Pressure & temperature

|
| In Europe its the easiest way to fail an IR checkride by
failing to
| compensate for the ISA deviation in the winter and being
deemed to have
| busted the limits
|
| Our FAA isn't that smart.

Or that anal.



"Bee" wrote in message
...
| akjcbkJA wrote:
|
| "Bee" wrote in message
| ...
|
| S Green wrote:
|
|
| Have a look at the cold weather adjustments needed for
instruments
| approaches. As the temperature deviates from ISA
whilst the altimeter
| could be reading 200' you will actually be well under
and enough to
| bust a checkride. For example, an ISA deviation
of -15C is an
| correction of 12'. approximately 4ft/1000ft for each C
of
| difference. In this case, -15C ISA is only a
temperature of 0C or 32F.
|
|
| That isn't done for instrument approaches in the U.S.
except for VNAV
| final segments on RNP SAAAR IAPs.
|
|
| In Europe its the easiest way to fail an IR checkride by
failing to
| compensate for the ISA deviation in the winter and being
deemed to have
| busted the limits
|
| Our FAA isn't that smart.

On Oct 21, 9:17 pm, Terence Wilson wrote:
On Sun, 21 Oct 2007 20:06:30 -0400, "An Aviator"

wrote:
Current temperature is not used to convert
pressure to altimeter settings.

Can you please elucidate? Isn't current temperature implicitly used if
we use a barometer to measure local pressure and then extrapolate the
pressure at sea level (to be used in the Kollsman window)?

The interpretation of the altimeter setting (Kollsman window) as the
sea level pressure is what causing this confusion. The altimeter
setting is just a number. It is not the sea level pressure. If you set
your altimeter correctly for your airport elevation, and go down a
hole to sea level, your altimeter will not read 0 ft. What it might
read will be greatly influenced by temperature, though. On a very cold
day it might read a negative elevation, and on a hot day will be read
a positive elevation. On rare occasions it might read 0 ft.

Therefore, temperature only affects what the altimeter will read at
altitudes other than the elevation to which it is corrected for. The
altimeter setting is only affected by the atmospheric pressure at your
airport elevation, and this is not affected by temperature.

On Mon, 22 Oct 2007 18:25:36 -0700, Andrew Sarangan
wrote:

On Oct 21, 9:17 pm, Terence Wilson wrote:
On Sun, 21 Oct 2007 20:06:30 -0400, "An Aviator"

wrote:
Current temperature is not used to convert
pressure to altimeter settings.

Can you please elucidate? Isn't current temperature implicitly used if
we use a barometer to measure local pressure and then extrapolate the
pressure at sea level (to be used in the Kollsman window)?

The interpretation of the altimeter setting (Kollsman window) as the
sea level pressure is what causing this confusion. The altimeter
setting is just a number. It is not the sea level pressure. If you set
your altimeter correctly for your airport elevation, and go down a
hole to sea level, your altimeter will not read 0 ft. What it might
read will be greatly influenced by temperature, though. On a very cold
day it might read a negative elevation, and on a hot day will be read
a positive elevation. On rare occasions it might read 0 ft.

Therefore, temperature only affects what the altimeter will read at
altitudes other than the elevation to which it is corrected for. The
altimeter setting is only affected by the atmospheric pressure at your
airport elevation, and this is not affected by temperature.

Thank you Sir! Your explanation has brought clarity to my foggy
noggin.

An altimeter setting used at an airport for landings,
instrument approaches, etc is taken at a known position and
true altitude. It is corrected for all know errors in
"standard atmosphere" and that leaves the aircraft altimeter
errors as the primary error to be concerned about.

When you are flying in the mountains, the altimeter setting
may be from a location 100 miles away, on the other side of
a range and in a different air mass.
The greater the difference between the reporting station
altitude and the aircraft indicated altitude and the
pressure altitude, the greater the error between indicated
and true altitude.

True altitude is what gets you across a mountain with a
known elevation. Indicated altitude keeps you from hitting
other airplanes.
At a certain altitude, 18,000 feet in the USA and as low as
3,000-5,000 feet in parts of the world, they go Flight
Levels, which are pressure altitudes. The lowest useable
Flight Level is adjusted for pressures below 29.92 [1013.2
mB] so that aircraft at a Flight Level will really be above
the indicated MSL altitude.



