low temperature correction (or not?)
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low temperature correction (or not?)
Hello all,
At the moment I'm in the middle of a discussion on a Dutch Forum for pilots about low temperature corrections for a baro altimeter reading.
I am used to make a correction for a low temerature to my baro altimeter reading for a DA or a MDA if the temperature is ISA-something.
In our company is a discussion whether you should do the same on all altitudes as depicted on your approach chart. Say MSA, initial altitudes, intermediate altitudes, etc.
We also make corrections for the above when flying over high terrain, especially when using a driftdown graph in order to cross the Alps. (my n-1 net level of is tyoically something like 10000 PA)
Now the problem is some guys who I discuss with disagree about the need to do so. They claim that my baroaltimeter already has some errors, so why bother and further they claim that if you have to calculate the correction due to low temperature in order to miss the mountain peaks when n-1, you should not be out there at all.
Has anyone some thoughts about this one?
Regards, skate
At the moment I'm in the middle of a discussion on a Dutch Forum for pilots about low temperature corrections for a baro altimeter reading.
I am used to make a correction for a low temerature to my baro altimeter reading for a DA or a MDA if the temperature is ISA-something.
In our company is a discussion whether you should do the same on all altitudes as depicted on your approach chart. Say MSA, initial altitudes, intermediate altitudes, etc.
We also make corrections for the above when flying over high terrain, especially when using a driftdown graph in order to cross the Alps. (my n-1 net level of is tyoically something like 10000 PA)
Now the problem is some guys who I discuss with disagree about the need to do so. They claim that my baroaltimeter already has some errors, so why bother and further they claim that if you have to calculate the correction due to low temperature in order to miss the mountain peaks when n-1, you should not be out there at all.
Has anyone some thoughts about this one?
Regards, skate
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I believe the correction for ISA-20 is 7% (i.e. the altimeter overreads by about 7%). It can get a little cooler than that (-5 degC on the surface) in NW Europe, but usually not too much.
Thus a 200 ft DH becomes a 215 ft indicated DH. A 1500 ft step down height becomes a 1600 ft indicated step down height.
Is the correction worth making? Well, there's an argument that the 15 ft gets lost in the tolerance of the instrument itself and the altimeter setting, after all it's only half a mbar/hPA. And the 1500 ft step down probably has a 1000 ft terrain clearance built in.
There's also an argument that you might as well adjust the figures for known calibration issues like temperature correction, and preserve the safety buffers for other eventualities.
It's a judgement call...
Thus a 200 ft DH becomes a 215 ft indicated DH. A 1500 ft step down height becomes a 1600 ft indicated step down height.
Is the correction worth making? Well, there's an argument that the 15 ft gets lost in the tolerance of the instrument itself and the altimeter setting, after all it's only half a mbar/hPA. And the 1500 ft step down probably has a 1000 ft terrain clearance built in.
There's also an argument that you might as well adjust the figures for known calibration issues like temperature correction, and preserve the safety buffers for other eventualities.
It's a judgement call...
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AIP Canada (RAC fig 9.1) covers this here. It recommends applying temp corrections under conditions "of extreme cold" to all altitudes on the approach chart. You are then supposed to inform ATC of the correction you are using. ATC vectored altitudes are temp corrected here.
The definition of 'extreme cold' relates to the airfield temp.
With regard to en route, I suppose if your drift down alt is less than 1500' above the rocky stuff then perhaps you shouldn't be up there
Any help?
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edited for persistently dyslexic keyboard
[This message has been edited by Code Blue (edited 03 June 2001).]
The definition of 'extreme cold' relates to the airfield temp.
With regard to en route, I suppose if your drift down alt is less than 1500' above the rocky stuff then perhaps you shouldn't be up there
Any help?
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edited for persistently dyslexic keyboard
[This message has been edited by Code Blue (edited 03 June 2001).]
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Definitely correct all altitudes below MSA and also MSA if you want to live forever!
See my small thread on one of the last pages of the Tartan Giants thread on the technical site for a bit of useless info from Boeing on the subject.
There is also a Boeing Bulletin on this subject covering the 707 to the 777 which all suffer from altimeter error in cold climes.
