Thrust Augmentation

Accelerating beyond limits: Thrust Augmentation

Afterburner
F-15 eagle Afterburner
Among the various types of jet engines, the turbojet engine is the one that produces the least amount of thrust for a given engine size. This is due to the fact that the thrust generated by a turbojet engine is only by the exhaust gases. Any aircraft equipped with a turbojet engine will be enabled with the basic thrust (nominal thrust). With the basic thrust, an aircraft will be able to perform all the conventional maneuvers like take-off, cruise, land etc. But there are certain circumstances wherein an aircraft has to perform special missions for which the basic thrust is not sufficient.

Missions which require excessive thrust

  • Take-off from a short runway or a reduced runway-Ex Naval vessels
  • A take-off on a hot summer day
  • Combat maneuvers-Ex- Dogfight

In order to perform all these maneuvers, a turbojet engine has to generate a large amount of thrust. Higher thrust can be generating by using a large-sized engine. But using a large-sized engine will have its own concurrent weight penalties. Hence, in such critical conditions, to meet the thrust requirements, a method called thrust augmentation is preferred. Thrust augmentation is a technique by which the basic thrust of a jet engine can be increased without increasing the size of the engine. Basically, there are two methods by which the thrust of a jet engine can be augmented.

Thrust Augmentation methods

  1. Afterburning Technique
  2. Water or Water-Alcohol Injection Technique

Afterburner Technique


Afterburner techniques

Afterburner is one of the widely used thrust augmentation technique by which the basic thrust of the jet engine can be periodically augmented to improve the thrust during take-off, climb and combat maneuver. Afterburner is the common method of thrust augmentation and is a characteristics feature of all supersonic aircraft. The use of afterburner is made possible due to the fact that the main combustion chamber consumes only 25 % of the total oxygen passing through the engine. As a result, the remaining 75% of the air can be burnt with additional fuel in a secondary combustion chamber located downstream of the turbine i.e., in an afterburner.

Thus afterburner may be defined as an auxiliary burner attached to the tailpipe of the jet engine. By using the afterburner, an additional fuel can be burned downstream of the turbine which increases the temperature of the exhaust gases and consequently the thrust of the exhaust gases. By using an afterburner, an additional thrust of up to 50% can be increased. Basically, an afterburner is nothing but a combination of a simple gas turbine engine and a Re-heater, where the expansion of the exhaust gases is accomplished in two stages and reheating the gases to the maximum permissible temperature between the stages.

Components of Afterburner

  1. Diffuser
  2. Spray Bars
  3. Torch Igniter
  4. Flame Holders
  5. Screech Liners
  6. Fuel Valves and Pumps
  7. Variable Area Nozzle


Working of the afterburner

Working of afterburner
Mig 23 Afterburner
During the working of an afterburner, the hot exhaust gases from the turbine will be made to pass through the diffuser, where the gases are first deswirled and diffused and then made to enter the combustion chamber of the afterburner. Simultaneously, the fuel will be injected into the afterburner through multiple fuel spray bars. Further, the combustion process is initiated with the help of torch igniters or pilot burners which are placed in the wake of the number of flame holders. The process of combustion results in the generation of hot exhaust gases. 

During combustion, the temperature of the exhaust gases increase rapidly and can reach up to 2200 C. Since, the temperature of the exhaust gases are very high, the flame is made to concentrate around the jet pipe axis, thereby maintaining a safe wall temperature. Most of the afterburner will be provided with a specialized liner which acts as both a cooling liner and screech liner. The liner is generally corrugated and perforated with thousands of small holes. This liner prevents the high frequency and amplitude pressure fluctuations resulting from excessive noise, vibrations and other combustion instabilities from causing physical destruction of the afterburner components. All engines incorporating an afterburner must be equipped with a variable nozzle in order to accommodate the large changes in the temperature produced by the afterburner.
During the non-afterburning operation, the nozzle will be in minimum position, but when it is switched ON, the nozzle will automatically open to provide an exit area suitable for the increased volume of the gas stream. The opening of the nozzle prevents any increase in the back pressure from occurring which tends to slow down the turbine as well as the compressor and will ultimately lead to the stalling of the compressor.

