The Physics of a Perfect Landing

 Contrary to popular belief, a 'perfect' landing isn't the smoothest or softest possible touchdown. In fact, relatively firm landings may be more desirable in certain situations. A perfect landing is demarcated by a stable approach followed by a flare, with the aircraft touching down in the designated landing zone at the correct speed and alignment. Upon landing, the weight of the aircraft should be transferred to its wheels, and available deceleration systems should be deployed immediately.


However, achieving this seemingly simple sequence proves to be far more intricate and convoluted than one may suspect. Each landing is influenced by interactions among wind speed and direction, airspeed, descent rate, aircraft weight, runway length, and weather conditions. A slight change in any of these variables can alter an aircraft's approach significantly.


Key Concepts

1. The Difference Between Airspeed and Groundspeed: Airspeed refers to the speed of the plane relative to the surrounding air, while groundspeed refers to the plane's speed relative to the ground.


2. Glide Path: Glide path is an airplane's intended trajectory towards the runway during its descent. It is denominated by an angle, generally 3°.



3. Descent Rate: Descent rate, also known as vertical speed, describes how quickly an aircraft loses altitude. It is measured in feet per minute.

4. Stabilised Approach: An approach in which the aircraft is flying within its prescribed parameters for speed, pitch, flight path, etc.

5. Flare: The maneuver that occurs directly before landing in which the pilot pitches the aircraft's nose up. 



6. Ground Effect: The change in the aircraft's aerodynamic behaviour when it flies close to the ground.

The Final Approach

1. Setup

A good landing begins before touchdown. The aircraft must arrive at the flare with the right speed, energy, and configuration, which can only be achieved with the help of a perfect setup.

During the approach, the aircraft has two important forms of mechanical energy- kinetic and potential:

(I) P.E. = mgh
(II) K.E. = ½mv²

Height must be managed during descent, while speed must be controlled and eventually dissipated during landing.

 If an airplane approaches too fast, it carries excess kinetic energy. The higher airspeed can cause it to float longer during the flare, while the higher groundspeed catalyses more energy needing to be lost to bring the aircraft to a halt.

Conversely, if an aircraft approaches too high, it needs to lose more altitude before reaching the intended touchdown zone. If the pilot cannot correct the flight path, the aircraft may land too far down the runway.



2. Flare

As the aircraft is approximately 10 feet above the runway, the pilot initiates the flare maneuver. He or she pitches the nose up, thereby increasing the wing's Angle of Attack (AoA), creates a greater lift coefficient. This allows the plane to maintain sufficient lift while reducing the airspeed and rate of descent, setting up a softer and safer touchdown.

As the aircraft nears the runway during the flare, ground effect comes into the picture. It reduces induced drag which allows the plane to maintain its speed easily. If the aircraft enters the flare too fast, it may float above the runway, causing it to touchdown far beyond its original touchdown zone.


3. Touchdown

After the flare, the aircraft's main landing gear makes contact with the runway first, followed by its nose gear. At this stage, two factors emerge as important- where the wheel touches the runway and how much vertical speed the airplane carries at contact.

The touchdown point is imperative because every metre the aircraft floats above the runway is one less metre of runway for the aircraft to stop safely.

 For example, on 7 August 2020, Air India Express Flight 1344 floated over the runway for an extended period of time at CCJ Airport, eventually touching down almost halfway along the runway. Tragically, with a wet runway and severe tailwinds, the aircraft overran the runway, claiming 21 lives.

4. Deceleration

After the aircraft touches down, it still carries immense kinetic energy. It needs to rapidly decelerate in order to be brought to a stop. Three systems play imperative roles in this process:

(I) Spoilers: Located on the wings, spoilers deploy to reduce lift and increase drag. They transfer the aircraft's weight onto the wheels and thereby improve braking effectiveness.

(II) Wheel Brakes: Wheel brakes use frictional force to convert the plane's kinetic energy into heat.

(III) Reverse Thrust: Reverse thrust redirects engine thrust to the direction opposite of motion, thus helping slow down the aircraft.

Kinetic energy is proportional to the square of speed, so even a diminutive increase in speed can make losing kinetic energy far more difficult.

Go Arounds

Sometimes, the safest landing is one that never happened. A go around is a maneuver in which pilots abort a landing attempt, climb back to a safe altitude, and prepare for another approach. 



A go around may be necessary for a plethora of reasons- an aircraft may be approaching too fast, it may be too high, it may be improperly aligned with the runway. Climatic conditions, like windshear, may cause go-arounds too. Instead of trying to correct an unstable approach close to the ground, which could be extremely dangerous, pilots opt for a go-around.

A go-around may be initiated in almost any phase of an approach- as the aircraft approaches the runway, during the flare, or even after it has briefly touched down.

Challenging Landing Conditions

Treacherous climatic conditions make an aircraft's approach even more complicated. Crosswinds push planes sideways, causing pilots to have to correct for drift and maintain runway alignment. Gusty winds cause sudden changes in airspeed and lift. This makes controlling the approach and flare even more difficult.


Another factor is rain. Wet runways have relatively lower friction, which reduces the effectiveness of braking. In certain severe cases, hydroplaning occurs, where water separates the tires from the runway entirely, resulting in an almost complete loss of traction.

In certain conditions, a firm touchdown is safer than a smooth or 'butter' landing. Prolonging the flare in order to achieve a smooth landing can cause the aircraft to float further down the runway, while a firm landing in the intended touchdown zone establishes wheel contact with the runway so that brakes and spoilers can become effective. This is majorly important on wet runways, where stopping distances may already be increased. In crosswinds, a controlled touchdown helps pilots establish directional control without redundantly increasing exposure to drifting winds.

Conclusion

While achieving a perfect landing may seem straightforward at face value, in reality, it's the result of several interacting factors, including a stable approach, a controlled flare, and touchdown at the conducive speed and location, complemented by effective deceleration. 

From managing kinetic and potential energy to dealing with crosswinds and wet runways, every stage of landing tests a pilot's skills and intuition. Sometimes, that means aborting landing and initiating a go-around.

At the end of the day, a perfect landing isn't one that passengers barely feel. It is the one that brings an aircraft safely to a stop.

Sources

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