Determining Cruise Altitudes

Higher altitudes enable aircraft to fly above most weather, ensuring a smoother ride for passengers.

Airplanes fly at high altitudes for a variety of reasons. To put it simply, the higher an airplane flies, the lower its fuel burn. Higher altitudes also allow the aircraft to achieve greater cruise speeds more efficiently. Furthermore, high altitudes ensure the aircraft is well away from the most severe weather which can lead to turbulence and icing. Aircraft altitudes vary, with commercial jets typically cruising between 30,000 to 42,000 feet in the stratosphere where the thinner air reduces drag and increases fuel efficiency. And, smoother air makes for a better ride as the aircraft flies above most weather. Aviation also uses different altitude types like True Altitude (above sea level), Absolute Altitude (above ground), and Pressure Altitude (based on air pressure) for safety and navigation. At higher altitudes, pilots are assigned different Flight Levels. For example, FL 350 means 35,000 feet above mean sea level. Also, higher altitudes call for standardized separation and pilots are assigned even or odd flight levels depending upon their direction. Eastbound flights fly at odd altitudes while westbound flights fly at even altitudes. With all of the above in mind, what are the main factors that determine the altitude of an aircraft in flight?

Service and absolute ceilings define an aircraft's constraints, ensuring safe, reliable performance.

When an aircraft is certified, a service ceiling is calculated by the manufacturer. This is typically defined as the highest altitude at which the aircraft can maintain a climb rate of 100 feet per minute while at maximum weight and using maximum continuous power. It’s a performance-based definition established during pre-certification test flights and is part of the Federal Aviation Administration’s (FAA)-approved Airplane Flight Manual. Service ceiling is reached when either the wings can’t provide enough lift or the engines can’t provide enough thrust to climb any higher. An aircraft’s absolute ceiling is its minimum possible altitude for level flight, where its engines provide just enough thrust to counter drag, but with zero extra power for climbing; it’s the absolute performance limit, often far above normal operating altitudes..

Optimum cruising altitude is selected for maximum fuel efficiency and a smoother ride for travelers.

In normal operations, airplanes are flown at an altitude which is called the optimum altitude. This altitude is considered the most efficient and leads to increased range and less fuel burn. The optimum altitude is determined in several ways. In its most basic derivation, it’s all about increasing the aircraft’s range. Large jetliners cruise using a Mach (speed of sound) number as a reference. As the airplane climbs, its speed or True Air Speed (TAS) increases, and the local speed of sound decreases. The combined result of this is an ever-increasing Mach number. Initially, the increase in TAS is quite beneficial as it allows the aircraft to cover more ground with less fuel burn. But, as the Mach number increases, there’s a corresponding increase in compressibility drag (drag due to the aircraft approaching the speed of sound). At some point, this drag increase can overcome the benefits of the increasing TAS and start to reduce the aircraft’s range. So, the altitude at which the effects of compressibility drag do not negatively affect the range of the aircraft is known as the optimum altitude.

The Flight Management System acts as the central interface to manage the aircraft's entire flight path.

The most optimum or efficient altitude is not only affected by aerodynamics. The environment plays a major role as well, particularly the prevailing winds and temperature. In modern aircraft, the Flight Management System (FMS) calculates the optimum altitude by considering these factors. For this, the pilots are required to input accurate data into the FMS. This includes entering cruise winds and updating the temperature for various altitudes. During the dispatch phase of flight, the pilots are provided data on forecast winds and temperature for normal cruise levels of the aircraft. The pilots then input this data into the FMS and, once in the air, the FMS calculates the most optimum altitude based on the inputed data.

The speed and direction of prevailing winds can significantly alter an aircraft’s in-flight progress.

It should come as no surprise that one of the biggest factors affecting an aircraft’s movements and altitude during flight is the wind. Seeing as aircraft very rarely travel in exactly the same direction as the wind, in order for an aircraft to maintain its desired course during flight, pilots must continually compensate for both wind direction and wind speed. In tailwind conditions, the aircraft gets a push from the winds, which increases its ground range. In a headwind, the opposite effect occurs as the headwind reduces the airplane’s ground speed. Thus, when accurate wind data is available, the FMS may give a lower optimum altitude because a favorable tailwind will result in greater range.

Federal Aviation Regulations mandate that pilots obey air traffic controllers in the interest of safety.

Step climbs or cruise climbs is a climbing technique whereby pilots initially remain at a higher or a lower altitude than the optimum altitude. As an aircraft's weight decreases, the drag reduces which increases the optimum altitude. In a long-range flight, the fuel burn results in large changes in weight which can keep modifying the optimum altitude throughout the flight. In the initial stages, pilots may cruise at an altitude which is lower than optimum. Most of the time this occurs because a heavier aircraft has lower climb rates, and a slow-climbing aircraft can be a nuisance to both pilots of other aircraft as well as air traffic control (ATC). Once enough fuel is burned off and the aircraft weight is reduced, pilots can initiate a climb to the optimum altitude. If the aircraft performance permits, a higher altitude than optimum may also be selected. This way, as the weight of the plane decreases, the aircraft can settle into its optimum altitude later in flight. This is the best option given all other conditions, such as weather and ATC instructions as it prevents the aircraft from being stuck at lower altitudes for the majority of its flight.

Inclement weather and turbulent air often affect aircraft from achieving their optimum altitude.

Pilots may not always obtain the optimum altitude for cruise on every flight. This mainly occurs due to ATC restrictions such as other aircraft occupying the altitude or airspace curtailments. Weather and turbulence are other factors that may prevent pilots from achieving desired optimum altitude. In such situations, pilots try to remain as close as possible to the optimum altitude. Generally speaking, remaining within 2,000 feet either above or below the optimum altitude will not affect cruise performance significantly.

The vagaries of cruise altitude refer to the dynamic factors influencing a plane's height, balancing fuel efficiency, air traffic rules, weather (winds, turbulence), terrain, aircraft performance, and air density. All these factors require constant adjustment for airliners flying between 30,000 to 40,000 feet, with pilots optimizing for tailwinds, avoiding storms, and ensuring separation, creating a complex interplay for smooth, safe flight. Flying is a complex science and managing the aircraft properly at all times is an art.

Until next time...safe travels.