How we fly by the minute: Highways at 30,000ft

It is a widely known fact that air travel is currently the safest mode of travel. This is due to the fundamental principles used in all modes of transportation: "Corridors". For instance, if you want to visit the shop down the road in your car, there are roads that will lead you there. Same for the railway industry. If you'd like to get to a certain destination, the appropriate train will take a specific route through the tracks. Similarly, for an airplane to get from its point of origin to its destination, there are 'corridors' or highways in the sky that it must follow. This ensures safety, efficiency and a strict structure each flight must follow, just like you in your car or the train or even a ship. Out on the road, they're called 'roadways', for trains they're called 'railways' & in Aviation they're called ... you guessed it .. 'Airways'. It is due to these carefully planned and organized airways in the sky that allow you to travel from your departure airport to your destination airport with the highest safety standard.

What is an airway?
According to the ICAO Annex 11 [1], an airway is a control area or portion thereof established in the form of a corridor. They are also referred to as "ATS Routes" in AIP (Aeronautical Information Publications).

To explain in simple words, Airway is an air route which starts from waypoint A and ends at waypoint B
For Eg: The Q1 Airway in India starts from the Mumbai VOR (BBB) and terminates at Delhi VOR (DPN).

Similar to how the Railway route from Mumbai to Delhi has several stations in between, an airway has several navaids or waypoints in a specific order which defines that route.
For Eg: Taking the previous example, the waypoints which define the Q1 Airway are :
  • BBB (Mumbai VOR) - Start
  • DPN (Delhi VOR) - End
Did you know, Mumbai - Delhi is the 3rd busiest air route in the world with 130+ daily flights!

As discussed before, an airway is defined by several waypoints in a specific order. These waypoints are nothing but a geographical location defined by latitudes and longitudes. A Waypoint can also be a geographical location of a navigational aid such as VOR or NDBs
Eg: For the Airway W141 is defined by
  • Waypoint BIA which has a geographical location of  131225N 0774356E 
  • Waypoint VEMBO which has a geographical location of  133530N 0785000E 
  • Waypoint TTP which has a geographical location of  133806N 0793349E 

Most of the modern airliners have a FMS/FMGS (Flight Management System/Guidance System) which store a database of these waypoints and airways. So , if the pilot enters W141 in the route section of the FMS , it will plot a route from location 131225N 0774356E to 1133530N 0785000E to 133806N 0793349E .

Now since the computers know the exact precise location of the planned route, pilots can fly the airplane to that exact location with help of autopilot. The computers are so advanced that they even give you the exact altitude and even the exact time at which you will cross the waypoint if you are climbing or descending.
Airways/ATS Routes has made flying from one airport to another airport very easy. Since there is a database of predefined airways and the waypoints, it makes it very easy to navigate from one airport to another. 

Advantages for the Air Traffic Controllers (ATC): 
In a busy airspace , most of the airplanes are modern jetliners which have a FMS and are capable of flying the ATS Routes very accurately. This helps the ATC to manage traffic as they are aware of the current position as well as the next intended location of the aircraft. So with the help of secondary radar systems , there is no ambiguity or confusion about the aircraft's location. Technology is so advance today that due to such sophistication , we can predict if there will a mid - air collision hours before it can actually occur.

Types of Airways/ATS Routes as per ICAO: 

1. RNAV5 : 
  • ± 5 NM for 95% of the flight time*
  • Route spacing – Low ATC intervention rate
    - 16.5 NM uni-directional
    - 18 NM bi-directional
  • Route spacing – High ATC intervention rate
    - 10 -15 NM
2. RNAV2 :
  •  ± 2 NM for 95% of total flight time*
  • Route spacing at least 8 NM
3. RNAV1
  •  ± 1 NM for 95% of total flight time*
* Tolerance in deviation from the airway. If its ± 5 NM that means that the airway has a width of 5nm which allows 2.5nm deviation from either side of the track.

