QNH correction for instrument approach

[tgflyer]

Did it get hot in here???

Let's take a deep breath, shall we?

The good old barometric altimeter is a considerable amount of limitations. That's a fact. For the pilots it means that it can give quite inaccurate readings.

In school we are taught how to deal with these limitations.

The problem is: when you open a book, that's approved by the authorities to be used as part of a distance learning course, and you read "QNH is corrected for temperature" it should make you wonder. At least our students did wonder.

RaAlt is a very useful tool, provided the aircraft has one of those.

And the problem (once again) that arose was: should a pilot, using a simple barometric altimeter, who's flying a non-precision IAP, adjust the QNH for temperature? Or did the appropriate MET office already do that?

Mind you, ATC creep, that I teach students with all kinds of dreams. Some want to fly for airlines, and others just want to take their little A/C for their own leisure and fly in IMC. Not everybody will have RaAlt.

I believe bookworm has stated quite nicely the possible problems encountered on a colder than ISA day.

BOAC,

no problem bud!
I have not been able to find any threads which would shed any light on this.
All the threads and posts I have found so far treat the typical Pressure vs True altitude issue, or the barometric altimeter limitations.

Regards,

TG

[DFC]

1. You never adjust a QNH or a QFE.

2. Where there are cold temperatures, you make adjustments to the minimum altitudes/heights acording to the tables provided or a simple 4% height increase for every 10deg C the temperature at the source is below standard.

3. Radalt is useless unless there has been an accurate terrain survey of the area that is being overflown. Some aerodromes may have done this for Cat 1 precision approaches but it is usually limited to CAT 2 and 3 procedures.

Imagine the variety of terrain below the final approach area on a non-recision approach, the cost of the survey and the ability to predict where the pilot will be and what the rad-alt will say when at minimums.

Regards,

DFC

[BOAC]

Originally Posted by DFC

You never adjust a QNH

- thanks DFC, of course correct and I have edited my previous post to avoid the wrong impression.

TG - here is one thread for you at least. You could also look at post#3 of the JT's tech log's 'sticky'

[ATC Creep]

Bookworm,

I am not trying to prove anything to anyone, discussing only, expressing my view which might be wrong - an ATC and a Private Pilot used to live and work by certain rules.

TG Flyer,

As an ATC I say, never adjust QNH! Do we agree that it would be a mess if everybody in the air adjust their own corrected QNH? You will immediately endanger safety for all around you and yourself! Vertical separation minima will be changed either way, reduced or increased! So, always fly the QNH ATC gives you - all aircraft would fly the same QNH, corrected or not, with small mistakes or without, but all use the same value! This is the point - same value!
It will be corrected for temperature on the ground - if you are flying in organized, well developed ATC system!

Good point is written by DFC, I fully agree.

As A PPL I have never had a need to think about possible temperature implications. Lucky me!

All together I enjoy this discussion. Flying safety is paramount!

Regards,

ATC Creep

[Wojtus]

QNH should be set as given by ATC. If you need correction due low temperature (typically below -15C), correct the MSA/MRVA altitudes on the charts - and REPORT it. Values shown at your altimeter are just virtual, not exact elevation over MSL (except in ideal ISA conditions). But we use these "virtual" values for separation and they are good for it until everybody has the same altimeter setting.

[tgflyer]

Just to make sure everybody understands me: I never, ever teach to adjust QNH for anything. And it is not what I was taught either.

I was just questioning the source in which it was stated, and whether somebody else (non-flying crew) was doing such adjustments without our knowledge.


BOAC,

thanks for the hints. I believe I have found an answer in it (at least my suspicion has been verified once again). (If you wish to read it, check out "The Chilling Result of Cold Temperature on Barometric Altimeters" or RAC 9)


Regards,

TG

[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!

ATC Creep

It was not my intention to criticise the accuracy of your writing in English, which is excellent. But in the context of the discussion about temperature errors, there is a significant difference between your definition and ICAO's. The QNH is measured at the airport, not measured at sea level and "corrected". That was the only point I was making.

[DFC]

OK. Lets look at what is measured, how it is corrected and how we end up with QNH or QFE.

I know there are electronic versions to day but lets look at the basic (old) system.

The pressure at the station is measured by a Barometer i.e. a column of mercury balanced by the pressure of the air.

