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Does a plane have brakes to stop or slow down while flying?
For example, if the pilots see a large flock of birds at some distance in front of the airplane, and they want to reduce speed suddenly. What will happen if an airplane doesn't have such brake?
An airplane can slow down and reduce its speed while in flight. The easiest way to do so is to reduce the amount of thrust that the engines are producing. This will produce an almost immediate reduction of the airspeed, especially if the plane is maintaining the same altitude.
There are also devices called air brakes and spoilers that can be further used to reduce speed. These, however, are never1 used in normal, level flight by passenger2 aircraft and are normally only used to reduce speed during the descent and landing phases of the aircraft3.
If an airplane reduces its speed too much, it will of course stall and start dropping precipitously, at which time the airspeed usually also increases again. The slowest speed an aircraft can maintain at a given altitude without stalling is listed in its flight envelope.
However in the scenario you mentioned (obstacles ahead), it's usually easiest to simply fly around the obstacle.
1: Never say never on the internet.
2: The situation is different in military aircrafts, you can see the approaching Su-27 using its airbrake in this video
3: The wheels of most aircraft also have brakes, similar to those found on automobiles. Obviously these can only be used when the plane is touching the ground. Lowering retracted landing gear will also increase drag. There are also thrust reversers on some aircraft that are used in the final stages of landing. Use of thrust reversers during flight can be disastrous although some older aircraft (including the C-17A) were designed to use thrust reversal to scrub speed in-flight.
Finally, some gliders and larger aircraft have drag or drogue parachutes to help slow them down, for example for use on short runways.
You can also "crab", which is the extreme opposite of being in "trim". Essentially, you turn the ailerons opposite the rudder and keep the nose down so it doesn't stall. The plane ends up going thru the air diagonally, presenting one side of the body to the air. That produces a lot of drag. It is a great way to lose altitude quickly in a small plane. It's normally done at low speed, and feels strange to passengers. Best to go gently at higher speeds. Too much exuberance might stress a weak airframe.
No a plane doesn't stop in midair, planes need to keep moving forward to remain in the air (unless they are VTOL capable).
What it can do is simply turn around or go over/under the obstruction.
VTOL means vertical takeoff and landing. It essentially means they can hover in place like a helicopter.
Many jets & gliders have spoilers or airbrakes.
They function by adding drag and, in some cases, reducing available lift.
When an aircraft descends, speed builds up unless the energy is dissipated in some way. Modern aircraft have fairly clean design which means it takes a while to 'bleed' off the energy. This is good for energy efficiency, but is problematic when you need to descend quickly, for example during an approach or flying around significant thermals.
Propellers often provide a natural speed brake when power is removed and so act as an airbrake, but jets & gliders do not have this option.
As mentioned in other answers, there is a minimum speed below which the aircraft cannot fly, however avoidance of issues such as birds, smoke, etc., is usually performed by change of direction or altitude rather than slowing down.
There are many planes fitted with air brakes. StuKas, for example:
To prevent the airplane from building up too much speed, dive brakes (large perforated flaps) are needed.
Similarly, flaps are/were used to "brake" when landing.
But: They won't be used to slow down because of "flying cows" "grazing" on the "air street", like cars would. Either a plane can turn to evade the birds, or it will have to go right through them. But then, if you can believe one Cpt. Sullenberger, you won't even see a bird strike coming like that...
Airplanes can slow down in a variety of ways. The most common is actually to pull the nose up and start climbing - it trades the energy of velocity for height and avoides the object in front of you. Think of what happens when you are riding a bicycle up a hill and stop pedalling.
Reducing the throttle allows the friction of the air the plane is moving through to slow the airplane as well. Think of putting a car in neutral while driving on level ground. If the pilot doesn't also raise the nose of the plane as it slows down, the loss of airspeed over the wings will result in a reduction of lift and the plane will start descending and the reduced altitude will help avoid an airborn object.
