On a balloon equipped with a blast valve, the blast valve is used for which purpose?

• A. Altitude control
• B. Fuel regulation
• C. Temperature control
• D. Auto-depressurization

Answer: (A)

Altitude control is achieved by venting hot air from the envelope. A hot air balloon rises because the heated air inside is less dense than the surrounding air. To descend, you need to reduce lift, and the blast valve provides a quick way to do that by dumping hot air out of the top of the envelope. Opening the valve lowers the interior temperature (relative to the outside) and increases the density difference in a way that reduces buoyancy, allowing the balloon to descend or hold a desired altitude.

This valve isn’t for regulating burner fuel; fuel management controls heating, not venting. It isn’t a direct temperature control device in itself, though releasing hot air does influence the interior temperature. And it isn’t an automatic depressurization feature—it's typically operated manually to fine-tune altitude.