If you are wondering how high jet planes fly and why they fly so high, this post has everything you need to know!
How high do commercial jets fly?
Most commercial planes fly anywhere between 35000 to 42000 ft (10,668m to 12800m). Getting to cruise altitude takes around 20-30mins. As the aircraft climbs, the temperature decreases. The temperatures at cruise level can be anywhere from -40 to -50degress celsius.
The reasons that planes fly so high are because of several different factors. Read on to find out what they are.
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Why do planes fly so high?
Modern jet aircraft are optimised to fly long distances at high speed as efficiently as possible (to reduce the amount of fuel they need). To allow the aircraft to fly fast and economically the aircraft need to reduce drag (or air resistance).
Generally speaking, as the aircraft climbs, the air becomes less dense and the total drag on the aircraft reduces. The reduction in drag means the aircraft is able to fly further and faster using less fuel. The aircraft is able to achieve a higher true airspeed.
Jet engines work best at higher altitudes. The benefit of the lower temperatures and reduced density at higher altitudes means jet engines can run at relatively high settings (around 80-90% of available thrust) without overspeeding or overheating.
There is also the advantage from an efficiency point of view. As the air is less dense, the engine has to do ‘less work’ and with colder air entering the engine, less fuel is needed, so the thermal efficiency increases.
Weather also dictates how high planes fly
Weather is a major factor in determining how high planes fly. Commercial jet aircraft normally want to take the fastest and most efficient route to their destinations.
To reduce flight times, commercial aircraft flight plans are optimised to take maximum advantage of tailwinds and try and minimise the effect of headwinds.
For this reason, if there was a strong jet stream at a lower level e.g. at 32,000 feet, it may make more sense for the aircraft to fly at the lower level to take advantage of the stronger tailwinds compared to flying higher e.g. at 40,000ft where the tailwind is much weaker.
At times, flying higher allows the aircraft to fly above most of any troublesome weather e.g. thunderstorms.
Passenger comfort influencing how high planes fly
If turbulence is expected, then the aircraft may descend and fly lower to get out of the turbulence. The other benefit of commercial jets flying slightly lower in turbulence is that this increases the margin between low speed (stall) and high speed (structural limit) windows.
You can imagine that in turbulent conditions, the aircraft speed may be difficult to control, so having an increased margin by flying lower may be safer.
Air Traffic Control determine how high commercial jets can fly
Air traffic control will also have a say in how high planes fly. Air traffic control is tasked with keeping aircraft separated and at a safe distance from each other. How they achieve this is by horizontal separation and also vertical separation.
A lot of the time high an aircraft can fly is dictated by air traffic control.
Depending on which direction you are travelling and the type of airspace, there may only be certain levels available to cruise at to maintain separation with other traffic coming the other way.
Terrain separation is another key factor in determining how high planes fly. Pilots need to remain safe and avoid terrain so where high terrain is an issue, they would have to fly a certain height above the highest point within a certain distance of the aircraft.
Aircraft typically fly 2000ft higher than the highest point in mountainous or inhospitable terrain or 1000ft above in most other areas.
Maximum altitude that a plane can safely fly?
The maximum altitude that a plane can safely fly is determined by a number of factors including
- what it was certified to be able to fly to
- pressurisation capability of the aircraft
- weight of the aircraft
- envionrmental conditions
Maximum altitude an aircraft is certified to fly
The maximum altitude an aircraft is certified to fly is based on performance limitations. The regulating authorities during the design of an aircraft will set criteria that the manufacturer would need to meet performance-wise to be certified to operate at a particular altitude.
This is called the service ceiling. Technically an aircraft may be able to fly above the service ceiling but performance may not be able to be guaranteed above this point. The service ceiling for a Boeing B737-800 aircraft is 41,000ft.
Two factors come into play for high altitude flights. The point at which the aircraft will stall gets closer and closer to the maximum speed the aircraft can fly (typically a structural limit). This is called coffin corner.
As we get closer to coffin corner, a small disturbance in the airflow could cause the aircraft to stall and there may not be enough thrust available from the engines to be able to come out of the stall.
Equally, close to coffin corner, a small disturbance in the airflow may push the airspeed of the aircraft over the maximum allowed airspeed causing structural issues to the aircraft.
Pressurisation capability of the aircraft
Most modern passenger jets are pressurised i.e. able to maintain reasonable cabin pressure inside the aircraft to allow people to breathe despite flying at an altitude of 40000ft or higher.
The cabin pressure is normally the equivalent of around 6000-8000ft elevation. There is a maximum level that the pressurisation system will work at. This is normally dictated by the structural strength of the fuselage.
The pressure outside is much lower than inside the cabin during high altitude flights. This creates a pressure differential. There will be a maximum structural pressure differential that the aircraft is certified to, and this will also limit how high the aircraft can fly.
Some aircraft are not pressurised, so although they may be able to climb above 10,000ft from a performance standpoint, without oxygen the occupants would not be able to fly any higher because of the risks associated with oxygen deprivation and hypoxia.
Most modern passenger jets start their flight off relatively heavy with all the fuel they will be carrying. As the flight progresses, fuel is used up and the aircraft becomes lighter.
As the aircraft becomes lighter, more thrust is available to allow the aircraft to climb higher. As the aircraft weight reduces, so does the stalls speed.
The margin between low-speed stall and high-speed increases which makes a climb favourable to take advantage of a higher true airspeed and reduce fuel burn.
For this reason, the optimal height an aircraft can fly will increase as fuel is used up and the weight reduces.
What happens if a plane flies too high?
If an aircraft flies too high, the aircraft will get close to coffin corner (the point at which the stall speed and high-speed buffet limit meet. Maintaining a safe flight close to coffin corner is difficult and could potentially lead to a high altitude upset where control of the aircraft is lost.
It is not unknown at a very high altitude for an aircraft to initially stall and during the recovery exceed the maximum permitted airspeed during a high altitude upset.
If you have any questions about how high planes fly, please leave a comment in the section below – we would love to hear from you!
Kudzi Chikohora is a B737 pilot with over 2,500 hours of flying in Europe. He holds a Master’s degree in Aerospace Engineering, is a chartered engineer, and is a member of the Royal Aeronautical Society.
Kudzi completed his pilot training via the self-funded modular pilot training route and created kcthepilot.com to share pilot training and aviation content.