What is the dutch roll?


What is Dutch Roll?

It’s the 7th of June 1967, the six-day war is unfolding on the ground. But our eyes stay fixed to the skies where the swept wing greats, the Mirage 3 battles it out with the MiG 21 in a dogfight which is sure to make it to the books. Diving into the battlefield from the cloud ceiling, Giora Romm in the cockpit of the Dassault Mirage 3 engages in a fight for air superiority against the MiGs. A few minutes in, he is seen with an enemy MiG on his tail shooting for the kill. He then sets the Mirage to rock about its axis, to dodge the gunfire and escapes the danger successfully. PAUSE.

Dutch roll- Mirage 3
Hey, wanna check out my cockpit?

What he did there,  is a brilliant maneuver in military aviation, called The Dutch Roll.
This might sound like a mouth-watering puff pastry which tests the prowess of some cute dutch bakers, but all it tests is the roll and yaw stability of an aircraft....which is way cooler.
Before we go into how a dutch roll happens, let’s look into a couple of crazy circus acts our aircraft can do:
Dutch Roll - Aircraft Moments
The rotational motions of an aircraft
Rolling (banking) – Aircraft rotation about the longitudinal axis. This can be brought about by one wing having a higher lift than the other wing. (ailerons jutting in opposite directions).  For example, if the left wing has more lift, right-wing tilt down and the aircraft rolls right 

Yawing- Aircraft rotation about the perpendicular axis. Rolling and yawing come as a package deal. So your aircraft rolls left. The lift vector which was initially perpendicular to the wing jutting upwards is tilted to the left. Thanks to vector algebra, this tilted lift vector has a horizontal component associated with it called Sideslip (somebody makes this a dance move already). This skids/ yaws the aircraft in the direction of the roll in our case yaws left. 

Pitching- Aircraft rotation about the lateral axis. Can happen when the elevators on both the wings are operated in tandem. We don't need pitching motion in the dutch roll so it isn't explained here. But feel free to check out how pitching works. Knowledge is boundless. 


Put on your imagination caps so I can paint you this three-dimensional moving picture.

Dutch Roll - Swept angle
Rolling right creating the horizontal sideslip
Assume you roll your Mirage right, and the lift vector tilts right. The horizontal component of the tilted lift, sideslip yaws the aircraft to the right. This yawing causes the free stream velocity or the velocity of the wind to hit the wing at an angle. Now, pay close attention to the sweep (Wing swept angle) of that beauty. (learn about swept back wings here: yay information) This angle at which the wind hits the wing causes more chordwise component of wind on the right wing. (lift which matters is only generated when the flow is parallel to the chord. Go learn about lift here: click diz ). So more air is flowing just how we want it (parallel to the chord) on the right wing and not as much on the left. This translates to more lift on the right wing than the left. Still with me? This unbalanced lift, higher lift on the right side and lower on the left creates another roll, except now it's to the left. The right wing with more lift brings with it drag. This is the lift-induced drag explained in our article on the types of drag. : Drag your butt over here 

Dutch Roll - Swept Wing
Yawing right, the relative wind hits the leading edge like this
This right wing drag pulls the plane right (yaws right). This is when the vertical stabilizer (fixed vertical wing on the tail end of aircraft) does its thing by reversing this yaw, bringing it lined up with the nose. Remember our Mirage is still rolling left? This induces a left sideslip. This gives a yawing to the left. And this goes on as a repetitive rocking and rolling motion until of course the rudders are employed to bring it back to stable flight. You can think of the Dutch roll as you when you're 5 drinks in, and the rudder is your sober best friend. 

Although military aircrafts are thrown into dutch roll intentionally, their commercial cousins go into a dutch roll due to turbulence or external disturbance. They are not built for such instability: because that's what this is,  an instability condition. Which is why we say fighter jets are built for maneuverability (aka instability) while commercial aircrafts are built for stable level flight. (if they weren't our flights would be one hell of a ride). In this constant tradeoff between stability and maneuverability, as a design engineer you have to ask yourself this question: 

Is your aircraft equipped to rock and roll? 

Thanks for reading!

What is the Magnus effect?

What is the Magnus effect?
Magnus Effect
They see me spinnin', they hatin'. 

Spinning Cylinders

In 1930, Plymouth A A 2004 flettner airplane used a bunch of spinning cylinders (flettner rotors) to achieve flight. This puts forth the question: CAN SPINNING CYLINDERS FLY?

