QNH correction for instrument approach
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QNH correction for instrument approach
Ladies and gents,
I apologise in advance if this happens to be the wrong place to ask.
Yesterday my colleague and I were confronted with a definition issue, or so we thought. During his class yesterday, using his manuals as usual, he came across the definition of QNH. I can not remember the exact wording, but it was something along the line of: QNH is also corrected for temperature. The students then asked whether this was correct or not and if so, how is it calculated. Our main concern is the instrument approach environment without the radar altimeter and the usage of the QNH given by ATC.
I have done a search within the forum, but I have not been able to find an appropriate answer.
Thanks!
TG
I apologise in advance if this happens to be the wrong place to ask.
Yesterday my colleague and I were confronted with a definition issue, or so we thought. During his class yesterday, using his manuals as usual, he came across the definition of QNH. I can not remember the exact wording, but it was something along the line of: QNH is also corrected for temperature. The students then asked whether this was correct or not and if so, how is it calculated. Our main concern is the instrument approach environment without the radar altimeter and the usage of the QNH given by ATC.
I have done a search within the forum, but I have not been able to find an appropriate answer.
Thanks!
TG
At airfields with iap's, you should find that the met office will have provided a correction chart for the met observers to apply; there is not just a temperature correction; it will vary according to the 'range' of the observed pressure eg if QNH is between say 1003 and 1010 there will be one correction say 0.5 mb, whereas if it's in the range 1015 - 1020 it could be a different correction eg 0.8 mb
NB These are just examples; I don't have the correction chart in front of me.
NB These are just examples; I don't have the correction chart in front of me.
It's semantics.
QNH is measured, in principle with an altimeter, at the aerodrome level, by looking at the subscale setting that causes the altimeter to show the aerodrome elevation. It doesn't need to be corrected for temperature, save for the correction for error in the instrument itself, which is likely to be very small.
As soon as you move above or below the level of the aerodrome, temperature errors creep into the indicated altitude, as the altimeter is calibrated to the ISA. If you move from an aerodrome at a considerable elevation down to sea level, you may find that the indicated altitude is not zero. Similarly, if you measure QNH at a sea level reference (Sea Level Pressure) and use it at your high aerodrome, you may find it to be incorrect. In that sense aerodrome QNH is sea level pressure "corrected" for temperature.
And any time you use a QNH at a level different from the level it was measured at, you may need to apply a temperature correction. In particular, in IAPs in cold conditions, the altimeter will overread so a correction for temperature is required at any altitude that is critical.
chevvron, are you sure that's not the conversion from QNH to QFF (Sea Level Pressure)?
QNH is measured, in principle with an altimeter, at the aerodrome level, by looking at the subscale setting that causes the altimeter to show the aerodrome elevation. It doesn't need to be corrected for temperature, save for the correction for error in the instrument itself, which is likely to be very small.
As soon as you move above or below the level of the aerodrome, temperature errors creep into the indicated altitude, as the altimeter is calibrated to the ISA. If you move from an aerodrome at a considerable elevation down to sea level, you may find that the indicated altitude is not zero. Similarly, if you measure QNH at a sea level reference (Sea Level Pressure) and use it at your high aerodrome, you may find it to be incorrect. In that sense aerodrome QNH is sea level pressure "corrected" for temperature.
And any time you use a QNH at a level different from the level it was measured at, you may need to apply a temperature correction. In particular, in IAPs in cold conditions, the altimeter will overread so a correction for temperature is required at any altitude that is critical.
chevvron, are you sure that's not the conversion from QNH to QFF (Sea Level Pressure)?
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chevron, thanks for your reply.
bookworm,
I do understand the temperature problems that arise while using the altimeter and it's indications.
The problem which was presented to me, was quite specific about the conditions, i.e. flying an instrument approach down to the MDA.
"And any time you use a QNH at a level different from the level it was measured at, you may need to apply a temperature correction. In particular, in IAPs in cold conditions, the altimeter will overread so a correction for temperature is required at any altitude that is critical."
Reading your post, it sounds as if while the approach is being flown and let's say that the OAT is ISA -20ºC, the pilot has to calculate the actual MDA. Is this correct?
