Altimeter Correction Cold WX OPS

[Mach]

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.

[hopharrigan]

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.

[mustafagander]

This topic had a big run here about this time last year as I recall it.

[Mach]

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.

[Mach]

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.

[MFALK]

There is also a correction chart for this in the A320 in-flight performance manual, so the correction is not in-built within the ADCs.

[rusty]

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.

[quid]

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).]

[hopharrigan]

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.

[mustafagander]

RTFQ
This topic got lots of space some time ago. RTFQ. http://www.pprune.org/ubb/NonCGI/frown.gif

[7times7]

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.

[bizjet pilot]

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.

[rusty]

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!

[rusty]

Oh yeah, sorry to take up more bandwidth here but in canada anyway ATC corrects your radar vector altitudes for you when it gets chilly so everybody in his space is on the "same page" so to speak.

[trunnion bolt]

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

[Mach]

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).]

[hopharrigan]

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.

[pterodactyl]

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).]

[Checkboard]

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).

[Canuck_AV8R]

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

[Mach]

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.

[pterodactyl]

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.

[hopharrigan]

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?

[rusty]

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.

[Jon]

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.

[Squawkbox]

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).]

[Don Coyote]

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).]

[Lime]

Finally some altimeter rules to whom it may concern:

"From warm to cold you never get old"

10°C ISA DEVIATION = 4% ALTITUDE CORRECTION

Thats all.

[hopharrigan]

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.

[pterodactyl]

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).]

[hopharrigan]

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.

[bizjet pilot]

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.

[Checkboard]

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).]

[hopharrigan]

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?

[bizjet pilot]

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.

[BEagle]

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!!

[Checkboard]

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

While I don't fly the 744, the Australian AIP requires altimeters in any IFR aircraft to agree within 70 feet. I don't know the specific tolerance for the 744, so I said if they were out of tolerance, they should have been written up. It was you who suggested that they were.

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!

[hopharrigan]

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.

[BEagle]

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??

[quid]

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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[bizjet pilot]

First of all, apologies to Hopharrigan for my being irritated. My problem not his. He's just being extra careful.

Yesterday I got the Boeing Flight Ops Tech Bulletin ATA Number 01, dated 1 November 1999. It applies to every single Boeing civil transport in existence. It's a pdf file (Adobe). Takes less than 60 secs to download.

Also found 3 page article entitled "Lies Your Barometric Altimeter Tells You" on page 98 of October 1999 issue of Business & COmmercial Aviation. (Same McGraw-Hill publishers who do Aviation Week).

Also urge Prunners, Hopharrigan included, to download Adobe file of Canadian advisory at canuck_av8r's website at Geocities.

I think that the abovementioned 3 written references take the wobble out of the stool, so to speak.

Best of luck, Hopharrigan, in your ongoing self-education. Like the rest of us.

[hopharrigan]

Thanks bizjet, I appreciate that.

I admit I do not know any longer what the difference is between Temperature Compensation and Temperature Correction. When this first came up for me about 10 years ago, I asked the Performance Engineers in the company I worked for at the time and they confirmed that the CADC did not need to be corrected for approach errors due to temperature. That was good enough for me. The matter came up again during an IP re-training course in another airline and the ground instructor said I was wrong when I repeated what I was told, but the next day he apologised and said he had researched it and I was right. Then again with this thread, I asked the performance engineers in yet another airline (I get around!) and they produced the documents supporting my view. They even offered to amend the Ops Manual which currently says the correction is required (I asked them not to do so, without further study).
I am impressed with a lot of what you guys had to say, and will look into it more. If I find anything definite, either way, I will let you know.

By the way I hold several licences, all ATP or equivalent, including the Aus, NZ, Singapore, British and US. I am proud of them all.

[Mach]

I believe we may be getting somewhere lads and lasses. Hop, will you promise me to look up that Boeing flight Bulletin number 1, OCT 99, on the Canadian web page that has just been mentioned, that ADOBE thing. Then please return here and tell the world Boeing is wrong. If my computer know-how was better I could have ended this thread two weeks ago by downloading that very bulletin to PPRUNE. Unfortunately, like you, my computer knowledge is basic.
Your 744 corrects temp error and my 707 doesn't. If you admit there is an error in barometric altimeters what is keeping these two machines apart, LUCK?
PLEASE read the ADOBE file and tell us we are wrong. No more long winded explanations of your theories please. Read the facts and return. I am happy to wait.

[Canuck_AV8R]

Here is the link to the Boeing .pdf
http://www.boeing.com/commercial/aeromagazine/erroneous.pdf

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Keep the shiny side up and the dirty side down.

Canuck Av8r
ICQ 26305263




[This message has been edited by Canuck_AV8R (edited 03 December 1999).]

[hopharrigan]

I couldn't find an icon for groveling, so I won't use one.
I have finally gotten answers for my queries about the temperature compensation on glass cockpit airplanes, and it was not what I expected. The compensation is in fact designed to ensure that the indication is correct for the actual OAT, thus the indication is, as so many of you insisted, NOT corrected for variations from ISA.
I WAS WRONG!!
Oh well, I learned something, even if you smarter operators did not need to. Maybe next time......

[pterodactyl]

As the great philosopher once observed. "There are none so blind as those who will not listen"