FAA Aircraft Dispatcher (ADX)Flight Planning and PerformanceEasy
An airport has a field elevation of 1,200 feet MSL. The current altimeter setting is 30.42 in. Hg. Using the rule of thumb that pressure altitude changes approximately 1,000 feet for each 1-inch change in altimeter setting from 29.92, what is the approximate pressure altitude?
- A1,700 feet
- B700 feet
- C1,200 feet
- D500 feet
Show answer & explanationAnswer & explanation
Correct answer: B. 700 feet
The altimeter setting (30.42) is 0.50 in. Hg higher than standard (29.92). Since higher pressure lowers the pressure altitude, subtract 500 feet (0.50 x 1,000) from field elevation: 1,200 - 500 = 700 feet.
Why the other options are wrong
- A. This results from adding instead of subtracting the correction.
- C. This is field elevation, ignoring the altimeter correction.
- D. This applies too large a correction (700 ft instead of 500 ft).
Pressure Altitude Correction
Pressure altitude is field elevation corrected for nonstandard altimeter setting; approximately 1,000 ft per inch of Hg difference from 29.92.
- Higher-than-standard altimeter setting lowers pressure altitude
- Lower-than-standard altimeter setting raises pressure altitude
- Rule of thumb: 1 in. Hg ≈ 1,000 feet
- Used for performance charts requiring pressure altitude input
Memory trick: High pressure pushes altitude DOWN; low pressure lets it rise