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low temperature correction

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low temperature correction

Old 7th Jan 2006, 18:59
  #41 (permalink)  
 
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Arrow Re: low temperature correction

Arctaurus, well as usual if in doubt follow the safest course of action, which in this case is adding 40 ft. However, if you check the design criteria for DA creation it includes corrections for the temperatures, but I couldn't find out down to which T is it good. I haven't experienced corrections for DA, with OAT UP to -30 C, flying in the northern USA. Maybe we were doing wrong, I am not sure. Logically, I agree with you it would makes sense, I am wondering why airbus didn't include it?
Cheers.
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Old 7th Jan 2006, 20:54
  #42 (permalink)  
 
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Re: low temperature correction

the minimum temperatures are associated with GPS/RNAV approaches. As you referred to earlier, there is a preprogrammed set of altitudes in the FMC database for the approach. As you can't (or shouldn't) overwrite these altitudes, there is a minimum temperature that these appraoches can be conducted in. If you were to fly the approach in VNAV (or whatever the vertical mode is in the Airbus world) you'd be low at all the IF/FAF/MDA etc altitudes.
CAT I/II/III approaches do not have this restriction.
Again, it's only the CAT I DA that is corrected.
CAT II/III DH's are based on a radar altimeter.

I think the lowest temperature I did an approach in was -53C. And guess who had to do the walkaround after,......Brrrr!!!
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Old 9th Jan 2006, 13:39
  #43 (permalink)  
 
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Re: low temperature correction

I guess the next question is - does anyone have any solid references to temperature compensation automatically incorporated into the DA during the design of a Cat 1 (barometric minima) approach system.

If so, is there a minimum temperature ?

Dartman; At -53 C, there is significant error, I wonder what the RAD ALT was indicating at the cat 1 barometric DA ?
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Old 9th Jan 2006, 16:46
  #44 (permalink)  
 
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Re: low temperature correction

Bumz Rush wrote:
Several Russian Airports, in Moscow give the corrected QFE setting, as perhaps there is no need to correct a setting that has been read directly from the reference mesuring device.

Next time I will compare the advised QNH, and QFE. Expect optortunity tomorrow....

So does the problem only manifest itself in NON controlled airports, that are working on QNH settings.
It's the other way around. QNH is derived from QFE(measured pressure at the station) and adjusted by adding 1 hPa pr 30 ft above MSL. Therefore both will be equally accurate in terms of giving us a reference for our altimeters to give us either alt(QNH) or height(QFE), and the temperature corrections apply to both.
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Old 10th Jan 2006, 13:24
  #45 (permalink)  
 
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Re: low temperature correction

I am a long way away from active involvement, but maany years ago we were asked to make temp corrections to decision heights/altitudes and to any step-down fixes inside the final app fix if the temp was ISA minus 15 or below For simplicity we were allowed to use airfield temp 0șC or below for airfields up to 1000ft amsl.

QFE is the actual airfield surface pressure and is not affected by air temperature. The error is engendered in the height gap between the airfield and the aircraft, and varies according to the size of this gap. If you used a false QFE to adjust for temp error at a 500ft decision height your altimeter would be still give false readings at heights above and below 500ft. I don't see how ATC could do this Of course, once you have made your decision at the correct point, with the calculated temp correction incorporated, your indicated altitude is still wrong, but with decreasing error down to touchdown. Your RADALT will be right.

QNH is calculated from QFE, the measured airfield pressure, down to an assumed msl on the assumption that ISA temperatures apply. You set QNH, and the altimeter then back-calculates, again using ISA, and displays field elevation accurately when you are on the airfield, regardless of actual temps. At any height above or below the airfield the altimeter is then subject to temp errors if ISA temperatures do not apply. As with QFE and decision heights, so with QNH and decision altitudes. The size of the temp error depends on the height gap between the airfield and the aircraft (and the temp, of course).

How would the aircraft systems generate a changing temp correction as the aircraft descended, keeping the indicated height correct? Is it possible?

Dick W
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Old 1st Feb 2006, 13:58
  #46 (permalink)  
 
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When flying in Canada we regulary corrected altitudes for temperature...especially NPAs! However, I did a stint in Europe for a while and also flew corrected altitudes including ATC Radar Vectored Altitudes, as we had been told that only Canada corrects RVAs for cold weather.

Obviously, we informed ATC when we corrected their altitudes.

My question is: Does anyone know if this is true? We never had any documentation to back up this claim. It just seemed prudent to apply the corrections in case it was...
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Old 1st Feb 2006, 15:13
  #47 (permalink)  
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temp correction

doubleoh:

I would agree that Canada provides the correct information.

RYR-738-JOCKEY:
Should have said "correct", not "corrected".

QFE is the reference, and QNH is computed based on ISA, and when the temperatre is different the errors appear. Also rememebr in mountainous terrain, the error can be of a large magnitude.

It is perfectly possible for a modern ADC to compensate, as it knows the actual temperatre, at all times, (less any computation lag), thus a corrected height could be displayed. However the lag at normal appraoch speds would be unacceptable.

Arctaurus:

At -53C at a sea level airport you are about 100ft below your displayed altitude, so the baro shows 200ft (cat 1), but you are at 100 (cat 2). The rad alt will show the correct height.
But remember, on a cat 1 approach the RAD alt is not the approved height measuring device.

I have been working in Siberia this winter, and these errors are noticable. BUT in Russia you land on QFE, so no problem.

Bumz
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Old 1st Feb 2006, 19:23
  #48 (permalink)  

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Question

Bumz_Rush,

ADC will only know actual temperature, not temperature below. That would make passing an inversion in the descent a bit interesting - suddenly you'd loose 1000 ft. of indicated altitude, notwithstanding the implication on obstacle clearance.

How would the ADC solve this problem?

Empty
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Old 1st Feb 2006, 21:11
  #49 (permalink)  
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Inversion layer

You are correct, but remember part of the "land init" is surface temperature. So it is possible to calculate something of relevence. The FMC knows the elevation/height plus isa.

I was passing thru a 1000 ft / sfc inversion layer a few weeks ago, from -10 to sfc of -30. Siberia.
The Auto throttle went ape, and was disconnected rapidly.
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