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Cold Wx Altimeter Correction???

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Here is what my company has to say about this:

Altimeter Correction in Cold Temperatures

General
◊​
Barometric altimeters are calibrated to
indicate true altitude under International
Standard Atmosphere (ISA) conditions.

◊​
If on a given day the temperature is
warmer than ISA, the true altitude will be
higher than indicated altitude. While
warmer temperatures will cause the
pressure altimeter to under-read in
comparison with the Radar Altimeter and
effectively increase the vertical path
angle to the runway for non-precision
approaches, greater than ISA
temperatures are considered safe for
obstacle clearance.

◊​
Conversely, on a day colder than ISA,
the true altitude will be lower than
indicated altitude. These errors increase
in magnitude as the altitude above the
altimeter setting source increases.

◊​
When considering obstacle clearance
with large deviations below ISA
temperatures, altimeter indication error
becomes very important. Unless altitude
corrections are applied, required obstacle
clearance cannot be assured.

WARNING

◊​
Should variation from a cleared altitude
(this does not apply to DA or MDA) be
required, approval from ATC must be
obtained prior.

WARNING

◊​
If database Non-Precision Approach
altitudes require manual correction,
Managed/VNAV vertical guidance shall
NOT be used.

◊​
In a number of regions, RADAR
vectored altitudes are temperature
compensated by ATC. In such case,
additional corrections are not required.
Refer to Flight Information publications
and/or query ATC.

Enroute Terrain Clearance​
◊​
Corrections must be applied to published
enroute Minimum Safe Altitudes any
time the OAT is ISA minus 15°C or
colder (0°C at Sea Level). Refer to the
Route Manual for applicable altitude
correction table.

◊​
Should variation from a cleared altitude
be required, approval from ATC must be
obtained prior.

Terminal Area Terrain Clearance
��​
DA or MDA

◊​
Corrections must be applied to approach
minima (DA or MDA) whenever the
OAT is ISA minus 30°C or colder (-15°C
at Sea Level).

��​
Other Approach Procedure Altitudes

◊​
Corrections must be applied to other
minimum Procedure Altitudes
(Procedure Turn, Final Fix &
Intermediate Fix, Missed Approach
Altitude) whenever the OAT is ISA
minus 45°C or colder (-30°C at Sea
Level). Refer to the Route Manual for
applicable altitude correction table.

◊​
In mountainous regions during any Cold
Temperature operations, corrections to
all approach procedure altitudes must be
considered.

RNAV (GNSS) Approach
��​
Published Limiting Temperature

◊​
Certain approach procedures planned for
use by the Airline (typically, RNAV (GPS)
approach procedures) will have a VNAV
Limiting Temperature (TLIM) published
on the chart. In this case, the approach
may be flown using the published
minimum altitudes unless the OAT at the
airfield is colder than VNAV Limiting
Temperature.

◊​
If the temperature on a given day is
lower than the VNAV temperature
limitation, the aircraft may only be flown
to the published LNAV MDA for that
approach.

The forumla I use for caluclating temperature corections, either cold or warm, is:

Altitude correction = Altitude above airport X 4% X (+/- ISA Deviation/10 )

For example with an ISA deviation of +25, the final fix crossing height for a published altitude of 1400' above the airport would be: 1400 X 4% X ( + 25/10 ) or 140 feet. So when the PNF calls out the altitude crossing the final fix as 1260 I know it is correct.

The other thing to think about in regards to this issue, as mentioned above, is the effect on descent gradients. On a warm day your vertical speed to maintain NPA paths will need to be higher. For ISA +25 the figure I use is about another 0.3 degrees, this equates to an extra 100 feet per minute required to maintain the path.

As with most people here who fly in the contiguous 48, none of this was taught or thought about. I only learned of it when coming to my present airline and they only started to publish information on it when we started service to Moscow.


Typhoonpilot

 
A Squared said:
It's not a QNE/QNH transition level issue. It has to do with the possiblility that an airplane with the correct (for the pressure) QNH might hit trees on approach because the altimiter was not adjusted for extremely low temps.
I remember an American Airlines jet clipped some trees going in to KBDL a few years ago. Seem to recall something about altimeter error. Was is very low temperature or low barometric pressure in the area?
 
gfvalvo said:
I remember an American Airlines jet clipped some trees going in to KBDL a few years ago. Seem to recall something about altimeter error. Was is very low temperature or low barometric pressure in the area?

