Altimeter 3066

A very high pressure settled in Pacific NW:

KPSC 020453Z 31006KT 6SM BR BKN007 BKN075 M07/M08 A3066 RMK AO2
SLP389 T10671078

That's the highest that I've ever remember seeing.

I always thought that the NW seems to have larger pressure swings
(probably more so in Alaska). We can go from 2960 to 3060 in about two
days.

BTW, FAR 91.144 restricts flight operations when barometric pressure
exceeds 31 inch mercury. Any idea why? (note this restriction applies
to VFR traffic a well).


M wrote:
A very high pressure settled in Pacific NW:

KPSC 020453Z 31006KT 6SM BR BKN007 BKN075 M07/M08 A3066 RMK AO2
SLP389 T10671078

That's the highest that I've ever remember seeing.

I always thought that the NW seems to have larger pressure swings
(probably more so in Alaska). We can go from 2960 to 3060 in about two
days.

"M" wrote in message
ps.com...

BTW, FAR 91.144 restricts flight operations when barometric pressure
exceeds 31 inch mercury. Any idea why? (note this restriction applies
to VFR traffic a well).


The Kollsman Window is only required to have a range of 28.00 to 31.00
inches Hg. So if barometric pressure is outside that range, you cannot
accurately set most altimeters.

John Severyn
@KLVK

I don't believe VFR flight requires accurate altimeter adjustable for
barometric pressure. 91.205 only requires such altimeter for IFR
operations.


J. Severyn wrote:
"M" wrote in message
ps.com...

BTW, FAR 91.144 restricts flight operations when barometric pressure
exceeds 31 inch mercury. Any idea why? (note this restriction applies
to VFR traffic a well).


The Kollsman Window is only required to have a range of 28.00 to 31.00
inches Hg. So if barometric pressure is outside that range, you cannot
accurately set most altimeters.

John Severyn
@KLVK

I don't believe VFR flight requires accurate altimeter adjustable for
barometric pressure. 91.205 only requires such altimeter for IFR
operations.


J. Severyn wrote:
"M" wrote in message
ps.com...

BTW, FAR 91.144 restricts flight operations when barometric pressure
exceeds 31 inch mercury. Any idea why? (note this restriction applies
to VFR traffic a well).


The Kollsman Window is only required to have a range of 28.00 to 31.00
inches Hg. So if barometric pressure is outside that range, you cannot
accurately set most altimeters.

John Severyn
@KLVK

Correct. But you still are required to follow the NOTAM in extremely high
or low barometric pressure conditions. If you read FAR91.144 quoted he

"Sec. 91.144

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.
(b) Waivers. The Administrator is authorized to waive any restriction issued
under paragraph (a) of this section to permit emergency supply, transport,
or medical services to be delivered to isolated communities, where the
operation can be conducted with an acceptable level of safety."

In other words, it says "Read the NOTAM" or "Get a Waiver". Normally the
NOTAM will restrict operation at certain altitudes so that safe altitude
separation can be maintained in spite of the inability to correctly enter
the barometric pressure.

John Severyn
@KLVK


"M" wrote in message
oups.com...
I don't believe VFR flight requires accurate altimeter adjustable for
barometric pressure. 91.205 only requires such altimeter for IFR
operations.


J. Severyn wrote:
"M" wrote in message
ps.com...

BTW, FAR 91.144 restricts flight operations when barometric pressure
exceeds 31 inch mercury. Any idea why? (note this restriction applies
to VFR traffic a well).


The Kollsman Window is only required to have a range of 28.00 to 31.00
inches Hg. So if barometric pressure is outside that range, you cannot
accurately set most altimeters.

John Severyn
@KLVK

"M" wrote in message
oups.com...
I don't believe VFR flight requires accurate altimeter adjustable for
barometric pressure. 91.205 only requires such altimeter for IFR
operations.


AIM 7-2-2 might answer your question about high and low barometric
pressures.
John Severyn
@KLVK

That is the highest pressure you can set on the Kollsman
window.



"M" wrote in message
ps.com...
|
| BTW, FAR 91.144 restricts flight operations when
barometric pressure
| exceeds 31 inch mercury. Any idea why? (note this
restriction applies
| to VFR traffic a well).
|
|
| M wrote:
| A very high pressure settled in Pacific NW:
|
| KPSC 020453Z 31006KT 6SM BR BKN007 BKN075 M07/M08 A3066
RMK AO2
| SLP389 T10671078
|
| That's the highest that I've ever remember seeing.
|
| I always thought that the NW seems to have larger
pressure swings
| (probably more so in Alaska). We can go from 2960 to
3060 in about two
| days.
|

Jim Macklin wrote:
That is the highest pressure you can set on the Kollsman
window.



I can hardly see how it should affect VFR flights. Additionally if the
pressure is higher than 31 inch mercury and the altimeter is set to
3100, the true altitude would be higher than what's indicated by the
altimeter. For VFR flights this would almost never introduce any
problems. (Even for IFR terrain separation this allows a bigger margin
for terrain clearance).

The only problem I could think of is the mode C requirement for
altitude above 10,000 MSL, if someone flying without a mode C x-ponder,
set their altimeter to 3100, on day with pressure higher than 31"
mercury, and flying right near 10,000 feet indicated on their altimeter
:-)

"M" wrote in message
oups.com...
Jim Macklin wrote:
That is the highest pressure you can set on the Kollsman
window.



I can hardly see how it should affect VFR flights. Additionally if the
pressure is higher than 31 inch mercury and the altimeter is set to
3100, the true altitude would be higher than what's indicated by the
altimeter. For VFR flights this would almost never introduce any
problems. (Even for IFR terrain separation this allows a bigger margin
for terrain clearance).

The only problem I could think of is the mode C requirement for
altitude above 10,000 MSL, if someone flying without a mode C x-ponder,
set their altimeter to 3100, on day with pressure higher than 31"
mercury, and flying right near 10,000 feet indicated on their altimeter
:-)


I thought the Mode C transponder reports Pressure Altitude and is basically
fixed at 29.92?