Average Plane Speed
How often have you sat aboard a jet airliner and wondered about the average plane speed and how it is arrived at? Why is it that different speeds are used at different stages of the flight and why do they climb to different altitudes each time you fly?
To answer this we have to look at the various factors that determine the answer.
The atmosphere in which you will be flying is a very fluid environment and just like the sea, has established currents. Also like the sea, it has varying pressures with the highest pressure being at the Earth’s surface and that pressure decreasing the further we get from the surface until we reach the near vacuum of space.
The currents or winds and the changing pressure plays a huge part in the planning of flights and the way they are carried out. Some winds are a constant feature of the atmosphere. On the surface, we know of the Trade Winds that blow along the Equatorial regions. These winds were counted on by the early sailing ships and were so named as they blew the early traders to and from their destinations.
Like the early traders, we still count on the wind to aid us in reaching our destinations more quickly. Since the advent of jet airliners in the 1950s which could fly much higher than their propeller ancestors, it was found there are very strong winds at those higher altitudes which were named the Jetstream. When flying with the Jetstream, one can easily add significant speed to the flight and reduce the flying time to the destination. The winds move slightly with the seasons but can be counted on to the extent that airlines schedule their flights taking into account a faster flight with the Jetstream and slower flight against the Jetstream.
Measurement of Aircraft Speed
When we ask the question, how fast is an aircraft going? There are several answers that can be given and it can be very dependent on the stage of flight the aircraft is in.
Average plane speed and Take-off
We are sitting on the runway in a shiny new Boeing 777 about to apply full power and commence our take-off run. We’ve done our calculations and with the weight of cargo and fuel, we expect the airliner to become airborne at, for example, 152 knots(nautical miles per hour).
Hold on a minute, what does that mean exactly?
Ok, the additional information we need is that the local wind on the runway is blowing in your face and you will take off into the wind. When you are taking off, you don’t care about how fast the wheels are spinning on the ground, you care about how fast the air is moving over your wings. For instance, if the wind is blowing in your face at 20 knots, you only need to achieve 132 knots ground speed before you can expect the aircraft to start flying. This makes for a shorter take-off run as you started with a bonus of 20 knots before you even applied engine power. If you decided to take-off with the wind in the other direction, you would start off with 20 knots of wind going the wrong way over your wings and therefore would require a longer take-off run. The result is you would take the tops off the car park shuttle buses on the perimeter road which is not approved.
So we have established that speed through the air is the governing factor of flight. This is measured and expressed as KIAS or Knots Indicated Air Speed. Simplistically this is measured by air rushing into a forward facing tube called a pitot head or pitot tube which channels the air into a bladder inside the Air Speed Indicator. The higher the pressure which is driven by the forward movement of the aircraft, the higher the bladder causes the dial to read. It is a little more complex than that but it gives the idea at least.
Now in the climb out phase, air traffic control will be aware of the flight plan you have lodged, however, their first priority is to get you into a traffic flow that will clear you from the airport area without banging into other flight traffic. You will be given an assigned altitude, compass heading and speed. At busy airports, this can be a long involved process and you may find yourself tracking all over the countryside, possibly even in the opposite direction to your intended destination.
During this phase of flight, the rule of thumb all over the world is that you must remain under 250 KIAS (Knots Indicated Air Speed). Remember this is your speed through the air and not across the ground, so if the same wind you had on the runway is still blowing at this level you will have a ground speed of 230 Knots if you fly against it, but if you turn around and fly with the wind you will be doing 270 knots ground speed.
The speed restriction is there to enable a safer control of aircraft in a constricted space. In some cases, if it is not busy, air traffic control may release you from the speed restriction and allow you to go off on your merry way.
Climb to Cruise Altitude
So long as the sky above you is not too congested you should get your clearance to climb to your desired cruise altitude and start on your actual journey. As we pass through 10,000 AMSL (Above Mean Sea Level) we can increase our speed from 250 KIAS to that recommended in our particular airliners manual. The rule of thumb is 300 KIAS.
You may wonder why we need to bother to climb to those high altitudes. Isn’t the view nicer down here where you can see something? There are a couple of answers to that:
Firstly, at higher altitudes, we can fly above most of the weather. This is a winner for the passengers who expect to have mostly smooth flying when they get on an aircraft. In the pre-jet days, aircraft were much more susceptible to the vagaries of the weather as they had to fly through storm clouds and the like which was very uncomfortable.
Secondly, the higher you climb, the thinner the air. This means an aircraft can pass though it with less air resistance and therefore can fly faster using less fuel. This not only makes the airline accountant happy, but also enables a long range aircraft to achieve that range. For example, if I loaded up my Boeing 777 with enough fuel to get from Singapore to London and then only flew at 10,000 feet of altitude. I would expect to be looking for an emergency landing site somewhere in Afghanistan as my fuel was about to run out.
