getting to grips with fuel economy(37)

 
If green dot is higher than these maximum recommended speeds, it is advised to hold in configuration 1 at “S” speed below 20000ft: keeping clean configuration coupled with a speed reduction would save fuel but would decrease the speed margins which are especially important in turbulent conditions.
 
5.5.3 LINEAR HOLDING

 
If holding is going to be necessary, linear holding at cruise flight level and at green dot speed should be performed whenever possible since total flight time will remain constant (cruise time is increased but holding time is reduced) and fuel flow is lower at high flight levels.
If A.T.C. informs 15 minutes before reaching a fix and that 10 minutes holding is expected. Two options are possible:
. The aircraft is flown 15 minutes at cruise speed and holds for 10 minutes at green dot speed.
. The aircraft performs the cruise to reach the fix at green dot speed and holds for the remaining time at the same speed.
 
 
 
 

 
 
 
 
 
 
 
Holding optimization

Cruise at green dot speed

Holding for remaining 
time at green dot speed

Holding at cruise

15 minutes at cruised

10 minutes holding 
at green dot  speed

ATC restrictions may not permit a cruise speed reduction at the cruise flight level, or permit a hold at the cruise flight level. The standard procedure would be to continue to the top of descent at cruise speed and descend to a flight level to join the stack. However if ATC permit a linear hold it can give significant fuel savings.
However the amount of savings is very dependant on the characteristics of the aircraft type. The increase in time in the cruise depends on how much slower green dot speed is compared to the normal cruise speed. This increase was much higher with the A320 than the A340. In addition, most aircraft, flying the same cruise distance at green dot speed actually uses a little more fuel at these altitudes.  The following table shows the gains due to cruising slower and spending less time in the hold at  the cruise flight level.
Advantages of a 15min linear hold at cruise altitude at Green Dot speed 
Aircraft type
 Weight kg
 Cruise Flight Level
 Cruise Speed
 Fuel savings kg
 
A300
 120000
 350
 0.8
　
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