By the Magnus effect, that is a possibility.

Magnus Effect

Almost a hundred years before this flettner aircraft took flight, in 1852, Heinrich Gustav Magnus was working on projectiles from firearms. The bullets seemed to take a deflected path rather than the destined path. Pissed off, he grabbed the bullet by the collar and asked it what the hell it was doing. “It’s the pressure difference man! Look I’ve got a family to feed ” said the bullet. Thus he proposed the Magnus effect. True story.

Fast forward to the 20th century, there came a revolutionary theorem barging into the abode of aerodynamics and that was the Kutta Jowkowski theorem. Kutta and Jowkowski sat together and whipped up some dank mathematics to prove our man Magnus’s effect.
As opposed to going into the mathematics of the Magnus effect, and analytically proving its existence, we’ll look into the physics of it and see what sort of sorcery it is.  
Magnus effect
Spinning cylinder
Suppose a cylinder is spinning in the clockwise direction. There’s going to be attached flow right above and below the cylinder creating a boundary layer. (layer where friction effects are felt). After this attached flow, there’s going to be flow separation in the aft of the cylinder (One could say after but Aircraft Nerds like cool words). This flow separation creates low pressure vortices. (The physics behind this attachment and separation has been elaborated in the article on Coanda effect linked below) Let’s step outside the boundary layer for a bit and examine the velocity above the cylinder. It’s going to be higher than the velocity below the cylinder. Why? Because it spins. Any spinning object rotates about it's axis by an angular velocity. This angular velocity is induced by the velocity acting tangent to the circle at every point. Over the top of the cylinder, this clockwise spin or rotation creates a tangential component of velocity which is added to the free stream velocity (velocity of the flow V which is a constant at a distance from the cylinder) creating a velocity Vtop. Why is it added? because the flow velocity and the spin velocity are in the same direction. And in the bottom of the cylinder the velocity is reduced from the free stream velocity (V) creating a lower velocity Vbottom. Why is it subtracted? Because the flow velocity and the spin velocity are acting in opposite directions. So the velocity over the top is greater than the velocity below. 

Magnus effect Angular velocity
Tangential velocity over top > Tangential velocity below

Bernoulli: “That makes the pressure over the top lesser than the pressure below the cylinder!”

What he said. So as mentioned in the linked article on Coanda effect, flow moves from high pressure to low pressure. This difference in pressure creates a force which pushes the flow in that direction. In our spinning cylinder, there’s high pressure below the cylinder and so the force acts upwards. This upward force is, drumroll, please…
Or Magnus force.
 Now look at the aircraft again, see those spinning cylinders separated by discs? They're flettner rotors. These flettner rotors take advantage of the Magnus effect to generate lift.
Random cricket fanboy: Curiouser and Curiouser..
Related image
Topspin on a cricket ball
Random cricket fanboy: This sounds an awful lot like topspin and backspin in cricket.
Right you are random fanboy, right you are.The Magnus effect explains the deviations of spinning balls in ball sports like golf, tennis, baseball and ofcourse cricket. In top spin the ball goes down because the Magnus force acts downwards. In backspin the ball rises.

Now you know how to throw a killer serve with physics.

Click here for more enlightenment: What is the coanda effect?

What is coanda effect?

Why are fluids clingy? Coanda Effect? 

Lift on an airfoil is a by-product of the attached flow over the airfoil. (this is explained in the article on lift linked below). But how do the air molecules know that they have to climb up the curvature and follow the contour of the airfoil. Is there a rule in nature which says that being clingy is socially acceptable behavior?
Yes, in fact, there is, and it's called the Coanda effect.
Image result for streamline flow airfoil
fluids- where an attachment is not a disorder

What is the Coanda effect?

It is the tendency of a jet of fluid (gas/liquid) to curve towards a surface. In other words, it's the attachment complex of a fluid. 

Why are fluids clingy? Let's delve into physics behind the Coanda effect.

Setting: a stream of water flows over a convex object (works on concave objects as well)

Fluid: anything that has molecules not too closely packed to be considered a solid.

Bernouilli equation: energy equation that says if pressure increases (pressure energy), velocity decreases (kinetic energy) and vice versa. So as to maintain constant energy.

