Optimum cruise alt

[chechi191288]

Hi, i was studying and I found the term "Optimum cruise alttitude"... and I don't know what it mean! Somebody knows what it means? Thanks

[Tom Goodrick]

For any aircraft, power or thrust is reduced with altitude but true airspeed, for the same power level increases slightly with altitude. The service ceiling is defined as the altitude at which full power or thrust only give a climb rate of 200 fpm (ft/min). The optimum altitude is the altitude that balances the best speed with the best fuel flow. But fuel flow is directly related to power (in piston engine aircraft). For economy cruise over a long distance, power is usually set at 55 to 60% of sea level max. The altitude at which this power gives the best true airspeed is the optimum altitude.

That is one definition. A second definition relates to jets and range of a particular flight.

For piston aircraft that do not have turbcharging, the optimum altitude is often near 6,000 ft where they can still develop 75% power.

For turboprop aircraft like the Beechcraft King Air, the optimum cruise altitude is generally above 20,000 ft.

Jets are more efficient when cruising near their max altitude (as determined by stability and control issues, generally near 40,000 ft, lower for airliners, higher for bizjets). But it may take 100 nm to get up and down. Obviously this does not work for short trips of a few hundred nautical miles or less. Thus the optimum altitude for a particular flight of a jet is the altitude at which the times to climb and to descend are reasonable portions of the total time and the time at cruise is enough to gain some level of efficiency. People have become famous for writing math and computer programs that figure this out. The pilot gets the cruise altitude from his flight department for each flight.

Best we can do is make an educated guess. Note the time, fuel and distance for climbs to various altitudes. Descent is generally made at idle for jets.

[Allan_Lowson]

Yesterday I flew home from Southampton to Glasgow in a Dash 8 400, which is about the longest route for turboprops within the UK, and the cruise altitude was 24,000 ft, with the descent starting parallel with Carlisle. The direct route in fs2004 is west of that actually used as they route you up the centre of the country.

While I remember, next week I'm off to Belfast from Glasgow, and the cruise altitude for that route is typically about 8,000 ft!

[johnl]

Alan, if you departed from rwy 20 and turned leftabout to head north you probably flew around (or over) my house. We don't really notice the Dash-8s here - it's the Clockwork Tristars on the Aldeney run that make the noise.

[Tom Goodrick]

I should add that the best altitude for a particular flight depends on the wind at altitude. Often that is a major factor.