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CarlosSi
Topic Author
Posts: 1055
Joined: Sat Jul 08, 2017 8:29 pm

### Cold temperatures and the flight levels

Curious, we know the pressure levels "condense" with cold temperature requiring altitude corrections when coming in on an instrument approach. I also assume the intervals are equidistant across all altitudes for a given temperature at sea level (and then decreasing at a particular lapse rate).

Could it be cold enough (much colder) at the flight levels that aircraft are no longer technically separated vertically by 1000ft? What's the correction here?

zanl188
Posts: 4051
Joined: Sat Oct 21, 2006 9:05 pm

### Re: Cold temperatures and the flight levels

Adiabatic cooling occurs as an aircraft climbs, this is due to the change in pressure. As an aircraft climbs into the flight levels all aircraft use a standardized altimeter setting to ensure separation. As an aircraft descends out of the flight levels local atmospheric pressure, from the arrival airfield, is used for the altimeter setting.

CarlosSi
Topic Author
Posts: 1055
Joined: Sat Jul 08, 2017 8:29 pm

### Re: Cold temperatures and the flight levels

zanl188 wrote:
Adiabatic cooling occurs as an aircraft climbs, this is due to the change in pressure. As an aircraft climbs into the flight levels all aircraft use a standardized altimeter setting to ensure separation. As an aircraft descends out of the flight levels local atmospheric pressure, from the arrival airfield, is used for the altimeter setting.

Mmmm yes I'm aware everybody uses 29.92 inHg above FL180, though I'm wondering if even at that same setting do the pressure levels "condense".

Basically asking if airplanes flying an IAP have to correct for cold temperatures if marked in the approach chart (and it is cold enough), does something similar apply to the flight levels if the temperature is much colder than usual (i.e., -100 deg F at FL270. Random number)

zeke
Posts: 17418
Joined: Thu Dec 14, 2006 1:42 pm

### Re: Cold temperatures and the flight levels

CarlosSi wrote:
Basically asking if airplanes flying an IAP have to correct for cold temperatures if marked in the approach chart (and it is cold enough), does something similar apply to the flight levels if the temperature is much colder than usual (i.e., -100 deg F at FL270. Random number)

No, while aircraft true altitudes are a function of temperature and pressure, we still fly at indicated 1000 ft separation. Yes it does overread the 1000 ft however it is not significant as the aircraft below and above are exposed to the same temperature profile at that geographic location.

There was talk of using GPS true altitude at all levels to remove barometric errors however the rate things change in aviation I cannot see that in my lifetime.

LH707330
Posts: 2641
Joined: Fri Jun 15, 2012 11:27 pm

### Re: Cold temperatures and the flight levels

CarlosSi wrote:
zanl188 wrote:
Adiabatic cooling occurs as an aircraft climbs, this is due to the change in pressure. As an aircraft climbs into the flight levels all aircraft use a standardized altimeter setting to ensure separation. As an aircraft descends out of the flight levels local atmospheric pressure, from the arrival airfield, is used for the altimeter setting.

Mmmm yes I'm aware everybody uses 29.92 inHg above FL180, though I'm wondering if even at that same setting do the pressure levels "condense".

Basically asking if airplanes flying an IAP have to correct for cold temperatures if marked in the approach chart (and it is cold enough), does something similar apply to the flight levels if the temperature is much colder than usual (i.e., -100 deg F at FL270. Random number)

Short answer is yes, at below-standard temps or weird lapse rates the actual differences will be less than the 1,000 feet indicated. That's not much of an issue, because even in extreme outlying cases it will probably still be 9xx feet.

Posts: 3790
Joined: Wed Mar 01, 2006 4:29 am

### Re: Cold temperatures and the flight levels

LH707330 wrote:
CarlosSi wrote:
zanl188 wrote:
Adiabatic cooling occurs as an aircraft climbs, this is due to the change in pressure. As an aircraft climbs into the flight levels all aircraft use a standardized altimeter setting to ensure separation. As an aircraft descends out of the flight levels local atmospheric pressure, from the arrival airfield, is used for the altimeter setting.

Mmmm yes I'm aware everybody uses 29.92 inHg above FL180, though I'm wondering if even at that same setting do the pressure levels "condense".

Basically asking if airplanes flying an IAP have to correct for cold temperatures if marked in the approach chart (and it is cold enough), does something similar apply to the flight levels if the temperature is much colder than usual (i.e., -100 deg F at FL270. Random number)

Short answer is yes, at below-standard temps or weird lapse rates the actual differences will be less than the 1,000 feet indicated. That's not much of an issue, because even in extreme outlying cases it will probably still be 9xx feet.

And what kind of temperature profile is the default for pressure-temperature conversion? I assume upper troposphere has to follow certain seasonal trends, so whatever profile is assumed, it will be only that accurate.

Posts: 3790
Joined: Wed Mar 01, 2006 4:29 am

### Re: Cold temperatures and the flight levels

zeke wrote:
CarlosSi wrote:
Basically asking if airplanes flying an IAP have to correct for cold temperatures if marked in the approach chart (and it is cold enough), does something similar apply to the flight levels if the temperature is much colder than usual (i.e., -100 deg F at FL270. Random number)

No, while aircraft true altitudes are a function of temperature and pressure, we still fly at indicated 1000 ft separation. Yes it does overread the 1000 ft however it is not significant as the aircraft below and above are exposed to the same temperature profile at that geographic location.

There was talk of using GPS true altitude at all levels to remove barometric errors however the rate things change in aviation I cannot see that in my lifetime.

Is there really an advantage of going to true altitude? Vulnerability to problems with external signal is a clear disadvantage from my perspective, though.
I can see terrain separation as a possible one, but there are only that many mountains extending past FL180.

LH707330
Posts: 2641
Joined: Fri Jun 15, 2012 11:27 pm

### Re: Cold temperatures and the flight levels

LH707330 wrote:
CarlosSi wrote:

Mmmm yes I'm aware everybody uses 29.92 inHg above FL180, though I'm wondering if even at that same setting do the pressure levels "condense".

Basically asking if airplanes flying an IAP have to correct for cold temperatures if marked in the approach chart (and it is cold enough), does something similar apply to the flight levels if the temperature is much colder than usual (i.e., -100 deg F at FL270. Random number)

Short answer is yes, at below-standard temps or weird lapse rates the actual differences will be less than the 1,000 feet indicated. That's not much of an issue, because even in extreme outlying cases it will probably still be 9xx feet.

And what kind of temperature profile is the default for pressure-temperature conversion? I assume upper troposphere has to follow certain seasonal trends, so whatever profile is assumed, it will be only that accurate.

According to this, the standard temp at the tropopause is 220K: https://en.wikipedia.org/wiki/Density_of_air

That means that density will change roughly 1% for every 2 degrees, but I'm not exactly sure how the temp will affect the reading of the altimeter, presumably at a lower rate. There's an ugly looking equation in there that mentions T and To, so there's got to be some kind of offset.

Most of what I can find concerns temp corrections at lower altitudes, e.g. for approaches: https://code7700.com/altimetry_temperat ... ection.htm

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