Pressure, altitude and temperature
 

I had a query at work today.

I am told that Flight minima [such as Minimum Decision ] Heights are
adjusted to take account of the temperature relative to the 'base station'
temperature when looking at QNHs [Pressure at sea level]

So in the case of Heathrow, which has a recorded standard temperature oif
+15c apparently, any change in temperature from that, needs manual
adjustment to find the 'true height' given a pressure setting.

I am told that a true height of 280ft would be adjusted by around 30ft if
the temperature was zero, to make an indicated height of 310ft.

As the temperatures go down, the indicated height goes up i.e the actual
height goes down
and as the temps do up, the indicated heights goes down but the actual
heights go up.

I thought, as the temperature goes up, the pressure goes down i.e 'Heat
Low', so for a given pressure setting, would your height go down which seems
to contracdict the above!

Any help anyone!

Phil

Pressure, altitude and temperature
 

Phil,

A formula for reducing the pressure from station level to sea level is as
follows:

M = 1000 * ( 10^m - 1 )

M is the correction in millibars.

m = h / (18429.1 + (67.53 * T) + (0.003 * h)

where h is altitude of station in metres (= ft / 3.28)
T is station temperature in C

As far as I can see, the altimeter uses the ICAO standard values of +15 C
and 1013.25 mbar for
the surface values.

Thus, for a station at 280 ft ASL, and a QNH of 1013.25, the pressure at 280
ft will be 1003.1 ( M = -10.2) mbar

If the temp was 0 C, M would be -10.7 mbar, making the pressure at 280 ft
1002.5 mbar for a MSL p of 1013.25. But the altimeter will indicate 1003.1
and 280 ft, But 1003.1 is now below ground level. The true height for the
1003.1 pressure will be 265 ft ASL.An allowance of +15ft to the altimeter
reading is thus required, and the ground will be encountered when the
altimeter is reading 295 ft.
Similarly, if the temp is 30 C, M would be -9.7 mbar, making the pressure at
280 ft 1003.6 mbar, while the true height of the pressure of 1003.1
indicated by the altimeter would be 295 ft. In this case the allowance will
be -15 ft to the altimeter reading.

When the air is colder than standard, pressure falls quicker with height,
and vice versa.

Any help?

--
Bernard Burton
Wokingham, Berkshire, UK.


Satellite images at:
www.btinternet.com/~wokingham.weather/wwp.html
"Phil Layton" wrote in message
. ..
I had a query at work today.

I am told that Flight minima [such as Minimum Decision ] Heights are
adjusted to take account of the temperature relative to the 'base station'
temperature when looking at QNHs [Pressure at sea level]

So in the case of Heathrow, which has a recorded standard temperature oif
+15c apparently, any change in temperature from that, needs manual
adjustment to find the 'true height' given a pressure setting.

I am told that a true height of 280ft would be adjusted by around 30ft if
the temperature was zero, to make an indicated height of 310ft.

As the temperatures go down, the indicated height goes up i.e the actual
height goes down
and as the temps do up, the indicated heights goes down but the actual
heights go up.

I thought, as the temperature goes up, the pressure goes down i.e 'Heat
Low', so for a given pressure setting, would your height go down which
seems
to contracdict the above!

Any help anyone!

Phil

Pressure, altitude and temperature
 

Phil,

It's exactly five years since I retired, so memory is fading.

We did not correct decision height (altitude) for temperature, but we did
make corrections at high levels for such things as terrain clearance (eg
descending into the alpine valley at Innsbruck).

But don't forget, in many cases decision height is a radar altimeter
measurement.

Jack

Pressure, altitude and temperature
 

"Phil Layton" wrote in message
. ..
I had a query at work today.

I am told that Flight minima [such as Minimum Decision ] Heights are
adjusted to take account of the temperature relative to the 'base station'
temperature when looking at QNHs [Pressure at sea level]

So in the case of Heathrow, which has a recorded standard temperature oif
+15c apparently, any change in temperature from that, needs manual
adjustment to find the 'true height' given a pressure setting.

I am told that a true height of 280ft would be adjusted by around 30ft if
the temperature was zero, to make an indicated height of 310ft.

That's correct, though the numbers look high. 0.35% per degC variation from
ISA would give about 15 ft. Details in AIP AD 1.1.2 para 3. It seems to
suggest that such corrections need only be applied in the UK when the
temperature is freezing or below.