I guarantee you there will be one pilot( at least ) who will poo poo this whole subject through their own ignorance on this thread.
Brace yourself!!!
See my small thread on one of the last pages of the Tartan Giants thread on the technical site for a bit of useless info from Boeing on the subject.
There is also a Boeing Bulletin on this subject covering the 707 to the 777 which all suffer from altimeter error in cold climes.
I guarantee you there will be one pilot( at least ) who will poo poo this whole subject through their own ignorance on this thread.
Brace yourself!!!
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Adjustments for low temperature should be made. If the adjustments are not made then be prepared for certain terrain features to trigger GPWS warnings. The Canadian Approach supplement for Jepp charts have a very handy correction chart. I keep it with my Whitehorse Yukon plates which we often use as an alternate. The deviations get greater with height above the altimeter source. A 2,000 foot clearance, normal temperatures, in mountainous terrain could very well become
only about a 400 foot clearance under extremely cold temperatures.
only about a 400 foot clearance under extremely cold temperatures.
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Skate,
In your discussion on the Dutch forum I wonder if any there are KLM Long haul pilots? If they are then have them check out the Fairbanks (PAFA) ILS 19R approach. Lufthansa Cargo, Condor, and Air France Cargo pilots also. On a normal cold winters day day in Fairbanks (-30C) a GPWS warning will be normal for this approach if no correction is made.
In your discussion on the Dutch forum I wonder if any there are KLM Long haul pilots? If they are then have them check out the Fairbanks (PAFA) ILS 19R approach. Lufthansa Cargo, Condor, and Air France Cargo pilots also. On a normal cold winters day day in Fairbanks (-30C) a GPWS warning will be normal for this approach if no correction is made.
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I was told that ATC in those areas that experience cold weather have already made allowances for the lowest temps thus intermediate altitudes need no correction. Or they will add to the published altitudes on extra cold days. However I have not seen any additions when I have flown in these conditions (never had a GPWS under these conditions either).
And yes, there are some of us who think a bit deeper and so will disagree!
If you add to your published altitude, tell ATC. If not you risk reducing separation.
On cold days on the field (which is the only place the correction will be accurate) it is not necessarily cold at altitude. Lapse rates vary, and inversions are possible even in cold climates, so a correction will not necessarily be required at intermediate altitudes. With modern airplanes you can check this, since the airplane can give you true outside temp all the time. Before making changes, check it! If you add a hundred feet to the minima when the temp above the field is actually close to normal, you risk an unecessary go around. OK if you have the fuel I suppose.
Far better in my mind to use the radio altitude to confirm the pressure readings. Knowledge of the terrain would be needed, but most pilots who operate into these conditions know their area well. Good pre planning will prevent surprises. The procedure in the Jep is simplistic and should be used with common sense. It will give you a rough correction but might replace one problem with another. Next time you go to one of these places, note the true OAT at various points and see for yourself. You will see that the temp on the ground is quite often much lower than it is in the air, especially above 2000 feet agl.
And yes, there are some of us who think a bit deeper and so will disagree!
If you add to your published altitude, tell ATC. If not you risk reducing separation.
On cold days on the field (which is the only place the correction will be accurate) it is not necessarily cold at altitude. Lapse rates vary, and inversions are possible even in cold climates, so a correction will not necessarily be required at intermediate altitudes. With modern airplanes you can check this, since the airplane can give you true outside temp all the time. Before making changes, check it! If you add a hundred feet to the minima when the temp above the field is actually close to normal, you risk an unecessary go around. OK if you have the fuel I suppose.
Far better in my mind to use the radio altitude to confirm the pressure readings. Knowledge of the terrain would be needed, but most pilots who operate into these conditions know their area well. Good pre planning will prevent surprises. The procedure in the Jep is simplistic and should be used with common sense. It will give you a rough correction but might replace one problem with another. Next time you go to one of these places, note the true OAT at various points and see for yourself. You will see that the temp on the ground is quite often much lower than it is in the air, especially above 2000 feet agl.
Bunch of information when this topic was discussed previously. The topic was archived in the Tech Log Archive:
Altimeter Correction Cold WX OPS
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Tech Log forum moderator
Altimeter Correction Cold WX OPS
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Tech Log forum moderator
From the discussion mentioned above:
That probably answers the bit you mention about ATC correcting for cold temperatures.
