Soft Altitude

[Flyman35]

if you have the following on the FMA (SPEED | ALTCRZ) at FL240 is the soft altitude mode engaged or it has to be MACH | ALCRZ becasue I read some where that for the soft altitude to be engaged the aircraft must be flying at MACH,so what is the correct answer?

[Cough]

Soft alt only with mach. FCOM 1.22.30 AP/FD vertical modes under the heading Soft altitude refers.

[Flyman35]

thanks a lot.

[Cough]

Jimmy,

I don't know what your pic proves. Yup, alt crz engages at any altitude, but soft alt only with a mach mode.

The quote from FCOM

On reaching the planned cruise altitude, ALT CRZ engages and A/THR maintains the speed/mach target.

Two minutes after ALT CRZ engages, if the mach mode is operative, SOFT ALT mode engages. This allows the aircraft to deviate ± 50 feet from the target altitude, thereby minimizing thrust variations and reducing fuel consumption.

[Jimmy Do Little]

Lesson learned... RTFQ

Focused on the Mode, and missed the "Soft Alt" part of the question.

Thanks for being gentle. You're completely correct. Now going to...

Cheers

[On-MarkBob]

Soft Alt

With the introduction of RVSM airspace, 'soft' altitude has mostly been replaced with 'soft' or 'lazy' autothrottle speed control, for obvious reasons. Both modes are essentially for saving fuel, and they do. Keeping the engines 'purring' at a constant speed is much better than having them 'hunting' to maintain a set airspeed/Mach number. 'Soft' altitude would allow the aircraft a certain 'lattitude' in pitch to either speed up or slow down without the engines having to do the same. It was removed in RVSM aircraft and now the aircraft is allowed to speed up and slow down but not climb or decend.

Rgs.

Bob.

[FlightDetent]

OMB: I must say that I cannot find any FCOM reference tu support your conclusion. Perhaps the vertical limits fo soft mode had been narrowed down to 50 ft in order to adhere to RVSM parametres?

FD (the un-real)