Altimeter Correction Cold WX OPS
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Altimeter Correction Cold WX OPS
Winter is here in ernest, but I was wondering why, it seems, that so few pilots are aware of the problems of altimeter error when the temperature is anything other than 15'C. The CAA exams do not really emphasize the importance and magnitude of the error, especially in cold air.
At -20'C with a platform altitude of say 2500' you will actually be only 2190'. Or at 5000' only 4380'. Really significant differances when you are operating into moderately high airfields on Non - Precision Approaches. Have you ever wondered why, when passing the LOM it's sometimes sixty or seventy feet out? I have yet to see any real guidance issued by the authorities as to how they would like us to use their tables, especially as ATC also do not appear to take this error into account when vectoring us around.
I suggest adding the corrections to MSA's, LOM's and Minima for a start, and then maybe onto radar minimum altitudes given by ATC.
Can anyone shed anymore light on this issue from either an ATC, pilot or CAA(JAA) point of view? My company don't even mention it in their cold Wx Ops Manual.
At -20'C with a platform altitude of say 2500' you will actually be only 2190'. Or at 5000' only 4380'. Really significant differances when you are operating into moderately high airfields on Non - Precision Approaches. Have you ever wondered why, when passing the LOM it's sometimes sixty or seventy feet out? I have yet to see any real guidance issued by the authorities as to how they would like us to use their tables, especially as ATC also do not appear to take this error into account when vectoring us around.
I suggest adding the corrections to MSA's, LOM's and Minima for a start, and then maybe onto radar minimum altitudes given by ATC.
Can anyone shed anymore light on this issue from either an ATC, pilot or CAA(JAA) point of view? My company don't even mention it in their cold Wx Ops Manual.
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Check with your techies. You will find that modern airplanes equipped with a CADC have temperature compensation and will read correctly no matter what the OAT. See also the requirements for RVSM airspace. If you have a pressure altimeter as a reference then the corrections will be needed.
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Hopharrigan,
Are you 100% sure that new gen. a/c compensate for temp. error automatically? You may well be right but I would imagine this would be extremely difficult to do as the input to the DADC,CADC or ADIRU's are plain old Pitot and Static ports. There is no input available to allow them to know airfield temperature which they would have to know to be able to calculate the error.
Any response to my simplistic overview of this problem is welcome.
Are you 100% sure that new gen. a/c compensate for temp. error automatically? You may well be right but I would imagine this would be extremely difficult to do as the input to the DADC,CADC or ADIRU's are plain old Pitot and Static ports. There is no input available to allow them to know airfield temperature which they would have to know to be able to calculate the error.
Any response to my simplistic overview of this problem is welcome.
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Done some homework, haven't got a clue how to put a pdf. file onto this network but take my word for it, all boeing a/c from the 707 to the 747-400 must make corrections to all altimeters (inc. EFIS altimeter tape) as there is absolutely no correction applied whatsoever by CADC's, DADC's or ADIRU's. Whichever 'techies' you've spoken to are wrong. For more info contact, Chief Pilot, Training, and Standards, Flight Crew Ops, Seattle. Tel No:206 655 0878 and ask for Flight Ops Technical Bulletin ATA Number: 01
Here's a quote from Boeing's Bulletin: "Pilots should note that for very cold temperatures, when flying published minimum altitudes significantly above the airport, altimeter errors can exceed 1000 feet, resulting in potentially unsafe terrain clearance if no corrections are made."
Maybe this rumour network just saved someones life.
Here's a quote from Boeing's Bulletin: "Pilots should note that for very cold temperatures, when flying published minimum altitudes significantly above the airport, altimeter errors can exceed 1000 feet, resulting in potentially unsafe terrain clearance if no corrections are made."
Maybe this rumour network just saved someones life.
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I fly for an airline based in western canada and temp corrections are standard approach briefing requirements (actually on checklist year round to jog your memory for when the temp does go to the minus side). No matter where you fly temp can have a drastic effect on your true alt and as deadly as a mis-set altimeter descending from transition.Our company has had jeppeson produce quik ref charts for diff temps and step down alts for all our mountainous terrain app's which we all use and appreciate.
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The altimeter correction is made depending on the altitude above the reporting source. How can a CADC know what the *source* is, to say nothing of how *high* it is?
I think we'd best listen to our Canadian friends, they are faced with this every winter.
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[This message has been edited by quid (edited 23 November 1999).]
I think we'd best listen to our Canadian friends, they are faced with this every winter.
