Departure airplane

Takeoff! An Aeroplane Takeoff from a Technical Perspective

Tim Takeoff
12.03.2019
4 pictures
7 minutes

The engines have started. Thousands of kilograms of the most diverse materials swing into action, apparently with the greatest of ease. The plane reaches the runway and the command issues from the cockpit: “Takeoff!” Now everything is a blur of motion until the final wheel leaves the ground. But what exactly is happening here? Let’s take a look at the takeoff process in slow motion.

Long before the flight crew reaches the aircraft, staff are working on the plane’s takeoff. The flight schedulers in the office are planning the entire flight in advance. This also, of course, includes the takeoff. In order to facilitate this process, every aircraft manufacturer provides extremely precise tables which allow all the circumstances to be taken into account in the calculations. Nowadays, these tables have been incorporated into a range of diverse software solutions. These performance programmes enable the planner and the crew to perform a complicated takeoff calculation in just a few minutes, taking into account factors like the length and the surface conditions of the runway, the wind, the temperature, the air pressure and specific local conditions.

Performance Calculation for Takeoff

Once the flight planner, known as the dispatcher, is more or less satisfied with the calculation, he or she passes on all the information to the crew. Once the crew have boarded the plane, they collect the final data on the weight, any changes of runway and the current weather. This information is used to finalise the calculation, which is then checked and signed off on by all members of the crew. On the way to the runway, the crew set the flaps to a previously determined position. These flaps tremendously increase the lift at low speeds (more about lift in this article), as well as adding considerable resistance. This additional resistance is far outweighed by the benefit of the extra lift, which enables the plane to take off at a much lower speed than it would with “clean” wings. Once on the runway, the plane is cleared for line-up before the clearance for takeoff is issued.

Callsign XY, Wind 210, 10 knots, runway 18 cleared for takeoff!

Cleared for Takeoff

In the cockpit, the wind is now cross-checked for the final time, the stopwatch is set, all the lights and the weather radar (see our article on weather radar here) switched on, and the parameters checked. Once everything is ready, the various airlines each have their own procedures. The most common one is as follows: The “Pilot Flying”, or PF, which could be the pilot or co-pilot, takes over the plane’s controls. The “Pilot Monitoring” (PM) monitors all the parameters, makes sure the correct thrust is being maintained, and keeps an eye out for any potential issues. Regardless of the “Pilot Flying” and “Pilot Monitoring” designations, the captain maintains control of the thrust levers from the very outset.

As soon as the captain sets the initial thrust, the engines roar into life. The brakes are released. In a strong side wind, the Pilot Flying uses the rudder in combination with the steerable nosewheel to keep the plane on what is known as the centre line. If the side wind is extremely strong, he or she will also use the aileron to try to keep the plane angled “into the wind”. As the wings – which are usually slightly v-shaped – are exposed to the full force of the wind, the wing facing the wind may lift. The Pilot Flying can, to a large extent, counteract this using the aileron. Once the plane reaches a certain speed, the nosewheel automatically deactivates. The Pilot Flying continues to control the direction using only the rudder.

Aircraft use a precisely calculated range of predetermined speeds. The most significant of these are known as V1, Vr and V2.

V1

The first – and very important – speed the plane reaches is known as “V1”. This is the maximum speed at which the takeoff process can still safely be aborted. Until this speed has been reached, the captain keeps his or her hand on the thrust levers. Modern jets are equipped with an automatic “reject takeoff” function. If something goes amiss and the pilot pulls the thrust levers back from their takeoff position, a rejected takeoff is automatically signalled. In a fraction of a second, the plane will apply maximum pressure to the brakes and operate the air brakes. Depending on the type of problem, the pilot can use reverse thrust to take the pressure off the brakes and provide the plane with a little braking assistance. The takeoff has been precisely calculated to ensure that a plane travelling at V1 speed can be brought safely to a standstill before it reaches the end of the runway, and the values are adjusted to take into account the wind and runway conditions. V1 is considerably lower if the runway is wet or contaminated in some way.

After V1 … Rotation Speed

As soon as the V1 speed has been exceeded, the plane must proceed with the takeoff. The captain immediately removes his or her hand from the thrust levers. Even if one of the engines fails, the aircraft can still achieve a safe climb. The next speed, “Vr”, is signalled by the Pilot Monitoring calling loudly “Rotate!” This lets the Pilot Flying know that the plane has achieved sufficient speed to lift the nose off the ground. The air current passing around the wings is strong enough to provide sufficient propulsion for takeoff.

The Pilot Flying begins to pull back on the control stick, causing the elevator to move upwards. The pressure on the rudder now forces the plane to rotate around its lateral axis – which is how the speed got its name. This rotation changes the angle of attack of the plane to the oncoming air current and immediately increases the lift under the wings. The jet overcomes the force of its weight, and is now governed entirely by lift.

In this video, “Captain Joe” explains the takeoff speeds in detail:

“Positive Climb!”

As the plane lifts off, the landing gear’s rolling resistance is immediately reduced to zero, providing additional acceleration. The next speed exceeded is V2. This speed is required for a safe climb in cases of engine failure and must be maintained at all costs. The pilot is responsible for maintaining the plane’s angle of attack to ensure that the aircraft travels neither too slowly nor too fast. The flaps and landing gear are still extended. When the Pilot Monitoring confirms a safe rate of climb by calling out “Positive Rate!” or Positive Climb!” to the Pilot Flying, he or she responds with the command to retract the landing gear: “Gear up!”

After-Takeoff Checklist

Retracting the landing gear eliminates the majority of the resistance, increasing the speed to a point at which the flaps can be slowly and gradually retracted. They were used solely to reduce the speed during the takeoff process and to ensure a safe climb during the first few minutes. Once the flaps have been completely retracted, the crew’s next activity is usually to read the After-Takeoff Checklist. This covers various points like the flaps, the landing gear, the air conditioning and other, aircraft-specific items. Depending on the circumstances, the Pilot Flying now engages the autopilot, or instructs the Pilot Monitoring to do so. The autopilot now assumes control of all the jet’s axes and engages automatic thrust.

Below 10,000 feet, almost all airspaces prescribe a maximum speed of 250 knots. Once this altitude has been reached, the jet can accelerate to its optimal climbing speed. This varies, of course, according to the aircraft’s weight and engine power, and the external conditions. The plane is now free to continue climbing to its cruising altitude. If you would like to learn more about engines or about the autopilot, please check out our articles on these topics. If you want to know which further preparations are necessary before take-off, we recommend you this article.

by Tim Takeoff

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