getting to grips with fuel economy(17)

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

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
 

 
 
 
 
 
 
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 technique (300kt/0.78). Climbing at 320kt/0.82 will burn 1% more fuel.
　
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