I believe that was blamed partly on an old altimeter setting plus AA at that time still used QFE which only the company station would provde. On a QFE setting the altimeter will read "0" on touchdown. Well, maybe. :)

DC
 
A Squared said:
It's not a QNE/QNH transition level issue. It has to do with the possiblility that an airplane with the correct (for the pressure) QNH might hit trees on approach because the altimiter was not adjusted for extremely low temps.

In CONUS, I agree. But not everyone on this board flies only in CONUS. In Europe or South America where local TL's are as low as FL030, it could be a problem. The upper atmosphere is more homogeneous than down low where you can get wide deviations from ISA with the passage of warm and cold fronts.
 
Just to add to the discussion there is also altimeter error caused by high winds.​

http://dcaa.slv.dk:8000/icaodocs/Doc%208168/ Text starts on page 242, tables pg 245



Part


111

- Section I. Chapter 4 111-1-4-5



4.5​


MOUNTAINOUS TERRAIN

- TERMINAL AREAS




4.5.1 The combination of strong winds and mountainous terrain can cause local changes in atmospheric pressure
due to the Bernoulli effect. This occurs particularly when the wind direction is across mountain crests or ridges. It is not
possible to make an exact calculation, but theoretical studies



(CFD Norway, Report 109.1989) have indicated altimeter
errors as shown in Tables 111-1-4-4 and 111-1-4-5. Although States may provide guidance, it is up to the pilot-incommand
to evaluate whether the combination of terrain, wind strength and direction are such as to make a correction
for wind necessary.
4.5.2 Corrections for wind speed should be applied in addition to the standard corrections for pressure and
temperature, and ATC should be advised.





 
MarineGrunt said:
Already required by the military. Often a significant factor up at RDR and MIB.

Airman's Information Manual, paragraph 7-2
http://www.faa.gov/ATPubs/AIM/Chap7/aim0702.html

Just scroll all the way to the bottom.

I don't understand why the table asks for hight above airport, instead of MSL altitude. Could someone explain this?

I got into a discussion about the cold weather altimeter errors today with a CFII student of mine. He says that the altimeter setting you get from ATC or AWOS/ASOS is corrected for non standard tempature, so that when you're flying an apporach you don't hit somthing, and that is why the table in the AIM refers to altitudes above the airport, not MSL.

Thanks
 
paulsalem said:
I don't understand why the table asks for hight above airport, instead of MSL altitude. Could someone explain this?

I got into a discussion about the cold weather altimeter errors today with a CFII student of mine. He says that the altimeter setting you get from ATC or AWOS/ASOS is corrected for non standard tempature, so that when you're flying an apporach you don't hit somthing, and that is why the table in the AIM refers to altitudes above the airport, not MSL.

Thanks

It's because the amount you are off due to cold temps is a function of how high you are above the reporting station (usually the airport, but not always), not above sea level. This ties into what your student said. Yes the altimeter setting is corrected for non-standard pressure and temp *at the station*. but that is only meaninful at the station.

When an altimeter setting is computed, whatever the pressure and temp is at the station, the setting is figured so that an altimeter adjusted to that setting will read the correct elevation *at the station*. If you're above the station, at non-standard temps, your altimeter will read incorrectly, even if though the correct setting is used. The reason for this is that the inner workings of the altimeter are constructed so that it "calculates" altitude from pressure based on a standard atmosphere. In a standard altitude a 200' difference in altitude is approximately a pressure difference of 0.2 inches of mercury. But on a very cold day, there might be 0.2 in.hg difference every 150 feet of altitude. SO a correctly set altimeter, if it senses a pressure that is 0.2 in.hg.less that the pressure at the reporting station, it will display an altitude 200' above the station, when in reality you are only 150 feet above it. As you get closer to the altitude od the station the error decreases, until when you are on the ground, the altimter reads correctly.

So, your student is right, but not really. the altimeter setting corrects for non-standard pressure and temp *at the altitude of the observing equipment*, but the higher you are above that, the more your altimeter will be off from true altitude.
 

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