Altimeter Correction Cold WX OPS
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These topics will still keep coming up because there are now nearly 8500 ppruners out there, so I think it's healthy to keep the discussions up, both old and new.
Hop, I think you must be winding me up in your beliefs on this matter, but if you are not please call John Cashman, Chief pilot at Boeing, on 001 206 655 0878. He will probably quote the Flight Operations Technical Bulletin number 01, dated October 22, 1999 applicable to, 707,727,737,757,767,777,747 and 747-400 aircraft. If you are working for a decent company they will have a copy of all these bulletins which I urge you to read. If you have a fax number I would be more than happy to send the bulletin to you.
By the way an SAS a/c in front of me the other day applied his correction and informed ATC on approach to a Norweigan airfield, they know their stuff as well.
The Brits will get there in the end, I have every faith.
Hop, I think you must be winding me up in your beliefs on this matter, but if you are not please call John Cashman, Chief pilot at Boeing, on 001 206 655 0878. He will probably quote the Flight Operations Technical Bulletin number 01, dated October 22, 1999 applicable to, 707,727,737,757,767,777,747 and 747-400 aircraft. If you are working for a decent company they will have a copy of all these bulletins which I urge you to read. If you have a fax number I would be more than happy to send the bulletin to you.
By the way an SAS a/c in front of me the other day applied his correction and informed ATC on approach to a Norweigan airfield, they know their stuff as well.
The Brits will get there in the end, I have every faith.
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Canuck_AV8R,
How did you do that? I have that page but couldn't push it through. Ah, I see now you have accessed the ACTUAL regulation. Good work. As you point out the important thing is that the RIGHT information gets through. Forceful promotion of incorrect information is not likely to result in safe operation.
Chaps, take Canuck_AV8R's advice and apply the correction when conditions REQUIRE it's use.
How did you do that? I have that page but couldn't push it through. Ah, I see now you have accessed the ACTUAL regulation. Good work. As you point out the important thing is that the RIGHT information gets through. Forceful promotion of incorrect information is not likely to result in safe operation.
Chaps, take Canuck_AV8R's advice and apply the correction when conditions REQUIRE it's use.
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I must admit this has been fun. I did not want to stir anybody up, but was simply pointing out that according to the airplane and systems manuals I have studied, there is a temperature correction applied to those airplanes using the CADC/ADIRU system and that therefore so far as I can see, the Altimeter reads True. Why else would temperature be an input to the altitude computer?
I freely admit that my opinion might be wrong, and would like to read something from someone who does know the systems.
The stuff about exactly how to correct the altimeter is very interesting, and for many airplanes will be necessary. But it MAY not be necessary for the airplanes under discussion here, and therefore MAY be irrelevant. The Authorities would expect you to know if it applies to your equipment or not.
It may be of interest that in discussing this with a B777 colleague, he is of the opinion that his altimeter is in fact not corrected, since he has seen errors at the OM he did not previously see in the B747-400.
I would love to discuss this with Boeing, but they have a policy of not talking to a pilot directly.
This forum is not for the technical education of the reader, since nobody's credentials can be checked. I see it as food for thought, and cannot understand why it elicits such a furious, strident response from some.
Maybe a form of road rage?
I freely admit that my opinion might be wrong, and would like to read something from someone who does know the systems.
The stuff about exactly how to correct the altimeter is very interesting, and for many airplanes will be necessary. But it MAY not be necessary for the airplanes under discussion here, and therefore MAY be irrelevant. The Authorities would expect you to know if it applies to your equipment or not.
It may be of interest that in discussing this with a B777 colleague, he is of the opinion that his altimeter is in fact not corrected, since he has seen errors at the OM he did not previously see in the B747-400.
I would love to discuss this with Boeing, but they have a policy of not talking to a pilot directly.
This forum is not for the technical education of the reader, since nobody's credentials can be checked. I see it as food for thought, and cannot understand why it elicits such a furious, strident response from some.
Maybe a form of road rage?
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Maybe the response is heated because it has killed people and will continue to unless the "new jet" people figure out just actually what that altimeter is indicating? I myself fly an "old jet" and know the problems with the altimeter system and have the information to correct for it. Hope you new generation aircraft A/C's find an answer before you do a charter over here.
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Hi All,
Very interesting topic here.
