Aircraft Loads, Stresses, and Structural Inspection Fundamentals

Aircraft structures are subjected to various types of loads and stresses throughout their operational life, both on the ground and in flight. Understanding these forces, how they are defined, and their effects on the airframe is crucial for pilot awareness and maintenance inspections.

Loads and Design Limits

Aircraft are designed to withstand specific loads to ensure safety during operations.

• Limit Load: The maximum load the aircraft is expected to experience during flight operations.
• Ultimate Load: The aircraft must be capable of withstanding 150% of the limit load before failure occurs.
• Exceeding the limit load is not recommended as it can cause irreversible bending or structural damage.

Types of Loads

There are three main categories of loads, though they often overlap:

• Static Loads: Loads that do not change in size, position, or direction (e.g., the weight of the aircraft when parked).
• Dynamic Loads: Loads that change in size, position, or direction (e.g., turbulence, manoeuvring, taxiing loads).
• Cyclic Loads: Repeated or fluctuating stresses and strains. Common examples include the forces of pressurisation on the fuselage and bulkheads (which follow a cycle per flight) and the impact loads on landing. The degradation from these loads is known as fatigue. Pilots must log landings to track when cyclic load limitations may be exceeded.

Stress and Strain

Forces acting on an aircraft, such as tension, compression, torsion, and shearing, can be described in terms of stress and strain.

• Stress: The internal force per unit area (Force / Area). It is typically measured in Pascals (Pa).
• Strain: The measure of deformation caused by the action of stress on a material (e.g., Change in Size / Original Size).
• Relationship: Any increase in force increases the stress, and any stress causes strain (deformation).

Stress Conditions in Different Scenarios

The distribution of stress on the aircraft changes depending on the flight phase:

• Level Flight: The wings generate lift and flex upwards. The upper surface is under compression (squashed) and the bottom surface is under tension (stretched).
• Ground / Hard Landing / Negative G: Gravity or negative forces cause the wings to bend downwards. In this case, the bottom surface is under compression and the top surface is under tension. Signs of this stress state include sheared rivets or damage on the wing surfaces.
• Manoeuvring (Turns): Entering a turn increases the load factor. This increases the load (stress) on the wing spars and roots, causing visible flex (strain).
• Flaps: In level unaccelerated flight, extending flaps does not change the load factor (it remains 1G). However, because the maximum load factor is fixed, the margin between the current load factor and the maximum load factor decreases as the bank angle increases.

Areas Prone to Stress & Inspection

Certain areas are more sensitive to loads and require particular attention during inspections:

• Wing Root: The maximum bending moment occurs at the wing spar and skin near the root.
• Pressurized Areas: Differential pressure produces hoop stresses. The fuselage skin near bulkheads and skin around windows are highly stressed areas.
• Fuselage Skin: The top and bottom skin should be checked, especially after a heavy landing, as loads from the landing gear can be transmitted to the fuselage.
• Visual Cues:
  • Wrinkles/Buckling: When thin skin panels are under stress, they may buckle, appearing as "wrinkles" when light hits them at the right angle.
  • Control Surfaces: Physical restrictions or changes in movement limits can indicate damage or deformation.