Crusing Altitude.

[Chananya Freedman]

Happy Tuesday,

I'm wondering why commercial Air Lines cursing altitudes are so high, why couldnt they be lower? Info would be appreciated.

[atcman23]

I don't think it has much to do with the fact an aircraft is operating for an airline.  However, your statement is still true.  But it is due to a few factors:

1) Length and duration of flight.  The further the destination is from their departure point, the higher the aircraft is going to fly to avoid other traffic.

2) Fuel Efficiency.  Jet aircraft are more fuel efficient at higher altitudes.

3) Weather.  Higher flying aircraft can get above the weather in most instances, perhaps even a cloud deck.

I think the biggest factors, though, are the length of the flight and fuel efficiency.  But you would not want a bunch of jets flying around at 14,000 - 16,000 feet with other aircraft that typically fly at those altitudes (pistons, etc.).  So, they fly higher to avoid traffic issues, reroutes, etc.  Again, fuel efficiency plays a role, too.

I'll bet there are others in the forum that have other reasons as well.

[tyketto]

Happy Tuesday,

I'm wondering why commercial Air Lines cursing altitudes are so high, why couldnt they be lower? Info would be appreciated.

A number of reasons they are so high, but just a few that come to mind:

1. congestion. Lower cruise altitudes would mean congestion at that particular flight level. you may have converging traffic, in trail spacing problems, and wake turbulence issues if they were lower, whereas there may be less at a higher level.

2. weather. Moderate to heavy chop at a given level may dictate a lower or higher level. Same with weather. Would you rather stay below the tops for the weather and ride through it, or go above it for a smoother ride?

3. Fuel. higher altitude = less oxygen = less fuel burn.

I'm sure there's plenty more, those those are the biggest.

BL.

[klkm]

I would say #1 is fuel, #2 fuel, #3 fuel, and #4 fuel.  The higher you are the less fuel you burn.  Some/most routes could not be flow if the plane was forced to stay too low.  As the plane burns more fuel they will request higher throughout the flight and try to stay right in the maximum efficiency range for fuel burn.  Because of this, dispatchers/airlines try to keep as little fuel on board as possible to allow them to fly the highest. 

[joeyb747]

I would say #1 is fuel, #2 fuel, #3 fuel, and #4 fuel.  The higher you are the less fuel you burn.  Some/most routes could not be flow if the plane was forced to stay too low.  As the plane burns more fuel they will request higher throughout the flight and try to stay right in the maximum efficiency range for fuel burn.  Because of this, dispatchers/airlines try to keep as little fuel on board as possible to allow them to fly the highest.  

100% Agreed.

The higher the cruise, the lower the burn, and the faster the aircraft can travel.

Take the B747-200 for example:
Below FL100 = 250 KIAS
Above FL100 and Below FL180 = 330 KIAS
Above FL180 = 0.70 Mach (336 KIAS)
Above FL240 = 0.77 Mach (330 KIAS)
Above FL280 = 0.82 Mach (326 KIAS)
Above FL330 = 0.85 Mach (305 KIAS)

(For those who may not be aware, "FL 100" is not 100 feet, add two more zeros to the "FL" number.
FL100 = 10,000 Ft, FL330 = 33,000 Ft.)

Typical cruise for the B747-200 series is FL350, or 35,000 Ft. At that altitude, 0.85 Mach = roughly 555 mph.
Mach 0.89 (587 mph) is reachable, but not as efficient. The airplane is certified for flight as high as FL450.

Weather would be the number two reason. Generally, the air is smoother up high.

[N9IIT]

In particular on West to East flights the jetstream can push an airliner quite well at FL300 and above.

[Hollis]

Technically, fuel efficiency is the primary reason. Interelated, but just as important is aerodynamic drag. Higher altitude = lower air density = lower airframe drag = more true airspeed (Mach #) for a given thrust. Hence lower total fuel consumption per hour, or mile.
Another factor is as previously mentioned, operational requirements for traffic control. I don't have the exact figures, but typically various types and associated speeds of those aircraft are assigned a band of altitude for cruising. Military aircraft operate at FL40 and above, commercial heavies between FL30 and FL40,, regional aircraft betweeen FL20 and FL 30, GA and lighter aircraft up to FL 20. As I said, those are typical, and correct me if I'm not accurate on those.

[jmcmanna]

Typically, it's correct that RJs cruise between 20 and 30, and larger aircraft cruise higher, but there is not a range of altitudes that aircraft are restricted to.  Many Business aircraft fly above FL410, and many military aircraft cruise below FL400.  It's usually a factor of distance, fuel economy, weight, and weather.