Water or water-alcohol injection technique

Pratt and Whitney JT3
Pratt and Whitney JT3
Water injection technique is one of the simplest methods of augmenting the thrust of a jet engine. This technique is mainly used during take-off and when there is a drop in the thrust due to the changes in the atmospheric pressure or temperature. Using this technique, power or thrust up to 30 % can be boosted for take-off. The thrust developed by the water injection technique is termed as Wet Thrust and the thrust developed without the use of water injection is termed as Dry Thrust.

During the working, the water-alcohol mixture or just water is added into the engine through a series of spray nozzles. The principle by which this method produces extra thrust is by creating a cooling effect. When water is injected into the compressor inlet, the temperature of the compressed air decreases which increases the density and eventually the mass of the air. This further increases the discharge pressure ratio at the exit of the compressor. When this cooled compressed air is passed into the combustion chamber, the turbine inlet temperature will be reduced. In order to increase the combustion temperature in the combustion chamber, the mass flow rate of the fuel has to be increased.

The water provides additional thrust in one of two ways, depending on where the water is added. Some engines have the coolant sprayed directly into the compressor inlet, whereas others have fluid added at the diffuser. When water is added at the front of the compressor, power augmentation is obtained principally by the vaporizing liquid cooling the air, thus increasing density and mass airflow. Furthermore, if water only is used, the cooler, increased airflow to the combustion chamber permits more fuel to be burned before the turbine temperature limits are reached, thus increasing the turbine inlet temperatures. Higher turbine temperatures will result in increased thrust.

Water added to the diffuser increases the mass flow through the turbine relative to that through the compressor. This relative increase results in a decreased temperature and pressure drop across the turbine that leads to an increased pressure at the exhaust nozzle. Again, the reduction in turbine temperature when water alone is used allows the fuel system to schedule an increased fuel flow, providing additional thrust.

Water alone would provide more thrust per kg than a water-alcohol mixture due to the high latent heat of vaporization and the overall decrease in temperature. The addition of alcohol  adds to the power by providing an additional source of fuel, but because the alcohol has a low combustion efficiency, being only about half that of gas turbine fuel, and because the alcohol does not pass through the central part of the combustion chamber where temperatures are high enough to efficiently burn the weak alcohol-air mixture, the power added is small.

The increased thrust results because of the cooling effect of the water or the increased flow through the fixed area turbine that effectively increases the operating pressure ratio of the engine. All of the preceding depends on where in the engine the water is injected. The water injection system is not without penalty. Water and the injection system are very heavy; there is a thermal shock to the engine, and compressor blade erosion can occur when the system is activated. An important limiting factor, compressor stall can also be a problem with water injection.

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Suggested article: How do pilots land on low visibility runway?

Instrument Landing System

How Airplanes land in Low Visibility and Bad Weather?


Have you ever wondered how pilots are able to land easily even in bad weather? No matter if its raining cats and dogs or the airport are covered in thick fog, pilots land the aircraft just as if it's another routine landing. The system which assists the pilots to land in such conditions is called "Instrument Landing System" or commonly known as "ILS".

Instrument Landing System


ILS is a system which uses 2 radio signals to provide vertical and horizontal guidance to the pilots during the approach to land. One radio signal tells the pilot if they are high or low and the second radio signal tells the pilot if they are left or right of the runway centerline. The ILS is categorized as a "precision" approach because it provides both horizontal and vertical guidance. In modern Jetliners, the radio signals are interpreted by the onboard computers and continuously give feedback to the pilot about their position. If the aircraft is in AutoPilot mode, then the computers correct themselves to maintain a stabilized approach.

Did you know, the first ILS was set up at the Berlin Tempelhof Airport (EDDI) in the year 1932.

As mentioned above, the ILS uses 2 radio signals which are independent subsystems to provide vertical and horizontal guidance. The system that provides vertical guidance is called "Glide Slope" and that provides horizontal guidance is called "Localizer

Glide Slope (GP)

The glide slope is a radio signal which provides vertical guidance during an ILS Approach. The Glide Slope guides the pilot to ensure that the aircraft follows a "glide path" which is approximately 3 degrees above ground and safely reach the touchdown zone of the runway. The Glide Slope has an operating frequency range of 329 Mhz to 335 MHz and the transmitter is located between 750 and 1,250 feet from the approach end of the runway and is offset 250 to 650 feet from the runway centerline

Glide slope antenna
Glideslope antenna
Fun Fact: A standard glide path angle is 3 degrees horizontally but London City Airport has a glide path angle of 5.5 degrees! 

