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altitude. One can wonder whether it is profitable to stay at this altitude, instead of
climbing to the first optimum altitude.
Assuming the standard climb laws, the crossover altitude can be derived. The
standard speed laws are tabulated in paragraph 5.2.2.
The next table shows the effect of flying at the crossover altitude instead of
optimum flight levels. The 1st optimum flight level has been chosen for the short
sectors, whereas longer sectors assume step climbs with FL 310, 350 and 390
being available. This assumes ISA conditions and a take-off weight for a typical
sector with max passengers and some freight (2500kg for the A320 family and
5000kg for the other aircraft).
Aircraft type Sector
Distance
Cross-over
altitude
Optimum
Flight Levels
Gained
time (min)
Increase in fuel
consumption
A300B4-605R 2000nm 29000 ft 310/350 7 1190kg
A310-324 2000nm 30000ft 350/390 3 2160kg
A318-111 1000nm 29000 ft 370 3 740kg
A319-112 1000nm 29000 ft 370 3 650kg
A320-214 1000nm 29000 ft 350 2 580kg
A320-232 1000nm 29000 ft 340 2 440kg
A321-211 1000nm 29000 ft 330 2 350kg
A330-203 4000nm 31000 ft 350/390 9 5040kg
A330-223 4000nm 31000 ft 350/390 9 5780kg
A330-343 4000nm 31000 ft 350/390 10 6380kg
A340-212 6000nm 29000 ft 310/350/390 17 10900kg
A340-313 6000nm 29000 ft 310/350/390 14 8410kg
A340-313E 6000nm 29000 ft 310/350/390 17 9310kg
A340-500/600 6000nm 29000 ft 310/350/390 18 2430kg
This table shows that flying at crossover altitude increases the fuel burn
significantly for a relatively small reduction in block time.
Getting to grips with Fuel Economy IN FLIGHT PROCEDURES
- 41 -
5.3.2.2 CRUISE OPTIMISATION WITH STEPPED CLIMB
5.3.2.2.1 Introduction
It has been shown that flying at non-optimum altitudes can cause significant
fuel penalties, and that the effect of fuel burn is to increase the optimum altitude.
The ideal scenario would be to follow the optimum altitude as in the climbing
cruise, but A.T.C. constraints, performance and buffet limits do not make this
possible. However, by changing the cruise level with step climbs, as the aircraft
gets lighter the aircraft will remain as close as possible to the optimum altitude.
5.3.2.2.2 Choice of Profile
Several parameters such as weather conditions, ATC requirements, may
influence any decision made by the crew with regard to three fundamental
priorities: maneuverability, passenger comfort, and economics.
This pertains to the choice of the cruise flight level that can be made
according to the three following climb profiles as shown below for an A340-642:
The Low profile initiates the step climb at the weight where the next
available flight level is also the optimum flight level at that weight. Consequently
the flight levels are always at or below the optimum. This has the advantage of
better maneuverability margins and generally a better speed as it is closer to the
crossover altitude.
Step Climb Profiles
Even Flight Levels Non RVSM
26000
28000
30000
32000
34000
36000
38000
40000
42000
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
Distance from Brake Release - nm
Altitude - ft
Low Profile
Mid Profile
High Profile
Optimum Altitude
Maximum Altitude
Maximum Altitude Optimum
Altitude
5 - IN FLIGHT PROCEDURES Getting to grips with Fuel Economy
- 42 -
The high profile initiates the step climb at the weight where the next
available flight level is also the maximum flight level at that weight. The flight
levels are mainly above the optimum and the aircraft will have decreased
maneuverability and fly slower.
The mid profile initiates the step climb at the weight where the specific
range at the next available flight level is better than that at the current flight
level. This enables the flight profile to remain as close as practically possible to
the optimum flight level. It is this technique that is recommended for best fuel
economy, and is also very close to that required for best economics.
It is interesting to note that, in this case, the Mid profile step climb is made
1140nm before the Low Profile step climb and 1520nm after the High profile step
climb.
The situation changes with odd flight levels:
Because of the different available flight levels, the step climbs are initiated
some 1500nm further than the even flight level step climb points. However the
relative merits of each profile remains the same.
With Reduced Vertical Separation Minima (RVSM) the difference between
flight levels reduces from 4000 to 2000ft and this enables the aircraft profile to
　
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