We are all used to hearing announcements of how high our aircraft is flying. But why is this? How high are aircraft permitted to operate, why do they do this, and what are the limitations of going higher? In this article, we will take a look at these questions.
Limits set for commercial aircraft
Each aircraft type has a limit set for its maximum permitted flying altitude, as well as a range for optimal operation. These limits guide the height that an aircraft flies at, rather than any legal or regulatory requirements. Although, of course, air traffic control in each airspace has authority over an aircraft’s altitude.
Most commercial aircraft are approved to fly at a maximum of around 42,000 feet, also known as a ‘service ceiling.’ For example, for the Airbus A380, this ceiling is 43,000 feet, while for the Boeing 787-8 and -9, it is 43,100 feet (41,100 feet for the 787-10).
To take another example – earlier 737s (up to the -500) have a ceiling of 37,000 feet. This ceiling was increased to 41,000 feet with the 737-600. For optimal operation, most aircraft will be guided to fly slightly lower than this, around 35,000 feet.
Concorde has been the only notable exception to this, with a maximum altitude of 60,000 feet.
Why fly so high?
There are several reasons why an aircraft would want to fly at such an altitude. Primarily it is for efficiency. At higher altitudes, the air is thinner. This produces less resistance, allowing the aircraft to fly faster using less fuel. Jet engines also operate more efficiently at a higher altitude, as they can be run closer to their maximum capabilities. There is a good discussion of this on the blog flightdeckfriend.com.
The exact height chosen for any flight will also depend on wind conditions. Aircraft will aim to take advantage of or minimize the adverse effects of high altitude jet stream winds and may adjust altitude to help with this. Pilots will also seek to reduce turbulence and weather effects.
Why don’t aircraft fly higher?
This then raises the question of why not fly higher than this? And why are there set ceiling limits? Although the thinner air creates efficiency advantages at altitude, there is a point where the air is too thin. Much thinner air means that the engines cannot produce enough trust, and the wings would not produce sufficient lift. There could also be problems restarting a failed engine at a higher altitude.
Also, there are also safety implications of flying at higher altitudes. In the event of cabin decompression, the aircraft will descend quickly to a lower altitude. This takes more time from a higher altitude and could endanger passengers. There are defined measures for the ‘time of useful consciousness (TUC)’ that describes how long a person would remain conscious. At 35,000 feet, this is 15 to 30 seconds; at 50,000 feet, this decreases to just five seconds. This time is vital to allow passengers to fit oxygen masks.
Some aircraft can fly higher
While commercial aircraft have ceilings set in line with both engine and TUC limits, private aircraft have more scope. Many smaller corporate jets are rated to a higher ceiling of 51,000 feet, for example. They often have larger engines relative to the aircraft size and weight which help achieve this.
Concorde has been the only significant exception amongst commercial aircraft. It was able to fly up to 60,000 feet due to the increased lift generated when flying at much higher speeds. At low altitudes, drag would prevent it from reaching its high speeds.
At these higher altitudes, Concorde carried increased risks for passengers from decompression. There were several features used to minimize this risk. It could descend much faster than other aircraft, due to its delta wing shape. It also had a system to assist with rapid emergency descent (some corporate jets also use a form of this). It is also why Concorde has much smaller windows – this would help slow a decompression if a window failed.