What is the lowest usable flight level when the altimeter setting is 30.28?

What is the lowest usable flight level when the altimeter setting is 30.28?

• A. FL 170
• B. FL 190
• C. FL 200
• D. FL 180

Answer: (D)

Higher-than-standard local pressure pushes the pressure surfaces upward, so every flight level sits at a higher true height above the surface than it would under standard pressure. With an altimeter setting of 30.28 inHg, which is about 1027 hPa, the difference from the standard 1013 hPa is roughly 14 hPa. Using the common approximation that 1 hPa corresponds to about 27 ft near the surface, that’s around 370–380 ft of extra height for a given level. Because of this shift, the same nominal flight level represents more true altitude above the terrain. To maintain the required vertical clearance, the lowest usable flight level moves up from the level you’d use under standard conditions. In this case, that shift places the lowest usable level at the next standard level up, so FL180 is the lowest usable option. In short, the higher local pressure raises the actual altitude of each flight level, making the lower level unusable for safe separation and leaving the next level up as the minimum usable one.

Higher-than-standard local pressure pushes the pressure surfaces upward, so every flight level sits at a higher true height above the surface than it would under standard pressure. With an altimeter setting of 30.28 inHg, which is about 1027 hPa, the difference from the standard 1013 hPa is roughly 14 hPa. Using the common approximation that 1 hPa corresponds to about 27 ft near the surface, that’s around 370–380 ft of extra height for a given level.

Because of this shift, the same nominal flight level represents more true altitude above the terrain. To maintain the required vertical clearance, the lowest usable flight level moves up from the level you’d use under standard conditions. In this case, that shift places the lowest usable level at the next standard level up, so FL180 is the lowest usable option.

In short, the higher local pressure raises the actual altitude of each flight level, making the lower level unusable for safe separation and leaving the next level up as the minimum usable one.