Originally Posted by
Check Airman
Thanks. I’ll see if I can get my hands on the FCTM. Most crews here would have an aneurysm if you suggested 500 below maximum.
The Cruise section of the FCTM describes how the FMC maximum altitude is defined. It is the lowest of:
- maximum certified altitude
- thrust limited altitude
- buffet or maneuver limited altitude
The -200 and -300 (
not the -300ER) tend to be thrust-limited at high altitude, and the maximum altitude is therefore determined by the available thrust rather than buffet limits. The FCTM states:
...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.
In other words, if you start maneuvering at maximum altitude, the speed may decay and there may not be sufficient thrust to recover without descending. LNAV supposedly provides a bank angle limiting function that protects the commanded bank angle from exceeding the current available thrust limit. In any other mode, the FCTM recommends flying at least 10 knots above the lower amber band, using bank angles of 10° or less when operating at or near maximum altitude.
Many people have a "feel good" buffer they apply to the maximum altitude, but there's nothing official published. Strictly speaking a buffer is not required in smooth air
provided maneuvering is limited to low angles of bank. However, given that it’s hard to predict if smooth air will continue, a buffer of about 1,000 ft below the FMC maximum altitude is reasonable at weights where the aircraft is likely to be thrust limited.