RTC – MOD-03 – Pt.04 – Why is the correct Barometric Pressure important
If 2 aircraft have different Barometric Pressures set they may think there is vertical separation between themselves, when there actually isn’t.
Although not exact, to work out a difference between pressures use the following below:
- 1 hPa = 30ft
- 0.01 InHg = 10 ft
- 1 mmHg = 40ft/12m
So as you can see from the above representation the difference between 1028 hPa and 995 hPa is 33 hPa
33 x 30 = 990ft
As pressure decreases with altitude we can extrapolate that a pressure setting of 995 hPa is above that of a pressure setting of 1028 hPa by 990ft
To put this into another scenario:
The visual circuit at Al Minhad is 1000ft on QFE 1002 hPa, an aircraft has been given permission to fly over the Visual Circuit 1000ft above. That aircraft is flying on 1013 hPa, what altitude to the nearest 500ft will the aircraft need to fly to ensure that it is 1000ft above the circuit?
To work this out we need to work out the difference between 1013 and the QFE at Al Minhad
1013 – 1002=11
11 x 30=330ft
So there is 330ft difference between the pressure settings.
The pressure at Al Minhad is also below 1013, meaning it is “at a higher elevation” by 330ft (Lower pressures are always “higher” because there is less substance above pushing down on that given point), meaning, an aircraft at 1000ft in the visual circuit at Al Minhad is 330ft above an aircraft flying at 1000ft on 1013 hPa.
Now we need to find an altitude to the nearest 500ft that clears the Visual Circuit by at least 1000ft. We can easily work out that at 2000ft 1013 hPa would only give 670ft clearance above the Visual Circuit. That means 2500ft on 1013hPa is the minimum altitude that the aircraft can fly above the Visual Circuit.