"Andrew Sarangan" wrote in message
ps.com...
| On Oct 21, 9:17 pm, Terence Wilson wrote:
| On Sun, 21 Oct 2007 20:06:30 -0400, "An Aviator"
|
| wrote:
| Current temperature is not used to convert
| pressure to altimeter settings.
|
| Can you please elucidate? Isn't current temperature
implicitly used if
| we use a barometer to measure local pressure and then
extrapolate the
| pressure at sea level (to be used in the Kollsman
window)?
|
| The interpretation of the altimeter setting (Kollsman
window) as the
| sea level pressure is what causing this confusion. The
altimeter
| setting is just a number. It is not the sea level
pressure. If you set
| your altimeter correctly for your airport elevation, and
go down a
| hole to sea level, your altimeter will not read 0 ft. What
it might
| read will be greatly influenced by temperature, though. On
a very cold
| day it might read a negative elevation, and on a hot day
will be read
| a positive elevation. On rare occasions it might read 0
ft.
|
| Therefore, temperature only affects what the altimeter
will read at
| altitudes other than the elevation to which it is
corrected for. The
| altimeter setting is only affected by the atmospheric
pressure at your
| airport elevation, and this is not affected by
temperature.
|
|

The lowest useable
Flight Level is adjusted for pressures below 29.92 [1013.2
mB] so that aircraft at a Flight Level will really be above
the indicated MSL altitude.

Is that really right? I thought it was just that (in the US), if a
flight level puts you below true 18000 feet MSL, it is not "available".

If flight level 190 puts you at 18200 feet MSL, it will be available,
but you'll still indicate FL 190, or 19000 feet.

No?

Jose
--
You can choose whom to befriend, but you cannot choose whom to love.
for Email, make the obvious change in the address.

The regulation, FAR
§ 91.121 Altimeter settings.
(a) Each person operating an aircraft shall maintain the
cruising altitude or flight level of that aircraft, as the
case may be, by reference to an altimeter that is set, when
operating-

(1) Below 18,000 feet MSL, to-

(i) The current reported altimeter setting of a station
along the route and within 100 nautical miles of the
aircraft;

(ii) If there is no station within the area prescribed in
paragraph (a)(1)(i) of this section, the current reported
altimeter setting of an appropriate available station; or

(iii) In the case of an aircraft not equipped with a radio,
the elevation of the departure airport or an appropriate
altimeter setting available before departure; or

(2) At or above 18,000 feet MSL, to 29.92" Hg.

(b) The lowest usable flight level is determined by the
atmospheric pressure in the area of operation as shown in
the following table:
http://ecfr.gpoaccess.gov/cgi/t/text...2.4.11&idno=14

(c) To convert minimum altitude prescribed under §§91.119
and 91.177 to the minimum flight level, the pilot shall take
the flight level equivalent of the minimum altitude in feet
and add the appropriate number of feet specified below,
according to the current reported altimeter setting:

To see the tables, click the link. The available flight
level change at the rate of 1,000 feet in 500 foot steps.



ATC will not assign FL that are not available. The problem
at high altitudes is caused by low pressure, but very high
pressure can and does restrict night and IFR flight because
the adjustment stops at 31" Hg.

91.159 VFR cruising altitude or flight level.

Except while holding in a holding pattern of 2 minutes or
less, or while turning, each person operating an aircraft
under VFR in level cruising flight more than 3,000 feet
above the surface shall maintain the appropriate altitude or
flight level prescribed below, unless otherwise authorized
by ATC:

(a) When operating below 18,000 feet MSL and-

(1) On a magnetic course of zero degrees through 179
degrees, any odd thousand foot MSL altitude +500 feet (such
as 3,500, 5,500, or 7,500); or

(2) On a magnetic course of 180 degrees through 359 degrees,
any even thousand foot MSL altitude +500 feet (such as
4,500, 6,500, or 8,500).

(b) When operating above 18,000 feet MSL, maintain the
altitude or flight level assigned by ATC.

[Doc. No. 18334, 54 FR 34294, Aug. 18, 1989, as amended by
Amdt. 91-276, 68 FR 61321, Oct. 27, 2003; 68 FR 70133, Dec.
17, 2003]



91.179 IFR cruising altitude or flight level.

Unless otherwise authorized by ATC, the following rules
apply-

(a) In controlled airspace. Each person operating an
aircraft under IFR in level cruising flight in controlled
airspace shall maintain the altitude or flight level
assigned that aircraft by ATC. However, if the ATC clearance
assigns "VFR conditions on-top," that person shall maintain
an altitude or flight level as prescribed by §91.159.

(b) In uncontrolled airspace. Except while in a holding
pattern of 2 minutes or less or while turning, each person
operating an aircraft under IFR in level cruising flight in
uncontrolled airspace shall maintain an appropriate altitude
as follows:

(1) When operating below 18,000 feet MSL and-

(i) On a magnetic course of zero degrees through 179
degrees, any odd thousand foot MSL altitude (such as 3,000,
5,000, or 7,000); or

(ii) On a magnetic course of 180 degrees through 359
degrees, any even thousand foot MSL altitude (such as 2,000,
4,000, or 6,000).

(2) When operating at or above 18,000 feet MSL but below
flight level 290, and-

(i) On a magnetic course of zero degrees through 179
degrees, any odd flight level (such as 190, 210, or 230); or

(ii) On a magnetic course of 180 degrees through 359
degrees, any even flight level (such as 180, 200, or 220).

