Jet airplanes are normally operated at high altitudes where cruise speed is dictated equivalent to the revolutions per minute (rpm) or Exhaust Gas Temperature (EGT) limits. In view of this, one would ask, “why are jet engines more efficient at higher altitudes?”.
Jet engines are more efficient at higher altitudes because the cold and less dense air at this altitude efficiently maximizes fuel burn.
The primary reason for operating jet engines in the high-altitude environment is because it is most efficient in that environment.
At high-altitude, the outside air temperature decreases for constant engine revolutions per minute (rpm) and true airspeed (TAS) causing a decrease in the specific fuel consumption of jet engines. Thus, the pilot is able to fly with optimum cruise speed at high-altitudes with the best fuel economy.
Sit back and relax as we enjoy this theoretical high altitude jet ride at optimum cruise speed.
Operation of Jet Engine at High Altitude
When a pilot takes off in a jet engine aircraft, the engine takes air from the front intake. As the plane climbs to a higher altitude, the air becomes less dense (that is, there is less mass of air per volume). Therefore, the pilot needs to move faster so that the mass of the air coming through the intake per second remains constant.
As the plane travels faster at the high-altitude, the compressor helps to accelerate this less dense air through a system of blades and then jams the air into a confined space. This compressed air is then passed into the combustor.
At the combustion stage, we have the combustor which is designed to allow compressed air to move slowly enough to mix with fuel to allow combustion.
After combustion, the hot air moves through a turbine and is finally ejected from the exhaust. Besides, the cold air at the high altitude helps the jet engine to burn more fuel without reaching extreme temperatures.
The Air Is Cooler
Jet engines work more efficiently at high altitude because the air is cooler. When heated, cool air expands more than warm air. Hence, the larger the expansion of the air when heated, the faster the aircraft moves because it is the expansion of air that drives the turbines of the jet engine which generates more power for lesser fuel burn.
Also, at high altitude, there is low drag because the density of air is now lower than it was at a lower altitude. Given the same thrust, this low drag causes the aircraft to fly much faster at high altitude than at low altitude.
Likewise, the quantity of energy needed to heat air to adequate temperature is comparable between both altitudes. The amount of power generated at high altitude is higher because the aircraft is flying at a much higher speed here than at low altitude (Power = Thrust x Speed).
Generally, the cooler and less dense air at high altitude the less the fuel to air mixture in the combustor, thereby improving fuel efficiency.
In addition, flying at a high altitude provides thermal efficiency for the engine.
Flying at a High Altitude Improves Thermal Efficiency
The thermal efficiency cycle of a jet engine is determined by the temperatures of the incoming air and the outgoing air. The lower the temperature of the incoming air, the higher the efficiency. Air at a high altitude has a lower density and lower temperature. The cold air at this altitude improves the thermal efficiency of the engine because less work is done to compress the incoming air.
The incoming less dense air flows at a faster rate into the compressor and is led into the combustion chamber where it is mixed with fuel.
In the combustion chamber, the fuel-air mixture burns at nearly constant pressure and in a restricted volume, causing the compressed gas to expand.
The density of the air decreases as it is heated up because it is subjected to constant pressure. The density ratio between the burnt gas in the combustion chamber and the unburnt gas at the intake is proportional to its temperature ratio, measured in absolute temperature.
Thermal efficiency increases with the lower temperature of the intake air and an increased difference between ambient (atmospheric) temperature and temperature of the engine gases.
As the aircraft climbs higher, it gets to a “sweet spot” of flying known as the optimum cruise altitude.
Optimum Cruise Altitude
The optimum cruise altitude is the altitude at which a maximum range speed is attained for a given thrust setting. Jet engine aircraft have a high optimum cruise altitude. At this optimum altitude, the quantity of oxygen is just perfect for the fuel/air mixture, and the air resistance is also mild enough.
However, the optimum cruise altitude varies over the period of a long-distance flight. It changes with atmospheric conditions and the weight of the aircraft.
At the optimum cruise altitude, operating cost is at the minimum when operating at the most economical mode because there is minimum fuel burn in the long-range cruise.
As efficient as flying at high altitudes are, certain precautions are taken by pilots and airlines to ensure the overall safety of both the aircraft and every human or cargo aboard.
