Boeing FMC RTA

[JammedStab]

Recently we were given a crossing time at a waypoint further along in our flightplan. This was entered as an RTA on the Progress page. Prior to reaching that waypoint ATC asked us if we could accept a higher altitude. There was no OPT or Max altitude to be found in the FMC. On a hunch I deleted the RTA time(and executed) and the two altitudes were displayed again. This feature was later found in an FMC guide that I have. Anybody know why this is in the FMC?.

[BOAC]

Don't forget Opt and Max will be dependent on cruise speed, which RTA changes. I assume Honeywell decided it was too 'difficult' to re-compute for a new Mach number at a higher level?

[Kefuddle]

The problem would probably be the speed change after the RTA which could result in flying above the FMC max level.

[Guru8904]

Assuming you were in VNAV PTH before selecting RTA, the FMC was flying a constant speed - FMC SPD regardless of wind changes. After you select RTA, the FMC now computes the speed required to make the RTA good and you see the A/T moving as and when required to compensate for wind changes. This is why sometimes with the selected RTA, you get the message RTA UNACHIEVABLE as the headwind/tailwind increases/decreases and the selected RTA moves out of the RTA bracket. From climb basics, OPT/MAX altitude both depend on the excess power available = (power available-power required). In case of a changing power setting, the FMC is unable to calculate the excess power available and hence no figures are available for OPT/MAX altitude.

[Skyjob]

Usually when an RTA is given this is for that given level the restriction.
By climbing usually the RTA for conflict resolution is no longer required and SHOULD be queried as generally this would not be applicable any longer.
Hence also FMC computes the RTA only for the current FL.

[JammedStab]

Originally Posted by Guru8904

Assuming you were in VNAV PTH before selecting RTA, the FMC was flying a constant speed - FMC SPD regardless of wind changes. After you select RTA, the FMC now computes the speed required to make the RTA good and you see the A/T moving as and when required to compensate for wind changes. This is why sometimes with the selected RTA, you get the message RTA UNACHIEVABLE as the headwind/tailwind increases/decreases and the selected RTA moves out of the RTA bracket. From climb basics, OPT/MAX altitude both depend on the excess power available = (power available-power required). In case of a changing power setting, the FMC is unable to calculate the excess power available and hence no figures are available for OPT/MAX altitude.

Thanks, it does make sense that Max is not calculable(if that is a word) because the thrust can be reduced from max to meet the RTA. One would think that perhaps OPT might be calculable.

[Guru8904]

From the FCTM:

Optimum altitude is the cruise altitude in still air for minimum cost when operating in the ECON mode, and for minimum fuel burn when in the LRC or pilot-selected speed modes.

I believe that while in RTA mode, we are neither in ECON nor in LRC. Hence, the OPT is not computable.
Calculable (able to be measured or assessed) is a word all right but I would rather use computable (capable of being computed).