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ananoman · January 25th, 2004, 10:12

[ QUOTE ]
Isn't that why you're supposed to change the altimeter setting to the local altimeter setting at the destination airport?

[/ QUOTE ]

If you set the correct altimeter setting, your altimeter will read the correct altitude when on the airport surface. This is not the problem. The problem is that if non-standard temperature exist, at any altitude above the surface your altimeter will be in error.

We have all heard that atmospheric pressure decreases by about 1" Hg per 1,000' increase in altitude. This is only true in a 'standard' atmosphere. If it is very hot, then the decrease in pressure will be less. It may for example only decrease .9" per 1,000'. This will not cause much of a problem, you will be slightly higher than you are supposed to be. The real danger is when it is extremely cold. If it is cold, the decrease in pressure will be more than 1" per thousand feet, and you will be low. (Remember your altimeter relies on the 'standard' lapse rate to gauge your altitude.) Enroute this is not a problem. Everyone else will have the same altimeter error, and separation will be maintained. During an instrument approach it has the potential to kill you.

In the AIM, section 7-2-3, there is a table "ICAO Cold Temperature Error Table", which tells you how much to increase your MDA over what the approach plates indicate in very cold weather. Using an example from the chart we can see that even if we set our altimeter to the correct altimeter setting, we can experience potentially fatal errors. If we are planning to fly a circling approach with a circling MDA of 1000' AGL and it is -50 C at the surface, then we will have to add 300' to our MDA. If we do not do this, we will be 300' low and could hit an obstacle or terrain. (Remember that the guaranteed obstacle clearance during a circling approach is 300'.)

VFR this would not usually be a problem, but it could be of interest if you plan on flying over mountainous terrain. Looking at the chart, if you set in the correct altimeter setting, then take off and climb to an indicated altitude of 5,000' AGL over the airport when the reported temperature is -50 C, then you will be 1500' low. This error decreases with an increase in temperature, and at +10 C, the error is only 90'.

January 25th, 2004, 10:12	#17
ananoman Senior Member Join Date: Dec 2002 Posts: 916	Re: True v Indicated Altitude [ QUOTE ] Isn't that why you're supposed to change the altimeter setting to the local altimeter setting at the destination airport? [/ QUOTE ] If you set the correct altimeter setting, your altimeter will read the correct altitude when on the airport surface. This is not the problem. The problem is that if non-standard temperature exist, at any altitude above the surface your altimeter will be in error. We have all heard that atmospheric pressure decreases by about 1" Hg per 1,000' increase in altitude. This is only true in a 'standard' atmosphere. If it is very hot, then the decrease in pressure will be less. It may for example only decrease .9" per 1,000'. This will not cause much of a problem, you will be slightly higher than you are supposed to be. The real danger is when it is extremely cold. If it is cold, the decrease in pressure will be more than 1" per thousand feet, and you will be low. (Remember your altimeter relies on the 'standard' lapse rate to gauge your altitude.) Enroute this is not a problem. Everyone else will have the same altimeter error, and separation will be maintained. During an instrument approach it has the potential to kill you. In the AIM, section 7-2-3, there is a table "ICAO Cold Temperature Error Table", which tells you how much to increase your MDA over what the approach plates indicate in very cold weather. Using an example from the chart we can see that even if we set our altimeter to the correct altimeter setting, we can experience potentially fatal errors. If we are planning to fly a circling approach with a circling MDA of 1000' AGL and it is -50 C at the surface, then we will have to add 300' to our MDA. If we do not do this, we will be 300' low and could hit an obstacle or terrain. (Remember that the guaranteed obstacle clearance during a circling approach is 300'.) VFR this would not usually be a problem, but it could be of interest if you plan on flying over mountainous terrain. Looking at the chart, if you set in the correct altimeter setting, then take off and climb to an indicated altitude of 5,000' AGL over the airport when the reported temperature is -50 C, then you will be 1500' low. This error decreases with an increase in temperature, and at +10 C, the error is only 90'.