Image result for pressure in a container
Pressure on the walls of a container
Hydrostatic pressure: The pressure that comes out of particles in the fluid bombarding against one another. The pressure at a point in a non-moving fluid is directly proportional to the height of the fluid above it. It can be thought of as the pressure exerted by the fluid on the walls that restrain it.
 Continuity equation- the equation that says an area is inversely proportional to velocity. (If you wanna remember this: picture a fat man run.
Image result for boundary layer
Viscosity prevails only in the boundary layer
Viscosity: the friction that exists between the layers of the liquid. When the fluid flows over boundary the boundary acts like it’s deadbeat husband, holding it back, not giving it the permission to go as fast as it can. So the layer above it has significant velocity decrease(for subsonic flow). The layer over that is a little faster, and so on, until it’s out of the boundary layer, and the fluid layer moves as fast as it can. Viscosity tells you all that you need to know about a dead weight, the further you move away from them, the better your life can be.

Entrainment: The movement of one fluid by another. (can exist between two layers of the same fluid at different velocities). You can visualize this concept as how an audience sways to a musician’s tunes. standing awkwardly not knowing what to do one minute, to rhythmically shaking what your mother gave you the next. It’s not magic, it’s just emergent behaviour….but yeah it’s pretty close to magic.

Movie:  Tiny little fluid particles are on their way over the curved surface. So at time t=0 they listen to inertia and they don’t curve downwards around the curve, they move on down the same line (they don’t get far but wait for it). This moving layer entrains the layers right below it (past the curve). This entrainment causes the layers to leave that region leading to a reduced pressure there. Why reduced pressure? where there is a lack of fluid particles (owing to the fluid layer eloping with it’s second cousin), there is a lack of pressure being exerted on the walls of the surface. (Like when you’re finally done with high school ,but you feel a pang of loss for some reason). At this point in time, there’s a higher pressure on top and lower pressure below, this pressure difference creates a force which acts downwards. Force gives you displacement so this facilitates the movement of the fluid to move from high P to low P curving downwards.

Crazed fan’s plot interpretation on the pretentious blog: All of this happens at t=0.0000000000001, and so you don’t really see them trying to move down the same line. They get curved by the fluid military (force arising from pressure difference), leaving the law of inertia in a blubbering mess. Valiant attempt, my little-wet friend.

Click over here, go on, I dare you. How do wings generate lift?

Do aircrafts have horns?

Do aircrafts have horns?

In a machine which has evolved over time to be as sophisticated as the modern day aircraft, you’d expect there not to be annoying horns like the ones that exist on its ground cousins. (no disrespect to cars but it had to be said). But they do exist.

Why? Why do they exist? – a sincere question from a concerned migraine

For one, It’s to communicate to the ground crew. An alert like a horn could get them to put on their headset.
Also, an aircraft is pretty big. It can be as long as 60 m which are 60 adult human legs put together.

 F-16 looking fly 

The cockpit needs to communicate the essential information received from the various parts of the airplane. Contrary to popular belief, it can’t be achieved by having monitoring minions in the engine yelling THE PLANE’S GOING DOWN. That’s not going to sit well with the anxious aunty who brought her own snacks in the economy cabin. So a series of sensors are set up at various critical parts of the aircraft and they transmit information via cables to light and sound signals on a panel in the cockpit called the master annunciator panel.

 master annunciator panel 
This panel has a set of lights – red, amber and blue. When a system has a fault, depending on the magnitude of the problem, the appropriate lights illuminate.

Red- Caution, needs immediate action (YOU.)

Amber- Warning, needs crew’s immediate awareness

Blue- advisory, to let crew know that something is working as it is supposed to

But it’s not just visual cues, the (CAS) crew alerting system sets up auditory and tactile cues as well just for good measure. Hold up, what cues?