On the MD-11 the CADC's DO NOT AUTOMATICALLY compensate for low temperature versus altimeter corrections!!
In fact ATC, if under radar control, WILL have allowed for the low temperature when giving you radar vectors for final approach to an ILS.
All NON precision approaches require pilots to add their own altitude allowances to procedural heights etc and MDA, according to the temperature.
Terrain clearance can be a problem with low temperature and barometric pressure so watch out and take care out there this winter!!
ATB,
Jon.
Very interesting topic here.
On the MD-11 the CADC's DO NOT AUTOMATICALLY compensate for low temperature versus altimeter corrections!!
In fact ATC, if under radar control, WILL have allowed for the low temperature when giving you radar vectors for final approach to an ILS.
All NON precision approaches require pilots to add their own altitude allowances to procedural heights etc and MDA, according to the temperature.
Terrain clearance can be a problem with low temperature and barometric pressure so watch out and take care out there this winter!!
ATB,
Jon.
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I work for a UK operator who is just starting its first winter of operations into Scandinavia, so this thread has really caught my attention.
My understanding of Airfield QNH is that an ISA calibrated altimeter at the Airfield is set to read Airfield elevation. The subscale pressure setting is the Airfield QNH that is passed to the Pilot. The QNH therefore must be corrected for temperature. Any further altimeter correction for temperature to approach minima should be calculated from height above the runway threshold only.
If this topic ran last year, can anybody recall the conclusions reached?
[This message has been edited by Squawkbox (edited 28 November 1999).]
My understanding of Airfield QNH is that an ISA calibrated altimeter at the Airfield is set to read Airfield elevation. The subscale pressure setting is the Airfield QNH that is passed to the Pilot. The QNH therefore must be corrected for temperature. Any further altimeter correction for temperature to approach minima should be calculated from height above the runway threshold only.
If this topic ran last year, can anybody recall the conclusions reached?
[This message has been edited by Squawkbox (edited 28 November 1999).]
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I may well be wrong but I thought all modern altimeters were corrected to a standard ISA lapse rate of 2 degrees a thousand feet and a standard ISA day of sea level and +15 C. Any temperatures below 15C result in the altimeter (which has been corrected to +15 at sea level) under reading. For very low temperatures this under read can be quite marked and consequently a correction has to be added to avoid flying into the granit.
To a certain extent you are all right. The altimeters are corrected to give a true height on an ISA day, buit also have to be corrected due to the unsafe under read on cold days.
[This message has been edited by Don Coyote (edited 28 November 1999).]
To a certain extent you are all right. The altimeters are corrected to give a true height on an ISA day, buit also have to be corrected due to the unsafe under read on cold days.
[This message has been edited by Don Coyote (edited 28 November 1999).]
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Mach asked how to do the correction in starting this thread and I guess he got his answer. Some good stuff was sent in.
I pointed out that modern airplanes may not need to be corrected because the CADC/(ADIRU) Altimeter reading is already corrected.
I flew in a 747-400 last week and noted the altimeters: One is from the ADIRU and the other is a (standby) Pressure Altimeter. On the ground, with the same altimeter setting and in the same airplane, I saw that at ISA minus 15 the Pressure Alt was overreading the main Alt by 25 feet. At minus 10 it was overreading by 5 feet, and at plus 15 it was underreading by 25 feet.
(In the air the differences are obscured by Position Error, quite severe in the B744.)
This is double what I would have expected so there could very well be other factors, but it tends to support the Boeing and Honeywell Technical Manuals.
I pointed out that modern airplanes may not need to be corrected because the CADC/(ADIRU) Altimeter reading is already corrected.
I flew in a 747-400 last week and noted the altimeters: One is from the ADIRU and the other is a (standby) Pressure Altimeter. On the ground, with the same altimeter setting and in the same airplane, I saw that at ISA minus 15 the Pressure Alt was overreading the main Alt by 25 feet. At minus 10 it was overreading by 5 feet, and at plus 15 it was underreading by 25 feet.
(In the air the differences are obscured by Position Error, quite severe in the B744.)
This is double what I would have expected so there could very well be other factors, but it tends to support the Boeing and Honeywell Technical Manuals.