New Day , New Tracks : 
There are few airway tracks in the world which change everyday. They are mostly situated over Oceans where is is limited radar coverage. They are designed in such a way that the aircraft have minimum headwinds and maximum tailwinds which increases the efficiency and reduces the flight time. Since the enroute weather cannot be same everyday , these kind of airway tracks are planned and published everyday. 2 of such Airway tracks are

  • NATS - OTS : North Atlantic Organised Track System.
    They connect the North America and the European Continent

  • PACOTS : Pacific Organised Track System
    They connect the Asia and the West Coast of North America

  • AUSOTS :  Australian Organised Track Structure
    They connect Australia with the Indian Ocean

References :
1. ICAO Annex 11 - Air Traffic Services
3. ICAO PBN Workshop

Picture Credits :
1. Wikimedia Commons
3. Flight Service Bureau®

What is Angle of attack?

What is Angle of Attack?

Angle of attack is one of the most important features in an aircraft or airplane because angle of attack effects the lift of the airplane. This angle of attack concept is also used when the plane has to takeoff or land. So, angle of attack is a very useful concept if applied properly in aircraft and airplane.

 What is this angle of attack?

It’s as simple as the name suggest that it is an angle and angle is measured between two lines. So first of all to define the angle of attack and understand it properly. We have to first know about the two lines between which the angle is measured. Here these two lines are known as vectors(It is a line segment with an arrow on one side of line which displays a quantity graphically representing the direction as well as magnitude of that quantity, it’s length of line segment is magnitude of that quantity and arrow facing is the direction of the movement of that quantity ) in technical terms. The first vector is pointing towards the relative motion direction between the body(airplane, aircraft) and the fluid through which it is moving(air). Now what does this relative motion direction means if we analyse the flight of an airplane or aircraft then we see that the direction of the plane flying and direction of the wind striking it is not the same so to sort out this problem we consider a relative motion vector between these directions. Second vector is the reference line (Centreline) of the body of airplane or aircraft. Sometimes the second vector is also taken as chord line of the aerofoil. Here aerofoil is the name given to a particular shape of wing used mostly everywhere in the airplane or aircraft, so the imaginary line passing through the leading edge and trailing edge of the aerofoil shape is known as chord line. The angle between these two vectors is known as angle of attack. It’s a bit confusing but don’t worry we will once again rethink it. The angle of attack is the angle between the chord line of the wing and the vector representing the relative motion between the aircraft direction of movement and the air direction of movement.

 What is the use of this angle of attack ?

As I explained in the first paragraph that it is used for changing the lift of the planes.

How does this angle of attack effects the lift of the plane?

To understand this we have to first understand that how the lift is generated. The magnitude of the lift generated by an object( in our case airplane or aircraft) depends on the shape of the object and how it moves through the air. The lift in the airplane or aircraft is produced by it’s wings. The angle of attack in simple words we can say that is the inclination of the wing to the flight direction of the plane. This has a large effect on the lift generated by the wing.
This phenomenon is used during the takeoff of plane by the pilot when an airplane takeoff, the pilot applies as much thrust( large forward push applied due to engines )as possible to make the airplane roll along the runway. But just before lifting off, the pilot rotates the aircraft  (the aircraft has three axes shown in figure below so the aircraft can be rotated about these three axes by controlling the horizontal & vertical stabilizer provided at the trailing edge of the aircraft) due to this the nose of the airplane rises, leading to increase in the angle of attack and producing the increased lift needed for takeoff.

The angle of attack is determined by calculating the length to width(L/D) ratio of the airplane or aircraft wing as well as body and kept as per requirement to get different outcomes like to achieve good glide , to achieve an increasing lift  etc.      

→For small angle of attack, lift is related to angle of attack
    Greater angle = Greater lift
→For larger angle of attack, the lift relation is complex

Thank you for reading!
Do leave us your valuable feedback!

Why is the fuel stored in the wings of the aircraft?

Why is the fuel stored in the wings of the aircraft?

Why is the fuel stored in the wings of aircraft

Where is the aircraft fuel stored?

Aviation fuel is an important aspect, as it accounts to the aircraft's performance during lift or take-off and also contributes to the additional weight of the aircraft that changes throughout the flight course of an aircraft. For these reasons, it is essential to pay attention to the storage of the fuel in the aircraft. Since history, fuels tanks have been installed in various regions of the aircraft body to attain maximum efficiency; Nose, Main body, Wings etc.

