.  
thrust limited altitude - the altitude at which sufficient thrust is available
to provide a specific minimum rate of climb. (Reference the Long Range
Cruise Maximum Operating Altitude table in the PI chapter of the QRH).
Depending on the thrust rating of the engines, the thrust limited altitude
may be above or below the maneuver altitude capability

.  
buffet or maneuver limited altitude - the altitude at which a specific
maneuver margin exists prior to buffet onset. This altitude provides at
least a 0.2g margin (33° bank) for FAA operations or a 0.3g margin (40°
bank) for CAA/JAA operations prior to buffet.

Although each of these limits are checked by the FMC, available thrust may limit the ability to accomplish anything other than relatively minor maneuvering. The amber band limits do not provide an indication of maneuver capability as limited by available thrust.
737-300 - 737-500 The minimum maneuver speed indication on the speed tape (as installed) does not guarantee the ability to maintain level flight at that speed. Decelerating the airplane to the amber band may create a situation where it is impossible to maintain speed and/or altitude because as speed decreases airplane drag may exceed available thrust, especially while turning. Operators may wish to reduce exposure to this situation by changing the FMC parameters (via maintenance action) to suit individual operator needs. Flight crews intending to operate at or near the maximum operation altitude should be familiar with the performance characteristics of the airplane in these conditions.
To get the most accurate altitude limits from the FMC, ensure the following entries are accurate:
.  
airplane weight

.
cruise CG

.  
temperature deviation at the cruise altitude.

Copyright . The Boeing Company. See title page for details.
FCT 737 (TM) October 31, 2006
737-300 - 737-500 For LNAV operation, the FMC provides a real-time bank angle limiting function on airplanes with FCM update 10.0 and beyond. This function protects the commanded bank angle from exceeding the current available thrust limit. This bank angle limiting protection is only available when in LNAV. On airplanes with earlier FMC updates, it is possible for LNAV inputs to exceed the capability of the airplane. This could result in a loss of altitude or airspeed.
737-600 - 737-900 For LNAV operation, the FMC provides a real-time bank angle limiting function. This function protects the commanded bank angle from exceeding the current available thrust limit. This bank angle limiting protection is only available when in LNAV.
For operations other than LNAV, fly at least 10 knots above the lower amber band and use bank angles of 10° or less when operating at or near maximum altitude. If speed drops below the lower amber band, immediately increase speed by doing one or more of the following:
.
reduce angle of bank

.
increase thrust up to maximum continuous

.
descend. Turbulence at or near maximum altitude can momentarily increase the airplane’s angle-of attack and activate the stick shaker. When flying at speeds near the lower

amber band, any maneuvering increases the load factor and further reduce the margin to buffet onset and stick shaker. FMC fuel predictions are not available above the FMC maximum altitude and are
not displayed on the CDU. VNAV is not available above FMC maximum altitude. Fuel burn at or above maximum altitude increases. Flight above this altitude is not recommended.

Optimum Altitude
Optimum altitude is the cruise altitude for minimum cost when operating in the ECON mode, and for minimum fuel burn when in the LRC or pilot-selected speed modes. In ECON mode, optimum altitude increases as either airplane weight or cost index decreases. In LRC or selected speed modes, optimum altitude increases as either airplane weight or speed decreases. On each flight, optimum altitude continues to increase as weight decreases during the flight.
For shorter trips, optimum altitude as defined above may not be achievable since the top of descent (T/D) point occurs prior to completing the climb to optimum altitude.

737 Flight Crew Training  Manual
Trip altitude, as defined on the FMC PERF INIT page, further constrains optimum altitude by reducing the altitude for short trips until minimum cruise segment time is satisfied. This cruise time is typically one minute, but is operator selectable in the FMC by maintenance action. For short trips, operation at the trip altitude results in the minimum fuel/cost while also satisfying the minimum cruise time requirement.
　
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