labbadabba
Pattern Altitude
So with pilots at FL180 flying at 29.92 how do you account for pilots flying at 17,500ft with a baro of 30.42?
Proper vertical separation at FL180
- Thread starter labbadabba
- Start date
MarkZ
En-Route
In your scenario, the VFR aircraft flying at 17,500 with a local altimeter of 30.42 would be flying at a pressure altitude of 17,000.
Conversely, had the same aircraft flown with a local altimeter of 29.42, the pressure altitude is then 18,000.
Which is why FL180 is not usable when local altimeter settings dip below 29.92.
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Conversely, had the same aircraft flown with a local altimeter of 29.42, the pressure altitude is then 18,000.
Which is why FL180 is not usable when local altimeter settings dip below 29.92.
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Cpt_Kirk
En-Route
Unusable altitudes are listed with their corresponding pressures in the FAR/AIM.
I have made the mistake of hastily filing FL180 when the altimeter was below 29.92". ATC asked if I would like 16,000 or FL200 and I picked one.
I have made the mistake of hastily filing FL180 when the altimeter was below 29.92". ATC asked if I would like 16,000 or FL200 and I picked one.
MarkZ
En-Route
It's not a mistake in my view. How can you remain certain the local altimeters won't drop below 29.92?
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MAKG1
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This isn't an issue at altimeter settings of 29.92 or greater. As Mark said, in your example, you've got an actual 1,000 ft difference in separation between the two. The higher baro actually helps in this case.
With low settings of less than 29.92, the controller is compensating by issuing a higher usable FL to IFRs. In that case, the concern isn't about maintaining sep from a VFR at 17,500. It's about maintaining the 1,000 ft sep from IFRs at 17,000 ft.
With low settings of less than 29.92, the controller is compensating by issuing a higher usable FL to IFRs. In that case, the concern isn't about maintaining sep from a VFR at 17,500. It's about maintaining the 1,000 ft sep from IFRs at 17,000 ft.
Last edited:
luvflyin
Touchdown! Greaser!
Yup. The other way to "picture" it is the plane at FL180 is at 18,500 when the altimeter is 30.42.