach at the destination airport
• Climb from the missed approach altitude to the cruise level
• Flight from the end of climb to the beginning of descent
• Flight from the beginning of descent to the beginning of the approach
• Approach
• Landing at the alternate airport
• When two alternate airports are required*, alternate fuel should be sufficient to proceed to the alternate which requires the greater amount of fuel.

Final Reserve Fuel

The final reserve fuel is the minimum fuel required to fly for 30 minutes at 1,500 feet above the alternate airport or destination airport, if an alternate is not required, at holding speed in ISA conditions.

Additional Fuel

“AMC OPS 1.255
1.6 … the minimum additional fuel which should permit:
a. Holding for 15 minutes at 1500 ft (450 m) above aerodrome elevation in standard conditions, when a flight is operated under IFR without a destination alternate, in accordance with JAR-OPS 1.295; and
b. Following the possible failure of a power unit or loss of pressurisation, based on the assumption that such a failure occurs at the most critical point along the route, the aeroplane to:
i. Descend as necessary and proceed to an adequate aerodrome; and
ii. Hold there for 15 minutes at 1500 ft (450 m) above aerodrome elevation in standard conditions; and
iii. Make an approach and landing, except that additional fuel is only required, if the minimum amount of fuel calculated in accordance with sub-paragraphs 1.2 to 1.5 above is not sufficient for such an event.”