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to FCOM/99 Duref Cible FCOMQRH to determine the OPT FL. FCOM and QRH charts are only provided for four different Mach numbers.
STEP CLIMB
Applicable to: ALL
Since the optimum altitude increases as fuel is consumed during the flight, from a cost point of view, it is preferable to climb to a higher cruise altitude when aircraft weight permits. This technique, referred to as a Step Climb, is typically accomplished by initially climbing approximately 2 000 ft above the optimum altitude and then cruising at that flight level until approximately 4 000 ft below optimum.
Step climbs can either be planned at waypoints or be optimum step points calculated by the FMGS. In order to determine the optimum location of the next FL change, the crew will use the OPT STEP facility on the STEP ALT page and insert the next FL. If predictions are satisfactory in term of time and fuel saving, the crew will insert it in F-PLN provided it is compatible with ATC. The inserted step climb is
set as a (S/C) geographic waypoint. It may be updated by pressing the UPDATE* prompt on the STEP page.
NORMAL OPERATIONS
CRUISE
The OPT STEP computation will be accurate if vertical wind profile has been properly entered. Refer to FMS USE of this section. Refer to FCOM/PER-CRZ-AEO-ALT-20 WIND ALTITUDE TRADE FOR CONSTANT SPECIFIC RANGE to provide valuable tables to assess the effect of the vertical wind profile on the optimum cruise flight level.
It may be advantageous to request an initial cruise altitude above optimum if altitude changes are difficult to obtain on specific routes. This minimizes the possibility of being held at a low altitude and high fuel consumption condition for long periods of time. The requested/cleared cruise altitude should be compared to the REC MAX altitude. Before accepting an altitude above optimum, the crew should determine that it will continue to be acceptable considering the projected flight conditions such as turbulence, standing waves or temperature change.
OPT FL follow up
The diagram above shows three step climb strategies with respect to OPT and REC MAX FL. Strategy 1 provides the best trip cost, followed by 2 then 3.
EFFECT OF ALTITUDE ON FUEL CONSUMPTION
Applicable to: ALL
The selected cruise altitude should normally be as close to optimum as possible. As deviation from optimum cruise altitude increases, performance economy decreases.
As a general rule:
? Flying 2 000 ft above optimum altitude results in fuel penalty of approximately 2.5 %
? Flying 4 000 ft below optimum altitude results in fuel penalty of approximately 5 %
? Flying 2 000 ft below optimum altitude results in fuel penalty of approximately 1.5 %
NORMAL OPERATIONS
CRUISE
Consequently, flying at a level other than OPT FL will adversely affect the trip cost.
fuel penalty versus OPT FL
FUEL MONITORING
Applicable to: ALL
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A330/A340 机组训练手册 FCTM Flight Crew Training Manual 1(55)