Safety Considerations for High Altitude Flights
There are specific aerodynamic principles that apply to safe high-altitude flight that must be understood by pilots, so as to drastically reduce the number of serious incidents and accidents accompanying the increase in high altitude flight by modern jet aircraft. This understanding is crucial when there is a need to recover from a sudden loss of control.
Practically, ‘high altitude’ operations are any operations at levels above FL250(25,000 feet)
. FL250 is the altitude above which aircraft certification requires that an overhead panel oxygen mask drop-down system has to be installed in the passenger cabin.
Above this FL250 altitude, the following features begin to take on increasing importance:
- The need to reduce airspeed range so that aircraft can remain controllable by the pilot.
- True airspeed (TAS) and aircraft momentum increase with altitude. Consequently, the ability of the aerodynamic flight controls to recover from an upset is reduced as altitude increases.
- In the event of depressurization, the period of useful consciousness for passengers deprived of oxygen reduces significantly.
- Passengers are exposed to slightly increased cosmic radiation at very high altitudes.
To check these challenges, pilots:
- Stay alert! Complacency can be costly in a high altitude environment. Late maneuvering of induced loads and external forces such as windshear and turbulence can result in an upset.
- Avoid flying at speeds at or below minimum drag speed (Vimd). Speeds below Vimd are on the backside of the drag curve, hence, they are unstable and could eventually lead to a stall or the sudden necessity to descend to regain airspeed.
- Monitor the outside air temperature.
- Are careful when assuming manual control at high altitude. Remember that there is less aerodynamic flight control damping due to the less dense air at high altitudes.
- Understand the difference between an approaching stall and a full stall. An approaching stall gives a stall warning when the critical angle of attack is about to be attained while a full stall happens when the critical angle of attack has been exceeded. The angle of attack is the angle formed when the wing’s chord meets the relative wind.
- Know the difference in the actions required to recover an approaching stall and those required to recover from a full stall.
- Recognize that, in the event of a full stall, altitude recovery comes after stall recovery. Altitude recovery can only be achieved after a successful stall recovery.
It has been established that a jet engine is more efficient at high altitudes partly because of reduced thrust.
However, it is often very hard for a jet plane to climb and turn at the same time, when at these high altitudes.
Therefore, little excess thrust should be made available for maneuvering when the need arises. Moreso, in a bid to achieve maneuvering; controllability and stability must not be compromised.
We have spoken a lot about flying at high altitudes, let us now consider if aircraft use more fuel at low altitudes.
Do Planes Use More Fuel at Lower Altitudes?
The “optimum” air/fuel ratio for an engine to operate most efficiently is determined by the stoichiometric ratio.
The stoichiometric ratio for a jet engine is 14.7:1, that is, 14.7 parts of air to 1 part of fuel. This ratio applies at all altitudes.
Interestingly, this 14.7 ratio coincides with the value of “standard” air pressure at sea level which is 14.7 pounds per square inch (psi). However, the exact air pressure at a given altitude varies with prevalent weather conditions.
Imagine that the air pressure drops to about 85% of the sea level pressure at 5,000 feet Mean Sea Level (MSL), that is about 85% of the oxygen content of fuel/air mixture.
This means that the stoichiometric ratio requires about 85% of the fuel to be combusted at 5000 feet MSL. However, at a higher altitude of 40,000 feet MSL, the atmospheric pressure reduces significantly to 21% of the fuel burned at sea level. This remarkable contrast clearly shows that a jet engine burns more fuel at lower altitudes and less fuel at higher altitudes.
There are various ranges of altitudes where the fuel and power consumption is enhanced for efficiency given the right airframe and engine combination. For most commercial airliners, that range is likely in the mid 30000 foot MSL (about 35,000 – 36,000 feet).
It has been observed that as airplanes rise higher in the sky, air pressure decreases. Most passengers, especially those traveling for the first time, generally begin to feel drained, dehydrated, or short of breath because of the decrease in air pressure, despite the pressurized cabins.
As uncomfortable as that experience may be, it is important to remember that the engines of these flying vessels operate most efficiently at a high altitude. They eventually serve our good because they get us to our destinations faster and maybe safer!