alaskan9974
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High Pressure Grounds 737's

Fri Jan 27, 2012 6:33 pm

Not sure how common this is? It happened at our local airport though. Apparently pressure was too high to set altimeters and the airline had to contact Boeing to get an exemption so they could fly all of their aircraft out of PAOT. A high pressure system has settled up here though, temperatures have been in the -50's. I copied a link to the article below.
http://www.thearcticsounder.com/arti...igh_pressure_grounds_planes_at_otz
 
roseflyer
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RE: High Pressure Grounds 737's

Fri Jan 27, 2012 7:05 pm

This does happen on occasion. It isn't that the airplane is not safe to fly, but rather the airplane is outside the envelope of the performance charts. It is more common in extreme heat. I've never heard of it due to pressure. I have heard of it happening due to low temperature. There are many limitations on an airplane and the performance charts have limits for reasons. Exemptions can be made and it looks like this was the case.

I can't imagine high atmospheric pressure pushing the airplane against one of its operating limitations like high temperature and low density can. However it can cause problems with various indicating systems as the whole pitot static system is quite sensitive.
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yeelep
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RE: High Pressure Grounds 737's

Fri Jan 27, 2012 7:41 pm

In this case, the altimeters could not be physically set. The adjustment tops out at 31.99in-hg.
 
Northwest727
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RE: High Pressure Grounds 737's

Fri Jan 27, 2012 7:45 pm

Quoting RoseFlyer (Reply 1):
I've never heard of it due to pressure.

Yes, pressure can exceed the charts, with a legal maximum of 31.00 inHg according 14 CFR 91.144. Reference AIM 7-2-2(a)(2)(a) and 7-2-4 as far as the procedures required when the barometric pressure exceeds 31.00 inHg.
 
yeelep
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RE: High Pressure Grounds 737's

Fri Jan 27, 2012 9:05 pm

Quoting yeelep (Reply 2):
The adjustment tops out at 31.99in-hg.

Meant to type 30.99in-hg.
 
alaskan9974
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RE: High Pressure Grounds 737's

Fri Jan 27, 2012 10:26 pm

Quoting RoseFlyer (Reply 1):
This does happen on occasion. It isn't that the airplane is not safe to fly, but rather the airplane is outside the envelope of the performance charts. It is more common in extreme heat. I've never heard of it due to pressure.

I understand it is still safe to fly I just found this interesting since it is the first I have heard of it happening. It is still in the -50's here. It is really cool to see the planes taking off with thick contrails from the exhaust, both piston and turbine powered aircraft have them.

[Edited 2012-01-27 14:27:47]
 
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Scooter01
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RE: High Pressure Grounds 737's

Tue Jan 31, 2012 11:11 pm

I hear on the news today that a planned exersise in northern Sweden, at F 21, flying the JAS-39 Gripen, has been cancelled for safety-reasons as a high-pressure system (some reports say the highest in 40 years) makes the altimeter and airspeed readings unreliable.

In Swedish: http://sverigesradio.se/sida/artikel.aspx?programid=98&artikel=4938381

Scooter01

Added info: I also understand that the cancellation is only for the JAS-39. The Finnish (F/A-18?) and Norwegian (F-16?) aircraft should be able to participate as planned.

[Edited 2012-01-31 15:50:50]

[Edited 2012-01-31 15:54:30]
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HAWK21M
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RE: High Pressure Grounds 737's

Mon Feb 06, 2012 6:07 am

Quoting RoseFlyer (Reply 1):
It is more common in extreme heat

I've heard of temperature limitations but pressure liminations is a first.

Quoting yeelep (Reply 4):

Meant to type 30.99in-hg.

any reason why the Altimeter has this limit & not higher when such situations can arise.
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BAE146QT
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RE: High Pressure Grounds 737's

Mon Feb 06, 2012 7:49 pm

Quoting HAWK21M (Reply 7):
any reason why the Altimeter has this limit & not higher when such situations can arise.

Just a guess... Maybe when the aircraft was designed and certificated, no-one foresaw that later models might be able to fly in those conditions?

At the opposite end, it would be interesting to know whether the WC-130s that wander around in hurricanes have an altimeter that can go below 29.40.

I hve neither a real 737NG nor a WC-130 to check, unfortunately!
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HAWK21M
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RE: High Pressure Grounds 737's

Tue Feb 07, 2012 9:34 am

Quoting BAe146QT (Reply 8):
Just a guess... Maybe when the aircraft was designed and certificated, no-one foresaw that later models might be able to fly in those conditions

Upgrading a model Altimeter should not be a tough ask though......
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pilotpip
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RE: High Pressure Grounds 737's

Wed Feb 08, 2012 7:29 pm

Quoting HAWK21M (Reply 9):

Upgrading a model Altimeter should not be a tough ask though......

Is it worth the cost of certification for an event that rarely happens? Temperature limitations are a great example and can be exceeded on both ends of the spectrum. I once sat in YEG for almost 8 hours waiting for it to get above -40 because it's a limitation on the 170/190.
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romeomike
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RE: High Pressure Grounds 737's

Wed Feb 08, 2012 10:59 pm

I've seen it at the low end as well. A Jazz Dash-8 told me they couldn't leave until the pressure got to 28.17. It was 28.12 at the time (still the lowest I've seen.)

Perhaps someone can shed some light on that lower limit -- 28.17 seems like a fairly random number to me, but I assume there's a reason.
 
Fabo
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RE: High Pressure Grounds 737's

Wed Feb 08, 2012 11:44 pm

Quoting romeomike (Reply 11):
28.17 seems like a fairly random number to me, but I assume there's a reason.

954hPa. Random, as well...
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