What happens if a passenger plane flies too high?
A look at what happens if a commercial passenger jet flies too high…
If a passenger jet flies too high it reaches a point called ‘Coffin Corner’. This is the point at which the aircraft’s low speed stall and high-speed buffet meet and the plane can no longer maintain its altitude which forces it to descend. For a regular passenger jet this occurs at a height of around 40,000ft – 45,000ft but it can be higher or lower depending on the aircraft’s weight.
A low speed stall is where there is not enough air passing over the wing to keep the aircraft flying and high-speed buffet is where the air over the wing is going fast enough to cause a shockwave which can result in aircraft control issues. At the altitude at which Coffin Corner occurs, the plane can’t speed up, slow down or climb; the only way to keep the aircraft flying safely is to reduce the altitude and go down. Pilots are aware of what the maximum altitude is and ensure they do not reach it.
What limits a plane’s altitude?
The maximum altitude of a passenger aircraft is limited by 3 factors and any one of these factors can be the limiting one on any given day depending on air temperature and aircraft weight.
The air at high altitude is very thin (less dense). It gets so thin that at a certain altitude not enough air can pass through the engine for it produce enough thrust to keep the aircraft climbing. The density of the air is dictated by the temperature. So, on hot days, the aircraft’s maximum altitude is lower than on cold days. The engine limited altitude is said to be the altitude at which the aircraft can no longer achieve a rate of climb of at least 300 feet per minute.
Cabin Pressure Differential
As the air at high altitude is so thin, it doesn’t contain enough oxygen for a person to survive if they had to breath it in. It’s also very cold reaching temperatures as low as -60c. Therefore, compressed heated air is supplied by the engines (before it’s mixed with fuel) to the aircraft cabin. By forcing air into the cabin, it increases the air pressure and allows passengers to breath normally.
This creates a difference in pressure between inside the cabin and outside environment which is contained by the aircraft fuselage. The higher the aircraft flies, the higher this pressure difference becomes. The maximum pressure differential between the inside of the aircraft and outside is about 9 PSI. This limit is reached when the aircraft reaches around 43,000ft. If the aircraft flew any higher, the maximum pressure differential could be exceeded and this could cause structural failure of the fuselage.
Coffin Corner / Aerodynamic Altitude
A margin of safety is applied to ensure coffin corner is never reached. Typically, a 1.3g margin is used to determine the aircraft’s maximum aerodynamic altitude. This altitude varies with the weight of the aircraft.
Your pilots are aware of which altitude is limiting on any given day and will ensure the aircraft is operated within these limits. Pilots are also given extensive training on how to recover from an event where any of the above issues occured.
If you found this article of interest then check out our page on ‘How Fast do Commercial Jets Fly‘.
How Fast Do Commercial Airliners Fly?
Just how quickly do passenger aircraft fly?
How fast do commercial passenger jets fly?
A typical commercial passenger jet cruises at a speed of about 400 – 500 knots which is around 460 – 575 mph. Generally speaking, the higher the aircraft flies, the faster it can travel.
It cruises at this speed at a height of about 36,000ft. This high speed can only be achieved at high altitude, which is one of the reasons why commercial aircraft fly so high.
Different Types of Speed Measurements
Speed can get a bit confusing when talking about an object moving through the air. You have a few types of speed; airspeed (and there a quite a lot of variations of airspeed) and ground speed.
What is Ground Speed?
Ground speed is the time it takes you to cover a certain distance over the ground. For example, when at cruise altitude, aeroplanes might have a ground speed anywhere between 300 – 600 nautical miles per hour.
Whilst passenger jets usually cruise at roughly the same airspeed, the wind can make a big difference to the speed at which the aircraft passes over the ground.
A tailwind pushes the aircraft along at a faster speed whilst a headwind slows the aircraft’s speed across the ground down down.
When a strong tailwind occurs, such as when crossing the Atlantic from West to East, the aircraft’s ground speed might reach exceed over 700mph.