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hopharrigan,
Don't know about the B747-400 but most aircraft performance manuals state the known error in the Standby Pressure Altimeter for various configurations and stage of flight. Part of the errors you observed would be position errors associated with the installation and the ability of the mechanism to accurately display the sensed pressure on the instrument. (hysteresis)
Let us be clear about this. When we fly at a given Altitude/ Flight level we are flying at a "pressure level" equivalent to that level if the atmosphere was standard. The only time our TRUE ALTITUDE and the indicated altitude are coincident is in the standard atmosphere.
The real significance of the DADC/CADC is in the ability to ACCURATELY SENSE and DISPLAY that PRESSURE. (called an Altitude or Flight Level). In cold temperatures close to the ground that pressure level is lower than that displayed by the DADC/CADC as a numerical figure.
RVSM requirements depend on a high accuracy of acquisition of the required pressure aka Flight Level and also digital Auto Flight technology of high sensitivity to capture and correct disturbances within very tight tolerances.
[This message has been edited by pterodactyl (edited 29 November 1999).]
Don't know about the B747-400 but most aircraft performance manuals state the known error in the Standby Pressure Altimeter for various configurations and stage of flight. Part of the errors you observed would be position errors associated with the installation and the ability of the mechanism to accurately display the sensed pressure on the instrument. (hysteresis)
Let us be clear about this. When we fly at a given Altitude/ Flight level we are flying at a "pressure level" equivalent to that level if the atmosphere was standard. The only time our TRUE ALTITUDE and the indicated altitude are coincident is in the standard atmosphere.
The real significance of the DADC/CADC is in the ability to ACCURATELY SENSE and DISPLAY that PRESSURE. (called an Altitude or Flight Level). In cold temperatures close to the ground that pressure level is lower than that displayed by the DADC/CADC as a numerical figure.
RVSM requirements depend on a high accuracy of acquisition of the required pressure aka Flight Level and also digital Auto Flight technology of high sensitivity to capture and correct disturbances within very tight tolerances.
[This message has been edited by pterodactyl (edited 29 November 1999).]
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Of course you are right, that is why I only gave figures for when the airplane was on the ground and at that time the book says there is no position error correction, so it is the only time the altimeters can be accurately checked. For that matter, they cannot be checked against each other apart from in normal level flight, so comparison on final is pointless.
I actually simplified the figues, by reducing them to the same datum. The actual PFD altitude for the first check was 20 feet while the standby was 100 feet, for example. This is in fact outside tolerance for the altimeter system, according to the FRM, but I would be willing to bet it has always been this way and not one other pilot in the company would be aware of this. Of course when the temperature was at a more normal value the standby altimeter was again within tolerance.
The company I work for says the correction for low temperature should be made, but it does not say how it is to be done or where. That allows the more pedantic among us to complicate the procedure to their heart's content. Most, however, do not have a clue and leave it alone. This does not worry me, since we do not operate old airplanes in the Frozen North and neither do we fly non-precision approaches in poor terrain. Very few of our guys have ever flown a circling approach, another time when corrections would be very important.
Most of our guys do not even know that the auto Antice system does not operate on the ground, which is way more scary.
I have an agenda of course, and that is to simplify the operating procedures. I see the altimeter correction as an unnecessary complication IF it is in fact not needed. If it IS, then we should all be doing it, and the right way.
Unfortunately, nobody has posted here who has the real technical poop for me to decide this for myself. And so far, my requests to the manufacturer for information deeper than what they have published in their technical manuals have been ignored.
Nevertheless I have been educated way to hell and I hope others have too.
I actually simplified the figues, by reducing them to the same datum. The actual PFD altitude for the first check was 20 feet while the standby was 100 feet, for example. This is in fact outside tolerance for the altimeter system, according to the FRM, but I would be willing to bet it has always been this way and not one other pilot in the company would be aware of this. Of course when the temperature was at a more normal value the standby altimeter was again within tolerance.
The company I work for says the correction for low temperature should be made, but it does not say how it is to be done or where. That allows the more pedantic among us to complicate the procedure to their heart's content. Most, however, do not have a clue and leave it alone. This does not worry me, since we do not operate old airplanes in the Frozen North and neither do we fly non-precision approaches in poor terrain. Very few of our guys have ever flown a circling approach, another time when corrections would be very important.