Localizer (LLZ) : 

The Localizer is a radio signal which provides horizontal guidance during ILS Approach. It guides the pilot to ensure that the aircraft is aligned with the center line of the runway. The Localizer has an operating range of 108Mhz to 112 MHz. The localizer has an antenna which is installed at the far end of the runway so that the center of the antenna is in line with the center line of the runway.

ILS Localizer
Localizer
Now that we know what is ILS, let us take a look at its different categories. But first, let us first learn a few terminologies.

Runway Visual Range (RVR)

The range over which the pilot of an aircraft on the center line of a runway can see the runway surface markings or the lights delineating the runway or identifying its center line. (According to ICAO Annex 6)

Decision Height (DH)

The Decision Altitude (DA) or Decision Height (DH) is a specified altitude or height in the Precision Approach or approach with vertical guidance at which a Missed Approach must be initiated if the required visual reference to continue the approach has not been established.
(According to ICAO Annex 6) [1]

ILS Categories

CAT 1 -      DH > 200ft            |  RVR >1800ft
CAT 2 -       DH = 100 - 200 ft | RVR > 1200ft
CAT 3A  -   DH < 100ft            | RVR > 700ft
CAT 3B -    DH < 50ft              | RVR > 150ft - 700ft
CAT 3C -    No Limit

You must be wondering that even though many airports have such sophisticated systems yet why does fog disrupts so many flights and cause delay. The answer is even though the airport is equipped with the ILS, not all pilots are trained to land in low visibility. This training is very intensive and expensive Imagine learning to drive a car blindfolded!

Autoland

Most of the modern jetliners can Autoland. "Autoland" term is used when the autopilot approaches, flares and lands the airplane on its own without any pilot's intervention. Usually, Autoland is performed in very low visibility where the airports are equipped with ILS 3+ category. Sometimes airplanes Autoland to maintain the currency (accuracy) of the onboard computer systems.  If the autoland systems lose their accuracy prior decision height, an error message is displayed on the Primary Flight Display and then the pilots either take the decision to continue the approach as an ILS CAT 2 approach or if the weather conditions aren't suitable then they discontinue the approach and land at the planned alternate airport.

Enjoy a video of low visibility CAT 3B cockpit View




References : 
1. ICAO Annexure 6

Picture Credits :
1. RF Wireless World
2. Wikimedia Commons

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Mid Air Failure of One Engine in a Multi-Engine Aircraft

Mid Air Failure of One Engine in a Multi-Engine Aircraft

Mid Air Failure of One Engine in a Multi-Engine Aircraft

Have you ever wondered what if one, two or three engines of the four-engine flight carrying you from New Delhi to Texas fails in mid-air? What would happen? Will the pilot keep flying the aircraft or it will give rise to some catastrophic landing? The answer is right here in the article.

Engine failure

You would be amazed to know that you may have flown in a flight having one engine failed in mid-air or on the ground while take-off or landing without your knowing. Though engine failures are accompanied by huge noise or vibrations sometimes they are not very much noticeable, but it is required that the pilot gets to know about the failure very quickly.  

A Quantas Airways Flight 32 faced a three-engine failure but still, it managed to land safely with all the crew members and passengers on board safely. Before discussing what actually happens when we face an engine failure in mid-air let me tell you about basic aerodynamic terms that would be used later in this article.

The V speeds



The term V-Speed is derived from the French word VITESSE, meaning speed.

In the world of aviation V-speed is the term used to define airspeed important and useful for an aircraft in different flying conditions. These V-speeds are calculated using the performance testing data.

Some V speeds that are required in the engine failure situation are as follows-
  • V1- The speed after which take-off should not be aborted.
  • V2- The speed at which the aircraft may safely be climbed even if one engine is inoperative.
  • Vmc- The minimum control speed is the speed at which the aircraft is still controllable even if one of the engines is inoperative. The Vmc speed is further divided into two categories Vmca and Vmcg, Vmca is the minimum speed at which aircraft can be controlled even if one engine goes down in mid-air. Similarly, Vmcg is the minimum speed at which the aircraft can be controlled even if one engine goes down at or near the ground.
  • Vcl- The minimum control speed of an aircraft at which the aircraft is controllable even if one engine goes down in landing configuration i.e. the landing gear is deployed and the aircraft is descending.