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Tech Log forum moderator
<font face="Verdana, Arial, Helvetica" size="2">Question: So, you are landing at Madrid, 2000' AMSL, DH 200', is the correction based on 200' of cold air, or 2200' of cold air?
Ans: If you are using the Madrid QNH (as I expect you would) then that QNH is not the "actual" pressure at sea level.
If it was you would require a 2200' correction, and indeed the altimeter would not read 2000' when sitting on the taxiway. The Airport QNH is adjusted, so that an altimeter reads the airport elevation when you are at the airport.
The correction from sea level up to the airport elevation has already been done for you, so you only need to apply a density correction to the height above the airport elevation. (ie 200' in the question).</font>
Ans: If you are using the Madrid QNH (as I expect you would) then that QNH is not the "actual" pressure at sea level.
If it was you would require a 2200' correction, and indeed the altimeter would not read 2000' when sitting on the taxiway. The Airport QNH is adjusted, so that an altimeter reads the airport elevation when you are at the airport.
The correction from sea level up to the airport elevation has already been done for you, so you only need to apply a density correction to the height above the airport elevation. (ie 200' in the question).</font>
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Have a look at http://www.firstnethou.com/fmcman/media/alt_corr_ch.pdf (Adobe PDF file)
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It is possible to fly without motors, but not without knowledge and skill.
— Wilbur Wright
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It is possible to fly without motors, but not without knowledge and skill.
— Wilbur Wright
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Hey Jambo Buana,
Could you please let us all know where this Boeing Bulletin is or post it on here as I too am pushing my company to implement this. Trouble is anything I say has little weight here as I am a 'foreigner' and what would I know- but something from Boeing would be useful
Could you please let us all know where this Boeing Bulletin is or post it on here as I too am pushing my company to implement this. Trouble is anything I say has little weight here as I am a 'foreigner' and what would I know- but something from Boeing would be useful
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The formula I use when flying in extreme cold(rare but it happens!) is
+/-4' per degree of ISA Devn x Alt/1000'
This gives answers VERY close to what you get on correction graphs that I've seen.It starts to have an error of approximately 100' when you apply it at extremes of ISA - 50 surface temp and a MSA(for instance of 8000'.
As an example consider a 400' minima on a Non Precision Approach at an elevation of 2000' and an OAT at the field of -15 deg C.
4 x (-26x2) = -208'
So what would the true altitude be if you flew to the minima with no correction ? SCARY HUH ! So you apply that correction AND set 400' on your RADALT and when you get to that minima of 608' INDICATED ALTITUDE your RADALT will say 400' AGL(assuming flat terrain).
It's important to note that the temperature of air at the aeroplane is not relevant, temperature at the airport is the important one.
Baro Altimeters are calibrated to be correct in ISA,it is virtually never ISA out there in the world.
Baro Altimeters are corrected for a few errors...temperature(as far as ISA Devn) is not one of them.
As we all know cold air is more dense than warm air, this is where the errors creap in.
Your faithfull Baro Altimeter measures the difference in air pressure between MSL and the altitude you are flying at...if it's really cold the height of the column of air required to give the same difference in pressure as would happen in ISA is much less.
The reverse is true of course in ISA+ but that is merely inconvenient and not going to kill you.
Chuck.
[This message has been edited by Chimbu chuckles (edited 06 June 2001).]
Edited to fix errors that crept in due to 0200 LT original posting time
[This message has been edited by Chimbu chuckles (edited 06 June 2001).]
+/-4' per degree of ISA Devn x Alt/1000'
This gives answers VERY close to what you get on correction graphs that I've seen.It starts to have an error of approximately 100' when you apply it at extremes of ISA - 50 surface temp and a MSA(for instance of 8000'.
As an example consider a 400' minima on a Non Precision Approach at an elevation of 2000' and an OAT at the field of -15 deg C.
4 x (-26x2) = -208'
So what would the true altitude be if you flew to the minima with no correction ? SCARY HUH ! So you apply that correction AND set 400' on your RADALT and when you get to that minima of 608' INDICATED ALTITUDE your RADALT will say 400' AGL(assuming flat terrain).