Airspeed has a few different variations. If an aircraft is sat still on the runway and has a 20 mph headwind, the aircraft already has an airspeed of 20 mph, despite the fact it isn’t actually moving. This is because airspeed is a measure of the speed of the air over the wing. The speed of the air travelling over the wing dictates how much lift the wing is producing, and it’s this lift that allows the aircraft to support its own weight and allows it to fly.
If an aircraft has a take off speed of 140 mph, but has a 20 mph headwind, the aircraft will only need to achieve a 120 mph ground speed before it is able to take off. Conversely, if an aircraft has a 20 mph tail wind, it would need to achieve a 160 mph ground speed in order to lift off the ground.
Pilots always make reference their airspeed rather than ground speed as it is the airspeed that keeps the aircraft flying. The groundspeed is a byproduct. In principle, if you had about a 140 mph headwind the aircraft could lift off the ground without moving forward!
The Speed of Sound
When aircraft get to between 25,000 – 30,000 ft, they reference their speed to a “Mach Number”. This is simply a percentage of the speed of sound. For example, a Mach Number of 0.80 is 80% of the speed of sound. This is not a fixed speed, as the speed of sound varies with the temperature of the air.
The speed of sound at sea level with an air temperature of 15 degrees celsius is 761 Miles Per Hours. This reduces to about 660 miles an hour at -57 degrees celsius when at 36,000ft.
When aircraft approach the speed of sound, shockwaves start to form which causes aerodynamic issues. Aircraft therefore have a maximum mach number they can fly at, which is why this becomes the reference speed.
The Price of Jet Fuel and Fuelling an Aircraft
A look at the cost of aviation fuel for commercial aircraft
How Much Does Jet Fuel Cost?
As of January 2021, the price of Jet A1 was approximately $450 per metric tonne. With a metric tonne being 1,000 KG or 2,204 lbs, this equates to about $0.45 / £0.33 per KG.
This price is about 34% lower than one year ago (January 2020) where the price of Jet A1 was approximately $650 per metric tonne or about $0.65 per KG.
Due to collapse in oil price bought about by the Covid-19 pandemic, at one point (May 2020), Jet A1 was as low as $200 per metric tonne which equates to around $0.20 per KG.
1 KG (kilograms) = 2.2 LBS (pounds)
1 MT (metric tonne) = 1,000 KGS
The price of jet fuel (known as Jet A1) is closely aligned the price of oil which varies on a daily basis. In May 2020, the price of Jet A1 was down 69% compared to the previous 12 months.
The price of jet fuel as of January 2015 is as follows:
- 170.8 Cents (US dollars) per Gallon
- 1 litre = 0.3125 pence (pound sterling)
- 1 litre = 0.40 Euros
It should be noted that it does not include the delivery of the fuel or the fee to actually refuel the aircraft. Therefore the price airlines actually pay for the fuel per kilogram will be higher than the figures outlined above, subject to the contract details. At present there is no tax on aviation fuel in Europe.
A Jumbo Jet (Boeing 747-400) flying from London to New York burns approximately 70,000 kilograms of fuel. As jet fuel has an approximate specific gravity of 0.85 (the measure of its density), this therefore equates to 82,300 litres.
Based on these figures, the cost of the fuel required to fly from London to New York being operated by a B747 Jumbo Jet is approximately £25,500 (€32,500), which based on 450 passengers, would work out at £57 (€73) per person.
Fuel Price Hedging
The prices airlines actually pay for their fuel varies substantially depending on what price they’ve “hedged” the fuel at. Hedging is where you agree a constant price for fuel for a set period of time into the future. This helps an airline to reduce risk and fixed costs which can be important for financial planning. For example, a fixed amount of fuel, lets say 5 million tonnes, might be hedged at $600 per metric tonne for 12 months. The airline will pay $600 per metric tonne regardless of any fluctuation in price of fuel during this time. If the fuel price goes up, the airline is protected from this rise whilst if there is a drop in fuel price, the airline will be paying more for fuel than it might have done.
