STRUCTURES The weight distribution of a fully loaded airliner at take-off is about : structure (including equipment and crew) 45%; fuel ( full tanks), 35%; and payload ( passengers and cargo) only 20%. 
Hence payload is at a premium and there is a critical need for light weight as well as strong and flexible structures. Airline aircraft are pressurized and are subjected to twisting, bending, compressing, stretching or shearing loads on every flight. The reduced ability to withstand repeated stress over time is 
called fatigue. The key concept to combat fatigue is called “fail safe redundancy”, otherwise know as the “belt plus suspenders” law. All critical areas are backed up by additional structures to prevent not only disastrous failure but to ensure that the aircraft will be able to land safely after a failure.
CONTROL The three axis of aircraft control are longitudinal, lateral and vertical.
Movement around the vertical axis is called yaw and is controlled by the rudder.
Movement about the lateral access is called pitch and is controlled by the rudders.
Movement about the longitudinal axis is called roll and is achieved by ailerons located at the outer end of the wings.
In some aircraft, there are devices called spoilers are used which “spoil” lift over the wings to support the aileron function and improve movement about the longitudinal axis.
Small adjustments necessary to control pressures are provided by devices are called trim tabs
In addition to ailerons, spoilers and trim tabs. the wing structure contains large panels called flaps . These devices are used to enhance lift and safely reduce the flying speed for landings and take-offs This in turn, reduces the runway distance required for landing or take-off.
PROPULSION Almost all airliners use gas turbine engines to provide propulsion or thrust. The process starts with air being pulled into the front of the engine and forced through series of spinning discs that generate compression. The compressed air is then mixed with fuel, and ignited creating combustion. The expanded air mass then rushes rearward with great force. This force ‘thrusts” the aircraft forward, hence the term.
The modern turbofan engine has a fan mounted forward of the engine which directs some of the incoming air flow to by pass the engine inlet and recombine at the exhaust. The rest of the air is drawn into a combustion chamber where fuel is added. and thrust is generate. The two flows combine to create the force that pushes the aircraft forward. These “by-pass” engines are much quieter and more fuel efficient than their predecessors.
For smaller airliners on shorter runs, the modern turboprop engine excels ( up to 350 miles and cruising speeds of 300 knots). In this case the power product, expelled from the hot section, turns a propeller through a series of reduction gears to produce the driving force. The modern turboprop engine combined with lightweight fibre reinforced plastic propellers result in a power plants that are up to 40% more efficient than the conventional jet engine.
POWER SYSTEMS 
Most onboard services require electrical power and this is typically supplied by engine driven DC (direct current) generators. Most of the airliner’s radios and instruments require AC power( alternating current), which is provided by onboard invertors.
Hydraulic power is used for the operation of landing gear,( or “undercarriage” if you are a Brit), wheel brakes and nose wheel steering. It may also be used for flying controls and flaps. Power is also available from an aircraft battery and an onboard APU (Auxiliary Power Unit). Onboard power must be adequate to carry full loads in keeping with the fail-safe “belt and suspenders” principle. On the ground, an external GPU (ground power unit) is provided to support aircraft systems when the engines are not running..
INSTRUMENTS and AVIONICS Virtually all airline flights are flown under IFR (Instrument Flight Rules) . What this mean is that procedures are followed that assume that the pilots have no visual reference outside the aircraft regardless of the actual weather conditions . In effect it assumes that the pilot is flying blind in the cloud even when such is not the case. The system requires that the pilot navigate the aircraft is in relation to a defined route systems, called airways. To accomplish this, an airliner must be equipped with a set of instruments called avionics which allow for “blind” flying that included radio contact with the ground based Air Traffic Control (ATC). Another tier of instruments on the aircraft monitors system performance by reporting various pressures, temperatures and flows.
To provide the pilot with information on the aircraft’s position are two generic categories of navaids are currently in use ;
The conventional system is Station Referenced Navaids which go under numerous acronyms including VOR, DME, ILS and NDB. In general terms, these devices provide the aircraft with a signal from a near-by ground station which is converted into useful navigation information by on board avionics.
Station Referenced Navaids are now being replaced by new devices which do not rely on local stations. Called, Earth Referenced Navaids, these new systems depend upon either a signals from space, (the US owned Global Positioning System, (GPS) GLEANS (the Russian system) or from distant ground stations (e.g., LORAN). To improve the accuracy of these new system so-called differential enhancements are being introduced. These devices improve accuracy by uploading signal corrections to the aircraft ( see diagram) .
PRESSURE and AIR CONDITIONING Almost all airline aircraft are pressurized. Typically cabin pressure is maintained at about 10,000 feet even though the aircraft may be flying at 40,000 feet Typically pressurized air is supplied to the cabin by air bled from the engine Air-conditioning systems provide for both the heating and the cooling onboard the aircraft.
ANTI –ICING Airliners are equipped for flying in all weather including icing. 
Protection against excessive ice formation is provided for the leading edges of wings and tail surfaces, propeller blades, engine intakes, cockpit windows and probes. Leading edge anti-icing is commonly provided by means of ducted hot engine air from the engine. Another system, used on turboprop powered aircraft involves inflatable rubber bladders on leading edges of the wings. Windscreen icing. that may obscure pilot vision, is combated by means of electrical heating pads in the windows.
COMMAND Taking charge of all this wonderful stuff are two folks who sit up front in what used to be called the cockpit but is now known as the flight deck. The Captain in the left seat and the First Officer in the right seat, don’t just do the flying, but they command and control the entire show including the many system aboard the airliner. Each pilot has access to the all controls and switches and the aircraft can be flown from either side The only exception being nose wheel steering, which is used to taxi the aircraft on the ground, which remains the sole preserve of the Captain.
In addition to the flying controls and dozens of switches and knobs , the modern flight deck features large TV like screens which display all the details of the various systems; the two primary displays are called:
EFIS (Electronic Flight Instrument System) which displays the aircraft’s flight status including vertical and horizon position as well as navigation and communications details It also displays onboard RADAR and TCAS( collision avoidance system).
EICAS (Engine Indicating & Crew Alerting System); details on various aircraft systems are electronically displayed including the status of electrics, hydraulics, air conditioning, fuel, controls, anti-Icing, doors and data control