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[This message has been edited by quid (edited 23 November 1999).]
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I flew the 737-200 for some years in the tropics and I saw that the altimeter, despite being fed by an ADC, had a consistent error, that I decided was due to the above ISA temperature. It is not dependent on the temperature on the ground, or at the airport, but the ambient temperature that the airplane is experiencing at the time.
Obviously there is no need to make any corrections for high ambient temperatures.
After some time I started to fly the B757 over the same routes and I saw that there was no longer a consistent altimeter error. Since I had already decided that the 737 error was due to temperature, this threw my theory off, so I checked the stats for the ADC and found that it did, in fact, correct for temperature. In other words the altimeter reading is for True Altitude. (but this was a long time agon in Pilot years and I do not have reference to those manuals now).
I have assumed the same for all subsequent airplanes I have flown and my observations tend to support this view, whether in expremely cold or hot conditions. Again, it is not dependent on the ground temperature, but the ambient. Using Ground temperature to calculate corrections for the approach is a crutch, as you would see if you examined it closely.
Never mind all that, is the correction important? And if it is, how is it applied? You do not want to go around the pattern adding 500 or 1000 feet to all the ATC assigned levels, or else YOU would be a danger to others who are not doing the same.
It is important that the correct altitude be used for the approach minima, and so a correction should be applied to that, but an examination would show that it is usually less than 40 feet even under the most extreme conditions for an ILS, and maybe double that for a non-precision approach.
How would you determine the error in practice? One way I have found to be effective is to check the altimeter at the OM or DME fix on a Glide Slope, and as I said, the later generation airplanes seem to be "spot on" at this point, while the older airplanes had an error equal to the temperature correction.
So, to follow my own advice, I went to the Techies. If you care to look at the Boeing Maintenance Manual B747-400 (one source), on Page 24 of Section 34-12-00 there is a statement (10.a) that says "Pressure altitude is derived from the static pressure input which is compensated for ambient temperature and corrected for static pressure source errors."
I do not rest my case, since there may well be other documents affecting this. in particular the way the data is transferred and customer options, and it may not be applicable to Airbus airplanes. The manuals for the ADC manufacturer details the way the pressure and temperature values are related, but it is in techie language and makes little sense to me.
Making the temperature correction, whether required or not, will not get you into trouble, so please do what YOU believe is right.
Obviously there is no need to make any corrections for high ambient temperatures.
After some time I started to fly the B757 over the same routes and I saw that there was no longer a consistent altimeter error. Since I had already decided that the 737 error was due to temperature, this threw my theory off, so I checked the stats for the ADC and found that it did, in fact, correct for temperature. In other words the altimeter reading is for True Altitude. (but this was a long time agon in Pilot years and I do not have reference to those manuals now).
I have assumed the same for all subsequent airplanes I have flown and my observations tend to support this view, whether in expremely cold or hot conditions. Again, it is not dependent on the ground temperature, but the ambient. Using Ground temperature to calculate corrections for the approach is a crutch, as you would see if you examined it closely.
Never mind all that, is the correction important? And if it is, how is it applied? You do not want to go around the pattern adding 500 or 1000 feet to all the ATC assigned levels, or else YOU would be a danger to others who are not doing the same.
It is important that the correct altitude be used for the approach minima, and so a correction should be applied to that, but an examination would show that it is usually less than 40 feet even under the most extreme conditions for an ILS, and maybe double that for a non-precision approach.
How would you determine the error in practice? One way I have found to be effective is to check the altimeter at the OM or DME fix on a Glide Slope, and as I said, the later generation airplanes seem to be "spot on" at this point, while the older airplanes had an error equal to the temperature correction.
So, to follow my own advice, I went to the Techies. If you care to look at the Boeing Maintenance Manual B747-400 (one source), on Page 24 of Section 34-12-00 there is a statement (10.a) that says "Pressure altitude is derived from the static pressure input which is compensated for ambient temperature and corrected for static pressure source errors."
I do not rest my case, since there may well be other documents affecting this. in particular the way the data is transferred and customer options, and it may not be applicable to Airbus airplanes. The manuals for the ADC manufacturer details the way the pressure and temperature values are related, but it is in techie language and makes little sense to me.
Making the temperature correction, whether required or not, will not get you into trouble, so please do what YOU believe is right.
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If I understand correctly, all a/c altimeters, be it old capsule type or CADC/ADC driven are calibrated to ISA conditions.
