Why do Altimeters Max out at 31.00"?

SierraPilot123 · October 13th, 2006, 11:46

When the atmosphere has been known to exceed this setting.

There has to be a reason or logic behind this right?

7-2-4. High Barometric Pressure

a. Cold, dry air masses may produce barometric pressures in excess of 31.00 inches of Mercury, and many altimeters do not have an accurate means of being adjusted for settings of these levels. When the altimeter cannot be set to the higher pressure setting, the aircraft actual altitude will be higher than the altimeter indicates.

Chris_Ford · October 13th, 2006, 12:12

How often have you seen an altimeter above 31? Perhaps wayyyyyyy up north, but I have never seen it even close. Highest I've ever seen is 30.83

SierraPilot123 · October 13th, 2006, 12:16

True. But there are quite a lot of detailed regs both IFR and VFR for when it does exceed 31. It seems much easier and more logical to extend the Kolhsman window to 32.

There must be an error or something else that is keeping manufactures from build one that reaches 31".

Quote:

Originally Posted by Chris_Ford

How often have you seen an altimeter above 31? Perhaps wayyyyyyy up north, but I have never seen it even close. Highest I've ever seen is 30.83

Dazzler · October 13th, 2006, 15:48

Maybe it's something to do with vertical spacing of aircraft.

Example:

If you're at FL180, your alt. setting is 29.92 and your altimeter is indicating 18,000 feet MSL.
Then you descend to 17,000 feet MSL, you would set your alt. to 31.00, and it would indicate over 18,000 feet MSL.

You've physically descended, but have climbed according to the altimeter.

seagull · October 13th, 2006, 22:35

Happens when it gets really cold. It's a factor for steam guages. Don't know about the other types, but the MD11 altimeter settings go well beyond that, so it's not a factor for us.

ananoman · October 14th, 2006, 21:04

I'll have to crank on the altimeter next time I fly and see how far I can get mine to go. In the 800XP we have a limitation that you can't take off or land if the pressure altitude is less than -2,000' or more than 13,000'.

One thing that has to be considered is the cost factor. If you have EFIS it probably doesn't cost any more to incorporate settings beyond 31". If you have steam gauges, you will be paying money for something you may never use. Those mechanical gauges have lots of little parts and reducing their range of adjustment would make things simpler overall. This usually means cheaper.

sopdan · October 16th, 2006, 21:43

Quote:

Originally Posted by SierraPilot123

True. But there are quite a lot of detailed regs both IFR and VFR for when it does exceed 31. It seems much easier and more logical to extend the Kolhsman window to 32.

There must be an error or something else that is keeping manufactures from build one that reaches 31".

My guess is that if they went up to 32", then the regs would simply be for when the alt. setting exceeds 32". They probably chose 31" because that would suffice for 99.999% of operations.

Ian J · October 17th, 2006, 09:48

Quote:

Originally Posted by Chris_Ford

How often have you seen an altimeter above 31? Perhaps wayyyyyyy up north, but I have never seen it even close. Highest I've ever seen is 30.83

I can't believe you actually remember your highest altimeter setting...

Chris_Ford · October 17th, 2006, 11:48

Quote:

Originally Posted by ChinookDriver

I can't believe you actually remember your highest altimeter setting...

When it takes you 45 seconds to set the altimeter, you have a tendency to remember these things

Not to mention I have a practically photographic memory

Ian J · October 17th, 2006, 11:58

Quote:

Originally Posted by Chris_Ford

Not to mention I have a practically photographic memory

Dang... must making studying and test taking pretty easy, lucky bastard.

Chris_Ford · October 17th, 2006, 12:12

Quote:

Originally Posted by ChinookDriver

Dang... must making studying and test taking pretty easy, lucky bastard.

What's studying? You mean when you go out drinking the night before the test?

/true story about the SAT

ananoman · October 17th, 2006, 20:57

I just checked, ours goes to 32.5" woo-hoo! Or something.

mbtrebbe · October 28th, 2006, 14:12

Quote:

Originally Posted by Dazzler

Maybe it's something to do with vertical spacing of aircraft.

