Why does the trachea not collapse when there is not much air? 

The trachea does not collapse when there is insufficient air due to the presence of cartilaginous rings.

The trachea, also known as the windpipe, is a tube-like structure that connects the larynx (voice box) to the bronchi, which in turn connects to the lungs. 

The trachea is made up of cartilage rings that surround and support the tracheal lumen (the air-containing space within the trachea). 

Even when there isn't much air moving through the trachea, these cartilaginous rings keep it open, allowing air to pass freely to and from the lungs.

The presence of cartilaginous rings aids in the prevention of the trachea collapsing, which can obstruct the flow of air into and out of the lungs and impair breathing.