As the temperatures go down, the indicated height goes up i.e the actual
height goes down
and as the temps do up, the indicated heights goes down but the actual
heights go up.

I thought, as the temperature goes up, the pressure goes down i.e 'Heat
Low', so for a given pressure setting, would your height go down which
seems
to contracdict the above!

The altimeter measures a pressure difference. Imagine two columns of air,
bounded by the same pressures top and bottom (say 1000 hPa and 900 hPa) but
at different extreme temperatures. The warmer column will be taller, the
cooler column will be shorter. The aircraft sits at the top of the actual
column, but the altimeter reads the same in both cases. For the warm column
it will underread the true height of the column, for the cool column it will
overread.

Similar concept to thickness -- the indicated altitude difference between
two constant pressure surfaces is the same. The actual thickness is a
measure of its temperature. Higher temperature means more thickness (true
altitude difference).

Julian Scarfe

Pressure, altitude and temperature
 

"Phil Layton" wrote in message
. ..
I had a query at work today.
(snip)
I thought, as the temperature goes up, the pressure goes down i.e 'Heat
Low', so for a given pressure setting, would your height go down which
seems
to contracdict the above!

Any help anyone!

Phil



dont know if this helps
http://www.srh.noaa.gov/elp/wxcalc/a...rsetting.shtml

Pressure, altitude and temperature
 

"Jack Harrison" wrote in message
...
Phil,

It's exactly five years since I retired, so memory is fading.

We did not correct decision height (altitude) for temperature, but we did
make corrections at high levels for such things as terrain clearance (eg
descending into the alpine valley at Innsbruck).

But don't forget, in many cases decision height is a radar altimeter
measurement.

Jack

Did you ever land at Sion in Switzerland, Jack ? I hear that there
are similar problems with terrain there.

Joe


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Pressure, altitude and temperature
 

Dear Bernard, Jack, Julian & Joe,

Thanks for the reply, I now fully understand the theory. The only wonder I
have is that in all my life as a controller, it never occurred to me to
adjust the altitude to take car of the temperature. I was at Heathrow when
the temp never got above -5c one day! [must have been between 1986 & 1991
!!]

I came across an advisory issued by the FAA on :
http://www2.faa.gov/avr/arm/ac91-xx.doc
which explains the phenonoma pretty well - but it does place the
responsibility on the air traffic controller.I need to investigate further
and may call Safety Regulation Group.

Julian. Thanks for the ref in the AIP - I'll have a look

I suppose this also means that if we have aneroid barometers at home, we
ought to set them at a temp of +15c, as for every deg different the temp is,
the reading will be slightly inaccurate ?

Regards

Phil

Pressure, altitude and temperature
 

"Phil Layton" wrote in message
. ..
The only wonder I
have is that in all my life as a controller, it never occurred to me to
adjust the altitude to take car of the temperature.

I think the key is that there are two reasons for wanting to measure the
altitude of an aircraft. One is for separation from other aircraft, the
other is for separation from terrain and obstacles. The former, which you
usually deal with, only requires that everyone uses the same datum -- it
would be counterproductive to make corrections. The latter is only
important on approach, and even then the effects are minimal, at least in
the London TMA.

Julian

Pressure, altitude and temperature
 

"Joe Hunt"
Did you ever land at Sion in Switzerland, Jack ? I hear that there
are similar problems with terrain there.

No, but I did land at Amsterdam (minus 6 feet? - can't quite recall)
hundreds of times!

Jack

Pressure, altitude and temperature
 

"Phil Layton" wrote in message
. ..
Dear Bernard, Jack, Julian & Joe,

Thanks for the reply, I now fully understand the theory. The only wonder I
have is that in all my life as a controller, it never occurred to me to
adjust the altitude to take car of the temperature. I was at Heathrow when
the temp never got above -5c one day! [must have been between 1986 & 1991
!!]

I came across an advisory issued by the FAA on :
http://www2.faa.gov/avr/arm/ac91-xx.doc
which explains the phenonoma pretty well - but it does place the
responsibility on the air traffic controller.I need to investigate further
and may call Safety Regulation Group.

Interesting how an "amateur" newsgroup such as our can contribute to safety.

Jack