Example:

If you're at FL180, your alt. setting is 29.92 and your altimeter is indicating 18,000 feet MSL.
Then you descend to 17,000 feet MSL, you would set your alt. to 31.00, and it would indicate over 18,000 feet MSL.

You've physically descended, but have climbed according to the altimeter.

which is one reason FL 180 is usually unuseable

flyguy · October 29th, 2006, 01:05

I don't know the reg off the top of my head, but a TFR is supposed to be issued for all flights except emergency, if the baro goes above 31.00. Altimeter setting also don't go below 28.00, but if the pressure is that low you probably shouln't be flying anyway as it would probably be a hurricane or something.

ETAV8R · October 30th, 2006, 03:42

Quote:

Originally Posted by Chris_Ford

What's studying? You mean when you go out drinking the night before the test?

/true story about the SAT

Thanks.
Nice to meet you lucifer

October 13th, 2006, 11:46	#1
SierraPilot123 Senior Member Join Date: Aug 2005 Location: Bay Area Posts: 384	Why do Altimeters Max out at 31.00"? When the atmosphere has been known to exceed this setting. There has to be a reason or logic behind this right? 7-2-4. High Barometric Pressure a. Cold, dry air masses may produce barometric pressures in excess of 31.00 inches of Mercury, and many altimeters do not have an accurate means of being adjusted for settings of these levels. When the altimeter cannot be set to the higher pressure setting, the aircraft actual altitude will be higher than the altimeter indicates.

October 13th, 2006, 12:12	#2
Chris_Ford Banned Join Date: Jul 2004 Location: Phoenix, AZ Posts: 7,329	Re: Why do Altimeters Max out at 31.00"? How often have you seen an altimeter above 31? Perhaps wayyyyyyy up north, but I have never seen it even close. Highest I've ever seen is 30.83

October 13th, 2006, 15:48	#4
Dazzler Senior Member Join Date: Jan 2004 Location: Minneapolis, MN Posts: 936	Re: Why do Altimeters Max out at 31.00"? Maybe it's something to do with vertical spacing of aircraft. Example: If you're at FL180, your alt. setting is 29.92 and your altimeter is indicating 18,000 feet MSL. Then you descend to 17,000 feet MSL, you would set your alt. to 31.00, and it would indicate over 18,000 feet MSL. You've physically descended, but have climbed according to the altimeter. __________________ CSEL-IA AGI IGI CFI CFII CFI Wage per hour = $10 Cost to maintain CFI privileges = $250 Watching a student do their first solo = Priceless

October 13th, 2006, 22:35	#5
seagull Old Skool Join Date: Sep 2001 Posts: 2,045	Re: Why do Altimeters Max out at 31.00"? Happens when it gets really cold. It's a factor for steam guages. Don't know about the other types, but the MD11 altimeter settings go well beyond that, so it's not a factor for us.

October 14th, 2006, 21:04	#6
ananoman Senior Member Join Date: Dec 2002 Posts: 916	Re: Why do Altimeters Max out at 31.00"? I'll have to crank on the altimeter next time I fly and see how far I can get mine to go. In the 800XP we have a limitation that you can't take off or land if the pressure altitude is less than -2,000' or more than 13,000'. One thing that has to be considered is the cost factor. If you have EFIS it probably doesn't cost any more to incorporate settings beyond 31". If you have steam gauges, you will be paying money for something you may never use. Those mechanical gauges have lots of little parts and reducing their range of adjustment would make things simpler overall. This usually means cheaper.

October 17th, 2006, 20:57	#12
ananoman Senior Member Join Date: Dec 2002 Posts: 916	Re: Why do Altimeters Max out at 31.00"? I just checked, ours goes to 32.5" woo-hoo! Or something.

October 29th, 2006, 01:05	#14
flyguy Old Skool Join Date: Apr 2002 Location: Denver, CO Posts: 2,094	Re: Why do Altimeters Max out at 31.00"? I don't know the reg off the top of my head, but a TFR is supposed to be issued for all flights except emergency, if the baro goes above 31.00. Altimeter setting also don't go below 28.00, but if the pressure is that low you probably shouln't be flying anyway as it would probably be a hurricane or something. __________________ "Roads?...Where we're going we don't need roads."

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