we all know by now that aircraft use lift to get off the ground and that lift is provided by moving airflow over and under the wings. The faster flowing air over the wing (so less pressure) and the slower moving air under the wing (higher pressure) create the lift.
Engines provide the thrust to get the forward motion to get the proper airflow in order to get the lift, but how does the airplane use the lift?
In order to use the lift the aircraft must be able to steer otherwise it would just go in any random direction. The steering is done by the flight controls. The flight controls move the flight control surfaces wich will have an effect on the aircraft. The center point of gravity is where the aircraft rotates around its axis to get horizontal (or pitch) movement. This center of gravity is roughly the area where u would want the lift to be created, in front of the center of gravity and the aircraft will want to go nose up and behind the center of gravity and the aircraft will want to go nose down. If the lift is created in front or aft of this center of gravity (usually slightly aft) it needs to be corrected by the horizontal stabiliser (horizontal stab trim) for level flight.
The horizontal stabiliser moves to steer the aircraft nose up or nose down, this is not a lift creating device, for example: in order to make the aircraft steer nose up the tail must go down to rotate the aircraft around the center of gravity, in this case the horizontal stabiliser must pull the tail down and and for aircraft nose down the tail must steer vice versa.
The stabiliser is not the only control for pitch, it is mainly used for trimming the aircraft for a level flight wich means that when u release the control collumn (control wheel) the aircraft flies level. When u push or pull on the control collumn to give the aircraft the command to rotate up or down the elevators move, creating the force required to push the tail down or up.
In aircraft with auto-trim systems the horizontal stabiliser will move when the input remains the same for a long period of time, this means that when let's say, the forward cargo compartment is too heavily loaded and the center of gravity is in front of the center of lift (aircraft wants to go nose down) u must pull on the control collumn to steer the aircraft in a level flight. If an autopilot with autotrim functions is engaged it will pull the control collumn back for a longer period of time, the auto trim system will move the horizontal stabiliser to a more nose up position so the stabiliser and the elevators are level again. This can also occur when for example one wing is too full with fuel, it get's too heavy and the aircraft continuously has to steer the opposite direction to keep the aircraft level. This can then be cancelled by the aileron trim system (aileron's will be explained a bit further down).
The stabiliser and elevator should be level compared to each other as much as possible to get the minimum fuel consumption (a nose down stabiliser plus a nose up elevator just creates drag and cancels each other's movements out). On landing this setting is in many aircraft different i.e. stabiliser trimmed to aircraft nose up and elevator to nose down in order to have more clearance for pulling up again when a go around is selected (go around is when the crew decides not to land the aircraft when the aircraft is allready in the approach, they will throttle up the engines, and pull the aircraft up again fly around and try again).
This is simply how the pitch control works, in some aircraft there is more in the system like microburst protection, force feedback (for powered steering elevators) to the flight crew and anti ice heating etc. but for now let's just consider the basics.
Then there is also roll of the aircraft wich obviously rotates the aircraft to the left or right of the center of gravity. This is done by the ailerons. Ailerons work just like a wing that provides lift. The airfoil of a wing is such that the topside is longer than the bottom side so the air over it must travel a longer way then the air under the wing therefor creating a pressure differential under and above the wing.
By moving the aileron down, u effectively make the top of the wing even larger and so the pressure differential above and below the wing will be even larger and the wing will move up. When u move the aileron up the top of the wing is effectively smaller so it will loose some of its lift.
When u want to make a roll, on one side of the aircraft the aileron will create more lift and on the other side the aileron will decrease lift and there u go.
On larger aircraft there are panels on the wing called spoilers wich have different functions, one of wich is to aid the ailerons in steering. If u are making a right hand turn and move the control wheel far enough to the right, some of the panels on the right hand wing will start to move up slightly to get rid of even more lift on that wing. Why have different systems?
The ailerons are all the way at the end of the wing and they create very sensitive motion with relatively low lift dumping. For the majority of the flight, the aircraft is in high speed mode (cruise) and u only need minute changes in direction and as little lift dump as possible because dumping lift is dumping energy and that is dumping fuel wich costs money and weight.
Then there is the rudder wich controls the yaw rate of the aircraft, works the same again as a wing, by moving the rudder to one side it increases the distance the airflow has to travel on one side of the rudder and so creates a pressure differential and steers the tail left or right around the center of gravity.
These were the three main directional controls, pitch, yaw and roll control but there is more inside an aircraft to control it's movement. Think about wheels/brakes, thrust reverser, ground spoilers.
When the aircraft is on the ground getting ready to take off, it is then selected into a so called 'take of mode', in this mode the aircraft looks for certain anomalities inside the aircraft like: 'are all the doors closed?' 'are the emergency exit's unlocked (unlocked but closed ofcourse)' 'is the parking brake released' 'are the spoilers not up (aircraft won't go very far with these lift spoiling panels up)' 'is the horizontal stabiliser in it's correct position' etc.
