Altimeter Overview

The altimeter is a basic flight instrument connected to the PIDOT static system.

Types of Altimeters

• Barometric Altimeter: Uses barometric pressure for readings.
• Radio Altimeter: Uses radio waves to measure height above ground.
• GPS/GNSS Altimeter: Uses satellite data for altitude determination.

Focus on Barometric Altimeter

• Measures vertical distance from a specified reference level.
• Reference level where altimeter indicates zero is critical.

Key Terminology

• Altitude: Vertical distance from mean sea level.
• Height: Vertical distance from a point on the ground.
• Elevation: Vertical distance from mean sea level to a point on the ground.
• Flight Level: Another term for certain vertical distances.

How Barometric Altimeter Works

• Utilizes static pressure to determine altitude; pressure decreases with altitude.
• High static pressure = low altitude; low static pressure = high altitude.
• Default calibration: 29.92 inches of mercury (1013 hPa).

Pressure and Altitude Relationship

• Standard sea level pressure: 29.92 inHg (0 feet).
• Example: If static pressure is 25.14 inHg, altimeter calculates altitude based on pressure difference.

Instrument Mechanics

• Altimeter contains an aneroid capsule sealed with a constant pressure of 29.92 inHg.
• As altitude changes, static pressure changes causing capsule to expand or contract, moving the indicators.

Adjusting Barometric Reference

• QNH: Current pressure at mean sea level; pilots adjust altimeter for accuracy.
• Altimeter adjustment required under non-standard pressure conditions.

Example Scenarios

1. Higher than Standard Pressure: Altimeter indicates lower altitude than actual.
   • Adjust to new QNH to correct.
2. Lower than Standard Pressure: Altimeter indicates higher altitude than actual.
   • Adjust to new QNH to correct.

Measuring Height Above Ground

• QFE: Pressure at the airport level used to indicate height above a specific point (e.g., airport).
• Pilots must adjust for QFE when measuring height.

Colesman Window

• Instrument feature allowing pilots to adjust barometric reference using the Baro Adjust knob.

Reading Altimeter

• Needles:
  • Triangular needle: Tens of thousands of feet.
  • Short needle: Thousands of feet.
  • Long needle: Hundreds of feet.
• Example Reading Interpretation:
  • Needles at 0 = 0 feet.
  • Needles between 0 and 1 indicate between 0 and 10,000 feet.

Altimeter Calibration Flags

• 10,000 feet flag indicates altitude above/below 10,000 feet for easier interpretation.

Types of Altimeters

• Simple Altimeter: For small aircraft at low altitudes.
• Sensitive Altimeter: More accurate at higher altitudes.
• Servo-assisted Altimeter: Incorporates motors for precise measurements.

Instrument Errors

• Errors can arise from mechanical wear and pressure measurement inaccuracies.
• Calibration Table: Used to apply corrections for altitude, airspeed, and configuration.

Static Port Blockage

• If static port is blocked, altimeter reading freezes at the last indicated altitude.
• Alternate static source is available to correct this situation.

Conclusion

• Understanding altimeter mechanics and adjustments is vital for accurate altitude readings in flight.