The use of flaps will increase both drag and lift of the wings. It will slow the airplane and cause a change in altitude which the pilot can manage by slightly lowering the nose as the flaps go down. However, flaps have a maximum extension speed (often well below cruise speeds) so they aren't practical for obstacle avoidance.
Spoilers disrupt the airflow over the top of the wings to reduce lift. They have a side effect of increasing drag but it's less of an impact than the loss of lift. The primary purpose of spoilers is to increase the approach angle to the runway without changing airspeed or attitude.
With this many options, the real answer to obstacle avoidance isn't slowing down: it's steering around. Going left, right, up or down to avoid the obstacle is much more effective and is what all pilots are taught and what all automated collision avoidance systems use.
There are 4 forces that act on a plane: thrust, drag, lift, and weight.
Planes slow down because of drag. Once power is reduced, drag is greater than thrust and causes the plane to slow down.
UPDATE
Sorry, I'm pretty new here and probably didn't include enough information in my original answer so here's a more thorough answer from the FAA Pilot's Handbook of Aeronautical Knowledge:
In steady flight, the sum of these opposing forces is always zero. There can be no unbalanced forces in steady, straight flight based upon Newton’s Third Law, which states that for every action or force there is an equal, but opposite, reaction or force. This is true whether flying level or when climbing or descending. It does not mean the four forces are equal. It means the opposing forces are equal to, and thereby cancel, the effects of each other.
[...]
For an aircraft to move, thrust must be exerted and be greater than drag. The aircraft will continue to move and gain speed until thrust and drag are equal. In order to maintain a constant airspeed, thrust and drag must remain equal, just as lift and weight must be equal to maintain a constant altitude. If in level flight, the engine power is reduced, the thrust is lessened, and the aircraft slows down. As long as the thrust is less than the drag, the aircraft continues to decelerate until its airspeed is insufficient to support it in the air. Likewise, if the engine power is increased, thrust becomes greater than drag and the airspeed increases. As long as the thrust continues to be greater than the drag, the aircraft continues to accelerate. When drag equals thrust, the aircraft flies at a constant airspeed.
In high performance tactical jets there are 3 ways to reduce airspeed that are quite commonly used. For example, in the US Navy the landing pattern is entered at 250 knots, 800 feet AGL and then a turn made to downwind, with a descent to 600 feet, while slowing to landing speed. For the A7E pattern speed was, depending on fuel weight, around 125 knots. To bleed off all that energy we used the "break," which is a high-g level turn. Of course 250 knots is SOP, but at times, one might come into the pattern at over 600 knots, in which case the break was an absolute necessity to get the aircraft to 125 knots.
The other common way to slow down while in flight was to point the nose up, trading energy for altitude. But perhaps the most efficient way of reducing speed was using the speed break. On the A7 this was a "barn door" that was extended below the aircraft with a button by the pilot. It was very good at bleeding off energy at high speeds. In fact, it was part of the "break" maneuver described above. When not needed it withdrew into the aircraft, and left a flush aerodynamic fuselage. Another use was during a dog fight, hopefully catching your opponent who was at your 6 o'clock off guard, and causing them to overshoot you. This was always a rather desperate move.
On one occasion, I approached the flight lead to join formation at a closure rate of around 200 knots. He was at 250 and I was at 450 as I rocketed towards him. I was close and extended the speed brake, while looking at my airspeed indicator needle drop. Well it wasn't quite dropping, more like kinda moving towards 250. I retracted the speed brake, and felt the aircraft stop decelerating, as I approached fast down his right wing line. At around 10 aircraft lengths away I pulled hard up, and slammed the stick over to the left, never letting up on the back stick. I emerged on his right wing in perfect formation.
By the way this maneuver, or something like it is also used in dog fighting to keep from overshooting an opponent.
One thing I haven't seen mentioned is putting some G on the aircraft. Most people never think of it, but increasing the G load on a jet slows it down incredibly fast.
"Can a plane *stop* while flying?"Really?? $\endgroup$