Most of our guys do not even know that the auto Antice system does not operate on the ground, which is way more scary.
I have an agenda of course, and that is to simplify the operating procedures. I see the altimeter correction as an unnecessary complication IF it is in fact not needed. If it IS, then we should all be doing it, and the right way.
Unfortunately, nobody has posted here who has the real technical poop for me to decide this for myself. And so far, my requests to the manufacturer for information deeper than what they have published in their technical manuals have been ignored.
Nevertheless I have been educated way to hell and I hope others have too.
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Downloaded the Canadian official document. ALso read article in OCtober 1999 Business and COmmercial Aviation (a McGraw-Hill publication.) ALso have photocopy of Jeppesen page. All specifically say that altimeters do not compensate for the effect of non-ISA temp on pressure lapse rate above the reporting QNH's elevation.
The terrain clearance you get on a published altitude presupposes a delta-pressure (pressure difference) between QNH reporting point and you flight altitude. That delta pressure varies with temperature. The ADCs, DADCs, and non-electric altimeters will not, cannot, compensate for a non-standard pressure lapse rate, which is the prinicipal threat resulting from the below ISA temp.
If you ask a mechanic, technician, or engineer, you may get a misleading answer. They don't design or fly the approaches; they don't read the charts.
It's the people who design the instrument approaches (FAA or CAA) or who fly them, who will be operationally familiar with this issue.
It is rarely a safety threat because most approaches are precision, and because intercept altitudes are rarely 4,000 feet above MDA. Also remember that when it is that cold, it is rarely poor visibility, so many approaches are done on a visual basis, at visual altitudes.
Most VOR approaches run about 1,500 feet above airfield for the intermediate segment, and about 500 above for the final segment, so the distortions are small.
Now, think about in a night approach on a moonlesss night. YOu're probably flying the instrument approach, yet you're possibly also less-than vigilant on the instruments. You might be a little shocked to see how low you are, AGL, at the published segment altitudes. You'll dismiss it as some sort of optical illusion. What we are telling you is: you actually are lower, reference the terrain. You are at the same pressure level; but due to the non-standard pressure lapse rate, the delta pressure gives you a smaller terrain clearance.
It's quite all right being dubious, but you may be going a bit too far here. Sorry to be a bit provocative, but may I ask when was the last time you turned out to be right when thousands of Canadian airline pilots believed otherwise.
The terrain clearance you get on a published altitude presupposes a delta-pressure (pressure difference) between QNH reporting point and you flight altitude. That delta pressure varies with temperature. The ADCs, DADCs, and non-electric altimeters will not, cannot, compensate for a non-standard pressure lapse rate, which is the prinicipal threat resulting from the below ISA temp.
If you ask a mechanic, technician, or engineer, you may get a misleading answer. They don't design or fly the approaches; they don't read the charts.
It's the people who design the instrument approaches (FAA or CAA) or who fly them, who will be operationally familiar with this issue.
It is rarely a safety threat because most approaches are precision, and because intercept altitudes are rarely 4,000 feet above MDA. Also remember that when it is that cold, it is rarely poor visibility, so many approaches are done on a visual basis, at visual altitudes.
Most VOR approaches run about 1,500 feet above airfield for the intermediate segment, and about 500 above for the final segment, so the distortions are small.
Now, think about in a night approach on a moonlesss night. YOu're probably flying the instrument approach, yet you're possibly also less-than vigilant on the instruments. You might be a little shocked to see how low you are, AGL, at the published segment altitudes. You'll dismiss it as some sort of optical illusion. What we are telling you is: you actually are lower, reference the terrain. You are at the same pressure level; but due to the non-standard pressure lapse rate, the delta pressure gives you a smaller terrain clearance.
It's quite all right being dubious, but you may be going a bit too far here. Sorry to be a bit provocative, but may I ask when was the last time you turned out to be right when thousands of Canadian airline pilots believed otherwise.
hopharrigan, if both of your altimeters on the ground were set to the same QNH, then they should have read the same figure (within tolerance). If they didn't they should have been written up. (Indeed, in Australia, ground altimeter cross checks are required in the AIP.)
As to whether or not glass cockpit altimeters have an in-built true altitude correction it should be obvious that they do not.
