Understanding Pressure and Density Height

Differentiating Between Pressure Height and Density Height

Definition: The altitude indicated when an altimeter is set to the standard pressure of 1013.25 hPa (29.92 inHg).
Calculation:
Use: Important for setting the correct altimeter reading above 18,000 feet Mean Sea Level (MSL).

Definition: Pressure altitude adjusted for non-standard temperature and humidity; this is the altitude the aircraft behaves in terms of performance due to these variations.
Significance: Higher density altitude implies reduced aircraft performance, affecting lift, thrust, and engine efficiency. It is a critical factor for takeoff and landing calculations.
Calculation:
Example:
- Pressure Altitude: 6,000 ft
- Outside Air Temperature (OAT): 10°C
- ISA Temperature at 6,000 ft: 3°C
\text{Density Altitude} = 6,000 + (7 \times 120) = 6,840 \text{ ft}

Definition: QNH is the altimeter setting used to obtain the elevation above mean sea level.
Procedure:
- Obtain current QFE (field elevation) pressure from the aerodrome.
- Measure airfield elevation in feet and convert using (1 , \text{mb} = 30 , \text{ft}) to find the pressure difference.
- Add this difference to the QFE to obtain the QNH.
- Example:
  - Elevation: 200 ft
  - QFE: 1023 mb
  - ((200 , \text{ft} / 30) + 1023 = 1030 , \text{mb})

Altimeter Basics:
- Set the altimeter to 1013.25 hPa.
- Adjust to the local QNH to read the precise elevation directly.
Verification:
- Ensure the altimeter reading aligns with published aerodrome data using charts or air traffic control information.

Procedure:
- Set altimeter to standard pressure: 1013 hPa.
- Calculate pressure height using the formula:
- Example:
  - Elevation: 1,800 ft
  - QNH: 1027
  \text{Pressure Height} = 1,800 + (1013 - 1027) \times 30 = 1,380 , \text{ft}

Always adjust for temperature deviations when calculating density altitude, as ISA deviations can significantly impact performance.
For every 1°C deviation from ISA, density height adjusts by approximately 120 feet.
Altimeter settings must be accurate, and periodic cross-verification with airfield data is essential, especially for controlled flight optimum performance.
Be aware of local regulatory guidelines regarding QNH settings, notably when operating within 100 nm of an aerodrome, to ensure precision in altitude readings.