Auditory cues or in simpleton's tongue: Horns. And Other stuff.
"Delta four five one, winds two eight zero at eleven, cleared for takeoff." Young pilot grant just having been given permission, takes off from JFK and heads east towards Heathrow. After cruising at 14,000 ft, he climbs to 15,000 ft and is given a single chime of validation. Grant sighs taking in a deep breath. Working on little sleep and overwhelmed with exhaustion he switches over to autopilot. Maybe a little shuteye? PULL UP! PULL UP! Woken with a jolt, he gains control of his senses enough to know that Delta 451 was fast descending to a nearing hill. Just as he begins taking action, CLIMB! CLIMB! As if a series of unfortunate events were to follow, a timed beep beep beep beep and a series of horns successively bombard his auditory cortex. Hanging on to the single thread which holds his sanity, Grant realizes he can’t do it alone. As the co-pilot, you need to step up to the occasion (considering you were watching in awe as almost every system was gloriously failing before your eyes). But how can you help, if you don’t know what the sounds stand for?
TCAS and GPWS- voice warnings like pull up! Climb! For traffic or impending collision, or warning the closeness to ground.

goddammit grant, you had one job

 (Cool fact: TCAS and GPWS have been nicknamed bitching betty and the first ever voice to be digitized for this system was Kim Crow)

Pressurization: A continuous horn is noised when there are cabin pressure issues.

Configuration: A horn at intervals or a beeping sound when the control surfaces like flaps, slats, stabilizer trim, or speed brakes are not properly configured before takeoff.

Landing gear: A horn sounds and appropriate gear position indicator lights illuminate when an unsafe gear configuration exists.

Altitude: A single chime, along with a visual cue, alerts pilots when they are leaving the current altitude or approaching a new one.

Autopilot disconnect: Various kinds of siren, klaxon, or chime sounds, accompanied by red warning lights, signal that the autopilot has disconnected.

Engine or APU fire: FARs require that engine and APU fires be indicated by a bell accompanied by red fire warning lights.

Overspeed: An over speed "clacker" sounds when a limiting mach or airspeed is exceeded.
There you have it. No time to waste, young pilot grant needs you. Godspeed.

Thanks for reading!

What is turbulence?

                                       What is Turbulence?

Now anyone who has a background in the field of aeronautics or has ever traveled by air has definitely come across the term 'turbulence'. It is generally associated with the unsteady flight of an aircraft. In the following article, we're gonna clear some commonly existing myths about turbulence and make you understand this chaotic phenomenon. 

Types of Flows:

Here we shall discuss the three types of air flows based on streamlines.
  • Laminar Flow - All the streamlines move smoothly over one another and also parallel. The flow is smooth.
  • Transient Flow - In this flow, the streamlines are time-dependent. Their direction varies over time.
  • Turbulent Flow - All the streamlines collide with one another. This results in an unpredictable and unfavorable distribution of streamlines.

What is Turbulence?

With relation to Fluid Dynamics, a flow is said to be turbulent if the streamlines do not flow smoothly and parallel to one another, but rather flow in a random and unpredictable manner and cause a significant change in the pressure.

With relation to Reynolds Number, a flow is said to be turbulent if its Reynolds Number is over 4000. 
In simple terms, turbulence is a random disturbance which is caused due to various phenomena that greatly affects the direction of the flow of a fluid (Air or Water) due to which random and unfavorable changes in energy occur. 

Diagrammatically it is represented as 

Turbulence can be segregated into further subgroups,

  • Clear-Air Turbulence It is caused by variations in the jet stream. It ramps up in winter, when the jet stream — zippy air currents in the Earth's atmosphere — migrates south, and often plagues flight paths over the Pacific.
  • Convective Turbulence - It is created by thunderstorms and often occurs in the summer.
  • Low-Level Turbulence -  It is associated with strong winds, terrain, and buildings.
  • Wake Vortex Turbulence -  It is defined as turbulence which is generated by the passage of an aircraft in flight. It generates from the tip of the wings. Because there is a pressure gradient between the two surfaces of the wings, the air from below flows up at the tip of the wings.
  • Mountain Wave Turbulence - The flow of air near mountainous regions is highly turbulent because the mountains act as an obstruction to the airflow.

            What Happens When a Plane Encounters Turbulence?

