772 and 773 FMC max alt margin
 

Assuming smooth air, how much margin do you like to give to the FMC maximum altitude on the 777?



I’ve seen some people say as high as 2000ft. I’ve been 900ft below max (can’t recall if it was a 200 or 300) and thought the buffer was sufficient.


At my company, everyone seems to have their own “feel good” number. It’d be great to find something written down that I can reference the next time it comes up.



Thanks

The reference you will find in the FCTM is related to maneuvering. Can't recall the numbers for the 777, but in general Boeing asks to limit the bank angle from 10kts above maneuvering speed (amber band). It's not the straight and level flying that is the issue, it is the banking and maneuvering.

Set the correct cg, limit the bank angle and you should be fine (ico turbulence you can always ask to descend if really necessary). Depending on how long you have to go and the local ATC environment, I've been 500ft below many times to avoid discussions further downroute (first come first served idea).

I've also been 100ft above, but that's a long story involving Chinese ATC with altitude restrictions at the entry waypoint and a subsequent unexpected speed restriction. Didn't feel uncomfortable, apart from the need to go into HDG SEL and limited bank angle.

Generally 1000ft. But partly depends if turbulence forecast. Don’t want to be sat in coffin corner with my hand on the speed brake for hours. If it’s smooth then wouldn’t be the end of the world 700-800ft or so. -200 and -300 have slightly different over bank protection. Think -300 more conservative?

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.

Thanks. I’ll have to see if the bank protection is mentioned in detail in our manuals. I don’t remember reading that.

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:
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.