Which type of metal used in aircraft construction does not contribute to a fire, except by sparking during a wheels-up landing?

The correct choice is based on the properties of steel as used in aircraft construction. Steel is predominantly a combination of iron and carbon, and while it can burn under extreme conditions, it is not considered a material that supports combustion in the way that some other metals do, such as magnesium.

In the context of aircraft, steel is used primarily for its strength and durability, particularly in structural components and fasteners. During a wheels-up landing, the possibility of sparks is related to the friction between the steel parts and the ground, but this does not imply that steel is a fire hazard in normal operational conditions.

In contrast, magnesium alloys, known for their lightweight and strength, are flammable and can burn intensely when ignited. Fiber-plus-plastic materials, while they may not contribute to a fire in the same way metals do, can still ignite under certain conditions. Aluminum alloys, while also relatively resistant to catching fire, can be involved in severe fire conditions due to their properties when heated or exposed to certain substances.

Thus, steel's characteristics make it the option that least contributes to fire under typical conditions, except in specific scenarios such as a wheels-up landing where it can create sparks.