It's important to note that the temperature of air at the aeroplane is not relevant, temperature at the airport is the important one.
Baro Altimeters are calibrated to be correct in ISA,it is virtually never ISA out there in the world.
Baro Altimeters are corrected for a few errors...temperature(as far as ISA Devn) is not one of them.
As we all know cold air is more dense than warm air, this is where the errors creap in.
Your faithfull Baro Altimeter measures the difference in air pressure between MSL and the altitude you are flying at...if it's really cold the height of the column of air required to give the same difference in pressure as would happen in ISA is much less.
The reverse is true of course in ISA+ but that is merely inconvenient and not going to kill you.
Chuck.
[This message has been edited by Chimbu chuckles (edited 06 June 2001).]
Edited to fix errors that crept in due to 0200 LT original posting time
[This message has been edited by Chimbu chuckles (edited 06 June 2001).]
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<font face="Verdana, Arial, Helvetica" size="2">As an example consider a 400' minima on a Non Precision Approach at an elevation of 2000' and an OAT at the field of -15 deg C.
4 x (-26x2) = -208'
So what would the true altitude be if you flew to the minima with no correction ? SCARY HUH ! So you apply that correction AND set 400' on your RADALT and when you get to that minima of 608' INDICATED ALTITUDE your RADALT will say 400' AGL(assuming flat terrain).</font>
4 x (-26x2) = -208'
So what would the true altitude be if you flew to the minima with no correction ? SCARY HUH ! So you apply that correction AND set 400' on your RADALT and when you get to that minima of 608' INDICATED ALTITUDE your RADALT will say 400' AGL(assuming flat terrain).</font>
4 x (-26x0.4) = -41'
That's because...
<font face="Verdana, Arial, Helvetica" size="2">Your faithfull Baro Altimeter measures the difference in air pressure between MSL and the altitude you are flying at...</font>
The station was at 2000 ft at the airport, the MDA is at 2400 ft, so the column of air to which the temperature correction applies is only 400 ft high.
(Flying NPAs into airports at high elevation with no weather reporting facilities is, of course, a different matter, and asking for trouble!)
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IVAN URGE,
Try the following address;
[email protected]
The guys name is Robert Root he should be able to help you.
Good Luck
Try the following address;
[email protected]
The guys name is Robert Root he should be able to help you.
Good Luck
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Bookworm,
With your caveats your right of course. My example was a worse case one of a NDB approach at an uncontrolled airport. Not wide body territory true but they are about for us Bizjet drivers.
I should have been more careful in my example.
Chuck
With your caveats your right of course. My example was a worse case one of a NDB approach at an uncontrolled airport. Not wide body territory true but they are about for us Bizjet drivers.
I should have been more careful in my example.
Chuck
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For Bookworm; the column of air over the airfield affects the accuracy of the indicated altitude if it has a standard lapse rate, surely? If the lapse rate was lower than normal then at altitude the oat might be closer to ISA and so the altimeter would not have such a large error as it would at the field elevation.
If the airplane was flying over an area of extreme cold on the ground, but the air above was, due to an inversion, at ISA, then the altimeter would be indicating a correct value with no errors due to low temperature. If it had the field QNH setting applied it would be above the indicated altitude.
Am I right? It is easy to confuse me when I really don't know what I am talking about.
If the airplane was flying over an area of extreme cold on the ground, but the air above was, due to an inversion, at ISA, then the altimeter would be indicating a correct value with no errors due to low temperature. If it had the field QNH setting applied it would be above the indicated altitude.
Am I right? It is easy to confuse me when I really don't know what I am talking about.
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We add 4% pr 10 degrees below ISA on all minimum altitudes. Furthermore, we add 500 ft pr 10 knots of wind exceeding 30 knots (max 2000ft) during the initial letdown. Strong winds over mountaintops may create a local lowpressure that can give you a nasty surprise. Yes, we fly in mountainous terrain. Yes, we fly in temperatures down to - 35 degrees celsius (sometimes even lower). Yes, I apply the corrections. Religiously. No, our ATC will not do these corrections automatically.