(3) When operating at flight level 290 and above in non-RVSM
airspace, and-

(i) On a magnetic course of zero degrees through 179
degrees, any flight level, at 4,000-foot intervals,
beginning at and including flight level 290 (such as flight
level 290, 330, or 370); or

(ii) On a magnetic course of 180 degrees through 359
degrees, any flight level, at 4,000-foot intervals,
beginning at and including flight level 310 (such as flight
level 310, 350, or 390).

(4) When operating at flight level 290 and above in airspace
designated as Reduced Vertical Separation Minimum (RVSM)
airspace and-

(i) On a magnetic course of zero degrees through 179
degrees, any odd flight level, at 2,000-foot intervals
beginning at and including flight level 290 (such as flight
level 290, 310, 330, 350, 370, 390, 410); or

(ii) On a magnetic course of 180 degrees through 359
degrees, any even flight level, at 2000-foot intervals
beginning at and including flight level 300 (such as 300,
320, 340, 360, 380, 400).

[Doc. No. 18334, 54 FR 34294, Aug. 18, 1989, as amended by
Amdt. 91-276, 68 FR 61321, Oct. 27, 2003; 68 FR 70133, Dec.
17, 2003; Amdt. 91-296, 72 FR 31679, June 7, 2007



Using a calculator, E6b, the true altitude should be
calculated to determine terrain clearance when the terrain
is high, the pressure low, the temperature and the altitude
required is high. The greater the altitude difference
between the location and altitude of the baro-setting and
the required true altitude, the greater the possible error.
AS the saying, high to low, look out below.

Deviation from "standard conditions" and changes in lapse
rates lead to errors.



"Jose" wrote in message
news | The lowest useable
| Flight Level is adjusted for pressures below 29.92
[1013.2
| mB] so that aircraft at a Flight Level will really be
above
| the indicated MSL altitude.
|
| Is that really right? I thought it was just that (in the
US), if a
| flight level puts you below true 18000 feet MSL, it is not
"available".
|
| If flight level 190 puts you at 18200 feet MSL, it will be
available,
| but you'll still indicate FL 190, or 19000 feet.
|
| No?
|
| Jose
| --
| You can choose whom to befriend, but you cannot choose
whom to love.
| for Email, make the obvious change in the address.

Jim Macklin wrote:
|
| In Europe its the easiest way to fail an IR checkride by
failing to
| compensate for the ISA deviation in the winter and being
deemed to have
| busted the limits
|
| Our FAA isn't that smart.

Or that anal.

Well, then our Air Force is anal. They worked with the FAA for a long
time on this issue, then gave up and implemented cold temp procedures
just for the Air Force. They almost lost a cargo bird at Tule,
Greenland in the late 1980s that got their attention.

The problem isn't usually with minimums. It is out on the initial
segments and sometimes the feeder routes as well. The higher above the
altimeter setting source, the larger the errors.

Jim Macklin wrote:




ATC will not assign FL that are not available. The problem
at high altitudes is caused by low pressure,

That is what they claim. I've been had more than once when they failed
to apply the minimum flight level adjustment. It is ultimately up to
the pilot to reject the inproper altitude assignment.

The first time it happened to me, I was given FL 180 near CVG during a
rain storm. I said unable (pressure 29.65, or so). They said a twin
beech was opposite direction at 17,000 and traffic above me at 190. So,
I was stuck. I saw the twin beach zoom by about 500 feet below me.

NY Center always gets it right. Not so with other centers.

I learned how to calculate True Altitude when I was a
student pilot, about 40 years ago.


"Bee" wrote in message
...
| Jim Macklin wrote:
| |
| | In Europe its the easiest way to fail an IR
checkride by
| failing to
| | compensate for the ISA deviation in the winter and
being
| deemed to have
| | busted the limits
| |
| | Our FAA isn't that smart.
|
| Or that anal.
|
| Well, then our Air Force is anal. They worked with the
FAA for a long
| time on this issue, then gave up and implemented cold temp
procedures
| just for the Air Force. They almost lost a cargo bird at
Tule,
| Greenland in the late 1980s that got their attention.
|
| The problem isn't usually with minimums. It is out on the
initial
| segments and sometimes the feeder routes as well. The
higher above the
| altimeter setting source, the larger the errors.

The local altimeter measures the weight of the air above it. When
temperature is higher, the air expands (less density), but it's the
same amount of air in a taller column. The atmosphere expands you see
and is not bound tightly by a blocade above (it is bound by gravity
actually). Anyway another item of interest is what you are setting the
altimeter to is "sealevel" barometric pressure. Imagine a hole in the
ground and the altimeter is lowered to the bottom of the hole (at your
airport). Of course they dont actually have a hole, they have another
way of adjusting it. Standard barometric pressure at sealevel is about
29.92. If you fly an airplane with a manifold pressure guage, it gives
the local "absolute" pressure (non-sealevel adjusted). Anyway consider
these principals and your knowledge will expand.