Why are many jet aircraft designed to cruise around FL350-370?

Question

If the flight distance permits, the B737-800 will cruise at FL350, the A320 a little bit higher...

^{Flying at FL370. Photo: Live from the Flight Deck by GolfCharlie232 (reframed)}

Elements such as time to reach cruise altitude, engine and airframe efficiency at this altitude, and acceptable cabin pressure surely influence the choice, but:

• What are exactly the important criteria used, and the compromises made by engineers?
• Would engineers choose other altitudes if the technology or any other current limits could be improved?

Answer 1

In a word, the tropopause.

Gas turbine engine efficiency improves with colder & denser air. As an airplane climbs through the troposphere, the density & temperature both drop, and the loss of density is more than offset by the lower temperature. Above the tropopause, however, the density continues to drop while the temperature holds (approximately) constant.

In the U.S. Standard Atmosphere model, this occurs at 36,089 feet. This represents a local (possibly global) optimum altitude for efficiency (and if there is a better optimum at a higher altitude, it's inaccessible for other reasons).

Answer 2

There are multiple factors that affect an aircraft based on its cruise altitude.

The cruise altitude directly affects the aircraft pressurization and aerodynamics. In order to keep the cabin altitude around 6000 to 8000 feet, the fuselage would have to withstand a higher pressure differential. This would require more material, and make the plane heavier. Newer materials used on the 787 and A350 handle these loads better, which allows a higher pressure differential and larger windows.

The altitude also affects the aerodynamics. Aircraft typically cruise at a certain Mach number at high altitudes. The indicated airspeed for a certain Mach number gets lower with higher altitude. This can result in lower drag, but poses challenges as the airspeed drops lower. See: What determines the maximum altitude a plane can reach?

Also, see the answer by Peter Kämpf for more detail about the aerodynamic aspects.

At higher altitudes, less air is available for the engines, reducing the available power. Along with this there are efficiency benefits. See: Why do jet engines get better fuel efficiency at high altitudes?

As the answer by Ghillie Dhu explains, those benefits stop increasing around FL360.

The lack of breathable oxygen and increased pressure forces on the fuselage caused by the lower air pressure also lead to greater risks, resulting in tighter regulations on aircraft intending to fly higher. These regulations will add additional cost to flying higher, affecting decisions on service ceilings.

§ 25.365

(d) The airplane structure must be designed to be able to withstand the pressure differential loads corresponding to the maximum relief valve setting multiplied by a factor of 1.33 for airplanes to be approved for operation to 45,000 feet or by a factor of 1.67 for airplanes to be approved for operation above 45,000 feet, omitting other loads.

So for planes operating over 45,000 feet, the structure must be able to withstand an even higher pressure load factor.

§ 25.1441

(d) The oxygen flow rate and the oxygen equipment for airplanes for which certification for operation above 40,000 feet is requested must be approved.

Planes operating over 40,000 feet must get special approval for the oxygen system.

§ 25.841

(2) The airplane must be designed so that occupants will not be exposed to a cabin pressure altitude that exceeds the following after decompression from any failure condition not shown to be extremely improbable:
(i) Twenty-five thousand (25,000) feet for more than 2 minutes; or
(ii) Forty thousand (40,000) feet for any duration.

The cabin altitude is not allowed to exceed 40,000 feet under any condition that isn't extremely improbable. The higher the plane flies, the more difficult this will be to certify.

Business jets tend to have higher cruise altitudes, because the smaller structure is easier to reinforce for higher pressures, and typically sees fewer load cycles. Efficiency is also not as important as speed and comfort, so some weight can be spared for the stronger fuselage. The higher altitude also affords more flexibility in routing since most other traffic including commercial airliners are cruising at lower altitudes. Business jets may also be certified under less-stringent regulations.

Answer 3

I know there is already an accepted answer, but some key facts are missing.

Mainly, the optimum cruise altitude is where thrust and lift requirements for both take-off and cruise balance well. An additional benefit is the cooler air which increases the efficiency of heat engines.

With increasing flight altitude, the airliner needs:

1. Bigger engines to create the needed thrust in thinner air
2. Bigger wings to create the needed lift

With the wings, the size of the tailplanes will also grow; this effect alone likely will weigh more than the beefing up of the fuselage structure for the increased cabin pressure. Flying higher will make almost all parts bigger and heavier.

Note that Mach 0.85 is a hard limit for efficient flight; airliners cannot compensate for lower density by flying faster. The only way to allow higher flight levels is to attach bigger wings and tails.

Another consideration is Breguet's formula: Jet aircraft have their optimum cruise lift coefficient at a value of $c_L = \sqrt{0.6\cdot c_{D0}\cdot\pi\cdot AR\cdot\epsilon}$, if we assume the thrust of high-bypass-ratio engines to vary with speed proportional to $\text{v}^{-0.5}$, which is a reasonable assumption. This means the airliner cannot fly higher by flying at a higher lift coefficient: This would decrease efficiency.

(Nomenclature: $c_{D0}$ = zero-lift drag, $AR$ = wing aspect ratio, $\epsilon$ = span efficiency, v = flight speed)

With the wing size and the engines needed for flight at Mach 0.82 in the tropopause (Mach 0.85 is really not as efficient; follow the link to find out why this is the quoted cruise speed for long-range airliners), the take-off distance is quite reasonable and approximately matches the airports which had been defined by NATO during the cold war. Flying any higher into the stratosphere would increase the aircraft's mass due to bigger engines and wings, but would not incur the efficiency gains of increasing cruise altitude in the troposphere, where temperature drops with altitude.

Conversely, picking a lower design cruise altitude would allow to make both wings and engines smaller, but this would translate into:

1. Higher take-off and landing speeds, and critical speeds during take-off due to the smaller wing,
2. Lower take-off acceleration due to smaller engines,
3. For twins: Not enough thrust during take-off when one engine fails,
4. Lower climb speeds, so it would take longer to reach cruise altitude, and
5. Not fully taking advantage of the cold air up in the tropopause.

Designing for a lower cruise altitude would translate into much longer runways and less efficient flight overall.

Designing for cruise in the tropopause is simply the sweet spot for airliner designers where all conditions match well and produce a balanced outcome.

Answer 4

As mentioned in the comment above, I'm not sure where you are getting the FL400 reference from but here are a few common reasons for flying higher (not necessarily FL400):

• Fuel savings (more efficient flying at higher altitudes -- see Link
• Higher true airspeed
• Get above any weather/turbulence/icing
• Better NAVAID/comm reception
• Better visibility
• More altitude for better gliding distance (yes, not a huge deal for airliners, but think SE)
• Less traffic

Some disadvantages could include:

• More susceptible to compressor stalls
• More altitude to lose if loss of cabin pressure/rapid decompression
• Worse weather to include icing
• Need to be RVSM-equipped (if flying up to FL410).