Working of the Rudder

The rudder is a control surface which is used to control the yawing action of the aircraft, the rudder is present on the vertical stabilizer present at the tail section of the aircraft. The working of the rudder is based on the principle that when the rudder moves towards left the airflow across the vertical stabilizer is affected and the tail of the aircraft moves towards the right.

working of aircraft rudder


Now we will discuss about the aerodynamic consequence of one engine failure in a two engine aircraft, but this concept is easily applicable to aircrafts having more than two engines.
Before jumping on the core concept consider two of your friends holding your hand and pulling you to run with them. Now consider one of them leaves your hand you will get a sudden jerk and yawing motion towards your friend who left your hand, the same situation happens in an aircraft when the thrust from one engine goes off suddenly the aircraft gets a sudden yawing motion towards the engine which is failed.

If this unbalanced thrust is not compensated the aircraft would yaw very rapidly and will get out of control in seconds. Now here comes the skills of the pilot you all have read about rudder earlier which is a control surface used to control the yawing motion of an aircraft. These rudders are used very carefully by the pilot and they are turned in the direction of the operative engine so that they can provide a balancing action to compensate the action of unbalanced thrust produced due to the failure of one engine.

Aircraft sideslip
For example the left engine of two the engine aircraft fails then we would get an unbalanced moment acting on the aircraft in a counterclockwise direction as seen from the top. Then the rudder must be deployed in the direction of the operative engine so that we get a moment acting in the clockwise direction. This counterbalancing action would make the aircraft stable even on a single engine.

Effects of engine failure

1. On the Runway

If the engine failure occurs during the take-off then the aircraft would get a yawing moment in the direction of the failed engine. This yawing moment can be controlled with help of rudders if the airspeed is at or above Vmcg, if not then the thrust produced by the operative engine must be reduced before using the aerodynamic control. If the airspeed is below V1 then the takeoff should be aborted. For more details check this article

2. Mid-Air failure

Apart from the yawing action produced by the engine failure discussed earlier there are two more effects produced by the engine failure one is the roll moment induced by the yawing action. This is the result of continuous yawing action toward the failed engine, which causes a decrease of lift in the retreating wing. The second penalty is the arriving from engine failure is the drag produced by the failed engine.
Now the drag produced in the turbojet engine is less as compared to that produced in a turboprop engine due to the wind milling propellers. The drag produced in turboprop engines is reduced by adjusting the propellers blades in the position so that minimum drag is produced by the failed engine.

In the conclusion, I would like to say that it is very much possible to fly a multi-engine aircraft with one, two or three engines failed. According to the new standards set by the FAA the aircraft prior to its usage undergoes a testing procedure in which it is tested to fly even if only 50% of the thrust is available while in mid-air or during the time of take-off or landing. As per the new guidelines the pilot has to land the aircraft to the nearby airport with the consent of ATC in case of an engine failure. 

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Suggested article: Flow visualization using pressure sensitive paints

Flow visualization technique - Pressure sensitive paints

Flow visualization techniques - Pressure sensitive paints

Pressure Sensitive Paint
Pressure Sensitive Paint on C-5 Galaxy Model
Flow visualization is concerned with a study of fluid flow near the surface of objects or test specimens. The technique is commonly used in wind tunnel testing and has many forms, ranging from smoke flow visualization, particle tracker method, oil flow visualization, Special Clay Mixture Method, Fluorescent Dye Technique, visualizing tufts and streamers stuck to a surface.

smoke visualization

Surface flow visualization is a well-established technique used to help in understanding flow fields, particularly complex, three-dimensional flows. Pressure sensitive paints are majorly used to determine the surface pressure of the test object and it is one of the main surface flow visualization techniques used.

Pressure Sensitive Paints are a just a coating, such as paints which behave as a fluorescent material under a particular wavelength with respect to different intensities depending on the external air pressure which is applied to its surface. PSP techniques have been applied widely to the study of high-speed flows. PSP is a non-contact technique. PSP is the first global optical technique that is able to give information on flow structures that cannot be easily obtained using conventional pressure sensors and provide non-contact, quantitative surface pressure visualization for complex aerodynamic flows.

