Adjustments to DA for altimeter correction

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pstan

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Stan
Let say your altimeter reads 45 feet high when on the ground. The altimeter source elevation is the same as your aircraft (so not a temp correction situation).

For an ISA day, would you make an adjustment to your Decision Altitude based on this knowledge?

Any recommendations from relevant country authorities?

thanks,

Stan
 
pstan said:
Let say your altimeter reads 45 feet high when on the ground. The altimeter source elevation is the same as your aircraft (so not a temp correction situation).

For an ISA day, would you make an adjustment to your Decision Altitude based on this knowledge?

Any recommendations from relevant country authorities?

thanks,

Stan
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Altimeter Errors
A sensitive altimeter is designed to indicate standard changes from standard conditions, but most flying involves errors caused by nonstandard conditions and the pilot must be able to modify the indications to correct for these errors. There are two types of errors: mechanical and inherent.
 
It is my belief that altitudes determined for instrument approaches take into account all sources of altimeter error, so the requirement that your altimeter be correct to within 75 feet of the field elevation in order to be legal for IFR takes altimeter error into account. Given that, you should fly all procedures so that your indicated altitude agrees with what is published in the procedure.
 
No. Set local altimeter setting and fly down to appropriate mins. Sometimes when it’s really cold out you have to adjust minimums and FAF altitudes but only when told to by the chart.
 
excellent, thanks jordane and david.

Stan
 
Separation only works when everyone uses the same method, even if there are some errors in that method.
 
Clip4 said:
One can also use the FAF while on the GS as a cross check for proper altimeter setting
http://155.178.201.160/d-tpp/1812/00156IL5.PDF
http://155.178.201.160/d-tpp/1812/00958R24.PDF
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You have to beware of temperature when doing that. The altimeter setting could be perfectly accurate and your altimeter could be perfectly accurate and the Glideslope could be perfectly accurate and you could be right on it. If it’s hotter or colder than standard, your indicated altitude is going to be different than the number on the Chart.
 
luvflyin said:
You have to beware of temperature when doing that. The altimeter setting could be perfectly accurate and your altimeter could be perfectly accurate and the Glideslope could be perfectly accurate and you could be right on it. If it’s hotter or colder than standard, your indicated altitude is going to be different than the number on the Chart.
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True if you fly in extreme cold, otherwise there is no adjustment required for temp and the cross check is valid. It is also your only altimeter source in a lost comm. situation.
 
Clip4 said:
True if you fly in extreme cold, otherwise there is no adjustment required for temp and the cross check is valid. It is also your only altimeter source in a lost comm. situation.
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...and your definition of “extreme cold” is anything below freezing, where your altimeter can be 90 feet or more off at the FAF.
 
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Clip4 said:
True if you fly in extreme cold, otherwise there is no adjustment required for temp and the cross check is valid. It is also your only altimeter source in a lost comm. situation.
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Yeah. No correction required unless it is required. The Snowflake on the Chart tells you when it is. I was just saying that comparing your indicated altitude to the altitude on the Chart at the FAF isn't always going to be accurate when cross checking as you said in post #7. It could be off significantly, hot as well as cold.
 
"Density" has no real influence on altimeter to (physical) altitude correction. It's a pressure correction except in extreme cases (and in those, it's still not "density" that's the issue).
 
MauleSkinner said:
...and your definition of “extreme cold” is anything below freezing, where your altimeter can be 90 feet or more off at the FAF.
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And when extremely hot. It's just off in the other direction which isn't going to run you into the rocks. It can create aircraft separation issues though if you follow the Glideslope below published altitudes at fixes beyond the FAF
 
luvflyin said:
And when extremely hot. It's just off in the other direction which isn't going to run you into the rocks. It can create aircraft separation issues though if you follow the Glideslope below published altitudes at fixes beyond the FAF
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True...unfortunately there's no chart for that...I gotta whip out the ol' whiz wheel and calculate true altitude. :eek:
 
luvflyin said:
Yeah. No correction required unless it is required. The Snowflake on the Chart tells you when it is. I was just saying that comparing your indicated altitude to the altitude on the Chart at the FAF isn't always going to be accurate when cross checking as you said in post #7. It could be off significantly, hot as well as cold.
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MauleSkinner said:
...and your definition of “extreme cold” is anything below freezing, where your altimeter can be 90 feet or more off at the FAF.
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upload_2018-11-22_21-7-6.png
 
All of this makes more sense when you remember that it’s not the job of the barometric altimeter to accurately measure your actual altitude off the ground (that’s the job of a radar altimeter).

It’s designed to make sure that if everyone says they are at a certain elevation in the sky then others can also be at that same elevation. The DA altitude correction is just addressing situations when the discrepancy between your actual altitude and what the altimeter says could, in theory, put you in danger of hitting something but it changes the DA, not the altimeter setting, for the reason described above.
 
Again, there is no DA altitude correction. The only thing that matters to the altimeter is the pressure. It's all it is capable of measuring. It assumes a standard pressure lapse rate which works well enough except in odd (very cold) situations.
 
flyingron said:
Again, there is no DA altitude correction. The only thing that matters to the altimeter is the pressure. It's all it is capable of measuring. It assumes a standard pressure lapse rate which works well enough except in odd (very cold) situations.
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I think the D we’re talkin about here is Decision, not density.
 
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