How can a pilot determine when pressure height has been reached?

The determination of pressure height is closely linked to the behavior of the envelope and the pressure of the contained gas. When the pilot is trying to assess whether the pressure height has been reached, observing the liquid in the air manometer provides valuable information.

As the aircraft ascends, the atmospheric pressure decreases. The air manometer measures the difference between the internal pressure of the gas in the envelope and the external atmospheric pressure. When the pressure height is reached, the liquid in the air manometer will show fluctuations — it will rise and potentially fall — indicating that the external pressure has decreased sufficiently. This fluctuation reflects changes in altitude and the corresponding adjustments the pilot must make to manage buoyancy and maintain the desired altitude.

In contrast, other options represent different scenarios that may occur during flight but do not accurately indicate the specific point at which pressure height is reached. For instance, the rising of the liquid in a gas manometer is more indicative of a pressure change rather than the condition of reaching pressure height. Likewise, an envelope deflating or reaching maximum expansion does not give a precise measurement related to pressure height specifically; rather, these are more associated with changes in handling and gas management.