The reading obtained from the barometer is corrected for;

1. Index error - the calibration of the index.

2. Temperature error - temperature at the barometer affects the mercury ie heat it up and it expands.

3. Latitude error - Remember that gravity is also acting on the mercury and of course gravity force varies with latitude

What you get from all that is the station pressure.

Take the station pressure and reduce it to sea level asuming ISA conditions exist between the barometer level and sea level and the resultant pressure is called QNH.

Since the altimeter assumes ISA atmosphere, if you place the altimeter beside the barometer, and set the QNH you obtained, you will see the altitude of the barometer on the altimeter because the barometer pressure has in simple terms been reduced to sea level and back again all at ISA so there will be no error in a perfect altimeter.

Now take that altimeter to 5000ft above the barometer (station) level.

The altimeter is still assuming ISA conditions exist between the station level and where it is. Unfortunately, most days this is not the case. When the temperature of the air between the station and the level of the altimeter is colder than standard then the altimeter will over read i.e. it will tell you that you are higher than you really are.

That is dangerous. Therefore, when the temperature at the station is significantly below ISA, there can be a significant dangerous error affecting the altimeter.

As a pilot or as a controller providing radar vectors (since both are responsible for terrain clearance), you must make an appropriate adjustment to minimum levels when the temperature at the station is well below ISA (see ICAO DOC 8168 for the appropriate adjustments)

I hope that that explains the situation.

Regards,

DFC

[2 sheds]

"As a pilot or as a controller providing radar vectors (since both are responsible for terrain clearance), you must make an appropriate adjustment to minimum levels when the temperature at the station is well below ISA (see ICAO DOC 8168 for the appropriate adjustments)"

"... a controller...must make..."? The whole implication is that it is the pilot's responsibility to determine any corrections and, if necessary, e.g. if it were to affect an assigned intermediate level, to inform ATC.

[DFC]

What I said was that the person responsible for ensuring terrain clearance is also responsible for applying appropriate corrections when the station temperature is well below isa.

That means the controller when they are providing radar vectors.

ie if the minimum radar vectoring altitude is say 2000ft and the temperature at the aerodrome is ISA -16 deg C then you can not descend an aircraft you are vectoring to 2000ft because their actual altitude will be below 2000ft.

The Reference is ICAO DOC 8168 Volume 1 -

This chapter deals with altimeter corrections for pressure, temperature and, where appropriate, wind and
terrain effects. The pilot is responsible for these corrections, except when under radar vectoring. In that case, the radar
controller issues clearances such that the prescribed obstacle clearance will exist at all times, taking the cold
temperature correction into account.

(My Emphasis)

The UK does not provide any corrections (see AIP GEN 3.5.5). Thus you as a radar controller have to come up with them when vectoring.

Regards,

DFC

[BOAC]

As DFC says, it is 'normal' for the controller to adjust any cleared level, as, for example, any level issued inbound to Innsbruck will be 'adjusted' by the controller for temperature. It is the pilot's responsibility to adjust other altitudes such as MSAs, MDAs, even DAs and DHs where necessary and any other altitudes he/she may compute/consider, including, for example, procedure platform altitudes, acceleration altitudes, EOI procedure altitudes and others. It is still wise to confirm in your own mind that the level issued by ATC is, in fact, 'safe'.

[ATC Creep]

ATC Creep

It was not my intention to criticise the accuracy of your writing in English, which is excellent. But in the context of the discussion about temperature errors, there is a significant difference between your definition and ICAO's. The QNH is measured at the airport, not measured at sea level and "corrected". That was the only point I was making.

Hello Bookworm,
Thanks for this. However, when a new, good quality electronic barometer arrives at the tower you have to set it – usually by using a PC. The barometer is “told” at which elevation is the airport touchdown zone + the height of the tower. It than measures the actual pressure for the point where it is and automatically calculates the above figures and shows ‘corrected’ pressure which we call QNH. Having some German blood in my veins I even calculated the height of the bench where it was located. The same principle is applied for QFE – minus elevation of TD.
If the calculation says that the instrument is located at 2800feet above MSL, than the actual pressure at the time would be +/-100mb lower than QNH. Adding 100mb to actual pressure you basically ‘correct’ the reading, correct? That’s where my definition comes from - maybe not accurate enough!
In stable weather conditions with increase of outside temperature the pressure would drop up to about 3mb around 2PM and then towards the evening it would slowly go back to where it was in the morning.
Problem with ICAO is English. Sometimes you have to read some sentences few times in order to understand the meaning – it’s too much English! That’s why I said that we are not all of English origin.
From the comments I see that everybody agrees that you may not change, calculate or manipulate ATC given QNH at all. And that is the main thing!
Regards,
ATC Creep

Two wrongs don't make a right, but two Wright's built an aeroplane!

[DFC]

yrvld,

Speak for yourself.

ICAO says that it is up to the controller to make the correction when they are vectoring you. Do you request a higher level than issued by the controller because you have worked out the correction and added this to;

a)The level issued by the controller?

or

b)The minimum level and it is clear that the level issued is below the minimum level?

If a) you are making a double correction i.e. the controller corrects and you correct it further. Very safe but unnecessary.

If b) You are simply crosschecking that the controller is doing what they are supposed to do - a very good idea but totally different from a).

Regards,

DFC

[BOAC]

Originally Posted by yrvld

Again, as BOAC said, as pilots we do not expect ATC to issue instructions based on such corrections

- you misquote me!

[tgflyer]

I am glad to see that this is not a very clear area!

Great discussion! Keep 'em coming!

TG

[ATC Creep]

Here is my humble contribution to the topic.

• Minimal safe altitude, MSA, is calculated with the following margins: for example, if mountain is high 6000 feet, add to that 1500 feet as minimal obstacle clearance plus 1500 feet safety buffer, safety margin. This is applicable within 25NM radius around the mountain. So, at any time you will have 3000feet buffer.

• If QNH drops below 1013, 25mb, only then MSA is upped by 500 or 1000feet – ATC does it!

• Enroute, where minimal FL are published along airways the same is applicable: if QNH drops below 1013, 25mb, min FL is raised to the next applicable FL.

This is from my ATC perspective.
I am not aware of any or haven’t experienced any pilot self-altered altitude in my 33 years career yet. It doesn’t sound right that any pilot would alter ATC clearance – without ATC approval! I believe that temperature problem is compensated by 1500 feet safety buffer.
I have no problem with compensated MDA or MDH – it has a sense.
But we learn everyday, don’t we!



Two wrongs don't make a right but two Wright's made an aeroplane!

[DFC]

I am not aware of any or haven’t experienced any pilot self-altered altitude in my 33 years career yet. It doesn’t sound right that any pilot would alter ATC clearance – without ATC approval!

No one is talking about alter ATC clearance - without ATC approval.

If the published minimum altitude is 5000 and there is a 200ft cold weather correction then if ATC clear us to 5000ft, we will respond that the minimum safe level is 5200 due temperature and request decent to that level or a level above.

In general where there is regular corrections, ATC are up to speed.

However, on a few occasions there can be corrections required in the UK during a cold spell and I am not sure that ATC make an allowance when vectoring.

Note that we are only talking about temperature corrections here. There are also corrections for windspeed when in the vicinity of high ground.

Again if the minimum level is 5000ft and the wind is such that an extra 5ooft is to be added, we will refuse an ATC clearance to 5000ft and give our reasons.

To confirm, no one changes the level without telling ATC and ensuring that a clearance is obtained. In fact there is a note in most ops manuals warning pilots that if they increase the level of the MSA, IAF, IF and FAF they must tell ATC. It is only in the case of the DA/MDA that we don't have to tell.

Regards,

DFC

[bookworm]

If the published minimum altitude is 5000 and there is a 200ft cold weather correction then if ATC clear us to 5000ft, we will respond that the minimum safe level is 5200 due temperature and request decent to that level or a level above.

Would you really? That's only about ISA - 10 degC. While the got-a-safety-margin-so-I'm-OK argument can be a slippery slope, I'm not sure I'd bother protesting at a 200 ft incursion into my 1000 ft obstacle clearance.

[DFC]

Nice to know that you are happy to break the law.

No lower than 1000ft above all obstacles within 5nm etc etc.

The numbers I used were for demo purposes and are not necessarly a real life correction.

Of course the levels are minimums. Any pilot can ask at any time to fly at a higher level.

Regards,

DFC

[vintage ATCO]

The UK does not provide any corrections (see AIP GEN 3.5.5). Thus you as a radar controller have to come up with them when vectoring.

In 40 years of controlling (in the UK) I have never heard of this, never been taught it and never done it. I can't find your AIP reference, DFC, say again?