Discussion Overview
The discussion revolves around the conversion of QNH (the altimeter setting) to altitude using QFE (the barometric pressure at the observer's location). Participants explore the relationship between these pressures and how to calculate altitude based on them, considering various equations and assumptions related to atmospheric pressure.
Discussion Character
- Technical explanation
- Mathematical reasoning
- Debate/contested
Main Points Raised
- Some participants clarify that QFE is the barometric pressure at the observer's altitude, while QNH is the pressure at the airport.
- One participant suggests calculating altitude by subtracting QFE from QNH and using the formula h = (QNH - QFE) / ρ, where ρ is the density of air.
- Another participant raises the question of whether the density (ρ) should be considered constant in these calculations.
- Variations in atmospheric density are noted, with one participant mentioning empirical equations for calculating pressure from altitude.
- There is a suggestion to use a specific equation for atmospheric pressure that can be applied up to 10 kilometers with a certain accuracy.
- Participants discuss the possibility of substituting QNH into a simplified pressure equation and the implications of changing units from inches of Hg to hPa or kPa.
- One participant emphasizes the importance of performing calculations in the given units before converting to the required pressure units.
- Another participant states that knowing one barometric pressure is sufficient to calculate altitude, referencing mean sea level pressure.
- Concerns are raised about the accuracy of using a standard sea level pressure of 101.325 kPa, with a participant explaining how airports derive QNH from local pressure readings.
- A formula is provided for calculating height based on pressure in inches of Hg, with a note that the resulting height should be in meters.
Areas of Agreement / Disagreement
Participants express various viewpoints on the calculations and equations involved, with no clear consensus on the best approach or the assumptions regarding density and pressure units. Multiple competing views remain regarding the methods for converting QNH to altitude using QFE.
Contextual Notes
Limitations include assumptions about air density being constant, the dependency on specific equations for accuracy, and the potential variability of sea level pressure in different locations.