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Optimum Altitude displayed in the FMC without approaching the 1.3g buffet
margins (approximately 30 degrees of bank to buffet onset). A common “rule
of thumb” had been you could fly 2000 feet above Optimum Altitude with
minimal degradation of aircraft performance. This “rule of thumb” technique
was used to achieve step climb profiles at the earliest possible time.
With the installation of 10.3 and higher software, the 1.5 buffet margin provided
at Optimum Altitude is reduced significantly when flying above the FMC
displayed Optimum Altitude. Additional reduction of this buffet margin may
occur from turbulence, temperature variations, increased drag, and airspeed
decay. These factors may influence aircraft performance to the point where
sufficient thrust is not available to maintain altitude. Flying above Optimum
Altitude also increases the fuel burn 1 to 2 percent.
Crew Action: Before requesting or accepting an altitude above Optimum, it is
required that crews check the 1.3 G Buffet Boundaries Low / High IAS chart in
the QRH to determine if the flight has the required operational airspeed
performance margins. If altitude changes enroute are difficult to obtain, some
thought should be given to selecting an initial cruise altitude based on maximum
thrust limits. Selecting a cruise thrust limited altitude is dependent upon the
cruise level temperature.
CRUISE PERFORMANCE ECONOMY
The dispatch computed fuel burn from departure to destination is based on
certain assumed conditions, i.e., takeoff gross weight, cruise altitude, route of
flight, temperature, wind enroute, and cruise speed. Cruise fuel burn will
increase when flying:
· Other than optimum altitude.
· Speed faster than planned or slower than long-range cruise if planned for
long-range cruise.
· Stronger headwind component.
· Unbalanced fuel.
Sec. 3 Page 138 737
Rev. 11/15/02 #41 Continental Flight Manual
· Improperly trimmed aircraft.
· Excessive thrust lever adjustments.
The higher the aircraft flies above optimum altitude, the more buffet margin is
reduced. Before accepting an altitude above optimum, determine that it is, and
will continue to be, acceptable as the flight progresses under projected
conditions of temperature and turbulence.
NAVIGATION
“Fix to Fix” direct navigation should be requested and utilized whenever
possible. This, combined with the use of FMC ECON CRUISE, will result in
the most economical cruise profile. While at cruise, both Nav Radios should be
operated in the AUTO position to allow FMC radio updating. Substantial
deviations from flight planned altitudes and/or airspeeds due to weather or ATC,
etc., should be analyzed with a combination of computer information,
conventional fuel planning, buffet boundaries, etc.
Ideally, an END OF DESCENT POINT within the terminal area of the
destination airport, including speed and altitude, should be inserted while at
cruise. All expected descent profile information should be programmed at
cruise altitude so as to minimize low altitude programming.
737 Sec. 3 Page 139
Flight Manual Continental Rev. 11/15/02 #41
HIGH ALTITUDE / HIGH SPEED FLIGHT CHARACTERISTICS
Aircraft Flight Characteristics
The aircraft exhibits excellent stability throughout the high altitude / Mach
range. Mach buffet is not normally encountered at high Mach cruise, even at
MMO. However, even in Mach buffet, control response is smooth and normal.
The aircraft exhibits a slight nose down trim change when accelerating to
speeds approaching MMO. However, control force changes are light and easily
managed. When the Mach trim system is operative, the nose down trim change
is nearly imperceptible except by referencing the control column position.
Trim Technique
If an “out of trim” condition is suspected, check engine parameters for
indication of unequal thrust. Set and maintain a balanced thrust condition.
Trimming the aircraft to counteract this condition results in further increase in
drag with its resultant loss of fuel mileage. Check fuel quantities for lateral
imbalance. Zero the aileron and rudder trim. If trim is required, hold the wings
level with the control wheel using the ADI for a reference. Apply rudder to
maintain heading. When heading is stabilized, trim out any force that is being
held on the rudder and the control wheel. The wheel should be approximately
neutral.
Inadvertent Rudder Trim
Inflight, inadvertent activation of the rudder trim by an observer seat occupant
or by an object placed in contact with the trim switch is recognizable in manual
　
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