Flight Levels In The Real World...

I am a Flight Sim pilot ONLY. So, please forgive my ignorance:o. Recently I wanted to fly the sim from Denver (KDEN) to Las Vegas (KLAS). Since I wanted to "fly" as realistically as possible, I went to Flight Aware for some tips regarding speed, altitude, etc. I noticed that a 737 flew the route at an altitude of 40,000 feet. Also, an Airbus flew the same route at FL 380.

 

My question is: Why do the pros fly so high for such a short distance (628sm). I always thought that the shorter the distance, the lower the altitude. :confused:

 

Thanks :)

P.S. Of course, there are mountains in the Denver area, and high elevations around Vegas...Is that it:confused:

I'll give it a try...

 

For the most part, jet engines tend to be more efficient at higher Altitudes, so I would surmise that Pilots will try to get up high pretty quickly. Also, since they're flying a flight plan using specific high altitude Jetways, they would want to get there quickly.

 

Be advised... I'm a neophyte when it comes to flying Tubeliners so there are probably better answers out there... one reason why I shy away from flying the Big Iron.

 

Not to say I would love to figure it out, though; Adjust the Weight & CG prior to Takeoff, then Climb straight out, or perform a step climb to Altitude? What about Descending for Landing... when do I start, and do I step or drop like a rock?

 

Eager to learn,

 

Alan :pilot:

 

Cubbies or Dodgers... Tribe or the Jays? Personally, I'd like the Cubbies vs. The Tribe...

For a jet aircraft, the reason is fuel flow. At lower altitudes (especially 10,000 and below) a jet gulps fuel like there's no tomorrow. A made-up example, but the general idea: A jet aircraft at 5,000 feet would burn its full load of fuel in perhaps an hour (fighters are much worse), while having perhaps six plus hours at 30,000 plus feet.

 

So it's cheaper and gives more fuel reserve at the end of the flight to climb until it's time to descend on the shortest flights, such as Denver to Colorado Springs. Somewhat longer flights might give five to thirty minutes at cruise altitude before descent.

 

But fuel burn is the main reason.

 

I always thought that the shorter the distance, the lower the altitude.

For avgas powered aircraft that comes close to being right, but it's still a trade-off between speed and fuel burned, with turbulence (or its lack) being factored in for passenger comfort.

 

However climbing 6,000 feet up then descending (in a gasser) for a 15 mile flight is a bit much. Common sense and work the numbers...

Thanks, Inuss: I think I got it. The lower the altitude, the thicker the atmosphere. Thicker atmosphere means more energy required. More energy required means more fuel use needed. Am I in the right ballpark? :confused:

Modern airliners are designed to fly at higher altitudes than the older models. An MD80, for example, on that route would have probably stopped at 30,000 or 32,000, unless they were light. As for the terrain, you're right, but the highest peaks are 'only' a little over 14,000.

 

Then, there are things such as 'single engine driftdown altitudes' and such over the mountains, but that's tomorrow's lesson :)

Actually, it's not the "more energy required," (it's actually less energy required) but less air available up high burns fuel at a lower rate, since the fuel-air mixture needs to remain in proportion regardless of altitude -- so less air is less fuel. This is also true in piston engines, but not quite to the same degree, in terms of fuel flow difference -- jets are very thirsty, especially down low. And they can get up high very quickly.

 

An F-33A Bonanza*, for example, at 75% power will burn 15.4 gph (gallons per hour) and at sea level will cruise at 160 knots TAS, while at 7,000 feet it will cruise at 171 knots, per the performance charts for a standard day plus 20ºC.

 

At 65% power it will cruise that same 160 knots at 7,000 feet while burning 13.3 gph (less dense air equals less drag equals not as much power/fuel needed for the same TRUE airspeed). At 55% power it can only get up to 155 knots (and that's at 12,000 feet), but it's also only burning 11.5 gph.

 

Turbo-charged aircraft (jets are not) compress the air before it gets into the engine, so can retain sea-level air density going into the engine up to some "critical altitude," which varies from one setup to another, perhaps 10,000 feet in one while maybe 20,000 feet in another. After that altitude is exceeded, the air density into the engine starts to decrease, but is still higher than in a "normally aspirated" (not turbo-charged) engine. Fuel flow varies proportionally with the air density.

 

Hope this helps.

 

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* From the F-33A Beech manual

It's interesting then that FSX flight planner will only plot an altitude just above terrain collision. Following on from the OPs question, does FSX aircraft cfg hold a fuel burn scalar depending on altitude?

The altitude "just above terrain collision" is the minimum it thinks you need, and is primarily to minimize the change of hitting the ground, but the planner expects you to change it to the altitude at which you want to cruise.

 

As to "burn scalar," I generally operate with unlimited fuel, so I don't pay much attention to that, but I'm fairly sure there's some sort of provision for that. You can check by filling your tanks (perhaps to 25% or 50%, to reduce the required time) then flying at, say, 5,000 feet until you run out of fuel. Check how long you flew. Then do the same for 35,000 feet and check the time.

I'll have to try that. If there isn't a calculation in the body of the Sim then it might be a good option in one of the weather ad-ons so that planning also includes weather vs fuel burn... just a thought.

There isn't a "burn_scalar" per se in the aircraft.cfg file. Having said that, there are entries you can make that control the fuel burn depending on power setting in the aircraft.cfg file. Most of the fuel vs altitude calculations are done in the .air file, however. In the .air file, they are sections 1502-1507. The calculations in them for engines are very complex, and complicated, but I am sure I can find references on what goes on in the FSDeveloper forums. I read them before, but it's been a while. I'd have to do a lot of review if *I* wanted to try and explain them.

In the aircraft.cfg, however, you have a few things you can try adjusting to make the fuel burn come out the way it should. Maybe not how you WANT, but how it should be.

In the [generalenginedata] section, the line to focus on is fuel_flow_scalar.

In the [turbineenginedata] section, focus on the lines fuel_flow_gain, thrustspecificfuelconsumption, and, if you have them active, afterburnthrustspecificfuelconsumption. If not, ignore that one. If they are not in the appropriate section, just add them in yourself. They do matter a lot.

Quite often, the thrust specific fuel consumption number, and afterburner number if applicable, can be found in the particular engine used's date sheet from the manufacturer. Great reference. I have used that many times with very good results. Obviously, all that refers to turbine engines only, not piston engines.

Does that answer your question, tgon?

Pat☺