The hedging is normally based on purchasing a set quantity of fuel. If the airline stops flying, such as due to the COVID-19 crisis and don’t use the fuel they’ve hedged, the airlines still have to pay for the fuel they hedged even if they don’t use it which can result in financial loses.
Why Would Both Engines Fail On A Commercial Passenger Jet?
A look at the reasons that a passenger aircraft’s engines might fail . . .
What could cause a double engine failure?
Any engine failure is a very rare occurrence, and a double engine failure extremely improbable. But is has happened. Here are some of the factors which have caused double engine failures in the past.
Birds can be very hazardous to aircraft. Flying through a flock of geese caused both engines to fail on US Airways Flight 1549 in 2009 that subsequently landed in the Hudson river in New York. A similar incident occurred in 2008 when Ryanair flight 4102 suffered around 90 individual bird strikes when flying through a flock of starlings on final approach into Rome Ciampino airport. Despite losing both the 737’s engines, the crew managed to land the aircraft on the runway. The aircraft was written off. There is a little a pilot can do to avoid birds other than try to manoeuvre the aircraft around them, but they often seen to late to attempt this.
Shutting down the wrong engine
It sounds difficult to believe, but it has happened. When there has been a problem with an engine, there have been examples of the crew shutting down the wrong engine as a result, leaving both engines failed. It’s not actually that difficult to do, especially when factoring in all the stresses and information sources. A famous example of this was the British Midland Flight 92 crash at Kegworth where 47 people died. Airlines have updated their procedures as a result and the engine shutdown process is now carefully monitored by both pilots.
A fuel leak, or running out of fuel will cause both engines to fail. Air Transat 236 ran out of fuel due to a leak approximately 65 miles from the Azores in 2001. The pilots successfully managed to glide the aircraft to an airbase on the island. In 1983 Air Canda Flight 143 also ran out of fuel when descending through 35,000 feet, due to a fuel miscalculation (the weight of the fuel was measure in pounds instead of kilograms). The pilots successfully managed to glide the aircraft to safety onto a closed runway.
Icing in the fuel tanks could stop the engines from receiving fuel. This happened to flight BA38 in 2008 when ice in the fuel lines caused a dual engine flame out on final approach into London Heathrow. This was found to have been caused due to a an issue with the Boeing 777 fuel system. The quick actions of the Captain in making the decision to retract some of the flaps reduced the drag of aircraft which saved everyone on board.
This is where the ignition of the fuel stops. This could occur in extreme turbulence or very heavy rain / precipitation.
This can damage the engines to the point that they flame out or stops the combustion process. In 1982, British Airways flight 9, a Boeing 747, lost all four of its engines due to ingesting volcanic ash. The aircraft glided outside of the ash cloud and managed to restart its engines before successfully landing in Jakarta.
Believe it or not, there is a checklist on commercial aircraft for entire separation of the engine from the wing. The engine is held onto the wing by a ‘sheer pin’ to ensure the engine separates from the aircraft and protect the aircraft structure in the results of the engine suffering a significant impact.
What speed does a Boeing 777 passenger jet take-off and land?
What speed does a Boeing 777 take off and land at?
The Boeing 777-200ER take off or rotate speed (VR) typically occurs between 130 – 160 knots (roughly 120-180 mph) depending on the weight of the aircraft. At a typical take-off weight of around 230,000 kgs, the take off speed would be approximately 145 kts which is approximately 165 mph.
B777 Landing Speed
A typical landing speed (or speed over the threshold known as VREF) at a landing weight of 190,000 KGS is approximately 135 kts or 155 mph.
To converts nautical miles per hours (knots or kts) to miles per hour (mph), multiply the knots by 1.15.
What effects Take-off and Landing Speed for a passenger jet?
The speed at which aircraft take-off and land depends on a number of variables. These include aircraft weight, air temperature, airfield altitude and pressure.
The flap setting will also influence these speeds with a larger flap setting reducing take-off and landing speed. Factors such as runway length, climb performance and weight influence the decision of what flap setting to use for take-off and landing.
Check out our article on what speed the B747 Jumbo Jet takes off and lands at.
