Altimeter confusion...

[Nealius]

BACK TO THE TOPIC AT HAND

 

In DCS I have a waypoint set for 20,000' MSL over some high terrain. QNH is 29.51, but my altimeter is set 29.92 so I am flying at FL200. The waypoint at 20,000 feet is still above me. Because the altimeter increased its indicated altitude when going from 29.51 to 29.92 shouldn't the waypoint be below me? Or is the "MSL" altitude in the ME not related to QNH at all? I mean, if I wanted to arrive at the waypoint with my altimeter showing exactly 20,000' what should my pressure be set to?

[Flamin_Squirrel]

QFE is NOT used routinely, except maybe by some hobbyists on tiny airports.

 

Lol ok, you keep telling yourself that.

 

BACK TO THE TOPIC AT HAND

 

In DCS I have a waypoint set for 20,000' MSL over some high terrain. QNH is 29.51, but my altimeter is set 29.92 so I am flying at FL200. The waypoint at 20,000 feet is still above me. Because the altimeter increased its indicated altitude when going from 29.51 to 29.92 shouldn't the waypoint be below me? Or is the "MSL" altitude in the ME not related to QNH at all? I mean, if I wanted to arrive at the waypoint with my altimeter showing exactly 20,000' what should my pressure be set to?

 

When you increased the pressure scale to 29.92, the altimeter read higher, so you descended to compensate, leaving the waypoint above you. I think that answers your question?

[Glamdring]

Imagine yourself turning the altimeter subscale from 29.92 down to 29.51, what happens to the reading on your altimeter? It goes down, hence to fly at 20,000' altitude you would need to go higher. If you want to be at exactly 20,000 MSL fly on the QNH.

Edited January 22, 2014 by Glamdring

[Nealius]

Imagine yourself turning the altimeter subscale from 29.92 down to 29.51, what happens to the reading on your altimeter? It goes down, hence to fly at 20,000' altitude you would need to go higher. If you want to be at exactly 20,000 MSL fly on the QNH.

 

Now I see. I'm having a hard time visualizing that.

[Glamdring]

Now I see. I'm having a hard time visualizing that.

 

Go Here

 

Stick 20'000 in to the Pressure Altitude field and select 29.51 as the QNH.

 

You will see that even though you are FL200, on the QNH you are only at 19617'

 

Hence to fly at 20,000' altitude you need to climb 383'

 

:thumbup:

[Nealius]

Thanks. It was mainly the climbing part that I had trouble visualizing.

[Headspace]

In the USA we don't use QFE, nor do the "Q" designations ever show up in any capacity that I'm aware of. Maybe someone who is ATP rated would know more.

 

It would be considered completely wrong and in fact you would bust any ride if you left with the altimeter showing 0 unless ground elevation at the runway were in fact 0 MSL.

[Tucano_uy]

I wish I had the clarity my instructors had, command of the English language and patience to explain it.

 

As an ICAO rated ATCo, with experience in various FIRs, I can say that the use of QFE for separations or anything else than internal cockpit stuff is unthinkable, and probably obsolete due to the development of onboard equipment (EGPWS, radio altimeter, FMS, etc). In my career I was never requested QFE even If I had it available at my fingertips.

 

I read a some posts above that the ATC could calculate the difference in altitude between traffic on QNH and QFE to work separations... well, no. The point is to have ONE standard.

 

Here are 2 pictures that may help understand it to those who still don't have it 100% clear.

 

http://www.skybrary.aero/index.php/Altimeter_Pressure_Settings

 

I strongly wish that DCS starts issuing QNH instead of QFE

 

 

 

SOME INTERESTING READING

AirbusSafetyLib_-FLT_OPS-SUPP_TECH-SEQ01.pdf

Edited January 23, 2014 by Tucano_uy

[Flagrum]

I am genuinely surprised and somewhat shocked that there exists so many different possibilities to measure one of the, or maybe THE most important datums concerning what is the essence of flying: the altitude.

 

Imagine road traffic signs. You drive your car and see signs like "max speed 100 km/h". A bit later: "max speed reduced by 10 m/sec" ... "max speed 66 mph" ... then "max speed: increased by -44 ft per deka-minute on mondays".

 

Now, how fast are you now?:cry:

[Flamin_Squirrel]

I am genuinely surprised and somewhat shocked that there exists so many different possibilities to measure one of the, or maybe THE most important datums concerning what is the essence of flying: the altitude.

 

Imagine road traffic signs. You drive your car and see signs like "max speed 100 km/h". A bit later: "max speed reduced by 10 m/sec" ... "max speed 66 mph" ... then "max speed: increased by -44 ft per deka-minute on mondays".

 

Now, how fast are you now?:cry:

 

Eheh. It's not that bad really, it all makes sense when you've had some practice.

 

Speeds in aircraft are a whole other topic; IAS, TAS, GS, Mach is a fun one to wrap your head around :music_whistling:

[Nealius]

I find the airspeeds easier to grasp than the altitude :lol:

[Flagrum]

That's why there are standards in aviation

... following this thread here (or rather, trying to), one could come to a different conclusion ... :o)

[cichlidfan]

... following this thread here (or rather, trying to), one could come to a different conclusion ... :o)

 

+1 :thumbup:

[Flagrum]

That has more to do with individuals than with professional aviation safety standards and/or airmanship.