The altimeter reads a pressure level as a standard (as posted by ptero.) That's why it is called a "Pressure altimeter"
Imagine if one aircraft were to fly a true altitude, and another ("old") aircraft was flying a pressure altitude. Both are told to maintain 5000, on an ISA-15°C day. The old aircraft would be flying at 5000 feet. The "new" aircraft would be flying at 5,300 feet (5000' true) - obviously reducing separation with anyone at 6000. It is not that the technology isn't capable of performing the correction, it is simply not standard.
How much does it matter? If you use jepps you will notice that the printed lowest safe altitudes allow 1000' terrain clearance below 5000', and 2000' terrain clearance above 5000'. This is to allow for the effect of true altitude at cold temperatures. So you are reasonably protected en-route (provided you don't bust the lowest safe, of course)
With radar vectors, you will be asked to descend to a pressure altitude, however the radar controller will have adjusted his radar lowest safe to allow for true altitude clearance, so you are reasonably protected there.
With a precision approach you will be following a glide path down to the runway. As long as you are on slope, you will be protected from terrain, however the OM crossing height check will show a difference between the true altitude printed on the chart, and the pressure altitude read in the cockpit. If you wish to conduct a "proper" altimeter check you will have to have performed the correction. (I fly in ISA+ temps, and I perform the correction in order to check this - it is just a quick mental calculation as shown in my previous post.) If you are on glide slope, however you will have terrain protection anyway.
When you get to the minima the correction is only occuring over 200 or so feet (CAT1), and so it is only of the order of 20' or so - even at really cold temperatures. Technically you will be busting the minima (assuming you are not visual) but a 20' error is probably not going to kill you.
The only place you really have to be on the ball is for a non-precision approach, in pretty cold temps. With a minima at around 800' AGL the correction (ISA-30°, say) will be on the order of (.8 * 30 * 4) 96 feet, so flying to the indicated (pressure) minima, will result in a bust of 100feet or so. Something a professional pilot should be aghast at.
It doesn't matter if you use a correction chart (probably the safest, as it is hard to make a mistake), use the true altitude correction on the Jep. CR-2 calculator, or do it in your head (using the formula in my previous post) but if you are conducting non-precision approaches in cold temperatures then you have to adjust the printed (chart) minima for true altitude.
[This message has been edited by Checkboard (edited 01 December 1999).]
As to whether or not glass cockpit altimeters have an in-built true altitude correction it should be obvious that they do not.
The altimeter reads a pressure level as a standard (as posted by ptero.) That's why it is called a "Pressure altimeter"
Imagine if one aircraft were to fly a true altitude, and another ("old") aircraft was flying a pressure altitude. Both are told to maintain 5000, on an ISA-15°C day. The old aircraft would be flying at 5000 feet. The "new" aircraft would be flying at 5,300 feet (5000' true) - obviously reducing separation with anyone at 6000. It is not that the technology isn't capable of performing the correction, it is simply not standard.
How much does it matter? If you use jepps you will notice that the printed lowest safe altitudes allow 1000' terrain clearance below 5000', and 2000' terrain clearance above 5000'. This is to allow for the effect of true altitude at cold temperatures. So you are reasonably protected en-route (provided you don't bust the lowest safe, of course
)With radar vectors, you will be asked to descend to a pressure altitude, however the radar controller will have adjusted his radar lowest safe to allow for true altitude clearance, so you are reasonably protected there.
With a precision approach you will be following a glide path down to the runway. As long as you are on slope, you will be protected from terrain, however the OM crossing height check will show a difference between the true altitude printed on the chart, and the pressure altitude read in the cockpit. If you wish to conduct a "proper" altimeter check you will have to have performed the correction. (I fly in ISA+ temps, and I perform the correction in order to check this - it is just a quick mental calculation as shown in my previous post.) If you are on glide slope, however you will have terrain protection anyway.
When you get to the minima the correction is only occuring over 200 or so feet (CAT1), and so it is only of the order of 20' or so - even at really cold temperatures. Technically you will be busting the minima (assuming you are not visual) but a 20' error is probably not going to kill you.
The only place you really have to be on the ball is for a non-precision approach, in pretty cold temps. With a minima at around 800' AGL the correction (ISA-30°, say) will be on the order of (.8 * 30 * 4) 96 feet, so flying to the indicated (pressure) minima, will result in a bust of 100feet or so. Something a professional pilot should be aghast at.
