What is considered an altitude STACK?

An altitude stack refers to a system of vertical separation between aircraft to ensure safe distance and avoid collisions in the airspace. The correct answer is a 10K-foot separation, which is commonly recognized as a standard altitude stack among various aircraft flying in commercial and military airspace. This level of separation provides adequate vertical distance between aircraft, accommodating varying altitudes and ensuring safety.

The 10,000-foot separation allows for a buffer zone that takes into account any potential altitude excursions or variations in climb/descent rates. Therefore, when managing air traffic and organizing aircraft into an altitude stack, this standard ensures optimal safety and effective use of airspace.

Other options, while they represent different vertical separations, do not reflect the established standard practice for altitude stacking in most operational protocols, making them less relevant in this context.