Barometric Pressure Changes

[Michael]

Hypothetical question:
Lets say you want to depart an airport at night VFR and you need to cross some high mountains 5-10 miles out. There is a front passing between the airport and the mountains. There is no other airport within 50 miles on your route. Your plan is to continue the climb out and pass over the mountains at 500 feet above them. class bravo shelf sits atop that. Can the barometric pressure change that dramastically and put you in danger of colliding with the terrain?

 

[Joe B]

Hypothetically, yes, but not likely. Regardless, I wouldn't think of crossing mountains at night with only 500' of ground clearance. Hit a downdraft and you could be in the rocks before you knew what happened. Get clearance to climb through the Class B, find a pass, or go around.

Regards,
Joe

 

[Skip Miller]

Hypothetical question:
....Can the barometric pressure change that dramastically and put you in danger of colliding with the terrain?

Not so hypothetical.

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001207X04839&key=1

This is not 500 feet but the stale altimeter setting was listed by the NTSB as a contributing factor. This is only 76 feet off. I remember that night well, a very powerful gust front came through and killed the power at our house at exactly the time this plane landed at KBDL 16 miles away (clocks stopped, easy to tell when it came through...). The BDL tower was closed because the winds had blown the tower window glass out of the metal frames before this flight came in.

-Skip

 

[ejensen]

I was IFR from St. George thru Las Vegas one night. Over Mesquite ATC started chewing on me for being 800' off altitude. Then they gave me the Vegas altimeter. I now showed 800' high. Coming the other way I wouldn't have wanted to be crossing with just 500' under me. Maybe that's why MEAs are 2000' above terrain in the mountains.

 

[AirBaker]

Just out of curiousity, the Baro settings from a flight now from San Jose to Phoenix direct, range from 30.01 to 29.88. For a .13 difference, that would put you off by 130 feet right?

 

[fgcason]

Close approximation: 0.01" Hg = 10ft thus 500ft = 0.5" Hg change. For 10 miles that 0.05" per mile. At 5 miles that's 0.1"/mile. That's a rather serious pressure gradient and I don't recall ever seeing one that steep. If there was one on a chart somewhere, it probably wasn't very stable. Possible? Probably not outside of some rare extreme conditions.
If the gradient is that steep, assuming one took off into that in the first place, one would have a lot of other things (high winds, severe turbulence) to worry about long before getting to the rocks.

 

[Ed Guthrie]

Hypothetical question:
Lets say you want to depart an airport at night VFR and you need to cross some high mountains 5-10 miles out. There is a front passing between the airport and the mountains. There is no other airport within 50 miles on your route. Your plan is to continue the climb out and pass over the mountains at 500 feet above them. class bravo shelf sits atop that. Can the barometric pressure change that dramastically and put you in danger of colliding with the terrain?

I would worry more about frontal generated winds and rotors on the mountain; 500' isn't enough buffer given the conditions you expect.

 

[naughtius]

As of now, we have a 0.50 inch difference between a plain airport's ATIS and a Mountain AWOS, here in Colorado.

 

[poadeleted20]

I would worry more about frontal generated winds and rotors on the mountain; 500' isn't enough buffer given the conditions you expect.

I'm with Ed. If the winds are strong enough to bugger the altimeter settings, they're going to grab you and toss you to the ground like a crumpled piece of paper at that altitude, and your altimeter setting will be irrelevant. You'd need to be at least 2000 above the rocks to be reasonably safe, and you aren't going to see 2 inches of pressure error.

 

[wsuffa]

I agree. Rare is the time that I'd fly at 500' over the mountain, and then only if I'm doing photos or other stuff under carefully controlled visual conditions.

I have heard ATIS broadcasts noting "caution: rapidly dropping barometer settings, check altitude with tower when on approach"

 

[gismo]

I'm with Ed. If the winds are strong enough to bugger the altimeter settings, they're going to grab you and toss you to the ground like a crumpled piece of paper at that altitude, and your altimeter setting will be irrelevant. You'd need to be at least 2000 above the rocks to be reasonably safe, and you aren't going to see 2 inches of pressure error.

You (and Ed, nice to see you agreeing on something for a change) are probably correct on the turbulence being a bigger issue then the altimeter error. That said I think it might be possible to have smooth airflow over a ridge under the right conditions (stability of the air itself being a big factor) with relatively high wind. And such a condition could lead to a wind velocity induced air pressure difference. IIRC the lee side would have lower pressure than air at the same level but away from the ridge and that would give an altimeter error which would put you closer to the ground.

All of this seems moot to me as I would never plan a 500 ft clearance on any mountain ridge at night, that's just too close for comfort unless the ridge was well lit, I was familiar with the area, and the visibility was excellent.