There are so many environmental factors that can affect the performance of an aircraft during take-off and landings. And of the most crucial of them is the temperature and altitude. High temperatures and high-pressure altitudes can reduce the performance of the aircraft to such degrees that some airplanes cannot operate from certain airports when the conditions go above certain limits.

A look at Density altitude

The density altitude is the normal pressure altitude corrected for temperature. The laws of physics dictate that temperature and density are inversely proportional. That is, when the temperature increases, the density decreases, and vice versa.

Before diving further into the topic, it is important to understand the ICAO international standard atmosphere or more commonly known as the ISA. This is the atmosphere standard used when aircraft get certified. The values defined by ISA are as follows:

  • A mean sea level temperature of 15 degrees Celsius.
  • A standard pressure of 1013.25 mb/hPa or 29.92 inches of Mercury.
  • A lapse rate of - 1.98 degrees Celsius per 1000 feet increment in altitude.

By looking at the ISA atmosphere model, it is quite clear that such an atmosphere cannot exist in many parts of the world. Getting back to density altitude, when the ISA conditions match with the actual conditions in an airport or an aerodrome, the pressure altitude of the airport and the density altitude is the same. However, if the conditions deviate from the ISA, the altitudes differ.

A LATAM aircraft and a Copa Airlines aircraft
Density altitude is decided by ICAO standards. Photo: Daniel Martínez Garbuno | Simple Flying.

For example, say an airport has an altitude or an elevation of zero (located at mean sea level). On a particular day, the temperature recorded at the airport is 35 degrees Celsius. This temperature means that the airport has a temperature that is 20 degrees above the ISA. In technical terms, it can be said that the airport has an ISA deviation of +20 or ISA+20 for short. What this implies is that even though the airport is at sea level, its altitude for an aircraft certified for ISA conditions is going to be higher. There is a simple formula to calculate this altitude which is the density altitude.

DA = PA + [ 120 x (OAT – ISA)], where:

  • DA is the density altitude in feet
  • PA is the pressure altitude in feet
  • 120 is the correction factor
  • OAT is the outside air temperature in degrees Celsius
  • ISA is the international standard temperature in degrees Celsius.

Now, plugging in the numbers for the airport mentioned, we can derive the density altitude.

DA = 0 + [120 x (35 – 15)]

DA = 2,400 ft.

This shows that even when the airport is at sea level, when conditions differ from that of ISA, the altitude felt by an aircraft is different. In this case, at sea level, the aircraft behaves as if it is magically taken to a field with an elevation of 2400 ft.

When it is hot, thus, the density altitude is higher, and the aircraft does not perform as per the book.

Density Altitude Chart
Density altitude chart. Photo: FAA via Wikimedia Commons

The same thing happens when the altitude of the airport is higher. As altitude increases, the reduced magnitude of gravity increases the distance between air molecules. This has a combined effect of reducing the pressure and density of the airport. In the upcoming paragraphs, we will look at the effect of hot and high conditions on aircraft performance.

Aerodynamic behavior of aircraft in hot and high environments

During the takeoff, the airplane lifts off the runway due to the wings generating lift. For this lift to be generated, it must speed up because with speed, the lift increases. The formula for lift is as follows:

L = Cl x ½ x rho x v^2 x A, where:

Cl = Coefficient of Lift

rho = density

v = True airspeed (TAS) of the aircraft

A= Area.

When the outside temperature is hot, the density is reduced. According to the formula above, this reduces the lift. To counter this, the aircraft needs to accelerate more on the runway. This requires more runway length. If the runway length available is not sufficient, the aircraft must be loaded lighter to reduce the amount of lift required, or the risk of running out of runway on the takeoff roll is a high possibility.

The effect of Density altitude on take off performance
The effect of Density altitude on take-off performance. Picture: National Weather Service.

Maximum brake energy speed

One of the things that need to be considered during a takeoff is a rejected takeoff. For this, good braking efficiency is required. Generally, cooler brakes are more efficient. When the outside temperature increases, the brakes naturally get heated up. And this can limit the take-off mass.