If you are flying in temperatures above ISA, air being less dense, your actual true altitude is higher for the same altimeter setting. The mates around you will also be enjoying the same situation in terms of terrain clearance, so we are not worried when flying in ISA+ conditions provided everyone is on the same alt setting.
But if the temperature is below ISA the opposite occurs. The Jepps reference manual has a page for temp correction below ISA.
We know every millibar equates to approx. 30' at lower levels, the question is for every degree C below ISA, what is the altitude correction wrt true alt. From the Jepps it appears to be quite small until when it get pretty cold then the error become significant Can't remember off hand though.
Don't really operate in those conditions but a kind Canadian pilot with us liked to share this knowledge quite often.
Way to go.
If you are flying in temperatures above ISA, air being less dense, your actual true altitude is higher for the same altimeter setting. The mates around you will also be enjoying the same situation in terms of terrain clearance, so we are not worried when flying in ISA+ conditions provided everyone is on the same alt setting.
But if the temperature is below ISA the opposite occurs. The Jepps reference manual has a page for temp correction below ISA.
We know every millibar equates to approx. 30' at lower levels, the question is for every degree C below ISA, what is the altitude correction wrt true alt. From the Jepps it appears to be quite small until when it get pretty cold then the error become significant Can't remember off hand though.
Don't really operate in those conditions but a kind Canadian pilot with us liked to share this knowledge quite often.
Way to go.
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Hoppharridan
Word of honor: ADC, DADC, or regular baro altimetry is not temperature compensated for published approach altitudes. This topic got thoroughly talked over last year on this website. I was initially of your view. A number of emails later, with a training captain at a Canadian jet operator, proved otherwise. Afterward, I found the ICAO chart quoted on page CA-19 in the Canadian section of the Jeppesen materials that precede CAnadian terminal charts. I recall that the most dangerous scenario was a non precision approach at a sea level airfield near a coastal range of mountains, or a non precision approach to an airfield with an exceptionally low MDA due to the complete lack of nearby obstacles. In both cases, since you lacka glideslope, you descend to a baro altitude, and the terrain clearance from which the MDA is based, no longer applies. The numbers were I remember 1,200 feet low at minus 40 celsius if you're about 4,000 feet above the altimeter reporting station, and about 120 feet low if you're at,, say, a 400 foot MDA at that flat airfield.
Ask the Canadians. They know.
Word of honor: ADC, DADC, or regular baro altimetry is not temperature compensated for published approach altitudes. This topic got thoroughly talked over last year on this website. I was initially of your view. A number of emails later, with a training captain at a Canadian jet operator, proved otherwise. Afterward, I found the ICAO chart quoted on page CA-19 in the Canadian section of the Jeppesen materials that precede CAnadian terminal charts. I recall that the most dangerous scenario was a non precision approach at a sea level airfield near a coastal range of mountains, or a non precision approach to an airfield with an exceptionally low MDA due to the complete lack of nearby obstacles. In both cases, since you lacka glideslope, you descend to a baro altitude, and the terrain clearance from which the MDA is based, no longer applies. The numbers were I remember 1,200 feet low at minus 40 celsius if you're about 4,000 feet above the altimeter reporting station, and about 120 feet low if you're at,, say, a 400 foot MDA at that flat airfield.
Ask the Canadians. They know.
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Bizjet your figures are about right on,with one app we do in the rockies at -40 at 5000'above the altimeter source if we fail to do the correction we may still clear the hill on the approach by 130'!!!! Hope those evergreens didn't grow too much in the last few months!
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Just a quick note as we are having alot of altimeter errors on one of our 737-400 A/C.
Capt and F/O are fed from the DADC and the stdby is a mechanical unit with an input directly from altenate however the input to the capt's Altimeter is corrected from the TAT probe
Capt and F/O are fed from the DADC and the stdby is a mechanical unit with an input directly from altenate however the input to the capt's Altimeter is corrected from the TAT probe
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Hopharrigan,
I think the correction in the Jumbo-400 manual to the altimeter is probably in ref. to position error and instrument error, but it is definitely not temp. error correction. Call the boss at Boeing, I gave you his number and do us all a favour, find out the facts! I don't want to sit as a pax on your a/c over the Alps when you lose one and end up descending to MSA with a Zurich temp. of -15'C, cos your going to bang into a hill. My flight Ops manager was a Canadian as well, these guys know their stuff and it's their bread and butter.
I spoke to a CAA guy last week who is an avionics project specialist, his opinion as well was like yours, in that you don't need a correction.