Aircraft Fuel tanks

Main reasons accounting the storage of fuel tanks in aircraft wings

  1. avoids wing failure, by preventing flutter of wings
  2. to control the position of the center of gravity
  3. increased payload capacity and safety
  4. unconventional reasons

Aircraft refueling


Aircraft wings are susceptible to flutter during the flight conditions due to various force and moments developed on the wings. Flutter is the random vibration of the aircraft wings due to the airflow over it. Flutter over larger magnitude is so dangerous that it can even result in total failure or collapse of the wing. When these wings are used as fuel tanks, the weight of the fuel helps reduce the flutterby providing rigidity to the wing.


Centre of gravity is an important factor that affects the flight dynamics and control. Incorporating the wings with fuel tanks keeps the center of gravity more or less in the desired position.
If the tanks are at the nose or tail of the aircraft, there will be a large change of momentum during the flight because of the fuel consumption.
The longitudinal center of gravity is important for an aircraft’s stability and control, and any large variation in its position is not desired for flying the aircraft.
As a method to counter this phenomenon, the fuel is first consumed from the center tank and then the wing tanks. On the other and, during refueling, the wing tanks are filled first and then the other tanks.


Putting the fuel in the Wings also means, you can put more passengers or cargo into the plane without making the plane any larger and not to mention increasing its stability.
It is much safer to store fuel on the Wings away from the fuselage where passengers are seated or the payload is stored, in case of emergencies.


Aircraft wings are meant for the factor of providing lift for the aircraft to fly. They support the entire weight of the aircraft in flight by creating what we call lift. The only factor that affects the performance and working of a wing is its outer shape. The lift causes bending stresses on the wing structure. The increase of the lift pulls the wings upwards, while the reduction of it pushes them downwards. This up and down motion can easily fatigue the wing structure and if unchecked could even cause a break up in the air.
A hollow and a solid wing will have the same characteristics i.e., same lift, and same drag at same flow velocities and angle of attack. That is why most of the Aircraft wings are made hollow, with some reinforcing structures inside the wing which is just enough to support the massive loads acting on the wings, like spars and ribs. Storing fuel in the wings reduces the stresses the wing experiences because the increase of lift would not make the wings go as high up as it did with a let’s say a hollow inside the structure. It is common practice to use up the wing fuel the last if there is fuel in the aircraft center tank or in some airplanes in the horizontal stabilizer. This way the aircraft weight is reduced by the use up of fuel and the wings have to support lesser weight when the fuel inside them is utilized. The placement of engines below the wings also helps to relieve stress.

 In general most of the wings are hollow, so it makes sense to use that space judiciously. Providing some other space for storing the fuel in the aircraft, while you already have empty space in the wings of the aircraft will make the overall aircraft much bigger and heavier, thus making it more expensive to operate and compromising its performance.

Thanks for reading!

Suggested article: How does airplane WiFi work?

How does airplane WiFi work?


How does airplane wifi works

As a passenger of a commercial aircraft, where you are travelling at a height of almost 37,000 feet and with a speed of 500 miles per hour you are too far from cell tower to receive any signal but there are some modern airliners which would help you to deal with this problem by providing in-flight Wi-Fi to keep you entertained throughout your journey. This article deals with different ways of getting an in-flight Wi-Fi connection.


There are mainly three different ways of getting an in-flight Wi-Fi, which differ from each other in bandwidth and internet speed. ATG system requires two antennas to be placed at the bottom of the aircraft, which receives signals from the ground-based cell tower. When ATG system is activated, the aircraft starts receiving signals from different cell towers and these signals are then provided to the passengers via Wi-Fi router.   
The disadvantage of using ground-based cell tower to receive signal is slow internet speed and internet instabilities. Passengers would not be able to use Wi-Fi while the aircraft is cruising over a sea or where the cell towers are not available. The speed offered by ATG is about 3 megabytes per second while a mobile equipped with 4G LTE system could provide speeds up to 35 megabytes per second.


This is a satellite-based technology which could provide internet speeds of up to 50 megabits per second. K stands for a German word “Kurz”, which means short. Here in this context it actually means shortwave frequency. U means that the frequency is lying “Under” the original NATO K-Band frequency range. Original NATO K-Band frequency lies between 20 to 40 GHz. A ground-based transmitter is used to send the signal to the satellite which is received by a special aircraft antenna, after being reflected from the satellite.
Airplane server decodes this signal and then distributes it with the passengers via the Wi-Fi router. If a large commercial aircraft like Boeing 737 or Airbus A320 is considered, where about 150 passengers would like to access Wi-Fi at the same time then this system would still be relatively slow. The problem of time delay arises as the signal has to travel to a large distance. If there are many aircraft transmitting on the same satellite then the internet speed would definitely go down.


