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A300/A310 1- 5kg 15kg
A320 3-5kg 8-13kg
A330 12kg 24kg
A340 30kg 50kg
These figures assume Full take-off thrust. The advantage of Conf 1+F
increase with reduced power take-offs.
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
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5.1.4 SPEEDS
During a non limiting full power take-off, the use of the higher speeds
appropriate to flex thrust instead of optimized speeds appropriate to the actual
temperature can reduce the fuel burn by up to 8kg.
5.1.5 FLEX THRUST
Compared to a full thrust take-off, flex thrust will generally increase fuel
burn. The increased time at low level offsets the slight reduction in fuel flow
induced by the lower thrust. Typical increases are as follows:
Aircraft Conf 1+F Conf 2 Conf 3
A300/A310 10kg 10kg 10kg
A320 1kg 5kg 5kg
A330 0 0 0
A340 5kg 20kg 25kg
5.1.6 NOISE FLIGHT PATHS
The effect of an ICAO type A noise flight path, with climb thrust selected at
800ft and clean up delayed until 3000ft is generally to increase fuel burn
compared to the standard take-off with power reduced at 1500ft. The actual
distance to a fixed height, say 5000ft, varies very little with configuration. The
main effect is the different altitude – speed history experienced by the engines.
Typical values are as follows:
Aircraft Conf 1+F Conf 2 Conf 3
A320 -4kg +5kg +2kg
A330 +100kg +100kg +115kg
A340 +90kg +130kg +125kg
5.1.7 COURSE REVERSAL
In the event that a course reversal is required after take-off, then much
distance can be saved using a lower initial climb speed. Suppose ATC require an
aircraft to maintain runway heading to 6000ft. A lower climb speed will achieve
this altitude earlier and thus reduce the ground distance and fuel burnt.
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 27 -
5.2 CLIMB
5.2.1 INTRODUCTION
Depending on speed laws, the climb profiles change. The higher the speed,
the lower the climb path, the longer the climb distance.
Climb profiles
Climbs are normally performed in three phases on a constant IAS/Mach climb
speed schedule at max climb thrust, as follows:
• 250 KT indicated air speed (IAS) is maintained until flight level 100, then
the aircraft accelerates to the chosen indicated air speed (e.g. “300kts);
• constant indicated air speed is maintained until the crossover altitude;
• constant Mach number is maintained until top of climb;
Cruise level
High speed
Low speed
Typical Climb Law
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
200 250 300 350 400 450 500
Speed - ktas
Altitude - ft
Crossover
altitude
Tropopause
250 kias
300 kias
M No 0.8
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
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The crossover altitude is the altitude where we switch from constant IAS
climb to the constant Mach number climb. It only depends on the chosen IAS and
Mach number, and does not depend on ISA variation.
During climb, at constant IAS, the true air speed (TAS) and the Mach
number increase. Then, during climb at constant Mach number, the TAS and the
IAS decrease until the tropopause.
To correctly evaluate the effects of climb techniques, climb and cruise flight
must be viewed in relation to each other. A short climb distance for example
extends the cruise distance; a low climb speed requires more acceleration to
cruise speed at an unfavourable high altitude. One has therefore to consider
sectors that cover acceleration to climb speed, climb, acceleration to cruise speed
and a small portion of the cruise to the same distance.
5.2.2 THE EFFECT OF CLIMB TECHNIQUE ON FUEL BURN
This evaluation has been made for all Airbus types, based on a climb to
35000ft, acceleration and cruise to a fixed distance. The assumed cruise speed
was 0.78 for the A320 family and 0.8 for the rest.
The reference climb technique is the standard technique given in each FCOM,
and is summarized below:
Aircraft types Speed law
A300-600 250kts/300kts/M0.78
A310 (GE) 250kts/300kts/M0.79
A310 (PW) 250kts/300kts/M0.80
A318/A319/A320/A321 250kts/300kts/M0.78
A330 250kts/300kts/M0.80
A340-200/300 250kts/300kts/M0.78
A340-500/600 250kts/320kts/M0.82
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 29 -
The following chart shows the variation of fuel burn with climb technique
over a given climb + cruise distance.
This shows that there is an optimum climb speed and max climb Mach
number that produces the lowest fuel burn. This happens to be the standard
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