New tower approach calls
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All Spooky said is right but he missed the calculations…
All these parameters are ways to adjust altimeter away from the standard/ pattern values (above describd).
Calculations to be made IRL and AFAIK:
Same airport (above) at 980ft, imagine ATC says QNH to be 29,32’’ mercury… thou, that altitude should be by the table as 28,86’‘, now one should get the diff from standard at SL (29,92’‘), so 29,92-28,86=1,06, now this result shoulld be added to the ATC’s value at AB’s altitude (29,32’'); 29,32+1,06=30,38. This is the correction to be adjusted to your altimeter.
Same goes to QNE but in hPA and their table/ATC values
QFE puts your altimeter even far away from it’s standard values and it´ll start to “think” that AB’s altitude is SL, telling you that that you are at 0ft when you’re on the ground of that airport. -
All Spooky said is right but he missed the calculations…
All these parameters are ways to adjust altimeter away from the standard/ pattern values (above describd).
Calculations to be made AFAIK:
Same airport (above) at 980ft, imagine ATC says QNH (Same goes to QNE but in hPA and their table/ATC values) to be 29,32’’ mercury… thou, that altitude should be by the table as 28,86’‘, now one should get the diff from standard at SL (29,92’‘), so 29,92-28,86=1,06, now this result shoulld be added to the ATC’s value at AB’s altitude (29,32’'); 29,32+1,06=30,38. This is the correction to be adjusted to your altimeter.there are no calculation to do (except if you want to correct temperature but you dont). Just plig the vlaue from ATC depending on what you want , QNH or QNE
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No?
So you just set altimeter to ATC values, right?
Meaning, ATC gives you SL new values…? -
I missed some calculations …. but not really the one you stated.
But as you want some, there you go, actually for 1 inch of mercury you have 1000 ft.
Foe 1 hPa you have 30 feet.
So when do you calculate corrections ? … actually never when ATC gives you radar vectors, because ATC vectors are always corrected for possible altitude errors.
Altitude errors even you have the correct altimeter setting stetted in …happens in very very cold temperatures .
Now for aircraft performances we do calculate DENSITY ALTITUDE, Density altitude is the altitude ( the one with your QNH) corrected for temperature.
You always calculate that …for performances for takeoff and for fuel consumption for navigation. This Density altitude is actually … can’t be indicated on your altimeter, the one on you altimeter is the indicated one.
Density altitude is calculated that way:
Pressure Altitude: 5000 ft
Temperature : 20 celsius ( this is in fact 15 degrees above the temperature that it should be if we were in standard atmosphere … remember , we always calculate everything from a reference base, which here is the standard atmosphere values …29.92 Hg at sea level and 15 degrees with a cooling rate of the air of 2 degrees per 1000 ft )
So your density altitude is going to be : 120 ( you subtract 120 ft of the pressure altitude for every degree celsius that the ambient temperature differs from standard atmosphere called the ISA) x 15 ( 15 is the difference between the actual temp at 5000 ft and the one we should have if we were in ISA and you add the result to your current pressure altitude :
(120 x 15) + 5000 = 6800 ft.
If you want to do it right … in the ISA settings it is not assumed that there is humidity, so we calculate the virtual altitude .
For that we use another info, which is the dew point temperature … dew point, is the temperature at which if an air mass is cooled down … condensation occurs, or if you prefer clouds or in some cases precipitation occurs ( snow, rain, etc )
For virtual altitude we have this thumb of rule :
Dew point temperature, less than 10 degrees -----> you add 1 degrees.( you add it to your above calculated density altitude)
11 degrees to 20 degrees ------> you add 2 degrees
21 degrees to 25 degrees ------> you add 3 degrees
There are other ways to calculate this more accurately, but since this is a method to use in the cockpit, it is far more friendlier from the original one, where you have to use numbers with decimals …
So yes, you just enter the ATC values in your altimeter and normally … for flights below 18 000 ft ( here in Canada) you get altimeter settings updates depending on if you are on a flight plan from the controller or if you are not on a flight plan … by getting weather info by switching radio channels to stations you encounter en route ( airports generally).
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He!
Thanks mate.
Have read about all that long while ago, but as not using it frequently couldn’t have it that freshly in mind.
Thou, the calculations I mentioned were only about altimeter references, and only aplicable if ATC gives you local air pressure base, and not local SL pressure base. -
You are welcome
But i don’t understand what you mean by the local air pressure …. because the local air pressure is actually the QNH … the only other one is the QFE …which we don’t use anymore.
We never do correction …to the QNH for our altimeter … especially when taken from the ATIS before takeoff.
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Am not explaining well…
What ATC should give you about QNH? (mercury pressure at SL, or at AB altitude) ?
You’ll need to do calculations if you receive values at AB local site, because you’ll need to adjust altimeter based on SL values…
In other way, if ATC gives you SL correction values, then those are what you want.
Or am I wrong somewhere…? -
Hummmmm
I think you are wrong, because what ATC gives you is the local pressure ….
The QNH you get can’t be wrong … an easy way to verify this … is not to take the QNH …put you can on your altimeter … adjust the altitude.
If you know the elevation of the Airport … you just rotate the knob until your altimeter indicates the elevation of your Airport. The QNH value read next to it …is the QNH …and it should be …the same as the one the ATC gives you.
I’m doing my Pilot Training right now …and i just did my IFR ( instrument Flying Rules) … and at no times you have to calculate something else when the QNH is given to you.