Regards,
TG
bookworm,
I do understand the temperature problems that arise while using the altimeter and it's indications.
The problem which was presented to me, was quite specific about the conditions, i.e. flying an instrument approach down to the MDA.
"And any time you use a QNH at a level different from the level it was measured at, you may need to apply a temperature correction. In particular, in IAPs in cold conditions, the altimeter will overread so a correction for temperature is required at any altitude that is critical."
Reading your post, it sounds as if while the approach is being flown and let's say that the OAT is ISA -20ºC, the pilot has to calculate the actual MDA. Is this correct?
Regards,
TG
Reading your post, it sounds as if while the approach is being flown and let's say that the OAT is ISA -20ºC, the pilot has to calculate the actual MDA. Is this correct?
It sounds as though you're familiar with that. And it's not a correction of the QNH as such, rather a correction for the limitations of the pressure altimeter.
The only sense in which QNH could be described as being "corrected for temperature" is if you consider it to be the sea level pressure "corrected for temperature". But that's putting the cart before the horse since QNH is measured and SLP for plotting on charts is subsequently deduced from it.
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Thank you bookworm for your detailed explanation.
To sum it up: as a pilot we still need to make appropriate corrections to get the true altitudes while flying an IAP.
Thank you once again!
Regards,
TG
To sum it up: as a pilot we still need to make appropriate corrections to get the true altitudes while flying an IAP.
Thank you once again!
Regards,
TG
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Found this a while back - maybe from Pprune ?:
Not my words. But thanks to the author anyway.
Pressure sensitive altimeters do not make allowance for ISA deviation and therefore Indicated Altitude = True Altitude ONLY on an ISA day. Since ISA days are as rare as rocking horse poop an altimeter will only read the published figure on the ILS chart (at the OM or DME check HT) that often as well....virtually never.
A pressure sensitive altimeter reads the height of a theoretical column of air at the top of which is the altimeter and the bottom of which is sea level....assuming QNH set not 1013.2mb...by measuring and comparing the local air pressure to the MSL datum pressure.
On a warmer than ISA day the column is taller (air less dense) and on a colder than ISA day it is shorter (air molecules more densely packed)
The magnitude of the difference between Indicated and True altitude depends on how far above the datum the altimeter is.
On warmer than ISA days the aircraft will be flying higher than indicated on the altimeter and as this is safer it has always been ignored in Australia because that is the predominant condition. This has led to a degree of ignorance on the whole subject in Australia. It follows that on a colder than ISA day the aircraft will be lower than indicated on the altimeter and this can be very dangerous but because the difference is very small at typical Cat 1 minimas there is a level of complacence.
Now consider the case of the ILS. The glideslope DME Ht check occurs at a predetermined distance along a, for all intents and purposes, angled 'surface' set a 3 odd degrees....say 5 DME/1500'. Lets 'freeze' the aircraft at that point. On a warmer than ISA day what will the altimeter read, higher or lower....and on a colder than ISA day?
By how much?
Can you reasonably do any meaningfull altimeter check without knowing?
Will the error be the same at the minima?
Lets look at the colder day as that is the dangerous one.
I have said above that on a colder than ISA day the TRUE altitude will be less than INDICATED altitude. Viewed from the pilot's perspective 'frozen' on the ILS above the altimeter is OVERREADING....it might be saying 1580' for instance on a really cold day. Once upon a time not so long ago the AIP prescribed adding that difference to the minima...you will shortly understand why that is dumb....the AIP doesn't say that anymore because at some point CASA learned how dumb it was....but I digress!
Clearly once you pass the OM or DME/HT check the only reference you have for height above the runway threshold elevation is the altimeter...at the 5 DME point above you WERE at 1500' (lets assume you were bang on glideslope and a MSL runway)
irrespective of what the altimeter said. As you approach the DH/DA your only reference is the altimeter...lets freeze the aircraft at the Cat 1 minima of 200'.
What is your TRUE altitude?
Here is a little 'rule of thumb' formula which will tell you. The most important thing to remember is the temperature datum is the airfield that gave you the temp via ATIS or whatever and that temperature as ISA devn is what counts NOT the ISA devn at cruise alt.
+ or - 4' per degree of ISA devn(ht/1000).
So applying this formula to the above ILS...lets assume temperature 0 degree giving an ISA devn of 15.
- 4 x 15(1.5)
-60 x 1.5= -90'
So if on the day of the above ILS it was 0 C on the ground and the altimeter said 1590' at 5 DME then the altimeter is accurate.
At the minima.
-4 x 15(.2)
-60 x .2 = -12'
At the minima if you descended to 200' Indicated altitude your TRUE altitude would be 188'.
If you followed the AIP as it was writ 20 yrs ago you would have descended to an indicated alt of 290' which would have been a TRUE alt of -60 x .29 (call it .30) which = 272' and possibly not got visual...you might even do it twice and then divert to your alternate for no good reason at all.
You can see that if it was 30C on the ground the whole situation reverses and becomes safer...at the OM you would be indicating 1410' for a true alt of 1500' and at the minima indicating 200' gives a TRUE alt of 212'.
Now apply the same formula to a MSA of 7000' on a 0C day and how much terrain clearance do you really have? On a 30C day? And the MDA on an NDB or VOR or GPS approach?
Now you will be able to predict what the altimeter will say at the OM or DME/Ht Check before Topd and having allowed for known altimeter tolerance (checked against the airfield reference point at point of deparature) be confident that when you call the altimeter check on the ILS you actually understand what you just said and it wasn't just ignorant BS.
Hope this helps.
DogGone
Not my words. But thanks to the author anyway.
Pressure sensitive altimeters do not make allowance for ISA deviation and therefore Indicated Altitude = True Altitude ONLY on an ISA day. Since ISA days are as rare as rocking horse poop an altimeter will only read the published figure on the ILS chart (at the OM or DME check HT) that often as well....virtually never.
A pressure sensitive altimeter reads the height of a theoretical column of air at the top of which is the altimeter and the bottom of which is sea level....assuming QNH set not 1013.2mb...by measuring and comparing the local air pressure to the MSL datum pressure.
On a warmer than ISA day the column is taller (air less dense) and on a colder than ISA day it is shorter (air molecules more densely packed)
The magnitude of the difference between Indicated and True altitude depends on how far above the datum the altimeter is.
On warmer than ISA days the aircraft will be flying higher than indicated on the altimeter and as this is safer it has always been ignored in Australia because that is the predominant condition. This has led to a degree of ignorance on the whole subject in Australia. It follows that on a colder than ISA day the aircraft will be lower than indicated on the altimeter and this can be very dangerous but because the difference is very small at typical Cat 1 minimas there is a level of complacence.
Now consider the case of the ILS. The glideslope DME Ht check occurs at a predetermined distance along a, for all intents and purposes, angled 'surface' set a 3 odd degrees....say 5 DME/1500'. Lets 'freeze' the aircraft at that point. On a warmer than ISA day what will the altimeter read, higher or lower....and on a colder than ISA day?
By how much?
Can you reasonably do any meaningfull altimeter check without knowing?
Will the error be the same at the minima?
Lets look at the colder day as that is the dangerous one.
I have said above that on a colder than ISA day the TRUE altitude will be less than INDICATED altitude. Viewed from the pilot's perspective 'frozen' on the ILS above the altimeter is OVERREADING....it might be saying 1580' for instance on a really cold day. Once upon a time not so long ago the AIP prescribed adding that difference to the minima...you will shortly understand why that is dumb....the AIP doesn't say that anymore because at some point CASA learned how dumb it was....but I digress!
Clearly once you pass the OM or DME/HT check the only reference you have for height above the runway threshold elevation is the altimeter...at the 5 DME point above you WERE at 1500' (lets assume you were bang on glideslope and a MSL runway)
irrespective of what the altimeter said. As you approach the DH/DA your only reference is the altimeter...lets freeze the aircraft at the Cat 1 minima of 200'.
What is your TRUE altitude?
Here is a little 'rule of thumb' formula which will tell you. The most important thing to remember is the temperature datum is the airfield that gave you the temp via ATIS or whatever and that temperature as ISA devn is what counts NOT the ISA devn at cruise alt.
+ or - 4' per degree of ISA devn(ht/1000).
So applying this formula to the above ILS...lets assume temperature 0 degree giving an ISA devn of 15.
- 4 x 15(1.5)
-60 x 1.5= -90'
So if on the day of the above ILS it was 0 C on the ground and the altimeter said 1590' at 5 DME then the altimeter is accurate.
At the minima.
-4 x 15(.2)
-60 x .2 = -12'
At the minima if you descended to 200' Indicated altitude your TRUE altitude would be 188'.
If you followed the AIP as it was writ 20 yrs ago you would have descended to an indicated alt of 290' which would have been a TRUE alt of -60 x .29 (call it .30) which = 272' and possibly not got visual...you might even do it twice and then divert to your alternate for no good reason at all.
You can see that if it was 30C on the ground the whole situation reverses and becomes safer...at the OM you would be indicating 1410' for a true alt of 1500' and at the minima indicating 200' gives a TRUE alt of 212'.
Now apply the same formula to a MSA of 7000' on a 0C day and how much terrain clearance do you really have? On a 30C day? And the MDA on an NDB or VOR or GPS approach?
Now you will be able to predict what the altimeter will say at the OM or DME/Ht Check before Topd and having allowed for known altimeter tolerance (checked against the airfield reference point at point of deparature) be confident that when you call the altimeter check on the ILS you actually understand what you just said and it wasn't just ignorant BS.
Hope this helps.
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Altimeter
Dear all,
We know that there are a few altimeters on board. Which altimeter is giving level/altitude what we read in Mode C in the label and what pressure that altimeter is set on?
Thanks,
We know that there are a few altimeters on board. Which altimeter is giving level/altitude what we read in Mode C in the label and what pressure that altimeter is set on?
Thanks,
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BD may have said this already in a long, and technical post (Bookworm may have over-simplified it in his)
Airfield QNH is measured by taking from a calibrated precission instrument the observed pressure at aerodrome level. This is then adjusted by the Met Observer to sea level pressure by applying a correction. The correction allows for deviation from the ICAO Standard Atmosphere (ISA) by taking into account prevailing temperature and any extremes of pressure.
This corrected value is given to ATC who will pass it to the pilot who should be able to set it without having to make a further correction. (The idea of using an old altimeter to obtain QNH is antiquated, it might still be used by some but IMHO they really should know better?)
Airfield QNH is measured by taking from a calibrated precission instrument the observed pressure at aerodrome level. This is then adjusted by the Met Observer to sea level pressure by applying a correction. The correction allows for deviation from the ICAO Standard Atmosphere (ISA) by taking into account prevailing temperature and any extremes of pressure.
This corrected value is given to ATC who will pass it to the pilot who should be able to set it without having to make a further correction. (The idea of using an old altimeter to obtain QNH is antiquated, it might still be used by some but IMHO they really should know better?)
Airfield QNH is measured by taking from a calibrated precission instrument the observed pressure at aerodrome level. This is then adjusted by the Met Observer to sea level pressure by applying a correction. The correction allows for deviation from the ICAO Standard Atmosphere (ISA) by taking into account prevailing temperature and any extremes of pressure.
This corrected value is given to ATC who will pass it to the pilot who should be able to set it without having to make a further correction.
This corrected value is given to ATC who will pass it to the pilot who should be able to set it without having to make a further correction.
The value given to the pilot is not the sea level pressure, it's the QNH. The sea level pressure is used only by meteorologists in plotting charts at a uniform level.
On a day with any temperature deviation from ISA, a particular QNH will result in a correct reading of altitude at only one level. The level that is chosen is the aerodrome level. Thus at any level above the aerodrome, an altimeter reading using the aerodrome QNH would be subject to temperature error, and the pilot must correct critical altitudes accordingly.
(The idea of using an old altimeter to obtain QNH is antiquated, it might still be used by some but IMHO they really should know better?)
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Where could we possibly find such allegations?
I mean, don't get me wrong, I completely understand the issue and the involved limitations of the altimeter, but I would like to be able to show the students where to read the information accordingly.
Pierre,
I wish your post would be the case, since that would mean that it would be easier for us to fly down to the minimums without having to worry about the true altitude. But how do you know it is true? Is there any manual where I could research?
Bookworm,
your post makes sense. And that is exactly the problem we encountered in class.
The question at this point is, what do we tell the students?
I thank you all for your replies so far!
Regards,
TG.
I mean, don't get me wrong, I completely understand the issue and the involved limitations of the altimeter, but I would like to be able to show the students where to read the information accordingly.
Pierre,
I wish your post would be the case, since that would mean that it would be easier for us to fly down to the minimums without having to worry about the true altitude. But how do you know it is true? Is there any manual where I could research?
Bookworm,
your post makes sense. And that is exactly the problem we encountered in class.
The question at this point is, what do we tell the students?
I thank you all for your replies so far!
Regards,
TG.
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ATCO 2,
The altitude indicated on the transponder and sent to an ATCO's SSR screen on the ground is pressure altitude i.e. the aircraft's flight level and so 1013 mb (OK, OK, hPa!) is the datum. This is the case regardless of what the altimeters are manually set to by the pilot.
V1R
The altitude indicated on the transponder and sent to an ATCO's SSR screen on the ground is pressure altitude i.e. the aircraft's flight level and so 1013 mb (OK, OK, hPa!) is the datum. This is the case regardless of what the altimeters are manually set to by the pilot.
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at zzzz a met office calibrated barometer is used to read air pressure, this reading is then manually corrected for the height of the barometer above aerodrome level (it sits about 30 feet up in the control tower) AND a small correction made depending on the outside air temperature. This is done bey reading a chart. The corrected pressure is then used on ATIS etc.
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Tgf - do not get confused between the corrections you (ATC) have to make and those which pilots have to make.
Yes, we have to correct for non-standard OAT, but we correct via the QNH which the aerodrome has provided as said above. We do our bits and you do yours and we both let each other get on with it!
If you are interested in 'our' corrections, we get enumerable posts (from people who do not 'search' first
) popping up about the corrections and they usually all flower in autumn/winter. Search for 'altimeter' or 'temperature correction' or 'cold weather' or similar and ye will find.
Edited to correct misunderstanding!
Yes, we have to correct for non-standard OAT, but we correct via the QNH which the aerodrome has provided as said above. We do our bits and you do yours and we both let each other get on with it!
If you are interested in 'our' corrections, we get enumerable posts (from people who do not 'search' first) popping up about the corrections and they usually all flower in autumn/winter. Search for 'altimeter' or 'temperature correction' or 'cold weather' or similar and ye will find.Edited to correct misunderstanding!
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QNH by definition is “barometric pressure measured at sea level, corrected for the elevation of an airport”. Add to that the height of a tower or other spot where instrument is located. The idea is to read correct elevation of the airport at the certain point.
Today’s barometers are very accurate. I used VAISALA, which has a temperature dial where ATC enters the present value of outside temperature and barometer automatically makes correction – if any! Having used this barometer for years, I noticed very few correction of not more than 1mb at the time. Not that we used this feature very often – to be honest!
What is important with QNH? First, to be reasonably accurate, and second, that ALL users in the same airspace use the same QNH value. Small inaccuracies related to Altimeter and barometer are irrelevant to a point. For my 33 years of controlling I haven’t heard any pilot correcting QNH for temperature – which doesn’t mean it didn’t happen - except us on the ground. To support this, very often aircraft transponders show 2-3 hundred feet variation to a level maintained and very often ATC reacts only if 300 feet or more is showed. Where is this coming from?
As stated above, my experience says that temperature correction is for max 1 mb, in another word 28 feet. 28 feet goes almost 11 times in 300 feet! Temperature discrepancy, reading error, aircraft altimeter and ground barometer permissible errors combined would never produce error bigger than 2mb and that is very pessimistic value- it would probably remain within 1 mb. 2mb equals to 56 feet! Would this endanger aircraft knowing how much safety is included in procedures and MSA? ILS CAT 1 is different story, there’s no room for 56 feet!
Question for pilots: are you guys going to use barometric altimeter in full IMC, say WX at minima, flying ILS CAT 1 to the minima of 200 feet? I don’t think so, too dangerous! Not only because of possible combined error but barometric altimeter is also very slow – or better said not fast enough to accurately indicate 200 feet. Beside that you can read altimeter only in 100 feet increments. Electronic altimeter, whatever name they are called today, wouldn’t care for outside temperature at all, you will know vertical distance from the runway in a foot – and that is what you are going to use!
Today’s barometers are very accurate. I used VAISALA, which has a temperature dial where ATC enters the present value of outside temperature and barometer automatically makes correction – if any! Having used this barometer for years, I noticed very few correction of not more than 1mb at the time. Not that we used this feature very often – to be honest!
What is important with QNH? First, to be reasonably accurate, and second, that ALL users in the same airspace use the same QNH value. Small inaccuracies related to Altimeter and barometer are irrelevant to a point. For my 33 years of controlling I haven’t heard any pilot correcting QNH for temperature – which doesn’t mean it didn’t happen - except us on the ground. To support this, very often aircraft transponders show 2-3 hundred feet variation to a level maintained and very often ATC reacts only if 300 feet or more is showed. Where is this coming from?
As stated above, my experience says that temperature correction is for max 1 mb, in another word 28 feet. 28 feet goes almost 11 times in 300 feet! Temperature discrepancy, reading error, aircraft altimeter and ground barometer permissible errors combined would never produce error bigger than 2mb and that is very pessimistic value- it would probably remain within 1 mb. 2mb equals to 56 feet! Would this endanger aircraft knowing how much safety is included in procedures and MSA? ILS CAT 1 is different story, there’s no room for 56 feet!
Question for pilots: are you guys going to use barometric altimeter in full IMC, say WX at minima, flying ILS CAT 1 to the minima of 200 feet? I don’t think so, too dangerous! Not only because of possible combined error but barometric altimeter is also very slow – or better said not fast enough to accurately indicate 200 feet. Beside that you can read altimeter only in 100 feet increments. Electronic altimeter, whatever name they are called today, wouldn’t care for outside temperature at all, you will know vertical distance from the runway in a foot – and that is what you are going to use!
QNH by definition is “barometric pressure measured at sea level, corrected for the elevation of an airport”.
Question for pilots: are you guys going to use barometric altimeter in full IMC, say WX at minima, flying ILS CAT 1 to the minima of 200 feet?
For the impact of temperature error, precision approaches with a glideslope independent of the altimeter are, in many ways, less critical than non-precision approaches where the altimeter is the only measure of level. A low DH is probably less important to correct than for example the FAF height. As an example, on an ISA-20 degC day, an indicated height of 1500 ft will actually be about 220 ft lower than that, a true altitude of less than 1300 ft. That's a substantial proportion of your 492 ft obstacle clearance before the FAF and 246 ft obstacle clearance after it.
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BOAC,
I am sorry, but I have must given you the wrong impression. I am a pilot and flight instructor. The questions above arose during a class of a colleague of mine. I have posted the problem here since it does involve us pilots and ATC.
ATC creep,
thanks for your thorough post. It's good to read from a controller's point of view.
As bookworm stated, if we do happen to fly a non-precision approach and we do not have a radio altimeter, then the temperature can be a problem for us.
And that exactly was the initial question. It is great to read all these posts. Thank you!
Regards,
TG.
I am sorry, but I have must given you the wrong impression. I am a pilot and flight instructor. The questions above arose during a class of a colleague of mine. I have posted the problem here since it does involve us pilots and ATC.
ATC creep,
thanks for your thorough post. It's good to read from a controller's point of view.
As bookworm stated, if we do happen to fly a non-precision approach and we do not have a radio altimeter, then the temperature can be a problem for us.
And that exactly was the initial question. It is great to read all these posts. Thank you!
Regards,
TG.
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Bookworm,
Honestly, not all of us are of English origin! Maybe not even you, I don't know! So, I apologize for inaccuracy in my writing (if any!). If you compare my definition as I was taught many years ago and that one of ICAO, what do you get?
Same **** different wrap! One gives QNH, another one elevation! Or other way round!
The point is I still don't believe that airline pilots would use barometric altimeter for precision approach as the only mean of establishing altitude or height! Without radio altimeter, I don't think that any serious company would allow aircraft to fly.
For years, airport where I worked was considered to have elevation of 2800 feet. Then they sent a WGGS team of experts equipped with many electronic gadgets, they measured everything and – surprise! They established that the elevation is actually 2920 feet! 120 feet difference, 120 feet the ground was closer than expected! I state: Should aircraft used barometric altimeter only, somebody would have crashed for all those years of operation in marginal weather! Full stop!
Non-precision approach is different story as minima’s are much greater. I worked at the airport, which had MDA 1000 AGL. This would give enough time and height to compensate for all possible errors of barometric altimeter, temperature compensation etc and still to go around safely! I really don’t know if crews ever applied temperature compensation. Would they have enough time to calculate in extreme weather conditions? Can you explain what would happen if temperature inversion is in place?
I also have spent many hours “flying” an airline flights from the third seat in the cockpit. Not that I knew everything what was happening in the cockpit, but I had impression that the crew used only radio altimeter in final stages, also selecting the bug at prescribed minima which would activate alarm at the minima – and I have experienced really bad weather conditions!
What would happen when you fly in uncontrolled airspace and land at unmanned aerodrome? We’re not talking only about well established ATC system in USA and Europe. Look at Africa. There’s nobody to give you temperature; QNH, hmmmm, what’s that!!!???? But, this is another topic!
Honestly, not all of us are of English origin! Maybe not even you, I don't know! So, I apologize for inaccuracy in my writing (if any!). If you compare my definition as I was taught many years ago and that one of ICAO, what do you get?
Same **** different wrap! One gives QNH, another one elevation! Or other way round!
The point is I still don't believe that airline pilots would use barometric altimeter for precision approach as the only mean of establishing altitude or height! Without radio altimeter, I don't think that any serious company would allow aircraft to fly.
For years, airport where I worked was considered to have elevation of 2800 feet. Then they sent a WGGS team of experts equipped with many electronic gadgets, they measured everything and – surprise! They established that the elevation is actually 2920 feet! 120 feet difference, 120 feet the ground was closer than expected! I state: Should aircraft used barometric altimeter only, somebody would have crashed for all those years of operation in marginal weather! Full stop!
Non-precision approach is different story as minima’s are much greater. I worked at the airport, which had MDA 1000 AGL. This would give enough time and height to compensate for all possible errors of barometric altimeter, temperature compensation etc and still to go around safely! I really don’t know if crews ever applied temperature compensation. Would they have enough time to calculate in extreme weather conditions? Can you explain what would happen if temperature inversion is in place?
I also have spent many hours “flying” an airline flights from the third seat in the cockpit. Not that I knew everything what was happening in the cockpit, but I had impression that the crew used only radio altimeter in final stages, also selecting the bug at prescribed minima which would activate alarm at the minima – and I have experienced really bad weather conditions!
What would happen when you fly in uncontrolled airspace and land at unmanned aerodrome? We’re not talking only about well established ATC system in USA and Europe. Look at Africa. There’s nobody to give you temperature; QNH, hmmmm, what’s that!!!???? But, this is another topic!
ATC Creep
I think that you are applying false logic to try to prove a - probably false - point about the use of radalts. If everyone was under the impression that your aerodrome elevation was 2800 ft, then the QNH value would have been determined on that assumption, albeit that it was later demonstated to be inaccurate. As long as the IAPs were based on that same datum, then they were perfectly safe.
I think that you are applying false logic to try to prove a - probably false - point about the use of radalts. If everyone was under the impression that your aerodrome elevation was 2800 ft, then the QNH value would have been determined on that assumption, albeit that it was later demonstated to be inaccurate. As long as the IAPs were based on that same datum, then they were perfectly safe.