To understand that we must first understand how a plane's wing generates lift. They effectively cause air on the upper surface to flow significantly faster than the air on the lower surface. The flow is mostly streamlined and laminar in nature. This causes the pressure below the wing to be higher than the upper surface, thus generating a lifting force. Now when the air flow turns turbulent, the pressure over and under the wing changes in a rather sudden and abrupt manner causing unfavorable lift distribution over the wing and hence resulting in the jerky motion of the plane. Plane crash due to turbulence is very rare. A pilot can effectively receive a prior warning before entering a turbulent air zone. Here are a few ways as to how,

  • Eyesight - The most obvious way is just by looking outside and observing the sky. Large billowing clouds, called cumulus clouds, indicated pockets of unstable air (the clouds are rising because the air under them is as well). If the pilots must fly through these clouds then it's a safe bet that there will be some turbulence.
  • Weather Radar - Just like using your eyes, except the radar can see further through the haze and other clouds. Typically this is useful for finding embedded thunderstorms, but it can also be useful to find areas of potential turbulence.
  • Communication - Pilots talk. Both to each other and to air traffic controllers. En route controllers frequently ask pilots for PIREPS (pilot reports), to build an accurate picture of the flight conditions at different altitudes. Often commercial aircraft will request to change altitudes or deviate around weather/ turbulence, so it is in the best interest of the controller to know ahead of time where the bad flight conditions are and have a game plan of how to route traffic. This makes it much easier for the controller to route traffic, rather than getting request after request from individual aircraft.

    A plane can also encounter turbulence when it transitions from subsonic speeds to supersonic speeds, but we shall discuss that later in our upcoming posts on Supersonic Flows.

    So, the next that you're traveling by air and you hear the captain announce "Hello ladies and gentlemen, strap on your seatbelts. We're gonna encounter some turbulence." Be calm and relaxed, there is absolutely nothing to fear. 
    Planes are engineered to take a remarkable amount of punishment, and they have to meet stress limits for both positive and negative G-loads. The level of turbulence required to dislodge an engine or bend a wing spar is something even the most frequent flier won't experience in a lifetime of traveling.

    Thank you for reading!
  1. Wikipedia
  2. Tampa Bay Times

Are commercial electric planes possible?

      Are commercial electric planes possible?

commercial electric planes
Electric Plane - Wikimedia

You may have heard about Tesla cars which run on electricity and have zero emissions. Ever wondered can the same be possible with planes? Well, the concept of electric aviation is not new. You may be astonished to know that the world’s first electrically powered flight took in 1883. Albert and Gaston Tissandier showcased the world’s first electrically powered aircraft by coupling an electric motor to an airship.

Why are electric planes needed?

The main reason for the development of electric planes is the adverse impact on the environment caused by conventional plane emissions. It is found that out of total Global Greenhouse emissions aviation CO2 emission is 2%. The amount of CO2 emission from aviation is growing by 3-4% per year. US-based aircrafts are responsible for 29% of all greenhouse gas emissions from commercial aircraft worldwide according to EPA. Also, the penalties on greenhouse gas emissions are increasing and stricter emission regulation standards are pushing electric aircraft development.

The European community has set pollution reduction targets of 75% CO2, 90% NOX, and 65% for noise. Electric planes are better because they have no direct emissions. Electric planes are also quieter in operation. There will be no noise produced by electric planes in airports and inhabited areas. The other fact is that the optimum performance of a jet engine is decreased when there is an increase in altitude and temperature. Electric plane’s performance does not depend on environmental factors. Since electric planes work on electricity they don’t carry flammable fuel and thus are safer than conventional planes.

How electric planes work?

In conventional planes, the jet engine sucks the air which is compressed or squeezed in a compressor. This increases the temperature and pressure of air which makes it suitable for mixing with fuel. The fuel is injected from injection holes in the combustion chamber which mixes with the incoming air. This air-fuel mixture is then lit. Now, this high-pressure, high-temperature mixture (fluid) passes through a turbine which reduces its pressure. Due to the reduction in pressure of this fluid, it's velocity increases and it leaves the jet engine with high velocity. This high velocity is responsible for generating the thrust and makes the plane move forward.
Jet Engine
    Jet Engine-Wikimedia
On the other hand, the working of electric planes is rather simple. When it comes to electric planes the power source need not be compulsory batteries. Solar power can be used as power source. In case of battery powered planes, the batteries run an electric motor that spins a propeller. This spinning produces the thrust required. Yup as simple as that! Electric engines are thus mechanically simpler than the jet engines. This reduces their maintenance and performance monitoring requirements.

Problems with electric planes

            The question now arises that even though electric planes have so many advantages why don’t we see any flying over our heads? The main problem is weight. Let me put it in this way. When you travel by train you don’t have limitations on the weight of luggage you carry. In the crowded country like India, people travel by sitting on train roofs. Although you cannot travel by sitting on plane wings, the fact is there is a limitation on the maximum weight of luggage for traveling through planes. Why is this so? 