BASICS AND WORKING OF PRESSURE SENSITIVE PAINTS

Pressure sensitive coating
Pressure sensitive paint coating
Pressure sensitive paint is a luminescent dye dispersed in an oxygen permeable binder. The dye undergoes excitation when they absorb light. This usually happens in the UV portion of the electromagnetic spectrum. After this, it returns to its ground state by emitting light, in the red portion of the spectrum. Also, we can make the dye to return to its ground state without emitting light, by interaction with an oxygen molecule. This process is known as oxygen quenching, which involves the non-radiative deactivation of an excited photo-active molecule(luminophore). 

Non-radiative processes include internal conversion to a different electronic state and then the release of heat or external conversion via contact with an external molecule, in this case, oxygen. Thus, the increase in pressure of the oxygen above the PSP causes an increase in the oxygen concentration within the binder, leading to a decrease in the intensity of the emitted radiation. As the pressure on the surface varies, the intensity of light emitted varies measured by the detector. This gives us a measure of variation of pressure on the surface of the test specimen.

Applications of pressure sensitive paints

  •  Aerodynamic testing- High-speed facilities such as shock tubes,  Ludwig tubes, and short-duration hypersonic flow often see the use of PSP for improved performances. Many unsteady flow-fields, oscillating jets from fluidic oscillators, resonant acoustics, oscillating airfoils, and unsteady flow in turbomachinery use Porus PSP formulations.
  •  PSP at Large Wind Tunnel- Large industrial transonic wind tunnels can incorporate the use of PSP for practical tests.
  • iii) PSP is a useful tool for research and development of aerodynamics, structural analysis, CFD code validation and so on.

Advantages of pressure sensitive paints

  • Less preparation time compared to installing an array of pressure taps.
  • The same model can be used for another testing since the PSP will not interfere with other preparations or setups.
  • Low-cost alternative
  • Superior spatial resolution
  • High accuracy (within 150 Pa of pressure tap measurements)
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Air Crashes that gave birth to the future technologies in Aviation

Air Crashes that gave birth to the future technologies in Aviation

Air Crashes that gave birth to the future technologies in Aviation


You have always found yourself reading and learning about new advancements in the field of aviation but have you ever wondered from where these new advancements come from, are all of them a result of a team of 100 members sitting in a boardroom and inventing new technologies. Not all of them come from the boardroom most of them are a result of unfortunate air crashes that have happened across the globe. Yes, air crashes are very painful but they have been the main reason behind some revolutionizing aviation advancements.

So come let’s discuss some of the major advancements that have arrived into the world due to some unfortunate air crashes in past twenty or thirty decades.

1. The invention of the Traffic Collision Avoidance System

TCAS
TCAS is a system in which both the aircraft in air communicate with each other with the help of transponders fitted on both of them, then a computer system on both of them calculates the altitude that the aircraft should gain to avoid collision with other aircraft. To know more about TCAS click here


Until the late 90’s only the Air Traffic Control (ATC) was responsible for regulating the air traffic up in the sky and avoid collisions between different aircrafts but in 1956, a TWA plane crashed into a United Airlines flight above the Grand Canyon. The incident was the first of many that illustrated the need for increased communication between planes. After that FAA was formulated and TCAS system was made mandatory for all the airlines to get fixed on their aircraft which has almost eliminated the cases of air midair collisions of two aircraft.

Though a major drawback of TCAS system came forward by another air crash which happened between a commercial flight and cargo aircraft in which both the aircraft were carrying a functional TCAS system on board but the protocol being followed by the pilot in commercial flight was to keep the decision of TCAS above ATC and that by the pilot in cargo flight was to keep the decision of ATC above TCAS, both the pilots followed the decisions as per the protocols and in the end both of the aircrafts found themselves at the same altitude and crashed into each other.

2. Weather Sensing Radars

You will fell turbulence in every other flight flying in airfield across the globe, not always the pilot is the reason behind those shaky and noisy flights. It is the weather outside which brings in too much turbulence and uneasy flights. Sometimes this weather turns so hostile that it may result in major catastrophic air accidents. Delta Airlines Flight 191, which crashed in 1985 while approaching Dallas-Fort Worth International Airport in a thunderstorm in an example of weather being the reason behind the air crash. Rain is another thing but if a pilot comes in a thunderstorm or a wind current it can pick and throw the aircraft on the ground like nothing.

