Estimation Formulas - True Airspeed, Descent Rate, Descent Altitude, Cloud Ceiling, and Drift Angle

2014-05-11 | Aviation Knowledge Notes
1. Cloud Base Ceiling
Cloud Base (ft) = 4 * (Surface Temperature - Surface Dew Point) * 100

Example: Surface temperature 25°C, dew point 20°C, Cloud Base = 2000 ft

2. Rate of Descent
Rate of Descent (ft/min) = Ground Speed * 5

Example: Ground Speed is 140 knots, Rate of Descent is 700 ft/min.

3. Estimating Altitude Based on Distance to Touchdown (3° Descent Angle)
Altitude (ft) = Distance (nm) * 300

Example: 5 nm * 300 = 1500 ft. This means when the aircraft is 5 nm from the airport, the altitude should be 1500 ft.
3 nm * 300 = 900 ft, and so on.

4. Track Angle Error TKE

Track Angle Error = XTK (Cross-track deviation) * 60 / Distance between two check points
For example, if flying 20 nm from A to B, and the cross-track deviation at point B is 2 nm, the Track Angle Error is 6 degrees.

The method above is suitable for mental calculation. Of course, if you have a scientific calculator, simply calculating the inverse tangent is easier. For example:
tan TKE = XTK / D = 0.1
Therefore, the calculator gives a TKE of 5.7 degrees.

5. True Airspeed and Indicated Airspeed

At low to medium altitudes, for every 1000 meters of altitude gained, True Airspeed is approximately 5% higher than Indicated Airspeed. This corrects for the error caused by changes in air pressure.
If IAS is 370 and flight altitude is 5000 meters:
TAS = 370 + 370 * 5 * 5% = 463

6. Wind Speed Conversion
1 m/s = 1.944 kt ≈ 2 kt
Example: A wind speed of 8 m/s is approximately 16 knots.

7. Maximum Drift Angle
The Drift Angle is maximum when the Wind Angle (WA) is 90 degrees. DA max ≈ 60 / TAS * Wind Speed
Example: True Airspeed 120 knots, Wind Speed 12 knots, Maximum Drift Angle is 6 degrees.

8. Drift Angle
DA = DA max * sin WA
Example: Magnetic Course (MC) is 90 degrees, TAS is 180 knots, Wind Direction is 135 degrees, Wind Speed is 40 knots.
Then DA max = 60 / 180 * 40 = 13 degrees
DA = 13 * sin 45 = 13 * 0.7 = 9 degrees

9. Ground Speed
GS = TAS + WS * cos WA = TAS + WS * sin (90 - WA)
Using the previous example, GS = 180 + cos 45 * 40 = 180 + 0.7 * 40 = 208 knots