Planes have a maximum takeoff weight. If the weight is more than this it cannot take off and will keep running on the runway (actually it may damage the plane and is dangerous). So how is this related to electric planes? As mentioned earlier electric planes can be powered by

  1. Battery Power
  2. Solar Power

The thrust required for the plane is very large and thus will require more numbers of batteries. This concept is related to energy density. Energy density is the amount of energy stored per unit volume. The energy density of batteries is far less than fuel. It means that for the same amount of energy generated more numbers of batteries will be required than the fuel. As more battery packages are needed to be installed the weight of plane increases. Li-ion battery energy density is 300 Wh/kg or 1.08 MJ/kg while the energy density of typical aviation fuel is 44.65 MJ/kg. Research is being done to develop batteries having energy density of 400 Wh/kg.

The other problem is that airplanes also have a maximum landing weight. In conventional planes the fuel is burnt till it reaches its destination and thus the weight of the plane is reduced. Also a plane on its way to the destination burns fuel due to which the weight of fuel carried by the plane gradually goes on reducing while in air until it completes its journey. This reduction of weight due to burning of fuel also increases the efficiency of jet engines.

This is not possible in case of batteries. Batteries once used remain in planes and contribute to the overall weight of the plane and as a result of this the plane has to fly with the weight of used batteries.

The other method of powering the electric plane is by solar power. The batteries generate power by solar energy and thus drive the motor. But we cannot fully rely on this as weather conditions are different throughout the year.

Attempts made by companies for electric planes

The disadvantages of electric planes haven’t stopped some companies to develop one. Although electric planes do exist but their weight carrying capacity is less which is mostly upto 2-4 persons per flight only. This no is far less than that of commercial conventional plane flights which has a to take on an average 150 passengers per flight.

German company Siemens has developed propulsion system for Extra 330LE electric plane. The plane has a very powerful drivetrain. The motor which weighs only 50 kilograms develops a power of 260 kw which is equivalent to 350 horsepower. The aircraft is able to reach a world record breaking speed of 340 km/hr. Although these numbers don’t sound impressive but the important thing is the plane burnt no fuel and emitted zero emissions.
  Siemens Extra 330LE electric plane
  Siemens Extra 330LE electric plane - siemens
Solar Impulse 2 is another such aircraft financed by Swiss businessman and pilot Andre Borschberg. As the name suggests this aircraft runs by power generated through solar cells. The batteries are used to store the power generated. It is a single seater aircraft and contains 17000 solar cells. These cells are mounted on the wings of the plane. The plane is still under testing.

 Solar Impulse 2 - Wikimedia
 Solar Impulse 2 - Wikimedia
The Airbus Group’s E-Fan is a two-seater electric aircraft and first took flight in 2014. It has a top speed of 220 km/hr. The E-Fan has been upgraded to E-fan Plus version which has a hybrid configuration for longer flight endurance. Hybrid implies that it uses an internal combustion engine in addition to its on-board lithium-ion batteries.
Airbus Group’s E-Fan - Wikimedia
With the current developments in technology will we be able to travel in commercial electric planes? 

Probably not so soon. As mentioned earlier the Solar Impulse 2 uses solar power. However its flight is made possible only because it is light in weight. The technology is still not ready to power commercial planes which are more heavier. In order to have electric planes in the commercial market, we need better batteries. The current batteries in the market are lithium-ion batteries which have replaced the lead-acid batteries. To power a plane we need batteries which deliver more power simultaneously being small and light in weight. Scientists say that those kinds of power delivering batteries will be available in a decade and only then we will be able to travel in electric planes having tickets cheaper than those running on fuel.

Thanks for reading!  

Why do planes dump fuel?

Why do planes dump fuel?

Why do planes dump fuel
Why do planes dump fuel? - Wikimedia

Fuel cost represents one of the biggest expenses for the aerospace and airline industries. On an average fuel costs account for 29% of all operating expenses. So, why do planes dump fuel? Before answering this let me tell you that this is not done frequently and is done only in emergency situations, like mechanical failure or a medical emergency.

In technical terms, fuel dumping is known as fuel jettison

Aircrafts have two types of weight limits: 

The maximum structural landing weight is always less than the maximum takeoff weight. Now one might think that taking off at a heavier weight would be more difficult as opposed to having to land at a lighter weight. However, this is not the case, as landing puts more stress on the plane.

