What is the lowest usable flight level for an altimeter setting between 28.91 and 27.92?

The determination of the lowest usable flight level is based on the altimeter setting. For settings within the specified range of 28.91 to 27.92 inHg, the standard procedure is to refer to the flight level thresholds for altimeter settings.

When the altimeter setting is less than 29.92 inHg, the lowest usable flight level is above the indicated threshold of FL180. Specifically, for altimeter settings below 29.92 but above 28.90, the lowest usable flight level becomes FL200. This allows for an adequate buffer from the terrain and ensures safe vertical separation from aircraft operating at lower altitudes.

In this situation, because the altimeter setting is within the range of 28.91 to 27.92, the correct response aligns with aviation guidelines, which specify that the lowest usable flight level is FL200. Therefore, the answer is correctly identified as FL200.