Altimeter settings provided for international aviation purposes and reported in whole hectopascals are rounded down when disposing of tenths of hPa (e.g., 1009.9 hPa and 1009.1 hPa are both rounded down to 1009 hPa).

• A. Rounded to nearest
• B. Rounded down
• C. Rounded up
• D. Truncated

Answer: (B)

Altimeter settings for international aviation are given in whole hectopascals with any tenths discarded, which is rounding down. In practice, that means you take the floor of the pressure value: 1009.9 hPa becomes 1009 hPa, and 1009.1 hPa also becomes 1009 hPa. This standardizes the values and provides a conservative margin for safety—using a slightly lower pressure value makes the aircraft’s altimeter read a bit higher, helping ensure clearance from terrain and obstacles.

Rounding to the nearest would sometimes push 1009.9 up to 1010 hPa, which could shorten the actual altitude and reduce safety margins. Rounding up would similarly push values higher. Truncating or rounding down are effectively the same for positive numbers in this context, but the phrasing here uses rounding down, matching the stated practice.