I'm confused about what determines the local altimeter setting. My
understanding is that there is a barometer in the tower that measures
the weight of a column air extending up into the atmosphere. What
factors affect its weight? Is it purely temperature variations caused
by uneven heating by the Sun or is there something else?

In a similar vein, I understand that winds are caused by pressure
discrepancies which lead to winds moving from high to low pressure
areas. Again, are the pressure differences caused by temperature only?

Thanks.

On Oct 20, 8:57 pm, Terence Wilson > wrote:
> I'm confused about what determines the local altimeter setting. My
> understanding is that there is a barometer in the tower that measures
> the weight of a column air extending up into the atmosphere. What
> factors affect its weight? Is it purely temperature variations caused
> by uneven heating by the Sun or is there something else?
>
> In a similar vein, I understand that winds are caused by pressure
> discrepancies which lead to winds moving from high to low pressure
> areas. Again, are the pressure differences caused by temperature only?
>
> Thanks.

Uneven solar heating is the primary cause of all weather on earth.
There could be other sources, such as fluctuations in the earth's
magnetic field and gravity of the moon, but solar heating is the
primary source.

Andrew Sarangan wrote:
> On Oct 20, 8:57 pm, Terence Wilson > wrote:
>
>>I'm confused about what determines the local altimeter setting. My
>>understanding is that there is a barometer in the tower that measures
>>the weight of a column air extending up into the atmosphere. What
>>factors affect its weight? Is it purely temperature variations caused
>>by uneven heating by the Sun or is there something else?
>>
>>In a similar vein, I understand that winds are caused by pressure
>>discrepancies which lead to winds moving from high to low pressure
>>areas. Again, are the pressure differences caused by temperature only?
>>
>>Thanks.
>
>
> Uneven solar heating is the primary cause of all weather on earth.
> There could be other sources, such as fluctuations in the earth's
> magnetic field and gravity of the moon, but solar heating is the
> primary source.
>
>
>
>
What about varying ocean temperatures?

On Oct 21, 4:51 am, Bee > wrote:
> Andrew Sarangan wrote:
> > On Oct 20, 8:57 pm, Terence Wilson > wrote:
>
> >>I'm confused about what determines the local altimeter setting. My
> >>understanding is that there is a barometer in the tower that measures
> >>the weight of a column air extending up into the atmosphere. What
> >>factors affect its weight? Is it purely temperature variations caused
> >>by uneven heating by the Sun or is there something else?
>
> >>In a similar vein, I understand that winds are caused by pressure
> >>discrepancies which lead to winds moving from high to low pressure
> >>areas. Again, are the pressure differences caused by temperature only?
>
> >>Thanks.
>
> > Uneven solar heating is the primary cause of all weather on earth.
> > There could be other sources, such as fluctuations in the earth's
> > magnetic field and gravity of the moon, but solar heating is the
> > primary source.
>
> What about varying ocean temperatures?- Hide quoted text -
>

Oceans are heated by the sun. This is why oceans near the equator are
warmer than at higher lattitudes. However, there could bea minor
effect from underwater volcanoes which are not directly related to the
sun, at least not since the earth separated from the earth billions of
years ago.

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)?

On Oct 21, 7:17 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 altimeter will read correctly at the place where the baro setting
was determined.

Above that, all bets are off! All those factors affect the lapse
rate.

And it's surprising how far it can be off.

Bill Hale

"Terence Wilson" > wrote in message
...
> 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 altimeter is calibrated using the ISA and the temperature under ISA is
15C.

Setting the altimeter to read airfield elevation in no way is a real
representation of what the sealevel pressure is precisely because no
adjustment is made for temperature.

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.

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.

"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

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.

| >
| > 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/text-idx?c=ecfr&sid=735566d496392bdca6824ea0705c2b9b&rgn=div8&view=text&node=14:2.0.1.3.10.2.4.11&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
...
|> 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.

"Doug" > wrote in message
ups.com...
> 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.

Standard pressure at sea level is 29.92 at 15 degrees C.

The weight of the air contrary to what you wrote above does vary hence high
and low pressure weather systems.

With low pressure air rises up the column diverges at the top and then
descends giving a high pressure somewhere else.

with a high pressure, the air converges high up, descends and then diverges
when it gets to the ground.

The principle of the altimeter is that it works on pressure, and pressure is
the force exerted by the air and one common force is weight.

When the air is less dense there are fewer molecules, the mass of air is
less and therefore the column of air weighs less, (mass x gravity) hence
the lower pressure.

when you understand these principles then you will understand a bit more
about altimetry, weather systems, high pressure, low pressure and winds all
of which are about the forces of air.

"Peter" > wrote in message
...
>
> "akjcbkJA" > 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
>
> Is this your personal experience?
>
> I've never heard of anybody here (UK) using anything other than the
> published minima, directly.
>
> One would be landing with the airport QNH set, and altimeter error
> caused by non-ISA temperature is negligible at more or less any
> airport in Europe that has an instrument approach.
>
> And at high altitute airports one would not be using the airport QNH.
> They have some other system. An altimeter is a pretty inaccurate
> device once well away from the ground.

