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Question 1

Aviation Instruments & Atmospheric Concepts - Complete Notes

1. Flight Level (FL) vs. Pressure Altitude (PA)
   • Flight Level (FL): Standardized altitude based on QNE (1013.25 hPa).
   • Pressure Altitude (PA): Altitude measured based on actual barometric pressure.

Altitude References:
• Height (AGL) - Measured from ground level using QFE
• Altitude (MSL) - Measured from mean sea level using QNH
• FL or PA - Measured using standard pressure QNE (1013.25 hPa)

2. Altimeter & Related Formulas

2.1 Vibrating Device in Altimeter
• Purpose: To reduce the effect of friction in linkage and improve accuracy.

2.2 Altimeter Formulas

Total Altitude (TA):
TA = PA + (dev x PA) / 250

Where:
• dev = OAT minus ISA
• ISA Temperature Formula:
ISA = 15 - (2 x PA) / 1000

Pressure Altitude (PA):
PA = Elev + [(QNE - QNH) x 30]
PA = (QNE - QFE) x 30

Elevation Formula:
Elevation = (QNH - QFE) x 30

3. Types of Altimeters
4. Simple Altimeter
   • Has one aneroid capsule
   • Basic altitude measurement
   • Prone to lag errors
5. Sensitive Altimeter
   • Has two or more aneroid capsules in series
   • Provides higher accuracy than a simple altimeter
6. Servo Altimeter
   • Uses an electrical pick-off device
   • Removes lag error
   • More precise altitude reading

Accuracy or Tolerance of Servo Altimeter:
• Plus or minus 30 feet at MSL
• Plus or minus 60 feet at 30,000 feet
• Plus or minus 100 feet at 40,000 feet

4. Different Types of Altitude (PITAD Mnemonic)
5. Pressure Altitude (PA):
   • FL (Flight Level) is derived from PA when using QNE (1013.25 hPa).
6. Indicated Altitude:
   • Whatever is displayed on the altimeter.
7. True Altitude (TA):
   • Altitude above MSL when corrected for temperature and pressure variations.
8. Absolute Altitude:
   • Height above the ground (AGL), measured using QFE.
9. Density Altitude (DA):
   • PA corrected for non-standard temperature.
   • Used for takeoff and landing performance calculations at airports.

DA = PA + (120 x dev)

Takeoff performance is highly dependent on air density.
• High air density leads to good performance
• Low air density leads to bad performance

5. Altimeter Errors and Their Impact

5.1 Instrumental Errors
• Caused by manufacturing defects in the altimeter.

5.2 Position or Pressure Errors
• Occurs due to incorrect static port positioning or interference from dynamic pressure.
• Can cause the altimeter to under-read in certain conditions.
• Dynamic pressure effect varies with true airspeed and angle of attack.

Important Rule:
• “High to Low, Look Out Below”
• If flying from high pressure to low pressure, the altimeter overreads, causing a lower-than-actual altitude.

6. Density Errors and Their Impact

Density altitude is affected by temperature and pressure.
• High pressure leads to low density altitude and good aircraft performance
• Low pressure leads to high density altitude and poor aircraft performance
• Low temperature leads to high air density and good performance
• High temperature leads to low air density and bad performance

Formula:
Density Altitude is inversely proportional to temperature at a particular elevation.

Density Altitude is inversely proportional to altitude.

7. Sideslip and Yaw Effect on Altimeter

Sideslip Effects:
• Left Sideslip: Aircraft nose shifts to the right
• Right Sideslip: Aircraft nose shifts to the left

Yaw Effects:
• Left Yaw: Aircraft nose shifts to the left
• Right Yaw: Aircraft nose shifts to the right

8. Lag Error and Hysteresis

Lag Error (Time Lag in Altimeter Reading)
• Altimeter takes time to react due to pressure changes at the static port.
• Aneroid capsule expands or contracts, and movement is transmitted via mechanical linkages.
• Typical lag in altimeter response is 4 to 5 seconds.

Hysteresis (Lag When Maintaining Level Flight)
• If an aircraft flies at a constant altitude for a long period, the altimeter adapts to that condition.
• Sudden climb or descent takes time to reflect.

Solution:
• Use a Servo Altimeter with an electrical pick-off device to eliminate lag errors.

9. Key Takeaways
   • Vibrating devices help reduce friction in linkage.
   • Types of altitude: PITAD (Pressure, Indicated, True, Absolute, Density).
   • Formula for PA, DA, and TA must include correction factors.
   • Takeoff performance is affected by air density and temperature.
   • Density altitude is inversely proportional to temperature and altitude.
   • Sideslip and yaw affect aircraft nose direction.
   • Altimeter has lag due to mechanical linkages (hysteresis effect).
   • Servo altimeter minimizes lag error with electrical corrections.