The weather sensor is not a very complicated device but is indeed a very important one, it senses the weather conditions near the aircraft and gives the information about those conditions in the cockpit so that the pilot can decide his flying direction accordingly. The major condition that the weather sensors detect are the wind shear, a wind shear is a condition in which the speed of air varies along the horizontal and vertical direction. Getting into a wind shear is very easy and you may never come out of wind shear until you are very lucky.

Aircraft weather sensing


3. Leg Space between seats

Though you may feel a bit cramped while traveling in the economy class flights but still they are under set standards by the international governing bodies. Due to increasing fuel prices, airlines started directing the airframe manufacturers to have a maximum number of seats in an aircraft which gave rise to aircraft having lesser leg room between the seats. Then came forward a tragedy in which British Airtours 737 caught fire before takeoff at Manchester International Airport in 1985 and passengers were unable to eject from the aircraft due to lesser space between seats which created a panic and most of them burnt to death.

The incident gave rise to research work around the globe and Canfield Institute came up with a research which proved that through the emergency doors were open but there was no space between seats as well as between seats and emergency doors for people to eject.

This gave rise to proper and stringent standards about the leg room between seats in an aircraft. 


Aircraft seats

4. Electric spark elimination

Everybody was stunned after hearing about the TWA flight 800 which exploded midair, A Boeing 747 carrying 230 people from JFK to Paris turned into a huge explosion killing all the people on board. After investigating the wreckage NTSB dismissed any possibility of any terrorist activity. Further investigations revealed that the aircraft caught fire due to the partially empty fuel tank which started burning when a spark was generated in an electrical wire.

The FAA after this incident gave out guidelines according to which all the potential spark producing wirings should be checked prior to flight and they should be changed if there is even a small chance of a spark. Boeing also worked on the issue and created fuel tanks having argon gas filled in them, argon being inert gas reduces the chances of fire in the fuel tank.

5. Jet Engines Capable of bearing bird strikes


There is no specific air crash behind this advancement, but everyone can relate to it remembering the heroics of Captain Sully who showed his heroics by landing US Airways flight 1549 in the Hudson River on 15th August 2009 saving the lives of 155 people on board. Captain and his second in command glided the aircraft to Hudson River and then landed the aircraft in the river. As the aircraft met a midair bird strike and lost both of its engines in midair.

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Researches were carried on by various agencies across the globe and then came an idea of developing an engine having fan blades which can withstand bird strike without losing the aerofoil shape of the fan blades as well as the thrust produced by the engine. Then within a span of certain years, fan blades were developed which were capable of bearing bird strikes without any damage to them. Later on, FAA has made it mandatory for all the new engines to undergo bird ingestion testing in which a jet engine is made to undergo a bird strike in testing range and its performance parameters are captured.

Aircraft bird strike


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Suggested article: Why do you hear reduction in engine noise after Take-off?

What is Flex Temperature?


WHAT IS FLEX TEMPERATURE?

What is flex temperature

Flex Temperature or  de-rated temperature

As it is not necessary that the cars should be accelerated at a particular speed, similarly it is not necessary that the aircraft should always take-off at its maximum thrust. As there is an optimal value of speed at which a car would give the maximum average, similarly to reduce the fuel consumption of an aircraft during take-off an optimal value of thrust is decided. Boeing calls this concept as de-rated temperature and most of the other airliners call this Flex temperature. This article deals with the step by step analysis of power setting during take-off and its relation with Flex temperature.

Calculations prior to a take-off:

Aircraft Take off


A commercial aircraft is not always applied with full power during take-off, although most of the smaller aircraft use full power to take-off. There is a sudden decrease in engine sound just after the take-off, which could easily be observed by a passenger sitting in it. This sudden decrease in the sound of the engine is due to the Flex temperature and climb power setting. Various factors are taken into account while performing take-off calculations. These calculations are done prior to every take-off. The pilot is provided with a load sheet containing all the important factors required for the calculations. ATIS (Automated Terminal Information Service) which is a recorded voice containing description about weather conditions and several other things to reduce the workload of ATC (Air Traffic Controller) is referred by the pilot for performing the calculations.

