Relevant Discussion: AIM 5-4-5, 7-1-11, 7-2-1 through 7-2-5, P/C Glossary, FAA-H-8083-16, AC 00-45, AC 91-70, AC 120-28D, Order 7110.65T
Everything Explained: Altimetry Around the World
Key Takeaways:
- Q codes define different altimeter settings: QNH is the local altimeter setting (altimeter reads field elevation), QNE is the standard pressure setting of 29.92 inHg/1013.2 hPa (altimeter reads pressure altitude), and QFE sets the altimeter to zero feet on the ground.
- Transition altitude and transition level delineate the airspace where pilots switch between local altimeter settings (QNH/QFE) and the standard pressure setting (QNE), typically 18,000 feet msl (FL180) in the United States and Canada, though these vary internationally.
- The transition layer is the airspace between the transition altitude and transition level, where aircraft will be using different altimeter settings depending on whether they are climbing or descending.
- Pilots must be aware of international variations in altimeter settings (e.g., hectopascals vs. inches of mercury) and potential use of meters instead of feet, requiring conversions.
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