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technique (300kt/0.78). Climbing at 320kt/0.82 will burn 1% more fuel.
However the following chart shows that this is obtained at the expense of
time.
Effect of Climb Technique on Fuel to 120nm
A300B4-605R ISA F/L 350 Weight 140000kg
-0.2%
0.0%
0.2%
0.4%
0.6%
0.8%
1.0%
1.2%
1.4%
1.6%
260 270 280 290 300 310 320 330 340 350
Climb Speed - kias
Fuel difference - %
0.76
0.78
0.80
Mach No
FMG
0.82
CI "0"
CI "100"
Effect of Climb Technique on Time to 120nm
A300B4-605R ISA F/L 350 Weight 140000kg
-4%
-3%
-2%
-1%
0%
1%
2%
3%
4%
5%
6%
260 270 280 290 300 310 320 330 340 350
Climb Speed - kias
Time difference - %
0.80
0.78
0.76
Mach No
FMGS
0.82
CI "0"
CI "100"
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 30 -
This time difference plot has the same characteristics for all Airbus aircraft,
with the best time being obtained at the highest climb speed and max climb Mach
number. Note that although a slow climb speed gets the aircraft to cruise altitude
earlier, this requires more acceleration to cruise speed and more cruise to a given
distance, making it slower overall.
The fuel difference plot characteristics vary with aircraft type. The A310,
A321 and A330 show similar characteristics to the A300 with a best fuel climb
speed of about 290 to 300 knots.
The A318, A319 and A320 show better fuel burn at the lower speed range
(260 to 280 knots)
The A340 shows better fuel burn at the higher speed range (310-330 knots).
The A310 and A340 are similar to the A300 in showing minimum fuel at a
max climb Mach number of 0.78. In fact 0.8 is better for the A340-500/600.
However the A320 family and A330 benefit from the lower Mach No of 0.76.
Thus the A320 family benefits from low climb speeds and the A340 from high
climb speeds. This difference arises from the different behavior during climb of
twin-engine and four-engine aircraft. Indeed, twin-engine aircraft have a higher
thrust than four engine aircraft, as they must satisfy some take-off climb
requirements with only one engine operative, compared with 3 engines operative
on the quads. This enables them to have a higher rate of climb than four engine
aircraft and reach cruise flight levels quicker.
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 31 -
5.2.3 CORRELATION OF FUEL BURN & TIME WITH CLIMB TECHNIQUE
The following chart shows the differences in fuel and time to climb and
cruise to a fixed distance with varying climb speed and max climb Mach number
relative to the standard technique.
This chart shows that the fastest technique (330/0.82) uses the least time
(-3.2%) and the most fuel (+1.5%) whereas the slowest technique (270/0.76)
uses the most time (+4.5%) and nearly the most fuel (+1.4%). The least fuel is
obtained using a 300/0.78 climb technique. Variation of climb technique can cause
a total variation of 1.5% and climb time by 8% for this aircraft.
Also plotted on the charts are lines representing the speeds selected by the
FMGS for various cost indices (CI). The left hand point of each line represents a CI
of zero (fuel cost priority) and the right hand point represents a CI of 100 (flight
time priority). It should be noted how the FMGS line approximates to the lower
boundary of the time - fuel difference plot.
The chart on the following page is for the A320 and shows completely
different characteristics.
Effect of Climb Technique on fuel and time to 120nm
A300B4-605R ISA F/L 350 Weight 140000kg
-6%
-4%
-2%
0%
2%
4%
6%
-1% 0% 1% 2% 3%
Fuel %
Time %
Mach No
FMGS
0.76
0.78
0.8
280
300
320
330
Climb Speed
KIAS
0.82 270
50kg
1 minute
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 32 -
The different mach numbers all coalesce together and the FMGS line forms the
common boundary. Climb speed increases from the left to the right. Least fuel is
obtained using a 0.76/280 technique. Mach No has little influence, but increasing
speed from 280 to 330kias decreases time by 6% and increases fuel by 6%.
Completely different characteristics are also shown in the next chart (A340-642).
Effect of Climb Technique on fuel and time to 200nm
A320-214 ISA F/L 350 Weight 70000kg
-6%
-4%
-2%
0%
2%
4%
6%
-2% -1% 0% 1% 2% 3% 4% 5% 6%
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