Why do planes dump fuel?

Fuel Jettison Duct
Fuel Jettison Duct - Wikimedia
One of the factors increasing the weight of an airplane is the fuel stored in it. Now when an airplane takes off, its weight is under the limit of the maximum takeoff weight. The amount of fuel in the plane is such that most of it will be consumed as it reaches the destination thereby decreasing the weight of the aircraft and bring it under the maximum landing weight. 

But in the unfortunate situation of an emergency in which the plane has to land suddenly it is not possible to do so without dumping the fuel . This is because the weight of the airplane is higher than its maximum landing weight and hence it is far too heavy for a safe touchdown. Thus fuel is dumped so that the weight of the plane becomes safe for landing.

In certain situations, dumping of fuel will prove to be cheaper than not dumping it. If an airplane is made to land at a weight higher than its maximum landing weight then it can cause structural damages primarily in landing gear and also to the airframe structure. Sometimes planes can suffer severe structural damages that can take months to repair or worse yet, be irreparable. The plane can even break apart on landing. Thus fuel dumping is not as wasteful as it seems.

Fuel dumping is a simple procedure. Fuel is stored in the hollow wings of the plane. The fuel is jettisoned from nozzles which are located in the wings. The pilot performs a three step process to engage the plumbing and start dumping fuel. 

Plane fuel tanks
Plane Fuel tanks - Wikimedia
Sometimes there occurs an emergency in which there is no time to dump the fuel. In such cases, the pilot has to take the risk of overweight landing. Jets flying with US airlines in the 1950s and early 1960s tended to have fuel dump systems. However, most of the planes developed today are designed by considering the maximum overweight landing. 

For example, Boeing 757 has no fuel dump capability as its maximum landing weight is the same as its maximum takeoff weight. Larger, wider body planes like the Boeing 777 and 747 can dump fuel.

What happens to the dumped fuel?

Since the fuel is dumped in the sky, do we need to hide under roofs when a plane goes over us to protect ourselves from it? Well, the answer to this is NO. Fuel dumping is usually restricted by altitude, that is, fuel cannot be dumped below a particular altitude generally 4500 feet (1371.6 meters). Dumped fuel flows behind the plane like a contrail

Most of the fuel evaporates into the atmosphere before reaching the ground. The evaporation of fuel depends on several factors like altitude, air temperature, and dumping pressure .Technical advancements lead to the development of systems which are installed in planes which help to vaporize the fuel stream to a large extent so as to aid evaporation. 

F111 burning dumped fuel
F111 burning dumped fuel - Wikimedia
Air shows sometimes include dump-and-burn which is a type of fuel dump in which the dumped fuel is burnt by using plane’s afterburner. The dumped fuel burns and produces a tail of fire. However, Dump-and-Burn is banned in the USA as fuel costs a lot of money and to dump it would be a criminal waste.

How long is the fuel to be dumped for?

So how does the pilot decide how long the fuel dump is to be performed until the plane reaches appropriate landing weight. This can be explained by an example
Boeing 777
Boeing 777 - Aircraft Nerds
For Boeing 777

Maximum takeoff weight = 247,200 kg
Maximum landing weight = 201,840 kg
Weight to lose = 45,360 kg or approx 46,000 (46 tons)

The fuel dumping rate for Boeing 777 is 2 tons/min

Fuel burn rate = 6 tons/hr
                       = 0.1 tons/min

Thus fuel dumping rate + Fuel burn rate = 2.1 tons/min

To lose 46 tons the time required will be 46/2.1 = 21.9 min

So the pilot will need to perform fuel dumping for approx 22 min to reach the appropriate landing weight.
Generally landing with less fuel is better than to land with more fuel. Fuel makes up a very large proportion of its total weight and it’s inefficient and uneconomic to fly around with unneeded fuel. The point of dumping fuel is to decrease the weight of the aircraft to the point where it is safe to land. Large planes with a full load are too heavy to land safely, however, if it is a serious emergency like a fire or something similar, it has to be done.  

So to answer the question, dumping fuel is not routine and though fuel dumping leads to wastage of loads of gallons, it needs to be done in emergency situations to protect the plane from any harm and also the people traveling in it.

Thanks for reading!