"it is not necessarily cold at altitude, a correction will not necessarily be required, and the temp on the ground is quite often lower", will not necessarily bring you down in one piece.
[This message has been edited by Scando (edited 08 June 2001).]
[This message has been edited by Scando (edited 08 June 2001).]
"it is not necessarily cold at altitude, a correction will not necessarily be required, and the temp on the ground is quite often lower", will not necessarily bring you down in one piece.
[This message has been edited by Scando (edited 08 June 2001).]
[This message has been edited by Scando (edited 08 June 2001).]
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Scando,
Our ATC only corrects radar vectored alts, we have to do the math for the others.
I don't have that many big hills around where I usually fly - I have heard that mountain wave can cause errors of up to 3000'. I haven't seen that mountain wave correction before. Do you have a source for those numbers?
Regards
CB
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edited 'cos I still can't spel
[This message has been edited by Code Blue (edited 08 June 2001).]
Our ATC only corrects radar vectored alts, we have to do the math for the others.
<font face="Verdana, Arial, Helvetica" size="2">we add 500 ft pr 10 knots of wind exceeding 30 knots (max 2000ft) during the initial letdown. Strong winds over mountaintops may create a local lowpressure that can give you a nasty surprise</font>
Regards
CB
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edited 'cos I still can't spel
[This message has been edited by Code Blue (edited 08 June 2001).]
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Scando. You make a good point, and I have no argument with you. But...
I fly only in radar environment and have never had a problem with terrain (using radio alt, visual etc). Maybe it is not necessary to take the actions you prescribe unless it is for flights into the smaller airports and when flying non-precision approaches. And of course for departures, something not often considered.
I am not concerned about you and other pilots who are aware, but for those who visit such areas only occasionally. I have never seen any other pilot in the airline I fly with even consider such things but have not considered them to be in danger since as I say we fly with radar coverage into major airports, when I would imagine the controllers take extra care not to put them into danger. I hope, anyway.
I would also imagine that if one of my colleagues did apply corrections he would not tell anyone, which at the least will give ATC grief.
I fly only in radar environment and have never had a problem with terrain (using radio alt, visual etc). Maybe it is not necessary to take the actions you prescribe unless it is for flights into the smaller airports and when flying non-precision approaches. And of course for departures, something not often considered.
I am not concerned about you and other pilots who are aware, but for those who visit such areas only occasionally. I have never seen any other pilot in the airline I fly with even consider such things but have not considered them to be in danger since as I say we fly with radar coverage into major airports, when I would imagine the controllers take extra care not to put them into danger. I hope, anyway.
I would also imagine that if one of my colleagues did apply corrections he would not tell anyone, which at the least will give ATC grief.
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bunyip wrote:
What matters is the "average" temperature of the column of air -- and I'm being a bit vague about what I mean by average. For every slice of the column, the temperature of the slice determines whether the pressure drop is higher or lower than ISA.
Say there was a 1000 ft of ISA-30 degC air and ISA above it. The pressure drop in that 1000 ft would be about 3 hPa (100 ft) more than ISA would predict. In the 1000 to 2000 ft band, the pressure drop in that 1000 ft would be exactly as ISA predicts (and the next 1000 ft, and the next...)
So for any aircraft flying above the 1000 ft of cold air, the altimeter would overread by 100 ft, whether it was at 1000 ft or 10,000 ft.
Hope that helps.
<font face="Verdana, Arial, Helvetica" size="2">If the airplane was flying over an area of extreme cold on the ground, but the air above was, due to an inversion, at ISA, then the altimeter would be indicating a correct value with no errors due to low temperature. If it had the field QNH setting applied it would be above the indicated altitude.</font>
Say there was a 1000 ft of ISA-30 degC air and ISA above it. The pressure drop in that 1000 ft would be about 3 hPa (100 ft) more than ISA would predict. In the 1000 to 2000 ft band, the pressure drop in that 1000 ft would be exactly as ISA predicts (and the next 1000 ft, and the next...)
So for any aircraft flying above the 1000 ft of cold air, the altimeter would overread by 100 ft, whether it was at 1000 ft or 10,000 ft.
Hope that helps.