FL CH vs. VNAV

[lou ross]

I read that an airline (which???) engages FL CH at 3000'. What is the advantage of engaging FL CH instead of VNAV?
Which mode (FMC) would burn less fuel to cruise: max climb power, 5% reduction, or 10% reduction? Thanks, lou ross.

[Pilot Pete]

VNAV is normally programmed for the various speed constraints during FMC setup, once in FLCH there is no reason why you couldn't put these speeds into the MCP window (directly into the Autopilot) and fly them that way (including ECON), you're just creating more work for yourself. The thrust management system (with the autothrottle engaged) will determine the power setting (in the 757 with 'climb 2' or 'climb 1' giving reduced climb thrust). With the speed protection in FLCH the a/c pitch will hold the speed.

There are always a number of ways to skin a cat, but some are more efficient than others!

PP

Sorry if this reads as patronising, but not sure on your knowledge level.

ps Just re-read your question and not sure if you meant FLCH at 3000' in the climb or 3000' in the descent.

We normally engage FLCH in the descent before 3000', especially in a busy TMA as VNAV refers to the route in the FMC and quite likely once on radar headings you will not follow your FMC route therefore VNAV predictions are not really going to be that acurate. Once in FLCH we start utilising the old grey matter and experience to judge height to lose and distance to run to achieve the desired profile.

[lou ross]

Thanks, Pete, that cleared up the confusion. lr.

[Ford Airlane]

Full Rating thrust will use the least amount of fuel in a climb to cruise altitude compared to Rating I/II.

:)

[mustafagander]

The concept of reduced thrust is to reduce the wear and tear on the engines.
Time/Temp/RPM combination is what does the damage. Reduce any of them and the engine's life is extended. In general, high TIT does damage to the hot section - can't be avoided, it's the nature of the beast.
It's a job for the management to work out the most cost effective way to operate - reduce thrust and extend engine life (reliability too!!) or use max rated thrust and save fuel.