Entering fake air temperature to fool FADEC:

FADEC
FADEC
After the calculations, there would be some suggested values of Flex temperature along with the various take-off velocities for the particular Flex temperature. This article would help you to understand the various velocities involved during take-off. The pilot would choose one of the suggested values of Flex temperature and feed that value to MCDU (Multi Function Control and Display Unit). Now FADEC (Full Authority Digital Engine Control Unit) assumes that the outside air temperature is the one provided by the pilot. Actually, a fake value of outside air temperature is entered in the MCDU by the pilot, so that the FADEC reduces the power which would help in preventing engine wear. FADEC is actually fooled by the value of Flex temperature as it thinks that the outside air temperature is very high, therefore to prevent damage of the engine at such expected high temperatures it commands the engine to work at a reduced power.   
  
Benefits of using Flex temperature:

TOGA thrust is the maximum thrust available with an aircraft to take-off. All take-offs should not be performed at the TOGA power setting as it results in more engine wear. The reason behind choosing the Flex temperature approach is to increase the engine life, which would reduce the chances of an engine failure. This approach of using de-rated power setting reduces fuel consumption as the fuel flow rate is less when Flex temperature is used.

Harms of using Flex temperature:

Use of Flex temperature is not allowed when runway surface is contaminated. The aircraft would be left with shorter stop distance in case of rejected take-off as in de-rated power setting the aircraft would be accelerated at a lower rate. The use of Flex temperature is also prohibited in the runways which are either at higher altitudes or which are short in length.

The concept of using de-rated power setting is actually how to balance shorter runway length in case of rejected take-off and engine life.

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Suggested article: How do pilot move aircraft from one place on ground to another?

Aircraft Taxi and Line up procedure

PROCEDURE FOR TAXI AND LINE-UP


Aircraft Taxi and Line up Procedure


Aircraft taxi is a procedure of moving aircraft from one place to another under its own power. But can an aircraft move during taxi in the way it wants? Or are there any rules which bound the movement of an aircraft in an airport. 

Procedure to begin Taxi:

The place of an airport where the loading and parking of an aircraft take place is called the gate position of that aircraft. When boarding of all the passengers is over and an announcement to fasten the seat belts is done, then it is the time for aircraft to start its taxi. Now after getting the clear signal by the pushback driver, a request to get a taxi clearance is extended to the ground and apron controller. When the ground controller gives the taxi clearance to an aircraft, then aircraft moves out of its gate position and the taxi lights are switched ON. Parking brakes are released and taxi along the assigned route is started.   

Procedures to be followed during Taxi:


Aircraft Taxi


Just as an aircraft starts to taxi it is slowed down to check whether the brake is working properly. There are various other checks including that of flight controls in which ailerons and all other flight controls are moved to check their working. There is a single engine taxi procedure for turboprops in which the only one of the propellers is spinning. These procedures could vary according to the type of aircraft and airliner. Taxi phase also includes the de-icing procedure where aircraft enters the de-icing pad for getting de-iced. During taxi, the maximum speed allowed is 30 knots. For preventing the nose-wheel from being collapsed turns are made at a speed not higher than 10 knots. Holding point is the last phase of the taxi. Holding point is the position where an aircraft waits for its turn to come on the runway. At this phase tower controller, which is responsible for giving line-up, take off and landing clearances are consulted.

Procedure for Line-up:

The taxi phase ends when the aircraft has received the line-up clearance from the tower controller. Tower controller asks the cabin crew if they are ready for departure. Once the crew has completed the Before Take-Off Checklist, they would respond to the tower controller: “Airliner XYZ ready for departure”. Line-up starts when the aircraft moves past the holding point and ends once the fuselage of the aircraft is in line with the centreline of the runway. There are many special clearances which are provided by the tower controller. For example, to prevent any disaster on the runway, the tower controller makes the crew aware of all the aircraft which would be landing before their line-up and which aircraft would already be there on the runway. These are the differences between Taxi and Line-up. Taxi is the phase to get clearance from the ground controller and reach the holding position just before entering the runway. Line-up is getting in touch with tower controller, enter the runway and get ready for takeoff.

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Some misleading terminologies:

There are some terminologies which sometimes confuse the pilot, these are mentioned below.
HOLD SHORT - According to this command of ATC (Air Traffic Controller), the crew should taxi the aircraft to the holding position and wait there.
LINE UP & WAIT – According to this command the aircraft should be made to taxi onto the runway, line up on the centreline, have the aircraft prepared for immediate takeoff, and expect takeoff clearance within seconds.
But many times the crew keep the aircraft at the holding position even after getting “Line-up and wait” command, which sometimes causes a delay in take-off.

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