How high is high enough

[Charlie01]

I just completed an around the world and the one thing ( I suppose there are a lot) that I could never figure is how high to fly. Do you just take the altitude the flight planner chunks out and like it or change it and hope for the best? It seemed to me that when I flew higher ( FL30-FL40) I would use a lot more fuel per hour , but my ground speed was a lot higher also. What is the point where you will get the best fuel economy, and does anyone know how to figure it. It may be simple , but it eludes me.

[ashaman]

The cruise level is the max practicable height you can reach and maintain in your flight, directly dependent on airplane (max ceiling), its load (both fuel and passengers) and on the traffic (though this last is usually a concern of Real Aviation and not simulated one, unless flying online) and, on short hauls, of climb time.

Example: a 744 has a max certified ceiling of 41.000 feet. Though you'll never be able to reach that high at max payload and fuel. On a long haul, you usually do a step climb. As in your first flight level will be a low one (depending on overall weight, even so low as FL 280 to start) and as you burn fuel you will climb higher up to (if you think it necessary up to max ceiling).

On shorter hauls (ie, less than 400nm) better declare a max cruise level following the old rule nm/FL (ie, flight distance 240nm == FL240 -- flight distance 140nm == FL 140/14.000 feet).

At least this is how I do it.

[Nav]

[quote]It seemed to me that when I flew higher ( FL30-FL40) I would use a lot more fuel per hour , but my ground speed was a lot higher also. What is the point where you will get the best fuel economy, and does anyone know how to figure it. It may be simple , but it eludes me.

[Nexus]

unless you are the owner of the aircraft operating manual (AOM) there is no chance for you, or anybody else to give you the ideal cruise altitude. Too many external factors.
But bizjets in general (I assume you flew the Lear) have no problems cruising in the FL400's, they arent as sensitive to weight restrictions as a 744. Isn't the ceiling somewhere around FL500 f

[beaky]

When in doubt, experiment. Best primary indicator is the fuel flow meter... compare it to indicated Mach to determine what altitude (and power setting) works best.
But remember: the engines may be more efficient at a given FL, but the higher you go, the stronger the wind, generally. So you could end up losing economy in the long run with a strong headwind... groundspeed is the third part of the equation.

[Charlie01]

Thanks guys,
     I didn't know about the mach number vs airspeed number thing. I did try cruising at different altitudes at the same airspeed and then comparing the fuel per hour usuage, but I couldn't come up with a definitive answer. I think part of my problem is that to cut down on flying time , I was going as fast as I could without setting off the overspeed light, hence poorer mileage.  I will read up on the specs., and try to do better next time around. I am pretty new at this , I still haven't figured out how to use the VOR, that may be my next project.  I worked on aircraft radio in USAF , and could pull black boxes on the nav stuff, but I never really took the time to figure how the nav stuff really worked, as it was out of my specialty. I think it is all one specialty now-avionics.
                                 Charlie

[Nav]

Minor point, Charlie, but on jets the Fuel Flow is usually shown in pounds, not US gallons. Conversion is about 6 pounds to the gallon.

As to VORs, quickest way to explain is maybe to quote from that RW tute of mine:-

"(For newcomers again

[vololiberista]

It seemed to me that when I flew higher ( FL30-FL40) I would use a lot more fuel per hour , but my ground speed was a lot higher also. What is the point where you will get the best fuel economy, and does anyone know how to figure it. It may be simple , but it eludes me.                                                    Charlie

I would use Mach numbers for airspeed above 20,000 feet, as these are computer-calculated and accurate  

Use Mach no. at or above FL240 because that is the altitude where Vmo=Mmo. If you are in a crash dive eg. because of cabin depressurisation you must begin to reduce the dive angle at 24,500ft otherwise the a/c will rapidly exceed Vmo by a significant amount resulting in airframe failure.
Vololiberista

[YSHT]

That actually seems perfectly correct to me. I am a real pilot and I know these things - listen up.

First - your fuel. OK, I am assuming you are flying a prop (low Flight Level) so when you reach thinner air, try leaning the mixture a bit (I think it's Shift+F2, check the kneeboard) because when the fuel is mixed with the air, it is blown out at a certain rate. When you have less air (due to density) the amount of fuel you use is excessive.

Second - Your groundspeed. I noticed someone else said that you have less air resistance. This is precisely correct, aided by wind and also your GS may appear higher because (here's where it gets technical)... The ASI reads your IAS by 'measuring the amount of air molecules that pass through the pitot in a given amount of time.' So, if you take fewer air molecules you get a lower Indicated Airspeed compared to the same True Airspeed. Whilst I'm talking about airspeeds, if anyone uses "Display TAS" in the Settings, turn it back to "IAS" - this is a far more useful figure because (For one reason) you will stall at an airspeed higher than the stalling speed if you use TAS for the same reason.