Various times, I have seen and felt myself that the plane always lands on the rear wheels, but not the front wheels. Why do the pilots land it on the rear wheels instead of the front? Does it have some advantage to it or is it a rule?
Is this required for all the planes (irrespective of size, type) to land only on the back wheels?
Aircraft land on the main wheels. For aircraft with nose wheel it is the back ones, but for aircraft with tail wheel (also called “tail-draggers”) it is the front ones. In either case the main wheels are very close to centre of gravity and carry most of the aircraft's weight, the nose or tail wheel only carries a small fraction of it.
The aircraft must land on wheels that are close to the centre of gravity (longitudinally). If they were not, the force on the wheels would create a moment that would violently pitch the aircraft. Actually, tail-draggers tend to bounce on landing a bit because there still is some moment left and in case of tail-draggers it pitches the aircraft up.
Early planes were almost all tail-draggers, because that layout is more robust, handles unpaved runways better and has less drag when the gear is fixed. It however provides poor directional control especially once the tail lifts off the ground during take-off roll. Later most designs started to use nose-wheel, because it provides better directional control, is directionally stable, makes the floor level when the aircraft is standing so loading is easier and visibility during taxi is much better and with invention of retractable landing gear the drag didn't matter any more.
Jan Hudec already gave an excellent description of why the gear configurations are designed as they are. Another important thing to note, though, is that because aircraft are designed to land on their main gear, the other gear (nose gear in tricycle configurations or tail wheel in tail draggers) is not designed to be able to take the force of a landing. Landing on the nose gear in an airliner will most likely cause the nose gear to collapse, leaving you in a situation like Southwest Flight 345:
Image Source: New York Daily News
One of the passengers on that flight recorded the landing and posted it on YouTube. While there were only a few minor injuries (mainly from using the slide at such a high angle, if I remember correctly,) the aircraft received such substantial damage that it was written off. Obviously, that is not a desirable outcome, so airlines usually prefer their pilots to land on the main gear instead.
By the way, when we talk about a tail dragger, we're talking about planes with a landing gear configuration like this:
Douglas DC-3. Image Source: Wikipedia
Whereas tricycle landing gear refers to the configuration you're probably used to seeing that looks like this:
Boeing 737. Image Source: Wikipedia
One quick comment regarding the previous discussion on tailwheel aircraft. Taildraggers actually have two landing techniques at their disposal.
The one most people envision, where the aircraft touches down on the mains before gently settling the tail down, is called a wheel landing.
There is, however, also the three-point landing, where the goal is to touch down with all three wheels at the same time and is usually the first landing technique taught, especially to pilots transitioning from tricycle gear aircraft.
The technique used on each landing is chosen based on a number of factors, among which:
If you want to learn more about aircraft and flying, I would encourage to look up FAA resources (especially the FAA Airplane Flying Handbook), as well as the trove of information on AOPA's website.
In an approach an airplane is generally descending between 500 and 1000 feet per minute, that's a minimum of 8 feet per second. That may not sound like much but just hitting the ground at that rate of descent would lead to a big bounce and possibly damage the airplane. It would certainly be uncomfortable for the passengers.
In order for an airplane to land softly, it must decrease its rate of descent, to do this the nose is raised in a maneuver called a flare. A nose-up attitude will, in a tricycle configured airplane, bring the main (rear) wheels in contact with the ground first.
Not all, taildraggers will land on the front 2 wheels.
Because then the plane doesn't wheelbarrow off the runway. Have you ever tried running with a wheel barrow? The smallest deviation of forward will get expounded and before you know it the thing will be off to the side.
Landing on the rear wheels will put the center of mass forward of the drag source so deviations left and right will self-correct. This plus differential braking will allow greater control over the aircraft.
Just for completeness, there's also the B-52, which has "bicycle" landing gear (both the front and rear are considered "main" landing gear). I believe the ideal when landing it is to have the front and rear gear touch down simultaneously1.
For any who care, here's a video of a B-52 landing, showing the nearly simultaneous touch-down of front and rear gear. As an interesting aside, to support crabbed cross-wind landings, both the front and rear main gear of the B-52 are steerable, so it can land (or take off, taxi, etc.) with the nose turned up to 15 degrees from the direction of movement. Here's another (from the front) showing a crabbed landing.
1. I originally wrote that as "all the gear", but that would be false. It also has tip protection gear that normally touch down quite a bit later. Its wings are quite flexible, so the tip protection gear don't normally touch down until it slows down enough that the wings are producing substantially less lift. If it's carrying a light fuel load, they might not touch even then.
Just when sitting still on the ground, the B-52's wings could/did flex by around 10 feet at the tip, depending on fuel load. In flight, they could flex upward still more for a total of around 17 feet of flex (over a wing span of roughly 165 feet).
I am going to answer this in another way.
Generally this is by design. The wheels are not the real issue, the angle of attack is.
When most aircraft land, they are "flaring" or pointing the leading edge of the wings as high as possible without getting into stall territory. This is especially true for hang gliders and parachute canopies, even though neither of these have wheels.
It is called flaring as this is what it is called when birds land. (Ducks, Pelicans etc.)
In this aspect, a nose wheel is going to be higher, and hence touch down last.
Even a tail dragger is looking at this until the last moment. It's really a powered glide.
A pilot will avoid a stall, look for "ground effect", and be mindful of recovery options.
The design of the landing gear reflects this strategy, if a commercial ariliner touched town nose first (levelish) it would be travelling faster and experience more of a shock, the gear would need to be much heavier. This type of pancake landing does not usually end well.