It doesn't matter if you use a correction chart (probably the safest, as it is hard to make a mistake), use the true altitude correction on the Jep. CR-2 calculator, or do it in your head (using the formula in my previous post) but if you are conducting non-precision approaches in cold temperatures then you have to adjust the printed (chart) minima for true altitude.
[This message has been edited by Checkboard (edited 01 December 1999).]
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Real great stuff here. Thanks guys.
Wait a minute, I hijacked this topic from Mach and I'm not going to give it up without a fight!
I do fly into cold conditions, in Alaska, Canada, Europe and the USA and I have paid particular attention to the altimeter displays. The reason I am proposing that the manufacturers of the airplane, (B744 in my case) and the Air Data Computer, put it in their manuals that temperature is automatically compensated, is because that is the only way the performance of the altimeters I use can be explained. If they were not compensated then I would see all the things you say I would.
If the airport is cold, that does not necessarily mean the air above it is also. The cold air holds less water vapour, the tropopause may be lower (in the Frozen North), and the lapse rate could be wild. It is not uncommon for a marked inversion to form just above the ground, especially in Canada,such that for approach and departure you could be actually above ISA. Using the airport temperature to compensate the altimeter manually does not make any sense if you have a better way...and modern airplanes have, through the CADC, accurate temperature, (TAT and OAT) at all times. Far better to use that, would you not agree? Or is it a principle to you that the CAA is always right?
Read on if you acknowledge that there are better ways to apply the correction than using the airfield temp.
So is it too hard a leap of faith that the Boffins have anticipated the need to have a more accurate way of presenting altitude and have incorporated automatic compensation? And if they have, that they put that information in their manuals?
Feel free to knock off now and check for yourself. See the references above.
And is it hard to believe that the CAA (etc) and those guys in your company who write the Ops Manuals are not aware or do not care about this change? They have to write their regulations for all airplanes after all and if you want to apply the correction when it may not be necessary, it will not hurt you, so go ahead.
At altitude, the airplane with the new computer equipment will have much more accurate altitude and speed displays, and it is not unreasonable for any temperature error to be smaller than the altimeter error of the older airplane. I hope I don't come close enough to a 707 to find out for sure. The deviations from ISA are small at altitude anyway, and so are the differences due to temperature.
As well as being accused of being at odds with "all the Canadians", trying to kill my passengers in Zurich (I have been there many times in the winter and not killed anyone yet), now I am being told how to write up defects on my airplane.
Still you (but not me, apparently) are entitled to your opinion.
By the way, do you know the altimeter tolerance for the B744?
I am not trying to change anyone's procedures, it would be far better for you to research it yourself if you have an interest. I am not disagreeing with anything written here, as it applies to a Pressure Altimeter.
I just read that the Universe is flat. Are we going to crap on the guy who suggested that too?
Wait a minute, I hijacked this topic from Mach and I'm not going to give it up without a fight!
I do fly into cold conditions, in Alaska, Canada, Europe and the USA and I have paid particular attention to the altimeter displays. The reason I am proposing that the manufacturers of the airplane, (B744 in my case) and the Air Data Computer, put it in their manuals that temperature is automatically compensated, is because that is the only way the performance of the altimeters I use can be explained. If they were not compensated then I would see all the things you say I would.
If the airport is cold, that does not necessarily mean the air above it is also. The cold air holds less water vapour, the tropopause may be lower (in the Frozen North), and the lapse rate could be wild. It is not uncommon for a marked inversion to form just above the ground, especially in Canada,such that for approach and departure you could be actually above ISA. Using the airport temperature to compensate the altimeter manually does not make any sense if you have a better way...and modern airplanes have, through the CADC, accurate temperature, (TAT and OAT) at all times. Far better to use that, would you not agree? Or is it a principle to you that the CAA is always right?
Read on if you acknowledge that there are better ways to apply the correction than using the airfield temp.
So is it too hard a leap of faith that the Boffins have anticipated the need to have a more accurate way of presenting altitude and have incorporated automatic compensation? And if they have, that they put that information in their manuals?
Feel free to knock off now and check for yourself. See the references above.
