Which rivet can replace a #4D rivet in aircraft metal structures?

A #4D rivet has specific dimensions and characteristics that determine its suitability for use in aircraft metal structures. When considering a replacement rivet, it is crucial to match the strength and application requirements.

The #5 AD rivet is appropriately sized and designed to sustain similar or better mechanical properties compared to a #4D rivet. The designation "AD" indicates that the rivet is made from aluminum alloy 2024, known for its high strength-to-weight ratio and excellent fatigue resistance, making it suitable for aircraft applications.

In contrast, the other options do not maintain the same balance of strength and compatibility. A #3 AD rivet would be smaller in diameter, potentially compromising the joint integrity. A #4 B rivet has different material properties and might not provide the necessary strength in applications requiring the specifications of an AD rivet. Similarly, although a #6 AD rivet is also made from a high-strength alloy, it is larger than required, which could lead to difficulties in installation and potential structural issues.

In summary, the #5 AD rivet is the most suitable replacement as it closely matches the required dimensions and strength characteristics to effectively substitute for the #4D rivet in aircraft metal structures.