91.144 Abnormally high barometric settings

FlyingMoose

FlyingMoose

Well-Known Member
Temporary restriction on flight operations during abnormally high barometric pressure conditions.

(a) Special flight restrictions. When any information indicates that barometric pressure on the route of flight currently exceeds or will exceed 31 inches of mercury, no person may operate an aircraft or initiate a flight contrary to the requirements established by the Administrator and published in a Notice to Airmen issued under this section.


I was just wondering if anyone has the actual reason as to why this FAR exists. What could the safety implications be?
 
I know yesterday we set a barometric low record here in the Twin Cities, equivalent to a Category 3 Hurricane. They were calling this massive storm we have here an "inland hurricane" because it resembled a hurricane so much (rain, high winds, crazy barometric pressure readings).
 
Mike Lewis said:
I know yesterday we set a barometric low record here in the Twin Cities, equivalent to a Category 3 Hurricane. They were calling this massive storm we have here an "inland hurricane" because it resembled a hurricane so much (rain, high winds, crazy barometric pressure readings).
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That's crazy because that is what got us discussing this topic. We were wondering if there was also a Low pressure cut off.
 
FlyingMoose said:
Temporary restriction on flight operations during abnormally high barometric pressure conditions.

(a) Special flight restrictions. When any information indicates that barometric pressure on the route of flight currently exceeds or will exceed 31 inches of mercury, no person may operate an aircraft or initiate a flight contrary to the requirements established by the Administrator and published in a Notice to Airmen issued under this section.


I was just wondering if anyone has the actual reason as to why this FAR exists. What could the safety implications be?
Click to expand...

Next time you are in your airplane see how high the altimeter can be set. If the pressure is higher than that, your altimeter isnt telling you how far away the ground is anymore.
 
Since the Mach scale is a function of both pressure and velocity, I wonder if the upper limit of the air pressure plays into the Mach speed rating of an aircraft.

That is, if you normally could do Mach 0.85 at 550 kts under normal air pressure but under high pressure you might be pushing Mach 1 at 500 kts, all other conditions being equal. (Disregard my math, as I don't feel like digging out my Physics textbook, but that's the general idea.)

I recall someone screwing up the pressure calculations in college and the professor responding "At that pressure, you're probably under water." :D
 
Mike Lewis said:
I know yesterday we set a barometric low record here in the Twin Cities, equivalent to a Category 3 Hurricane. They were calling this massive storm we have here an "inland hurricane" because it resembled a hurricane so much (rain, high winds, crazy barometric pressure readings).
Click to expand...

Lowest I saw yesterday was 28.32
 
FlyingMoose said:
Temporary restriction on flight operations during abnormally high barometric pressure conditions.

(a) Special flight restrictions. When any information indicates that barometric pressure on the route of flight currently exceeds or will exceed 31 inches of mercury, no person may operate an aircraft or initiate a flight contrary to the requirements established by the Administrator and published in a Notice to Airmen issued under this section.


I was just wondering if anyone has the actual reason as to why this FAR exists. What could the safety implications be?
Click to expand...

Basically what they said boils down to the top of the Kollsman window is 31.00"
 
I was looking at my altimeter when all that was pushing through the Midwest. The bottom of my Kollsman window was 28.00".
 
Polar742 said:
Basically what they said boils down to the top of the Kollsman window is 31.00"
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In most aircraft types. MD11 can set it much higher or lower, but, because other aircraft are limited, we have to use those limits until we get on final.
 
braunpilot said:
I was looking at my altimeter when all that was pushing through the Midwest. The bottom of my Kollsman window was 28.00".
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I suppose to be technical and hypothetical, the limit was established to be 31.00" eons ago, but (and please correct me if I'm wrong here due to some technical impossibility) there's nothing to say that the limit could not be set at 31.50" or whatever other arbitrary number was desired is there?


FL180 becomes unusable when the pressure is BELOW 29.92, not above.
 
OldTownPilot said:
And likewise for the same reasons FL190 is not available when the pressure is below 28.92"
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yup


4-5-4. LOWEST USABLE FLIGHT LEVEL
If a change in atmospheric pressure affects a usable
flight level in your area of jurisdiction, use
TBL 4-5-2 to determine the lowest usable flight
level to clear aircraft at or above 18,000 feet MSL.

TBL 4-5-2
Lowest Usable FL
Altimeter Setting Lowest Usable FL
29.92” or higher 180
29.91” to 28.92” 190
28.91” to 27.92” 200
REFERENCEFAAO
JO 7110.65, Para 9-3-2, Separation Minima.

4-5-5. ADJUSTED MINIMUM FLIGHT
LEVEL
When the prescribed minimum altitude for IFR
operations is at or above 18,000 feet MSL and the
atmospheric pressure is less than 29.92”, add the
appropriate adjustment factor from TBL 4-5-3 to the
flight level equivalent of the minimum altitude in feet
to determine the adjusted minimum flight level.

TBL 4-5-3
Minimum FL Adjustment
Altimeter Setting Adjustment Factor
29.92” or higher None
29.91” to 29.42” 500 feet
29.41” to 28.92” 1,000 feet
28.91” to 28.42” 1,500 feet
28.41” to 27.92” 2,000 feet
4-5-6. MINIMUM EN ROUTE ALTITUDES
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same reason why standard temp/pressure is 15 degrees C/ 29.92...it was an number that "they" decided to pick, there is no significance for why these are "standard".

they just picked a number 31.00" ok thats the highest....next
 
Patrick said:
I suppose to be technical and hypothetical, the limit was established to be 31.00" eons ago, but (and please correct me if I'm wrong here due to some technical impossibility) there's nothing to say that the limit could not be set at 31.50" or whatever other arbitrary number was desired is there?


FL180 becomes unusable when the pressure is BELOW 29.92, not above.
Click to expand...
I don't fly up into the FL...ever....my C210 isn't turbocharged and I don't have O2 so being Part 135 I'm restricted to 10,000 and below. Otherwise I agree, although I do think that they did take the time to look up the lowest and highest settings then do a statical analysis of where 95% of all altimeter settings are found. Just my .02, YMMV.
 
FlyingMoose said:
Temporary restriction on flight operations during abnormally high barometric pressure conditions.

(a) Special flight restrictions. When any information indicates that barometric pressure on the route of flight currently exceeds or will exceed 31 inches of mercury, no person may operate an aircraft or initiate a flight contrary to the requirements established by the Administrator and published in a Notice to Airmen issued under this section.


I was just wondering if anyone has the actual reason as to why this FAR exists. What could the safety implications be?
Click to expand...

If I remember right, this has to do with the maximum altimeter setting possible on the actual altimeter. I remember I had somewhere around 3090 in Juneau last winter. Our EFIS could handle it, but the backup altimeter couldn't.
 
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