How much fuel are aircraft required to carry?
How do you know you’ve got enough?
How much fuel are passenger jets required to carry?
The minimum amount of fuel which a passenger jet must carry is set out by regulators such as EASA and the FAA. Airlines are actually required to carry substantially more fuel for a flight than is required to get from A to B in case anything unexpected happens such as an airport closure or aircraft emergency. Commercial flights typically carry at least one hour’s worth of additional fuel on top of that required to get to their destination, but this is often increased by the pilots depending on the circumstances on the day.
Airlines must comply with the regulatory stipulations with regards to carrying fuel. Most authorities policies are broadly similar and are detailed in each Airlines operating manuals.
Under EASA regulations (although FAA and other authorities are very similar) the Captain must ensure he has the following minimum fuel before departure:
- Trip Fuel
- Diversion fuel or 15 mins holding fuel if flight is planned with no alternate
- Reserve Fuel
- Contingency Fuel
- Taxi Fuel
- Additional Fuel
Fuel required from the start of take-off, through climb, cruise, descent and approach to touchdown at destination, assuming departure on the SID from the assumed runway and arrival using the STAR for the assumed arrival runway and routing based on the forecast wind.
Fuel required from go-around at destination, climb, cruise, descent, approach and landing at the selected alternate airport. This is normally calculated at the planned landing weight minus contingency fuel.
If no alternate is planned for the flight then the diversion fuel figure must be replaced by 15 mins holding fuel at 1500ft above destination airfield in standard conditions.
Is the minimum fuel required to be present in tanks at at the alternate airfield (or destination if no planned alternate). The figure is calculated based on 30 mins of fuel holding at 1500ft in clean configuration at planned landing weight.
This is carried to cover unforeseen variations from the planned operation. For example different winds / temps from forecast or ATC restrictions on levels and speed. It can be used anytime after dispatch (once aircraft moves under its own power). It cannot be planned to use before. More likely it is used for delays on departure or arrival.
Contingency Fuel should be the higher of (i) or (ii) below:
a. Not less than 5% of the TRIP FUEL required from departure to destination; or
b. If an En-route alternate is available and selected, not less than 3% of the TRIP FUEL required from departure to destination; or
c. An amount of fuel sufficient for 20 minutes flying time based upon the planned trip fuel consumption; or
d. Statistical Contingency Fuel (SCF).
ii. An amount to fly for 5 minutes at holding speed at 1500 ft clean at Planned Landing Weight.
The minimum contingency fuel to be carried must not be below 5 minutes at holding speed at 1500 ft clean at Planned Landing Weight, even for the purpose of an LMC fuel reduction
This is fuel for APU burn on the ground, engine start and taxi out. Most airlines use statistical data to calculate this by using the taxi time in minutes.
Additional fuel is planned and loaded if the existing total fuel is not sufficient to cater for an engine failure (2 engines in 4 engine aircraft) or de-pressurisation at the most critical point along the route. Fuel planning must allow a descent and trip Fuel to alternate airfield, hold for 15 mins at 1500ft and make an approach and land.
Most airlines will work the total fuel required, which is presented to the pilots, through their flight planning system. The pilots will then make a decision as to whether they require any ‘extra’ fuel. There could be many reasons for requested additional fuel such as weather, ATC delays, an increase in passenger numbers or a technical defect.
The final decision as to how much fuel should be carried for a flight is always the responsibility of the Captain of the aircraft. The Captain will discuss the requirements to take any extra fuel with the First Officer prior to the flight commencing.
If you find this page of interest, check out are article about how much fuel a Jumbo Jet burns.
What speed does a Boeing 747 Jumbo Jet take-off and land at?
A look at the speed the at which the Boeing 747 takes off and lands
What speed does a Boeing 747 take off at?
A fully loaded Boeing 747 ‘Jumbo Jet’ on a normal long haul flight would take off at a speed of around 160 knots which is 184 mph. The calculated take-off speeds vary depending on environmental conditions, runway length and weight.
What speed does a Boeing 747 land at?
