typhoonpilot
Daddy
- Joined
- Feb 24, 2002
- Posts
- 1,381
Here is what my company has to say about this:
Altimeter Correction in Cold Temperatures
General
Altimeter Correction in Cold Temperatures
General
◊
Barometric altimeters are calibrated to
indicate true altitude under International
Standard Atmosphere (ISA) conditions.
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.
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.
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
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
(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.
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.
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.
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.
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).
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.
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.
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.
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
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