Aircraft Structural Strength - Equal to or Better

Sir Robert Seppings, Surveyor of the British Navy from 1813 to 1832 made the famous comment: "Partial strength produces general weakness". What this means is that if you strengthen part of a structure you might end up weakening the whole structure.

Or the opposite, you might be able to strengthen a structure by weakening part of it. Sounds incredulous? This is exactly what was done by Fokker on the D8. When the wings kept breaking, Fokker, solved the problem by reducing the thickness of the rear wing spar, thereby decreasing spar strength but more importantly, increasing wing flexibility.

Strengthening part of a structure creates stress concentrations in adjacent areas. Evening out the stress improves the entire structure. Patches, Gussets, and straps, added to existing structures might weaken the structure if not properly analyzed using modern elasticity techniques. The old aircraft mechanic rule of "make it Equal to or Better considers static strength only and can lead to considerable loss of fatigue strength.

If you strengthen a portion of the aircraft you change the load path, this in itself may weaken the structure.

If you strengthen a portion of the aircraft you change the natural frequency, this in itself may weaken the structure.

If you strengthen a portion of the aircraft you may suffer a loss of fatigue strength compared to the original structure.

For example, when repairing a crack, a doubler is often riveted to the surrounding skin. It has been found that "overall fatigue life can be drastically reduced by increasing doubler thickness". FAA-AIR-90-01 page 5. "Repairs to Damage Tolerant Aircraft" This is why repairs need to be based on engineering analysis. It took three horrible aircraft accidents* to conclude that the continued safe operation of aging aircraft requires proper engineering, proper inspection based on engineering principles, and proper repairs based on Damage Tolerant engineering principles. A simple, Equal to or Better concept proved deadly.

*

Comet Disaster - Importance of fatigue strength in aircraft design
Aloha Disaster - Importance of inspectability in aircraft design to prevent fatigue failure.
Japan 123 Disaster - Importance of proper repair to prevent fatigue failure.

Transport category aircraft are now repaired, modified, and inspected based on Damage Tolerance per FAR 25.571. The Damage Tolerance philosopy is being extended to all aging aircraft as a method of evaluating airworthiness in high-time fatigue structures. "in-service structural safety of these aircraft is being managed through inspection programs based on a damage tolerance philosophy."*

What this means is that the static strength concepts of Equal to or Better are replaced by an engineering evaluation of crack growth and residual strength.