A 747 ‘Jumbo Jet’ would typically land at a speed of about 145kts-150kts (166mph-172mph), depending on the landing flap setting selected.
Most airlines and aircraft have a facility to de-rate thrust (or use assumed temperatures) for take-off. This occurs on runways where the aircraft has extra performance in hand e.g. the aircraft does not need the full length of the runway to take-off. Large commercial aircraft rarely use their full engine power for take-off as most runways at large airport long enough to support a reduction in thrust. Take-off thrust might be as little as 75% of the maximum thrust.
Taking off with reduced thrust reduces the temperature the various components in teh engine are exposed to. This in turn can significantly reduce engine wear and tear and thus maintenance costs, and ultimately extend the life of the engine.
Additional Speed for Landing
The greater the flap setting, the lower the take-off and landing speed. Jumbo Jets typically land at Flap 25 or 40. Pilots generally fly the approach speed faster than the actual touchdown speed to allow a speed ‘buffer’ in case of any airspeed fluctuations. This additional speed is ‘bled off’ during the flare or plain terms, the excess speed reduces as the nose is raised just before touchdown. In light wind conditions we would add 5 kts to the approach speed but in strong wind conditions up to 20kts can be added.
B747-400 Maximum Take-Off Weight
The xaximum take-off weight for the B747 is 396,000 KGS / 875,000 LBS
B747-400 Maximum Landing Weight
The Maximum landing weight for the B747 is 285,000 KGS / 630,000 LBS
Ever wondered how much fuel a Jumbo Jet burns? This page might be of interest.
How much fuel does a Jumbo Jet (Boeing 747-400) burn?
What fuel does a jumbo jet use between London and New York?
How Much Fuel Does a Jumbo Jet Burn?
The four engines of the Boeing 747 Jumbo Jet burn approximately 10 to 11 tonnes of fuel an hour when in the cruise. This equates to roughly 1 gallon (approximately 4 litres) of fuel every second. It can carry a maximum of 238,604 liters of fuel and it has a range of about 7,790 nautical miles.
A Jumbo Jet (Boeing 747-400) flying from London to New York burns approximately 70,000 kilograms of fuel. Jet fuel has an approximate specific gravity of 0.85, which therefore equates to 82,353 litres.
Therefore, the cost of the fuel (based on 1 litre costing 31 pence) required to fly from London to New York is approximately £18,500 (€23,600). The cost of fuel for a a jumbo jet carrying 450 passengers, would work out as about £41 (€52) per person.
The worldwide Jumbo Jet fleets are getting smaller with none now in operational service with UK airlines. Both British Airways (BA) and Virgin Atlantic retired their B747 fleet earlier than planned due to the reduction in travel demand as a result of the COVID-19 pandemic in 2020. Some airlines continue to operate the B747-8, the latest passenger version of the Jumbo Jet, such as Lufthansa. Boeing plans on stopping production of the Jumbo in 2022.
Did you find this article interesting? You might be interested in: How much does jet fuel cost?
Why do airliners fly so high in the sky?
Why do aircraft fly at high altitude? Is it to save fuel?
How High Do Passenger Jets Fly?
The average commercial passenger jet aircraft cruises at an altitude between 30,000 and 42,000 feet (ft) (9,000 – 13,000 meters). This means that aeroplanes usually fly between 5 to 7 miles up in the air. It typically takes around 15 to 30 minutes after take-off for the aeroplane to reach this altitude. The temperature of the air at this altitude is very cold, typically around -50 to -65C.
Why Do Commerical Aircraft Fly So High?
The reason aeroplanes fly so high is due to improved fuel efficiency. A jet engine operates more efficiently at higher altitude where the air is much thinner, allowing an aircraft to travel faster whilst at the same time, burning less fuel.
Flying above 30,000ft also has the benefit of allowing the aeroplane to fly above most weather systems making it more comfortable for the passengers.