Your "rear wheels first" observation pertains to aircraft with "tricycle" landing gear. For various reasons, the landing gear of such aircraft is configured so that they sit more-or-less level on the ground (when parked or taxiing). Such an aircraft usually cannot take off or land in such a level attitude. This is because the wings won't generate enough lift unless the aircraft is at or near cruising airspeed. To minimize wear and tear, danger, and runway length, takeoff and landing is performed at the lowest possible speed, allowing for a safe margin above stall. To fly at such speeds, the wing needs a high angle of attack, which means the aircraft has to be pitched into a nose-high attitude. To make the landing a positive transition, aircraft "flare"; essentially pitching up higher and higher and slowing down more and more while just above the runway until the wing essentially "stalls" - loses lift. At this point, the aircraft settles onto the runway at the slowest possible speed and should stay in contact with the runway (not "bounce" or "balloon" up). All this amounts to touching down on the main gear first.
It is certainly possible to touch down on the nosewheel first - a term called "wheelbarrowing". This is very dangerous. Firstly, it could only occur if the approach was flown too fast - more speed means more forces and things going wrong more quickly. Second, the aircraft is touching down on a point ahead of the center of gravity. The slightest sideways motion will create a lateral moment that could (and probably would) spin the plane around into a crash.
Tricycle vs taildragger
Is this required for all the planes (irrespective of size, type) to land only on the back wheels?
All aircraft touch the runway on their main gear, and the main gear is at the level of the wing. Adding a wheel aft or forward of the main gear leads to two configurations with a noticeable difference: The angle between the wing and the horizontal plane when the aircraft is on the ground.
Those with a nose wheel and a low angle, the vast majority of today's aircraft, and all commercial airliners. This gear configuration is named tricycle.
Those with a tail wheel and a high angle, a category which was used at the beginning of aviation. This gear configuration is named taildragger.
The advantage of the tricycle is the stability on the ground, thanks to the engine being located between wheels. The inconvenient is that takeoff cannot take advantage of the weight of the engine to help rotate the aircraft. Elevators are more solicited. If the rotation is not executed correctly, a tailstrike can occur.
Tricycle aircraft landing
Does it have some advantage to it or is it a rule?
It is a technical requirement. When landing, the aircraft must touch the runway with a very small vertical speed, the glide path must slope gently down. The weight of the aircraft pulls it down at an accelerating rate (free fall), the aircraft must oppose this acceleration by generating lift.
Lift is proportional to airspeed and to the angle between the direction of the aircraft and the direction of the wing (the wing median line, named the chord line). This angle is named the angle of attack. The angle of attack obviously depends on the aircraft attitude (pitch angle).
When landing a tricycle aircraft the pitch angle must be relatively high, so that the nose gear doesn't touch the runway before the main gear. At the same time a fair amount of lift is generated due to the correspondingly relatively high angle of attack, as depicted in the left hand side of this picture:
In contrary, if we wanted to touch with the nose gear, we would need to strongly decrease the pitch angle. Such attitude automatically decreases the angle of attack, and the associated lift (right hand side above) which is already low due to the low speed required for landing.
If the lift is insufficient to counteract the gravity acceleration, the aircraft descends too quickly, and the vertical speed at touchdown creates a force on the gear exceeding its mechanical strength. Damages occur.
This is the reason airliners, which are tricycle aircraft, don't land on the nose wheel. However this leads to another question: Why do we use tricycles instead of taildraggers? Looking at the first picture, we see it would be uneasy to board and deplane, due to the floor slope, but there is more to know, maybe another good question to ask.
its is called a flare, it is a 5-10 degree pull to slow the aircraft down and put the rear wheels on the ground.
The Rear landing gear is put down first because there at the center of mass (CoM), It is also smoother and safer. It is safer because the front landing gear is not made to hold the whole wait of the aircraft. (EX. the airbus A380 has many Rear Gear but only 1 front gear(http://avimotive.com/wp-content/uploads/2016/01/a.jpg))
It is smoother because It slows the sink rate.
While there is certainly a lot of truth in the above answers they all sort of treat the question in a fashion similar to: "Why does water freeze at the top and work down? Because otherwise fish would freeze." While there are a number of reasons why landing on the back wheels works better the single most important one is that when airplanes land their nose is higher than their tail so their back wheels are closer to the ground. The reasons for this is because as planes slow down their wings generate less lift. They have to do something to make their wings generate enough lift despite the airspeed reduction so as to keep flying else a sub-optimal condition involving bent metal and maybe fireballs will occur. They do this by pointing nose up which increases the wings' angle of attack and by changing the shape of the wing (flaps and slats) to make the wings generate greater lift at the expense of introducing more drag. By the time that they get close to the ground they have slowed as much as they dare which means that the speed alternatives are being used as much as possible. That means that the aircraft is leaning back as much as it safely can and flaps are fully extended. Right at the point that a safe landing is guaranteed then the greater danger to the aircraft's safety shifts from falling out of the sky to hitting the ground at too fast a speed so the pilot does things to dump speed. That means leaning the plane back just enough to increase air resistance but not enough to make it bob up in the air. A good pilot does this just a very few feet above the ground and adjusts things so that the plane levels out for a few brief seconds as the speed bleeds off. A not so good pilot briefly gains altitude and then the plane falls out of the air from 10 or 20 feet up and the passengers get to experience three or four very short flights before the plane finally settles down on the ground. All of this puts their back wheels closer to the ground which means that's where the rubber meets the road.