The logistics of managing a long range flight are quite complex. The object of the exercise is to take as much payload as we can and carry it over the distance required. Obviously for long range flights we need a significant amount of fuel which will make up a large proportion of our weight at take-off and initial climb out. You may have noticed on long haul flights you have been on, that you might climb to an altitude around 30,000 feet to start with and then after a few hours you may then climb to a higher altitude possibly approaching 40,000 feet. There are two reasons for this:
Firstly, in the initial stages of flight with full fuel tanks the aircraft is too heavy to climb economically and safely past the early 30,000s. Doing so would burn more fuel trying to achieve the higher level. It could also put the aircraft in an unstable flight phase where a stall might be possible.
Secondly, pilots may change the altitude of the aircraft during flight from time to time to either make use of more favourable tail winds or to avoid unfavourable head winds.
Speed in the Cruise Phase of Flight
Once your aircraft reaches a certain height, the effectiveness of the ability to measure speed as KIAS (Knots Indicated Air Speed) begins to diminish. The air is now so thin that it can no longer provide accurate readings on the Air Speed Indicator. This is where speed starts to be measured differently.
Most aircraft and modern airliners particularly, have their speed controlled by the autopilot. A speed is selected, 300 KIAS for example, and the aircraft happily flies with the auto pilot applying or reducing thrust to maintain the desired 300 KIAS. When the aircraft achieves an altitude of around 25,000 feet, and this varies slightly from aircraft to aircraft, the speed is automatically changed from KIAS (Knots Indicated Air Speed) to a Mach number.
What is a Mach Number?
A Mach number is an expression of speed relative to the speed of sound. For example, Mach 1 equals the speed of sound. Mach 0.5 is half the speed of sound, Mach 2 is twice the speed of sound. On top of that we need to add the complexity of the air temperature. The speed of sound is not a constant value, but depends on the air it travels through for its’ speed. To illustrate this let’s take it to its’ extreme.
We know that in the sea, or water in general that sound travels long distances. Whales can communicate over long distances with their songs. The water molecules are dense and therefore will transmit the sound readily. At the opposite end of the spectrum, we can go into space and find that that it is almost silent. In the near vacuum there are few molecules available to help conduct sound.
This is why when you ask, what is the speed of sound? The answer will be 761.1 miles per hour / 661 knots / 1,225 kilometres per hour, with the qualifier being, at 15 degrees Celsius at sea level. This relates to the pressure of air which is governed by the altitude and by the temperature.
Using this knowledge we can understand that the higher you fly, the lower the speed of sound becomes. If you look at the speed of sound at sea level and compare it with that at around 40,000 feet, you would see that it is around 90 knots slower at that height than at sea level. The fact that the temperature is much colder at 40,000 feet, around minus 56C, means that it is not as slow as it might be if the temperature was the same as at sea level.
The only airliner to achieve greater than Mach 1 is the Concorde which was capable of Mach 2. This airliner was specifically designed to fly through the sound barrier as it used to be known. It took many attempts to break through this so called barrier as it calls for a totally different aircraft design. As an aircraft approaches the sound barrier, shock-waves start to build up on various surfaces of the aircraft. These have an adverse affect on the aircrafts’ forward movement and can negate any advantage of flying more economically through thinner air. If you persist on going faster still and get closer to the speed of sound, you will start to feel the aircraft start to buffet more and more violently until you reach a catastrophic failure of the air-frame and the aircraft breaks up.
Every aircraft comes with a Do Not Exceed speed, which indicates the air-frame is not built to sustain the possible pressures of those high speeds.
Transitioning to Mach Number
We are climbing through the mid 20,000s of feet of altitude and our auto pilot throttle control clicks over from KIAS to Mach. It may be around Mach .50 or so depending on conditions and how many knots we were doing. Each airliner will have a maximum allowable Mach number and a cruise Mach number. The cruise mach number is used to maximise the performance so we get the most economical flight results as well as keeping our aircraft within safe operating parameters. Too fast and we could bring on the buffeting which could break up the aircraft. Too slow and we could bring on a stall as the wing struggles to provide lift in the thinner air.
Typically most airliners operate in the Mach 0.71 to 0.85 range depending on the design. To see the average plane speed for any of our featured aircraft be sure to look in the menu at the top of the page and select the Specs page for your desired airliner.
With the current flight information systems that most airlines offer, it is possible to see how fast you are flying and lots of other interesting statistics as you travel along. I always get a kick when we have a following wind to see how high the ground speed can get up to. Getting over 1,000 KPH always feels like a bonus to me.
Thanks for stopping by to find out a bit more about average plane speed. As you can see it is quite a complex answer to what appears to be a straight forward question.
I’d love to hear about your flight experiences, how fast have you gone? how high have you gone?