Might be ... but what I really and in all seriousness meant was, how can it be that there can be a discussion about this AT ALL? Isn't there a single source that defines how such things are to be handled? I was always under the impression that FAA or whoever would document such things in a document/book and then it is "the law".

 

Like:

Q: "QFE? QNH? Err ...?"

A: "Here, see page 1013, chapter 25 - there, paragraph §2992 says: '...'"

 

Yes, I do realize that regulations might differ from country to country - similar to perhaps road traffic regulations ... but as I mentioned earlier - this is such a basic thing for every aviator ...

[Nealius]

I was actually searching all over for such FAA "laws" regarding altimeter settings. The only thing I could find was that all altitudes on charts are given in MSL unless otherwise specified; also all controlled classes of airspace are defined in MSL. Which then implies that the altimeter must be set to QNH.

 

It's hard enough finding any American info on QNH since most of the time it's just called "pressure" or"sea-level pressure." Ask a GA pilot (in my area, at least) what QNH or QFE are and they'll just look at you funny.

[Glamdring]

In the UK there is no set rules for altimeter use. Obviously if you are flying inside controlled airspace then ATC will be controlling you by issuing Altitudes and Flight Levels, therefore QNH and SPS will be used. Airline SOPs usually have their own procedures for when to change from QNH to SPS and vice versa.

 

Outside controlled airspace a pilot can set his altimeter to whatever he wants, but he should be aware of certain things...

 

Danger areas, military Training Areas and Controlled Airspace will be shown on charts using Altitudes or Flight Levels and the pilot is still required to avoid them no matter what setting he is using.

 

If in receipt of a radar service he may be asked to stay above/below a certain altitude. Again it is the pilot's responsibility to adhere to this no matter what setting he is using.

[Flamin_Squirrel]

Might be ... but what I really and in all seriousness meant was, how can it be that there can be a discussion about this AT ALL? Isn't there a single source that defines how such things are to be handled? I was always under the impression that FAA or whoever would document such things in a document/book and then it is "the law".

 

Like:

Q: "QFE? QNH? Err ...?"

A: "Here, see page 1013, chapter 25 - there, paragraph §2992 says: '...'"

 

Yes, I do realize that regulations might differ from country to country - similar to perhaps road traffic regulations ... but as I mentioned earlier - this is such a basic thing for every aviator ...

 

ICAO (international civil aviation organisation). The FAA, CAA etc will follow standardisation laid down by them to a large degree, but it's possible to deviate. Often there are good reasons. QFE is widely used by the RAF and GA in the UK and works well for us. Somewhere like the states it wouldn't be appropriate because it wouldn't be practical to set QFE at the many airfields they have at a high elevation.

[Flamin_Squirrel]

It IS a very basic thing indeed Flagrum. And with a little bit of logic, knowledge and reasoning it's very clear that QFE is not something you should use, because it's less safe, it's implies the pilot is stupid or lazy (wanting to read round numbers instead of actual values) and it's just setting your instrument INcorrectly on purpose.

 

I don't know where it says so in official documentation. But it's basicly common knowledge amongst professional pilots. Everything's based off of QNH and the standard altimeter setting. It's probably hard to find in regulations because it's basicly misuse of your altimeter.

 

Please, enough of the trolling. Give it a rest.

[Nealius]

I'm not sure who's trolling when the three countries that most frequently violate ICAO regs are the US, UK, and Russia :megalol:

 

(Or so I've been told).

[Glamdring]

I'm not sure who's trolling when the three countries that most frequently violate ICAO regs are the US, UK, and Russia :megalol:

 

(Or so I've been told).

 

Nobody "violates" ICAO regs. They are guidelines. In the UK we have our own rules that do differ from ICAO regs in certain areas, but for good reason. We aim to to keep in line with ICAO as much as we can, only just last year we changed from the UK-Only ILS phraseology "When established on the localiser, descend with the glidepath" back to the ICAO "Cleared ILS Approach".

[Nealius]

That's the first time I have ever heard of a regulation being a guideline....I wasn't aware the pirate code worked in aviation ;)

[Flamin_Squirrel]

Absolutely no trolling intended. Just answering a question.

 

There are 60,000 licensed civil pilots in the UK, plus the military ones, most of who will have, or do, operate using QFE. It works for us so we use it. If you don't want to that fine too, but you're we're not 'lazy' or stupid or any of the other offensive adjectives you've used so far.

[Nealius]

Can you two untangle your panties, please? At this point it's just a pissing match.

[Glamdring]

That's the first time I have ever heard of a regulation being a guideline....I wasn't aware the pirate code worked in aviation ;)

 

Each country makes their own rules regarding their airspace. In the UK the document is called the ANO (CAP393)...

 

http://www.caa.co.uk/docs/33/CAP393.pdf

 

Knock yourself out ;)

Edited January 23, 2014 by Glamdring

[Nealius]

I understand the local rules (FAA in the US, JACB in Japan, etc.) but how do they interact at international airports where incoming pilots are on ICAO rules? I can't imagine it being pleasant having FAA/ANO/JACB rules clashing with ICAO rules in the same airspace. I'm mainly thinking radio phraseology here, since that's the only bit I'm familiar with.