Does anyone remember any CAA question in the ATPL's that refers to Altimeter temp. error, whether in Perf. A or Instruments. I don't and they are talking about 35' clearances on obstacles, no way!
Less than 10% of the pilots I work with know about this problem area, and certainly none of the CAP 509 people, so what is going on?
CAA reply please.
[This message has been edited by Mach (edited 26 November 1999).]
I think the correction in the Jumbo-400 manual to the altimeter is probably in ref. to position error and instrument error, but it is definitely not temp. error correction. Call the boss at Boeing, I gave you his number and do us all a favour, find out the facts! I don't want to sit as a pax on your a/c over the Alps when you lose one and end up descending to MSA with a Zurich temp. of -15'C, cos your going to bang into a hill. My flight Ops manager was a Canadian as well, these guys know their stuff and it's their bread and butter.
I spoke to a CAA guy last week who is an avionics project specialist, his opinion as well was like yours, in that you don't need a correction.
Does anyone remember any CAA question in the ATPL's that refers to Altimeter temp. error, whether in Perf. A or Instruments. I don't and they are talking about 35' clearances on obstacles, no way!
Less than 10% of the pilots I work with know about this problem area, and certainly none of the CAP 509 people, so what is going on?
CAA reply please.
[This message has been edited by Mach (edited 26 November 1999).]
Guest
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There are some remarkably closed minds out there.
If the tech manuals of Boeing and Honeywell are right, then the correction is made automatically, in other words modern airplanes use True Altitude on the primary display. If they are right, then the correction is not needed; but if you want to save the lives of the passengers bound for Zurich, go ahead and apply it. I pronmise I will not be offended. I am certainly not going to change the way I think because of some strident opinion I read here, and neither should you.
I am sure the Canadians flying the older airplanes in the frozen North know all about the error and apply it intelligently.
If you are personally not convinced that the airplane and flight instrument manufacturers know what they are doing, or that there may be some difference in the way the information is actually presented in your airplane, by all means apply the correction and more power to you!
But if you are going to fly 500 or 1000 feet higher than ATC assigns you, please tell them, so they might put me out of your way.
If the tech manuals of Boeing and Honeywell are right, then the correction is made automatically, in other words modern airplanes use True Altitude on the primary display. If they are right, then the correction is not needed; but if you want to save the lives of the passengers bound for Zurich, go ahead and apply it. I pronmise I will not be offended. I am certainly not going to change the way I think because of some strident opinion I read here, and neither should you.
I am sure the Canadians flying the older airplanes in the frozen North know all about the error and apply it intelligently.
If you are personally not convinced that the airplane and flight instrument manufacturers know what they are doing, or that there may be some difference in the way the information is actually presented in your airplane, by all means apply the correction and more power to you!
But if you are going to fly 500 or 1000 feet higher than ATC assigns you, please tell them, so they might put me out of your way.
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hopharrigan and others.
We have been around this at length previously and it developed into a point scoring exercise then as now.
The best authoritative explanation which addresses all concerns as well as ATC assigned altitudes and adjustments can be found in Jeppesen - Canada - Air Traffic Control page CA 9-9. At the head of the page calibration of altimeters is addressed.
Further I quote:
1. IFR altitudes may be either accepted or refused.(applies anytime,anywhere). Refusal in this case is based on the pilot's assessment of temperature effect on obstruction clearance.
2. IFR assigned altitudes accepted by a pilot shall not be adjusted to compensate for cold temperatures, ie. if a pilot accepts "maintain 3000" an altitude correction shall not be applied to 3000.
3. Radar vectoring altitudes assigned by ATC are temperature compensated and require no corrective action by pilots.
4. When corrections are applied to a published final approach fix crossing altitude, procedure turn or missed approach altitude, pilots should advise ATC how much of a correction is to be applied.
The page incorporates the table, ATC and Pilot responsibilities and a practical example of using the table.
[This message has been edited by pterodactyl (edited 24 November 1999).]
We have been around this at length previously and it developed into a point scoring exercise then as now.
The best authoritative explanation which addresses all concerns as well as ATC assigned altitudes and adjustments can be found in Jeppesen - Canada - Air Traffic Control page CA 9-9. At the head of the page calibration of altimeters is addressed.
Further I quote:
1. IFR altitudes may be either accepted or refused.(applies anytime,anywhere). Refusal in this case is based on the pilot's assessment of temperature effect on obstruction clearance.
