Long Takeoff Ahead

High elevation airports have longer runways because
in thin air aircraft need more groundspeed to take off.

By Juhana Rossi

Closer you get to the sky, getting aloft becomes trickier. This is because at higher altitudes thinner air reduces an aircraft’s capability to take off in two ways.
First, thinner air provides less for an aircraft. Hence, an aircraft to take off needs to greater speed to become safely airborne. Second, at higher altitudes there is less oxygen compared to lower altitudes. This degrades a jetliner’s engine performance because the engines have less oxygen to burn. These two factors mean that an aircraft taking off needs a longer takeoff run to attain a greater takeoff speed at a high elevation airport than at a low elevation airport.
Airbus A350-900 is a modern twin-engine widebody jetliner. Assuming that it is taking off in a full load from Helsinki airport in Finland to a typical commercial flight to Denver airport in Colorado, it needs a 6,043-feet long takeoff run to achieve takeoff speed of 146 knots for a safe takeoff.

A Mile-high takeoff run is 65% longer

Takeoff run calculations are for an Airbus A350-900 aircraft based on following assumptions: take-off weights for hypothetical commercial flights between the two airports, flap settings for both take-offs identical and take-off power set at maximum.

Runway lengths to scale, widths no to scale. Source: Finnair and Denver & Helsinki airports

Helsinki airport is lies just 179 feet above the sea level, so its elevation has no pratical effect on aircraft takeoff performance. In contrast, Denver airport is situated on a high plain on the fringe of the Rocky Mountains at an elevation of 5,430 feet. Its high elevation degrades aircraft takeoff performance markedly. In fact, to accommodate the high elevation, Denver airport longest runway is 16,000 feet long. It is the longest runway at a commercial airport in the United States. For a hypothetical return trip from Denver to Helsinki, the aircraft would require a 9,954-feet long takeoff run for a takeoff speed of 162 knots. Takeoff speed refers to the VR or rotation speed which is the speed at which the pilot starts to lift the aircraft off the ground to initiate flight.

Finnair's Airbus A350-900 widebody jet is 219 feet long and can carry more than 300 passengers. Photo credit: Masakatsu Ukon

Elevation's effect on aircrafts’ takeoff performance manifests itself in the average length of runways at airports at different elevations. Airports at higher elevations have longer runways on average than airports at lower elevations.

There is no official definition for a high elevation airport. Often, a high elevation airport is defined as an airport that lies at an elevation 8,000 feet or greater. Based on information in the ourairports.com database, there are 37 medium-size or large civilian airports in the world that meet this criterion. For a more nuanced breakdown of airports in accordance to their elevation, in this article airports with an elevation of 8,000 feet or greater have been defined as ultra-high elevation airports. They are mainly located in the Andes in South America and in China.

High elevation airports are defined in this article as airports with an elevation between 5,000 and 8,000 feet. Both ultra-high and high elevation are relatively few. They constitute only 4% of medium-size and large airports in the ourairports.com database. In all, there are 4,769 medium-size or large airports in the database The highest elevation airport in the world is now Daocheng Yading airport in China. It lies at an elevation of 14,472 feet, higher than the summit of Mount Whitney, the tallest mountain in the contiguous United States.

More info about the author can be found behind this link. Data and notebooks used to gather it can be found in this repository.