Aircraft with flightspoilers can raise these spoilers in flight to decrease lift and increase drag to reduce altitude. Flying too slow with an aircraft or with too little lift will cause the aircraft to stall. Stalling means that the boundary layer of air that is supposed to run smoothly over the wing let's go too early causing the wing to lose some of it's lift. All commercial aircraft and most private aircraft have flaps installed.
These flaps increase the wings ability to create lift at lower airspeeds. Some commercial aircraft even have leading edge devices (flaps and slats on the forward side of the wing to create lift at lower airspeeds and at high angle's of attack 'the angle of the aircraft compared to the airflow'). Leading edge devices are classified as Leading edge slats and leading edge flaps, the leading edge slats are on the outboard, these slats can still create a lot of lift at very low speeds and very high angle's of attack and are on the outboard to give the ailerons a good airflow over the wing for roll control. The leading edge flaps are on the inboard side of the wing close to the fuselage and are not as effective as slats. If the aircraft angle of attack gets too high the aircraft now starts to stall, the wingroot wich creates most of the aircrafts lift loses much of its boundary layer (before the outer part of the wing where the ailerons are) and the lift is reduced and the aircraft will naturally want to move nose down. This is exactly what we want, the aircraft moves nose down to regain airspeed as to regain lift whilst not losing all control of the ailerons.
Krueger flaps on the inboard of the wing
LE slats on the outboard of the wing.
The aircraft stall management computer or stall warning computer ofcourse knows the aircraft specifications. This means that the computer knows just exactly what the aircraft can do at what angle of attack at wich flap setting and at what airspeed with wich weight (weight information comes from the flight management system where the flight crew enter the aircraft's weight excluding the fuel, the fuel information goes into the F.M.C. by the fuel totalizer system wich measures the amount of fuel onboard).
The stall management computer (SMC) uses inputs coming from the Flight Management Computer, flap position and the angle of attack sensors and the air data computer. By the fuel data from the flight management computer the stall management computer knows it's weight, it knows the airspeed from the air data computer and the angle off attack and flap setting. It now has all the basic 'need to know' items to calculate a stall situation. The Yaw Damper computer will also want to know how much the aircraft weighs in order to reduce or increse the strength of the yaw damping deflection of the rudder.
When the stall management computer sences a impending stall it will first start to operate the stick shakers wich litterally shake the flight control collumn's in the flight deck to attract the attention of the flight crew to the stall situation. some aircraft have incorporated a stick pusher system wich even pulls the control collumn forward with great strength commanding the aircraft to nose down.
Underspeed and overspeed also have warnings on the flight deck instruments and on the aural warning module. The operation speeds can be viewed as a vertical bar on the PFD (primary flight display) or ADI (attitude/direction indicator) wichever is installed.
In the take off mode If one off the take off configuration check items doesn't check out then as soon as the flight crew advances the thrust levers an aural warning (take of configuration warning) will sound and the crew will abort the take off. Most of the time this is just retarding (pulling back) the throttles, slow down and turn off the runway to go back to the aircraft stand for maintenance. If something were to fail however when the aircraft is thundering down the runway, the flight crew get's a fail caution on a central display in the flight deck and depending on the seriousness of the defect (seriousness is indicated by amber or red color) depending if the V1 speed is reached or not the flight crew will abort the take off by pulling on the thrust reverser handle and a rejected take off (RTO)will be initiated automatically. This means that a number of things will happen with the aircraft but for the passengers this means an unbelieveable fast stop in wich u can easily break bones if not strapped into a seatbelt properly. This RTO can have disastrous effect on the aircraft, scrapping maingears, brakes, even scrapping the entire aircraft depending on the force.
This RTO has a great deal to do with the autobraking system (u can set autobrake to light, medium, heavy braking and RTO wich is 'anchor's overboard'), the autobrake and antiskid systems work together to slow the aircraft down to a certain deceleration rate of wich the RTO mode is the fastest deceleration rate. When the thrust reverser handle is pulled the engines will direct fan air (aircraft with bucket type thrust reversers reverse core air too) forward effectively turning the engines around (partly) and slowing the aircraft down, with the aircraft at high speed, the spoilers will come up (in this situation all the spoiler panels will come up and not just the one that would come up in flight to aid the ailerons) and they will come up all the way losing all the lift and putting the entire aircraft weight on the wheels making braking of the wheels extra effective. The brakes will get so hot that they will be bright red and fire is often the result, all this heat will make the thermal fuses melt inside the wheel assemblies so the wheels will deflate rather then burst (see picture). The autobrake system tells the anti skid system how much deceleration rate it wants and the anti skid system will in turn operate the brakes to get the maximum out of them like a ABS system in your car.
When the aircraft's thrust reversers are not working the aircraft may only depart if the destination (this ofcourse includes any emergency divert airport) has long enough landing strips and no icy weather conditions.
Southwest airlines, Thrust reversers failed to deploy on landing, aircraft skidded off the runway.