But i think you are confusing things , QNH is mercury pressure … the only thing ATC gives about QNH is … its value …nothing more, nothing less.
And the QNH when entered in your altimeter setting window … gives you the Airport elevation.
But maybe you can help me to help you in telling me what you mean by AB altitude … if you mean Airbase by that , and do you mean by SL correction values ?
Do you have real flying experience ?
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The QNH you get can’t be wrong …. an easy way to verify this … is not to take the QNH …put you can on your altimeter … adjust the altitude.
If you want to be 100% acurate … you have do add a temperature correction… your altimetrer is calibrated for 15°C … so informations is not always 100% exact.
The Pressure altimeter is calibrated to indicate true altitude in the Standard Atmospheric conditions. Due to continually heating and cooling, the atmosphere at any given point is seldom at the temperature of Standard Air.
In fact, the only time you can be certain that the altimeter indicates true altitude is when the aircraft is on the ground at the airport for which the current altimeter setting is set on the sub-scale of the altimeter.
When an aircraft is in flight, it can be assumed that the altitude indicated on an altimeter is always in error as a result of temperature variations.
The amount of error depends on the degree to which the average temperature of the column of air between the aircraft and the ground (Altimeter setting Location) differs from the average temperature of the standard atmosphere for the same column of air.
If the actual temperature of the air column in which the aircraft is flying is COLDER than the standard, the true altitude of the airplane above sea level will be lower than the indicated altitude. Conversely, if the temperature is WARMER than standard, the true altitude will be higher.
Obviously, since all altimeters in the same area are equally affected by temperature error, we don’t apply any corrections for traffic avoidance purposes.
However, for terrain purposes, it’s a different story.
Quote:
| Do you take the 4% of the 18000 ft or take the 4 % of the 2000ft (height)??? I always thought you had to take the 4% of the 18000ft but then somebody argued that the first 16000ft isn’t air but mountain and can therefore not be compressed due to the lower temperature. Does anyone know which I should take and why?
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I was talking for the QNH on the ground … and yes it is never 100% accurate … i meant by that that when you get QNH before takeoff … you don’t calculate anything else.
And yes for abnormal temperatures, when it gets very cold …and it gets vey cold here lol …we have altimeter correction charts.
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Even though the altimeter can become inaccurate for other-than-standard temperatures, pilots NEVER calculate a correction when entering a pressure value into the Kollsman window. Pilots always enter exactly what is told to them by ATC (or, in real life, ATIS/AWOS) without any corrections. This ensures that all aircraft in an area have the same frame of reference when flying at specific altitudes.
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Thank you … looks like we’re 3 pilots here happy landings to Dee-Jay and RISCfuture.
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Concur with the consensus here. QNH/QNE are the standards for baro altimeter settings. I’ve never seen QFE used, though it’s theoretically more useful in flat areas. Quite frankly, QFE is useless if you have a RADALT on board.
Of trivia, apparently QFE was once more widely used in the UK. We had a Kriegsmarine officer in my class in flight school who said the way they remember QFE was “Queen F***s Easy.”
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loooooooooooooooooool … yes my instructor back in France told me it was used by the old old pilots lol
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SL is Sea Level
AB, yes AirBase.
Ok if you’re fresh on this subject, it might be some mistaken from my understanding, thou still think not being clear enough…If you want to adjust altimeter to airbase altitudes pressure correction and assuming it’s not the tabled value… ex:
AB altitude is 980ft, but pressure is not at table values of 28,86’‘… so you need the new one, let’s say it’s 29,32’‘…
So you’ll need to adjust accordingly you altimeter for that, but you’ll need to adjust regarding Sea Level value which is the base reference, you will need to do the mentioned calculations in order to find SL new values… based on ATC (AB altitude) values.
29,92’’ (table SL value) - 28,86’’ (table 980ft value), then add this diff. to ATC’s value (29,32’‘)… resulting in corrected local AB, but at SL new value of 30,38’’
After doing calculations you’ll have a new SL value of 30,38’’ to adjust altimeter and not ATC’s value.
Am I wrong?All this looses meaning if ATC gives you QNH values at SL and not AB’s altitude… see?
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All ATC is based on an altimeter setting from MSL. When you get one from a tower it comes from their direct reading instrument. When you get it from anywhere else it comes from a certified weather reporting person and is what is used for the next one hour or if there is a rapid change they will put out a special report. Usually an aviation weather reporting station is within 100 to 150 miles apart and these altimeter settings are given by ATC enroute facilities. When aircraft climb to certain altitudes (each country establishes these base altitudes) then everyone goes to a standard altimeter setting. Here in North America that is 18,000 feet MSL which then becomes known as a “flight level”, Since all aircraft above 14,500 are contolled by ATC it simplifies communications. Also, ATC is then tasked with being responsible for altitude assignment when the mercury falls below 29.92 and FL180 becomes unusable. I spent 18 years as an air traffic controller an can usually explain what happens fairly well.
Lumper
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Oh crap, more European sh*t to relearn, we don’t use this QFE/QNE/QNH terminology in the US…
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Sure we do. We just say “Local Altimeter” and QNH is implied.
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@Home:
Sure we do. We just say “Local Altimeter” and QNH is implied.
Yea I know but we never say it… Now we have to say it in BMS…
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In real life if you get it wrong or forget, things like this could happen - if your into aerobatics of course: