Why do Altimeters Max out at 31.00"?

S

SierraPilot123

Well-Known Member
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.
 
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
 
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".


Chris_Ford said:
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
Click to expand...
 
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.
 
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.
 
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.
 
SierraPilot123 said:
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".
Click to expand...

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.
 
Chris_Ford said:
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
Click to expand...
I can't believe you actually remember your highest altimeter setting...

:)
 
Dazzler said:
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.
Click to expand...
which is one reason FL 180 is usually unuseable
 
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.
 
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