And is it hard to believe that the CAA (etc) and those guys in your company who write the Ops Manuals are not aware or do not care about this change? They have to write their regulations for all airplanes after all and if you want to apply the correction when it may not be necessary, it will not hurt you, so go ahead.
At altitude, the airplane with the new computer equipment will have much more accurate altitude and speed displays, and it is not unreasonable for any temperature error to be smaller than the altimeter error of the older airplane. I hope I don't come close enough to a 707 to find out for sure. The deviations from ISA are small at altitude anyway, and so are the differences due to temperature.
As well as being accused of being at odds with "all the Canadians", trying to kill my passengers in Zurich (I have been there many times in the winter and not killed anyone yet), now I am being told how to write up defects on my airplane.
Still you (but not me, apparently) are entitled to your opinion.
By the way, do you know the altimeter tolerance for the B744?
I am not trying to change anyone's procedures, it would be far better for you to research it yourself if you have an interest. I am not disagreeing with anything written here, as it applies to a Pressure Altimeter.
I just read that the Universe is flat. Are we going to crap on the guy who suggested that too?
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Hopharrigan's postings on this topic, arguably the latter ones mostly, remind me of the saying among lawyers:
If the law is against you, argue the facts.
If the facts are against you, argue the law.
If they both are against you, shout and pound the table.
Recommend we close this thread. There's obviously nothing else we can do.
If the law is against you, argue the facts.
If the facts are against you, argue the law.
If they both are against you, shout and pound the table.
Recommend we close this thread. There's obviously nothing else we can do.
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Despite all the PITCHBLOTs etc. that we were taught as young Bloggses, all altimetry systems have only 3 basic errors:
1. They're not perfect machines.
2. They're installed in a moving aircraft rather than being free-standing in space.
3. They are only calibrated for International Standard Atmospheric conditions and have no idea of the actual values of lapse rate prevailing directly at and below their position.
You can reduce mechanical imperfections with, for example, an electronic display. You can also reduce installation errors by using a clever Air Data Unit or Central Air Data Computer. The OAT feed merely assists in reducing the errors in the 'physics' of the aircraft installation. However, unless you point-plot the actual environmental lapse rate in the atmosphere in which you are flying - which is obviously totally impossible - you cannot, Hophorrigan, correct AUTOMATICALLY for non-ISA atmospheres. This doesn't matter when everyone is flying on 1013 (or 29.92 for those still using quaint old inches) because everyone will have the same error due to non-ISA atmospheric conditions. But, near the ground in cold temperatures it is absolutely essential that temperatur error correction is applied to ALL altimeters. If that means your indicated altitude will differ from a published STAR altitude in order to achieve safe separation between metal and stone, ADVISE ATC - they won't mind!!
Incidentally, the meat bombs who made such a tragically nasty pink stain jumping out over the South Pole probably learned rather too late about temperature error correction IMHO - they were much lower than their altimeters said when they pulled their ripcords!! Hophorrigan - listen to those in this thread who are trying to protect you and your passengers from an unexpected close encounter of the worst kind with terra firma!!
1. They're not perfect machines.
2. They're installed in a moving aircraft rather than being free-standing in space.
3. They are only calibrated for International Standard Atmospheric conditions and have no idea of the actual values of lapse rate prevailing directly at and below their position.
You can reduce mechanical imperfections with, for example, an electronic display. You can also reduce installation errors by using a clever Air Data Unit or Central Air Data Computer. The OAT feed merely assists in reducing the errors in the 'physics' of the aircraft installation. However, unless you point-plot the actual environmental lapse rate in the atmosphere in which you are flying - which is obviously totally impossible - you cannot, Hophorrigan, correct AUTOMATICALLY for non-ISA atmospheres. This doesn't matter when everyone is flying on 1013 (or 29.92 for those still using quaint old inches) because everyone will have the same error due to non-ISA atmospheric conditions. But, near the ground in cold temperatures it is absolutely essential that temperatur error correction is applied to ALL altimeters. If that means your indicated altitude will differ from a published STAR altitude in order to achieve safe separation between metal and stone, ADVISE ATC - they won't mind!!
