Pressure Pattern Flying is a method used to calculate the Drift experienced by the aircraft.
It is a process of calculating the aircraft drift by comparing the heights between pressure altimeter when set to 1013.2 h.p. and Radar altimeter and using general rules.
Some general rules are as follows
It is assumed that the Geostrophic winds blow along Isobars (equal Pressure) at high levels.
It is further assumed that
In the Northern Hemisphere the wind blows in Clockwise direction in a High Pressure area and in an Anti clockwise direction in a Low Pressure area.
In the Southern Hemisphere the wind blows in Anti Clockwise direction in a High Pressure area and in a Clockwise direction in a Low Pressure area.
ZN = K × (D2-D1) ÷ TAS
Where
ZN = Crosswind Component
K = 21.47 ÷ Sine Mean Latitude
D = Z – ZP
Where Z = RADIO/RADAR ALTITUDE
ZP = PRESSURE ALTITUDE
The drift can be calculated by calculating the D value, which can be calculated by using Radio altimeter and Pressure Altimeter readings.
Example
- In a flight Aircraft with a TAS of 230 kts at 40° North Latitude.
Following readings were taken
At 1900 hrs
Pressure Altimeter 10,000 feet Radio Altimeter 10,200 feet
At 2000 hrs
Pressure Altimeter 10,000 feet Radio Altimeter 9,700 feet
To calculate
At 1900 hrs
D1- Z – ZP or 10,200 – 10,000 = 200 feet
At 2000 hrs
D2- Z – ZP or 9,700 – 10,000 = – 300
Since we know that the value of D is decreasing with the time (value of D2 is lower than D1), it can be found that the Aircraft is traveling from an area of High Pressure to an area of Low Pressure.
Since the Aircraft is traveling in the Northern Hemisphere and in a Low Pressure area the wind blows in the Anti Clockwise direction, therefore an Aircraft will observe Starboard drift while passing through this area.
On using the value in the calculations
ZN = K × (D2 – D1) ÷ TAS = 21.47÷Sine 40 × (-300 – (200)) ÷230 = 72.61 or roughly 73 N.M.
This shows that the Aircraft has drifted 73 N.M towards Starboard/right side of the track.
On further calculating using the One in Sixty Rules the Drift angle and Closing Angle can also be found.
The same Aircraft has traveled for one hour getting ground speed of 230 kts with 340 N.M distance to go to reach the base station.
Track Error = 60 ×Distance off Track ÷ Distance Covered
Using 60 × 73 ÷ 230 = 19° Drift towards Starboard
Closing Angle = 60 × Distance off Track ÷ Distance To Go
Using 60 × 73 ÷ 340 = 12.88 or 13° Closing Angle.
Therefore to reach the station the Drift Angle has to be added to the Closing Angle to get the Correction Angle.
19° +13° = 32°
Therefore the Correction to be applied is 32° towards port.
If the Aircraft was flying a heading of 200° it must fly a heading of 168° to reach the station, provided the Winds remains the same as experienced earlier in the flight.