2. IFR assigned altitudes accepted by a pilot shall not be adjusted to compensate for cold temperatures, ie. if a pilot accepts "maintain 3000" an altitude correction shall not be applied to 3000.
3. Radar vectoring altitudes assigned by ATC are temperature compensated and require no corrective action by pilots.
4. When corrections are applied to a published final approach fix crossing altitude, procedure turn or missed approach altitude, pilots should advise ATC how much of a correction is to be applied.
The page incorporates the table, ATC and Pilot responsibilities and a practical example of using the table.
[This message has been edited by pterodactyl (edited 24 November 1999).]
From my posts in the previous thread on this topic:
Variations from ISA temperature start becomming really applicable at or below ISA-15°. If it is not represented graphically in your Ops manual, you can use the 'rule of thumb':
+4 feet for each -1°C temperature variation from ISA, per 1000' altitude above the airport (QNH datum)
The airport QNH is adjusted so that it is accurate at the field, it is only the airspace above the field elevation that needs to be corrected for density variation.
e.g. For a 200' Cat I minima at, say ISA-20°, this produces a correction of (4 x 20 x 0.2) 16ft. Round it up to 20ft. and, remembering the rhyme: "ISA low? Watch out below!", set the published Decision Altitude PLUS 20ft. on the minima bug.
At an OM crossing height of say 1300ft AGL the variation is much larger (4 x 20 x 1.3 =)104ft. so your crossing height, on glide slope, would be more like 1404ft. indicated above the field elevation.
This assumes that the ISA-20° is constant throughout the atmosphere, and not affected by an inversion, which would mess up the calculations.
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).
Variations from ISA temperature start becomming really applicable at or below ISA-15°. If it is not represented graphically in your Ops manual, you can use the 'rule of thumb':
+4 feet for each -1°C temperature variation from ISA, per 1000' altitude above the airport (QNH datum)
The airport QNH is adjusted so that it is accurate at the field, it is only the airspace above the field elevation that needs to be corrected for density variation.
e.g. For a 200' Cat I minima at, say ISA-20°, this produces a correction of (4 x 20 x 0.2) 16ft. Round it up to 20ft. and, remembering the rhyme: "ISA low? Watch out below!", set the published Decision Altitude PLUS 20ft. on the minima bug.
At an OM crossing height of say 1300ft AGL the variation is much larger (4 x 20 x 1.3 =)104ft. so your crossing height, on glide slope, would be more like 1404ft. indicated above the field elevation.
This assumes that the ISA-20° is constant throughout the atmosphere, and not affected by an inversion, which would mess up the calculations.
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).
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For those interested please check the link below. I have posted a scan of the relevant page from the A.I.P. Canada. Remember when calculating the correction required it is the "height above the altimeter source" that is important. Another thing to bear in mind is that the 100NM safe and Minimum Sector altitudes only give you 1000' clearance from the highest obstacle within 5 miles (at least here in Canada). When flying VFR it is even worse the Minimum quadrangle altitudes depicted on the 1:500,000 scale VFR charts only provides 100 m (328') clearance above obstacles in that grid square. In the USA it is only 200'.
After spending every winter for the last 10 years making these corrections all I can say is "Hi to Low, Lookout Below", as far as I am concerned there is not an altimeter in existance that can automatically make the required corrections. If this were the case then all "modern" aircraft flying the high flight levels in winter where it is usually below ISA would all be flying at non assigned ATC altitudes. What if Honeywell uses a different correction logarithim to Sperry, there are just too many variables. This is the reason we use a standard altimeter setting in the flight levels, so that all aircraft in a particular geographic/meteorological area are using the same setting and therefore are prone to the same error.
http://www.geocities.com/canuck_av8r/
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Keep the shiny side up and the dirty side down.
Canuck Av8r
ICQ 26305263
After spending every winter for the last 10 years making these corrections all I can say is "Hi to Low, Lookout Below", as far as I am concerned there is not an altimeter in existance that can automatically make the required corrections. If this were the case then all "modern" aircraft flying the high flight levels in winter where it is usually below ISA would all be flying at non assigned ATC altitudes. What if Honeywell uses a different correction logarithim to Sperry, there are just too many variables. This is the reason we use a standard altimeter setting in the flight levels, so that all aircraft in a particular geographic/meteorological area are using the same setting and therefore are prone to the same error.
http://www.geocities.com/canuck_av8r/
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Keep the shiny side up and the dirty side down.
Canuck Av8r
ICQ 26305263