It is also a satellite-based system which uses frequency in the upper region of the native frequency i.e. 20 to 36 GHz. “ViaSat” is the satellite wireless service provider which uses “ViaSat-1” satellite to power Ka-Band. It promises to speed up to 70 Mbit/s to all aircraft and is the fastest Wi-Fi service available to the airliners.
Some aircraft use a hybrid receiver, which could switch between Ka-Band and Ku-Band depending upon the best signal strength available. This system is very useful for having a stable internet connection which could even be used to stream movies from Netflix.

Reasons for the ridiculous price for these services:

But these services are provided at a ridiculous price. There are many reasons for the high prices of a flight with onboard Wi-Fi. There is a bulky antenna which is fitted on the top of an aircraft, which functions in a manner similar to that of a TV satellite dish with an additional feature to move and adjust itself with the nearest satellite. It is very expensive to install and maintain this bulky antenna and it also increases the fuel consumption as it results in extra weight and causes hindrance in aerodynamic characteristics.   

Thanks for reading!

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Thrust Augmentation

Accelerating beyond limits: Thrust Augmentation

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.

Thanks for reading!

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Aerial Refueling: Nine Decades of Sheer Innovation and Precision

Aerial Refueling: Nine Decades of Sheer Innovation and Precision

Aerial Refueling is the best thing since sliced bread! Most millennial folks would agree with me on this and for some of you out there who think it is a neologism, do not be alarmed for it isn’t. I assure you, this is going to be as thrilling as you vicariously sky-diving out of a Cessna. But before we dwell deep into the realms of Aerial Refueling, we must first incorporate and inculcate core fundamental aspects of Airport Fueling, Aviation Fuels, and the origin of Aerial Refueling, a concept that is also referred to as Air Refueling, In-Flight Refueling (IFR), Air-to-Air Refueling (AAR) or Tanking. We shall even examine some of the Aircraft used by the Indian Air Force, US Air Force, and the Royal Air Force.

Before writing this article, I had pondered upon what a day in the life of a Fueler at a ground station would be like, let alone develop a perspective on how the scenario would be like mid-flight. And thus, I had stumbled across this intriguing video of Poonai, an Allied Aviation employee who supplies aviation fuel to JetBlue's very own Airbus A380 aircraft. I paid close attention to his immaculate communication with the fuel dispatchers and the pilot and observed how critical protocols (such as Safety protocols, Fuel plans, Inspection procedures, and Record Keeping) were mandatory and imperative to a stable balancing of fuel in the Aircraft’s tanks. The aviation fuel used to carry out the task was the Jet-A Fuel. Jet-A and Jet A-1 fuels are normally used in commercial airliners. Some of the military based fuels used are JP-1, JP-2, and JP-3, etc. These military aviation fuels have higher freezing points that are correlative to the aircraft that fly at higher altitudes. Thus, aviation fuels play a crucial role in the flight efficiency of an aircraft. Fuelers such as Poonai work for approximately 8-14 departures each day depending on the fuel loads carried out and the entire fueling procedure takes about 40 minutes to execute. If you’d like to see the entire procedure, I’d suggest you click on the link below.. immediately!

There are some days that aren't that stressful as a fueler. Take an example below..

What is Aerial Refueling?

After reading the ground procedures of Airport Fueling, ask yourself this, “What would the experience be like mid-flight?”
Back in 1909, a British weekly magazine by the name of Punch had also come up with a humorous illustration (given below) questioning whether Aerial Refueling was a probable feat.


It makes you want to giggle as well as contemplate right? Well, we can now address the definition of Aerial Refueling. It is defined as the method of transferring fuel from one aircraft (the tanker) to another aircraft (the receiver). In the backdrop of World War II, many had witnessed the use of Aerial Refueling in order to carry out various missions. Yes, it is pretty fascinating so as to how people had managed to quickly understand the methodology and immediately use it to their benefits in the time of war. And in the exact words of Richard Barke,

 The highest art form of all is a human being in control of himself and his airplane in flight, urging the spirit of a machine to match his own.
The earliest attempts of Aerial Refueling had taken place in the United States of America. On June 27th, 1923, two Airco DH-4B Biplanes of the United States Army Air Service had conducted the world’s first Tanking. Capt. Lowell Herbert Smith and Lt. John P. Richter was one of the first Airmen to complete the In-Flight Refueling operations. On August 27-28, 1923, it was Capt. Lowell Herbert Smith himself who set an endurance record of more than 37 hours of flight with the help of mid-flight refueling. There was a total usage of 2,600 Liters of Aviation Gasoline, 140 Liters of Engine Oil, and with the help of two other D-4B Aircraft Tankers.