There is a speed called VMBE (maximum brake energy speed). This is the maximum aircraft speed at which the brakes fail to stop the aircraft during a rejected takeoff. When trying to take off from a hot aerodrome, the VMBE reduces because the brakes are already hot. This is accentuated by the fact that the aircraft must accelerate more on the runway for take-off. More acceleration equals more kinetic energy, which needs to be dissipated by the brakes.

When it comes to landing, hot and high conditions result in increased true airspeeds, which also increases the ground speed of the aircraft. This makes the aircraft move faster relative to the ground, requiring the pilot to steepen his approach angle to keep up with the correct glide path.

The higher speed also means that once close to the runway, the aircraft may tend to float. And once on the ground, more runway is required to stop the aircraft as more speed needs to be bled off. As mentioned previously, the heated-up brakes are less efficient, and this adds to the degraded stopping performance of the aircraft.

Boeing YAL-1 landing
Hot and high conditions require more landing distance. Photo: US Federal Government via Wikimedia Commons

The effect of hot and high conditions on jet engine performance

The jet engines perform by sucking in air and compressing it. When subject to hot and high conditions, the reduced density of the air means that the engine must work harder to provide the required thrust or power.

Most jet engines are limited by the maximum amount of pressure they can develop and by the maximum temperature it is subject to. When the outside temperature is cold, the turbines do not need to work that hard as the air is already dense. Consequently, in these conditions, the engine reaches its pressure limit before it reaches its temperature limit.

In hot and high conditions, the reduced density requires the engines to put in more effort to generate the same thrust. While doing so, it reaches its temperature limit before it reaches its pressure limit. Due to these reasons, engine manufacturers usually flat rate their engines. This means that they give a guarantee to the engine user that up to a certain temperature, their engine can develop its maximum thrust without reaching either its pressure or temperature limit.

Flat rating of jet engines.
How jet engines are flat rated. Picture: Airbus

For example, an engine might be flat rated to 33,000 pounds of thrust to a temperature of 40 degrees Celsius. This implies that the engine can generate 33,000 pounds of thrust with no losses up to 40 degrees Celsius. Above this temperature, the engine can no longer provide its maximum thrust.

Why is this important? If you are in the Canadian North with a temperature of -45 degrees Celsius, then there is no issue. But if you are trying to take off somewhere in the Middle East with a scorching temperature of plus 50 degrees Celsius, then there is something to think about. Taking the above engine as an example, at 50 degrees Celsius, during the take-off, the engine is operating 10 degrees above its flat rated temperature. This means, that to protect the engines from over-temping, the thrust must be reduced. This may reduce the amount of mass that can be lifted, reducing revenue.

How to combat high and hot conditions

The most common method utilized by airports is to increase the runway length. One famous example is the Denver International airport in the United States, which has a 16,000 ft long runway. As Denver has an elevation of 5000 ft and experiences higher temperatures, its density altitude can get very high, which pushes the aircraft to their limits. Therefore, having a long runway allows aircraft to have more space to roll during take-off. It also gives more margin to stop in case of a rejected takeoff and during landings.

Denver International Airport
Denver International Airport's Runway 16R/34L is one of the longest runways in the world. Photo: Denver International Airport

Some aircraft manufacturers have come up with things like thrust bump options, whereby the pilots can rev up the engines a little more if the basic engine does not provide the required performance. Airbus used this on their A320s, A330s, and A340-300s. It is available as a customer option.

Some manufacturers have also developed variants of the same aircraft built purely to increase performance. The Dash 8 Q200 by de Havilland, for example, is fitted with the same engines as its heavier Q300 variant. This was done to increase the performance of the aircraft and to allow it to operate in high and hot altitude airports.

Maldivian Dash 8 Q200
Photo: Anas Maaz I Simple Flying.

From the piloting perspective, when faced with harsh conditions, it is important to consult the performance section of the aircraft operating manual. Most large aircraft have graphs and tables for various conditions above ISA, which can be applied so that those correct performance parameters can be applied.