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departure fuel price ratio is higher than the transport coefficient K.
Pa > K
Pd
Thus carrying extra fuel may be of value when a fuel price differential exists
between two airports. Graphs in the FCOM assist in determining the optimum fuel
quantity to be carried as a function of initial take-off weight (without additional
fuel), stage length, cruise flight level and fuel price ratio. The following graph is
an example for an A320.
However the needs for accurate fuel planning is necessary to avoid arriving at the
destination airport overweight. This could result in the economic benefit being
eroded or negated due to extra hold time or circuits.
Getting to grips with Fuel Economy 4 - PRE-FLIGHT PROCEDURES
- 21 -
4.6 AUXILIARY POWER UNIT
The Auxiliary Power Unit (A.P.U.) is a self-contained unit which
makes the aircraft independent of external pneumatic and electrical power supply
and environmental conditioning.
A.P.U. fuel consumption obviously represents very little in comparison with
the amount of fuel for the whole aircraft mission. Nevertheless, operators have to
be aware that adopting specific procedures on ramp operations can help save fuel
and money.
On the ground, A.P.U. fuel consumption depends on the A.P.U.
type load and the ambient conditions. The minimum is when the APU is in the
RTL(ready to load) condition. As additional loads, such as Electrical Loads(EL) and
Environmental Conditioning System (ECS), are connected, the fuel consumption
increases as shown in the following table (ISA, SL conditions).
Aircraft Type APU
Model
RTL RTL
Max EL
Min ECS
Max EL
Max ECS
Max EL
A320 family 36-300 70 kg/hr 85 kg/hr 105 kg/hr 125 kg/hr
A320 Family 131-9A 75 kg/hr 95 kg/hr 115 kg/hr 125 kg/hr
A330, A340 331-350 120 kg/hr 140 kg/hr 175 kg/hr 210 kg/hr
A340-500/600 331-600 160 kg/hr 180 kg/hr 225 kg/hr 290 kg/hr
A.P.U. specific procedures to save fuel have to be defined by the
operators. One extra minute of A.P.U. operation per flight at 180 kg/hr fuel flow,
means an additional 3000 kg per year per aircraft. This will also result in
increased maintenance costs.
They have to choose between using ground equipment (Ground Power Unit,
Ground Climatisation Unit, Air Start Unit) and the A.P.U. This choice depends on
several parameters and each operator needs to determine the benefits at each
airport and at each turnaround.
Such parameters can include length of turnaround, ambient conditions, cost
of ground connections, time delay to get connected, suitability and quality of
ground equipment, passenger load, local noise restrictions, etc.
For a long turnaround or night stop the G.P.U. is the best choice as time
considerations are not prevailing. It saves both fuel and A.P.U. life. So operators
are advised to use ground equipment if of a good quality, and to try to conclude
agreements with airport suppliers to get preferential prices.
However, for a short turnaround (45 minutes on average), the use of
A.P.U. may be preferable to limit A.P.U. start cycles and improve reliability, even
if it is not fully used during the turnaround. It is better to operate with A.P.U. at
RTL than to shut it down and perform a new start cycle soon after shut down.
Lack of suitable ground power may also require the use of APU. The use of APU
4 - PRE-FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 22 -
may also be preferable to avoid excessive hook up charges or to reduce
turnaround time.
Some airport regulations restrict the use of the APU to a defined time prior to
departure time and after the arrival.
For extremely short turnarounds, complete engine shut down would have
a cyclic cost impact, and therefore the turnaround could be made without APU.
However a main engine can sometimes not meet the ECS demand in high load
conditions (hot days).
The disconnection of ground equipment supplies and the start of A.P.U. must
be coordinated with A.T.C. pushback/slot requirements. A one-minute
anticipation in each A.P.U. start will lead to a significant amount of fuel saving
during a year (2000 to 4000 kg depending on A.P.U. types).
Engine start up should also, if possible, be carefully planned in conjunction
with A.T.C. If pushback is delayed, it is preferable to wait and use A.P.U. for air
conditioning and electrical requirements. Engine start time is critical, and the
engines should not be started until ready to go.
The following table assuming typical engine fuel flows, shows extra fuel
consumption by using one engine instead of the A.P.U. for 1 minute, assuming
maximum electrical load and minimum ECS:
Extra fuel when using Engine instead of APU
　
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