Incidentally, the meat bombs who made such a tragically nasty pink stain jumping out over the South Pole probably learned rather too late about temperature error correction IMHO - they were much lower than their altimeters said when they pulled their ripcords!! Hophorrigan - listen to those in this thread who are trying to protect you and your passengers from an unexpected close encounter of the worst kind with terra firma!!
by reducing them to the same datum. The actual PFD altitude for the first check was 20 feet while the standby was 100 feet, for example. This is in fact outside tolerance for the altimeter system, according to the FRM
The comment about the altimeters reading the same regardless of whether or not one is corrected for temperature applies because the aerodrome QNH has been corrected for temperature up to the airfield elevation.
If you are sitting at the airport, and you have the airfield QNH set, then all altimeters have to read within tolerance regardless of whether or not one is corrected for temperature!
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I agree that this subject is going around in circles. I do not claim that a correction is unnecessary to an altimeter for low temperatures, and all the great stuff included here should be considered as valid. My point was solely that so far as I am aware, and I base it on the reference I gave before,as well as practical experience in real cold and real hot conditions, that the ADIRU as fitted to the Boeing airplanes I am familiar with, have temperature compensation. I do not know what that really means, since I cannot get more information from the manufacturers, and it could well be that it is a partial compensation. It seems reasonable to me that the engineers would have looked at things like Approach requirements and set it up to fully compensate at normal landing altitudes, and to have uncompensated readings at cruise. it is a computer after all.
Why this should cause such angst among you I do not have any idea, since I have repeatedly said you should follow your own opinions.
With regard to the altimeter correction, the FRM is the Fault Reporting Manual and it gives the correct tolerances for writing up the instruments. It is not 70, or 75 feet, as most will claim, since that is for pilot reference to establish that the baro setting is right. If the check fails, the reason is probably that the baro setting or the parking position height reference is wrong.
Yes, the flight I commented on was out of tolerance, but I thought it reasonable to do a couple of flights and observe it before writing it up, and since it was ok on the next leg, I did not pursue it further. It would have been released under the MEL anyway. So sue me.
And the point of the altimeter correction for temperature is to find the True Altitude. For that you need to know the ambient temperature for the little bit of air you are currently in. The temp on the ground, or even a few hundred feet below you, is irrelevant. Using the ground (airfield) temp to correct the altimeter is a second-best way to do it, accurate only if the lapse rate is standard. But if that is all you have, go ahead.
These are opinions, not facts.
Why this should cause such angst among you I do not have any idea, since I have repeatedly said you should follow your own opinions.
With regard to the altimeter correction, the FRM is the Fault Reporting Manual and it gives the correct tolerances for writing up the instruments. It is not 70, or 75 feet, as most will claim, since that is for pilot reference to establish that the baro setting is right. If the check fails, the reason is probably that the baro setting or the parking position height reference is wrong.
Yes, the flight I commented on was out of tolerance, but I thought it reasonable to do a couple of flights and observe it before writing it up, and since it was ok on the next leg, I did not pursue it further. It would have been released under the MEL anyway. So sue me.
And the point of the altimeter correction for temperature is to find the True Altitude. For that you need to know the ambient temperature for the little bit of air you are currently in. The temp on the ground, or even a few hundred feet below you, is irrelevant. Using the ground (airfield) temp to correct the altimeter is a second-best way to do it, accurate only if the lapse rate is standard. But if that is all you have, go ahead.
These are opinions, not facts.
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Since we're talking about 'opinions' - in my opinion you don't appear to have sufficient understanding of altimetry to make valid statements on this thread, Hopharrigan. Do you know the difference between temperature compensation and temperature error correction?? Perhaps you should go and have another look at your theory book before pestering Boeing with your queries?? You do have a theory book, don't you?? Or did you just do the FAA tick test for your licence??
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hopharrigan-
>>Why this should cause such angst among you I do not have any idea, since I have repeatedly said you should follow your own opinions.<<
In a nutshell, I guess it's because over 8000 pilots may read this thread, and if just ONE of them doesn't use the correction properly, it may lead to a tragedy someday.
You will never be in trouble by applying the correction, but if you're wrong.........?
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>>Why this should cause such angst among you I do not have any idea, since I have repeatedly said you should follow your own opinions.<<
In a nutshell, I guess it's because over 8000 pilots may read this thread, and if just ONE of them doesn't use the correction properly, it may lead to a tragedy someday.
You will never be in trouble by applying the correction, but if you're wrong.........?
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