Its in the JAR instrument test standards and features within the learning
objectives for the ATPL Met exam.

Work it out. The difference is small on approach and greater at high
altitude but the examiner would be looking to see whether you have taken
account of the fact that temperature does make a difference.

In JAR land some say they are anal, others would take the view that they do
it properly.

"Peter" > wrote in message
...
>
> "S Green" > wrote
>
>>Its in the JAR instrument test standards and features within the learning
>>objectives for the ATPL Met exam.
>>
>> Work it out. The difference is small on approach and greater at high
>>altitude but the examiner would be looking to see whether you have taken
>>account of the fact that temperature does make a difference.
>>
>>In JAR land some say they are anal, others would take the view that they
>>do
>>it properly.
>
> Is the pilot supposed to get the ATIS to get the surface temp and work
> it out there and then?
>
> What is the typical correction, for an airport at say 1000ft AMSL
> which is ISA+10?

+40 feet. Being the height in thousands * 4 * ISA deviation

In this case 1 * 4 * +10

Temperatures - well you could get the temperature from the ATIS but you
also having sitting in front of you on the temperature gauge too. I am sure
the examiner will be looking at that and doing the mental calculations.

With a tolerance of +50 - 0 for at the decision height, it does not take
too much working out that you could blow the tolerances when its either much
warmer or much colder than ISA.

Say you are planning to fly the check ride a bit too close to the DH. On a
cold day, you would be below the DH.

On a very warm day, you could well above it and outside the tolerance
especially if you decided that you would aim to be about say 40 ft above the
DH. A 30 degree C day would have you actually higher than indicated at say
252 ft and busted.

When the temp is less than ISA the aircraft will be lower than indicated
height by 4ft /1000 for each degree c different.

So at ISA dev -15 C at 200 ft above touch down, the actual height will be
188.
ISA -25 C at 200 ft above touch down, the actual height will be 180.

Its not hard to find places in Europe in winter where the outside
temperature is ISA -25 C after all, thats only -10 C.

Hence why tables are produced to help pilots especially in cold climates to
make allowance.

Here is a helpful presentation - I have flow into all three of the field
used as examples http://williams.best.vwh.net/smxgigpdf/smxgig2000.pdf

Another one from Transport Canada and I suggest you scroll down to fig 9
where there is a correction table laid out. The numbers differ from mine
because they round up so my 12 ft, they round up to 20 ft.

The correction at 10,000 feet MSL is zilch when you include
the altimeter of the reporting station, it isn't the
altitude above sea level, it is the amount of altitude
between the aircraft and the field elevation, which will be
only 2,000, the errors below that altitude are corrected and
cancelled when they take the altimeter setting.


"S Green" > wrote in message
...
|
| "Peter" > wrote in message
| ...
| >
| > "S Green" > wrote
| >
| >>Its in the JAR instrument test standards and features
within the learning
| >>objectives for the ATPL Met exam.
| >>
| >> Work it out. The difference is small on approach and
greater at high
| >>altitude but the examiner would be looking to see
whether you have taken
| >>account of the fact that temperature does make a
difference.
| >>
| >>In JAR land some say they are anal, others would take
the view that they
| >>do
| >>it properly.
| >
| > Is the pilot supposed to get the ATIS to get the surface
temp and work
| > it out there and then?
| >
| > What is the typical correction, for an airport at say
1000ft AMSL
| > which is ISA+10?
|
| +40 feet. Being the height in thousands * 4 * ISA
deviation
|
| In this case 1 * 4 * +10
|
| Temperatures - well you could get the temperature from
the ATIS but you
| also having sitting in front of you on the temperature
gauge too. I am sure
| the examiner will be looking at that and doing the mental
calculations.
|
| With a tolerance of +50 - 0 for at the decision height,
it does not take
| too much working out that you could blow the tolerances
when its either much
| warmer or much colder than ISA.
|
| Say you are planning to fly the check ride a bit too close
to the DH. On a
| cold day, you would be below the DH.
|
| On a very warm day, you could well above it and outside
the tolerance
| especially if you decided that you would aim to be about
say 40 ft above the
| DH. A 30 degree C day would have you actually higher than
indicated at say
| 252 ft and busted.
|
| When the temp is less than ISA the aircraft will be lower
than indicated
| height by 4ft /1000 for each degree c different.
|
| So at ISA dev -15 C at 200 ft above touch down, the actual
height will be
| 188.
| ISA -25 C at 200 ft above touch down, the actual height
will be 180.
|
| Its not hard to find places in Europe in winter where the
outside
| temperature is ISA -25 C after all, thats only -10 C.
|
| Hence why tables are produced to help pilots especially in
cold climates to
| make allowance.
|
| Here is a helpful presentation - I have flow into all
three of the field
| used as examples
http://williams.best.vwh.net/smxgigpdf/smxgig2000.pdf
|
| Another one from Transport Canada and I suggest you scroll
down to fig 9
| where there is a correction table laid out. The numbers
differ from mine
| because they round up so my 12 ft, they round up to 20 ft.
|
|

The 40 foot error is from a altitude of 10,000 feet if the
pressure was based on sea level readings.