Engine Efficiency, Airspeed & Density
Modern jet engines on commercial passenger aircraft (referred to as Turbo-Fans), are most efficient when they are operated at high altitude. This is because jet engines gain an efficiency benefit when they are run at close to their maximum RPM limit or maximum (exhaust) temperature limitations. At lower altitudes, the engines can only be run at maximum thrust during take-off or perhaps the climb or else the aircraft would quickly exceed its maximum speed limitation. If you tried to fly straight and level at 10,000ft with more than 70% thrust set, you would quickly overspeed most commercial jet aircraft. At 70% thrust, the engine isn’t running very efficiently.
As the aircraft climbs in altitude, the jet engines produces less thrust (as the air is thinner), but it maintains a high compression ratio and thermal efficiency. As the air is thinner, the plane is able to achieve a much higher True Air Speed (TAS) than lower down, meaning the aircraft travels much faster whilst the engines burn less fuel.
The higher the altitude the less dense the air, or put in a different way, the thinner the air is. Therefore there is less resistance (or friction) to stop the aircraft moving through the air. We call this resistance “drag”.
Here’s an example to demonstrate how the air thins out as you get higher; imagine moving your hand through water and golden syrup. If you want to move your hand through both of these liquids at the same speed, you need much more effort to move your hand through the golden syrup than the water. This is the same principle with an aircraft flying at a higher altitude when compared to a lower altitude.
If you found this article of interest, check out our page on How fast do planes fly?
What happens if all the plane’s engines fail in the air?
If all the aircraft’s engines fail, will the plane still fly or will it fall out of the sky?
Can a plane fly if all its engines have failed?
A passenger aircraft will glide perfectly well even if all its engines have failed, it won’t simply fall out the sky. Aircraft are designed in a way that allows them to glide through the air even with no engine thrust. In fact, the chances are that if you’ve flown in a plane, you’ve seen it effectively glide at some point during the descent to land.
Aircraft are able to fly through the movement of air passing over the wings and as long as this process continues the aircraft will continue to fly. If both engines fail, the aeroplane is no longer being pushed forwards through thrust, therefore in order to keep the air flowing over the wings, the aircraft must exchange energy through losing altitude in order to maintain forward airspeed.
The aircraft doesn’t have to lose altitude particularly rapidly to keep flying and therefore it both engines were to fail a high altitude, the aircraft may have as much as 20 – 30 minutes of airbourne time to find somewhere to land.
How Far Can a Jet Travel with all its Engines Failed?
A passenger jet could glide for up to about 60 miles if it suffers a total engine failure at its cruising altitude. Here’s an example. A typical commercial aircraft has a lift to drag ratio of around 10:1. This means that for every 10 miles it travels forward it loses 1 mile in altitude. If an aircraft is at a typical cruise altitude of 36,000 (which is 6 miles up) and loses both engines, it can therefore travel a forward distance of 60 miles before reaching the ground. Therefore, if such an incident occurs within 60 miles of a runway, the aircraft could potentially be landed safely.
US Airways Flight 1459
Rest assured, dual engine failure is almost unheard of. We all know about the story of US Airways flight 1459 landing in the Hudson River in New York after both its engines were destroyed by birds, but that really was exceptional – and everyone survived thanks to the quick actions of the actions of the pilots Captain Chesley Sullenberger (‘Sully’) and First Officer Jeffrey Skiles.
Air Transat Flight 236
One other exception was Air Transat Flight 236. The plane had a fuel leak causing both engines to fail at approximately 65 nautical miles from Lajes Air Base in the Azores. With an average descent rate of 2,000 fpm, the aircraft glided without power to the airbase where the crew carried out a successful landing about 17 minutes after the last engine failed.
Gliding Every Flight
The lower the engine power, the less fuel the engines burn. On most flights pilots try and burn as little fuel as possible and part of this process involves descending the aircraft towards the destination airport at idle (minimum) thrust. When the thrust is at it’s minimum setting, it isn’t really producing any meaningful thrust at all so the aircraft is effectively gliding. Therefore you will have experience the aircraft gliding on almost every flight you have been on!
If you found this article of interest, you may find our article about how both engines on a passenger jet can fail to be worth a read.