An aircraft is build up from a number of major components: fuselage, wings, empennage, undercarriage and one or more piston or turbine engines hanging from the wing, fuselage or on the nose.
Each of these components consists of hundreds sometimes thousands of individual parts and they all are flying in very close formation looking like an airplane.
All these components need to be designed and constructed in such a way that they are able to withstand all possible loads an aircraft may encounter during its lifetime of flying.
The problem designers face is to keep the weight as low as possible but at the same time it must be strong enough to withstand all expected and unexpected loads too and with a healthy safety margin of at least 50%.
And at the same time the whole airplane should be easy to manufacture without difficult techniques so that build time and construction cost, operating and maintenance cost remain as low as possible for the manufacturer and owner to be competitive in the market and become a success.
Aircraft are designed to be able to take a number of loads from standstill to very high speeds at low level and high altitudes with ambient temperatures of +30°C to -56°C. These loads take the form of maneuvering, gusts from turbulence, fuel and cargo weight and last but not least pilot induced control surface loads.
All these various loads have their effects on the airframe. They will try to bend, stretch (tensile load), twist (torsional load) or shear parts or even the whole aircraft. The construction must handle all these loads safely during the life expectancy of the structure.
Below a list of the most common loads encountered in and on a structure of an aircraft. Note that propeller loads are discussed in a separate section, you may follow the next link to read about propeller aerodynamics.
Deflection of control surfaces induce a load on the wings and tail planes of the airplane. More so when the pilot must compensate for turbulence or is manoeuvring around trying to keep the flight smooth. To compensate and keep the loads low, ride the turbulence (go with the flow).
When in straight and level flight the lift generated of an aircraft is equal to its weight. But when turning or otherwise manoeuvring the lift increases, sometimes more than twice as high. Especially in 60° banked level turns were generated lift must be twice the weight for the aircraft not to loose altitude.
Air turbulence causes changes in direction and speed (velocity) and these will create gust loads on the aircraft, within a short duration of time. These rapid loads can exceed the maximum load factor the aircraft is certified for and cause the skin to buckle or even worse: the structure can fail completely eventually if the load is too high continues for too long.
Sometimes a landing is not as smooth as we would want too, intentionally or not. But the aircraft needs to take care of that too without falling apart. Rough runway surfaces also create loads on the undercarriage and these forces must be handled too without structural damage, within designed limits.
Apart form the above mentioned loads there are also fuel and cargo loads the airframe structure must be able to handle. Where cargo is held in the fuselage the fuel sits normally in the wings and with increasing fuel load the wings will bend and place a limit on the maximum an aircraft can carry. Designers have come up with the term: maximum zero fuel weight.
This is the maximum weight of the aircraft including payload without fuel and it means that any weight above this must be fuel due too previously mentioned limits.
This is not so much a load on the airframe as it is on the engine and its subcomponents. Engine internals as, turbine blades, combustion chambers, pistons and cylinders, valves and exhaust are under thermal stresses causing fatigue in the long run if the designer does not take this into account during component design.
The airframe of an airliner will see ambient large temperature variations in certain regions on the planet when flying at FL360 and landing in tropical areas. Causing challenges for the manufacturer and paintjob.