When does the transition level change in aviation?

The transition level is a crucial concept in aviation that informs pilots about the altitude above which the use of standard pressure (29.92 inHg or 1013.25 hPa) is mandated for vertical separation between aircraft. The correct scenario for when the transition level changes is when the QNH (the local barometric pressure) falls below 980 hPa or when an aircraft is within a specific proximity, such as 20 nautical miles of a specified point, typically an airport or significant navigational landmark.

When the QNH decreases below a certain threshold, such as 980 hPa, it indicates that atmospheric pressure is lower, prompting a need for an adjustment of the transition level to ensure safe vertical separation between aircraft operating at different levels. This is critical for maintaining safe operations in congested airspace and ensuring that all aircraft are adhering to a common understanding of altitude references.

The proximity requirement of 20 nautical miles to a specified point ensures that adjustments to the transition level can be made in anticipation of ongoing changes to air pressure, thus providing pilots with adequate information to manage their altitudes safely and correctly as they approach critical airspace areas.

In contrast, the other choices do not accurately reflect the criteria for changing the transition level. For example,