Operations Carried Out

Generally speaking, in the armed forces, Aircraft to Aircraft refueling is a procedure that is customized to provide an extended range of a tactical aircraft that receives the fuel in order further improve its combat potential. This procedure is sometimes susceptible to various problems mid-flight and thus, situational awareness and strict protocols must be catered to in order to react to contingencies. The two main procedures used in Air-to-Air refueling are Boom and Probe-Drogue

Boom /Flying Boom

This aviation manoeuvre involves a telescopic tube that transfers aviation fuel from the tanker to the receiver's receptacle. This telescopic tube is controlled by the aircrew and the operator who generally goes by the term 'Boom Operator' is responsible for administering the flight control surfaces in order to achieve smooth refueling. The term boom, flying boom, and boom operators is generally used in the United States Air Force. Countries such as India generally use the Probe and Drogue method. Given below is a perfect example of a Flying Boom Aerial Refueling mission that is carried out by Utah Air National Guard member Sargent Major Jason Blood, the Boom Operator on board the KC-135 Stratotanker (a tanker extensively used by the U.S Air Force). The receiving aircraft is the infamous C-17 Globemaster III.

Important Note: Flying Boom refueling operations are carried out on one aircraft at a time but at a faster pace.

A boom operator's Boom Pod  used for Aerial Refueling operations

Probe and Drogue

As the name suggests, this mid-air refueling method uses a probe (a versatile retractable, protruding arm placed on the Aircraft's nose or fuselage) and a drogue (a basket-like structure that resembles a shuttlecock; it is connected to a flexible hose with a narrow valve). The probe is always on the receiving aircraft and the drogue (sometimes operated by a boom operator just like in flying boom or situated inside the wings of an aircraft) on board a tanker aircraft. In order to carry out this operation, the tanker and the receiver must be aerodynamically in sync with each other along with the drogue/hose protruding out of the aircraft based on stable air conditions. The pilot of the receiving aircraft controls the flight in order to establish a connection with the probe and the basket-like structure. In order to vividly explain the concept, we can examine the following probe and drogue mission carried out by the Russian Air Force. The given task is carried out on a fleet of Sukhoi aircraft (Su-24, Su-30, and Su-34) with the assistance of an Ilyushin IL-78 Tanker. The IL-78 Tanker is also utilized by the Indian Air Force as well.

What are the differences between Flying Boom and Probe-Drogue methods of Aerial Refueling?

Compared to Probe and Drogue, Flying Boom method is applied for a larger fuel consumption of an aircraft. But unlike the probe and drogue, flying boom cannot be used on slower moving rotorcrafts as it moves at higher speeds. Also, Probe and Drogue method can be carried out on two different aircraft simultaneously as shown in the figure below.

The Indian Air Force generally refer to this process as MARS (Mid Air Refuelling System). As you can see, we have two Sukhoi MK-30 Aircraft in sync with one another and with the assistance from the IL-78 Tanker.

More images from around the world
The Sikorsky CH-53E Super Stallion helicopter carrying out a Probe and Drogue operation.

A retractable refueling boom on board the McDonnell Douglas KC-10 Extender ready for operation,

The Royal Air Force's very own Victor K2 Tanker, a strategic air bomber is used for various in-flight refueling operations. If you carefully observe the different hoses protruding out of the aircraft, you will notice two of them are smaller compared to the big one. The smaller ones are utilized for smaller aircraft and the bigger belly hose is used to supply fuel to larger aircraft.


If you'd like to see more exciting videos on Aerial Refueling procedures, click on the links below!

Until then, stay Turbocharged!

Air Refueling procedures between MC-130J Tanker & UH-60 Helicopter

A-10 Thunderbolt II Refueling

B2 Aerial Refueling Procedures

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
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!


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

Thanks for reading!

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