An E6b manual true altitude procedure will show the method
that should be used. The true altitude calculation is based
on the difference between altitudes, not MSL altitude.


"Peter" > wrote in message
...
|
| "S Green" > wrote
|
| >+40 feet. Being the height in thousands * 4 * ISA
deviation
| >
| >In this case 1 * 4 * +10
|
| I don't understand this. You are landing with the
*airport* QNH set on
| the altimeter. How can the altimeter reading be 40ft out
when it's
| reading 200ft, purely due to non-ISA temp?

On Oct 26, 5:07 pm, "S Green" > wrote:
> "Peter" > wrote in message
>
> ...
>
>
>
>
>
>
>
> > "S Green" > wrote
>
> >>Its in the JAR instrument test standards and features within the learning
> >>objectives for the ATPL Met exam.
>
> >> Work it out. The difference is small on approach and greater at high
> >>altitude but the examiner would be looking to see whether you have taken
> >>account of the fact that temperature does make a difference.
>
> >>In JAR land some say they are anal, others would take the view that they
> >>do
> >>it properly.
>
> > Is the pilot supposed to get the ATIS to get the surface temp and work
> > it out there and then?
>
> > What is the typical correction, for an airport at say 1000ft AMSL
> > which is ISA+10?
>
> +40 feet. Being the height in thousands * 4 * ISA deviation
>
> In this case 1 * 4 * +10


You are applying the calcualtion incorrectly. At a DH of 200ft, you
would be only (200/1000)*4*(+10) = 8ft in error, not 40ft.

The error of +40 ft would be correct only if you are at 1000 ABOVE the
airport, at which point the 40ft is insignificant.

At very large temperature shifts, (ISA-50) for example, the error
could become significant, which is why special procedures are
published for cold areas. For all other areas, you are really
splitting hairs.

If this is the only thing the examiner is worried about during a
checkride, I'd say candidate is doing a fine job.

On Oct 27, 1:12 pm, Andrew Sarangan > wrote:
> On Oct 26, 5:07 pm, "S Green" > wrote:
>
>
>
>
>
> > "Peter" > wrote in message
>
> ...
>
> > > "S Green" > wrote
>
> > >>Its in the JAR instrument test standards and features within the learning
> > >>objectives for the ATPL Met exam.
>
> > >> Work it out. The difference is small on approach and greater at high
> > >>altitude but the examiner would be looking to see whether you have taken
> > >>account of the fact that temperature does make a difference.
>
> > >>In JAR land some say they are anal, others would take the view that they
> > >>do
> > >>it properly.
>
> > > Is the pilot supposed to get the ATIS to get the surface temp and work
> > > it out there and then?
>
> > > What is the typical correction, for an airport at say 1000ft AMSL
> > > which is ISA+10?
>
> > +40 feet. Being the height in thousands * 4 * ISA deviation
>
> > In this case 1 * 4 * +10
>
> You are applying the calcualtion incorrectly. At a DH of 200ft, you
> would be only (200/1000)*4*(+10) = 8ft in error, not 40ft.
>
> The error of +40 ft would be correct only if you are at 1000 ABOVE the
> airport, at which point the 40ft is insignificant.
>
> At very large temperature shifts, (ISA-50) for example, the error
> could become significant, which is why special procedures are
> published for cold areas. For all other areas, you are really
> splitting hairs.
>
> If this is the only thing the examiner is worried about during a
> checkride, I'd say candidate is doing a fine job.- Hide quoted text -
>
> - Show quoted text -

Here is a table that lists altimeter errors due to nonstandard
temperatures:
http://www.faa.gov/airports_airtraffic/air_traffic/publications/ATpubs/AIM/Chap7/aim0702.html

Maybe the FAA isn't so dumb. They are anal, I'd give you that.

It just could be that the worst case errors are already factored into
the
minimums to begin with; no adjustment required.

At 500' AGL, the errors aren't zero but they would be small.

Isn't that how you would do it? We like simple.

Bill H.




On Oct 26, 10:04 am, "S Green" > wrote:
> "Peter" > wrote in message
>
> ...
>
>
>
>
>
>
>
> > "akjcbkJA" > 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
>
> > Is this your personal experience?
>
> > I've never heard of anybody here (UK) using anything other than the
> > published minima, directly.
>
> > One would be landing with the airport QNH set, and altimeter error
> > caused by non-ISA temperature is negligible at more or less any
> > airport in Europe that has an instrument approach.
>
> > And at high altitute airports one would not be using the airport QNH.
> > They have some other system. An altimeter is a pretty inaccurate



> > device once well away from the ground.
>
> Its in the JAR instrument test standards and features within the learning
> objectives for the ATPL Met exam.
>
> Work it out. The difference is small on approach and greater at high
> altitude but the examiner would be looking to see whether you have taken
> account of the fact that temperature does make a difference.
>
> In JAR land some say they are anal, others would take the view that they do
> it properly.- Hide quoted text -
>
> - Show quoted text -