New tower approach calls
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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:
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Flyway … here this is official document, from Canadian Transport Office.
1.5 pressure alTimeTer
1.5.1 General
The pressure altimeter used in aircraft is a relatively accurateinstrument for measuring flight level pressure but the altitudeinformation indicated by an altimeter, although technically“correct” as a measure of pressure, may differ greatly fromthe actual height of the aircraft above mean sea level orabove ground. In instances of aircraft flying high above theearth’s surface, knowledge of the actual distance between theaircraft and the earth’s surface is of little immediate value tothe pilot except, perhaps, when navigating by pressure patterntechniques. In instances of aircraft operating close to theground or above the highest obstacle en route, especially whenon instruments, knowledge of actual ground separation or ofApril 7, 2011
“error” in the altimeter indication, is of prime importance ifsuch separation is less than what would be assumed from theindicated altitude.
An aircraft altimeter which has the current altimeter settingapplied to the subscale should not have an error of more than±50 feet when compared on the ground against a knownaerodrome or runway elevation. If the error is more than±50 feet, the altimeter should be checked by maintenance asreferenced in AIR 1.5.2.
1.5.2 Calibration of the Pressure Altimeter
Pressure altimeters are calibrated to indicate the “true”altitude in the ICAO Standard Atmosphere. The maximumallowable tolerance is ±20 feet at sea level for a calibratedaltimeter. This tolerance increases with altitude.
The ICAO Standard Atmosphere conditions are:
(a) air is a perfectly dry gas;
(b) mean sea level pressure of 29.92 inches of mercury; mean sea level temperature of 15°C; and
(d) rate of decrease of temperature with height is 1.98°C per1 000 feet to the height at which the temperature becomes-56.5°C and then remains constant.
1.5.3 Incorrect Setting on the Subscaleof the Altimeter
Although altimeters are calibrated using the StandardAtmosphere sea level pressure of 29.92 inches of mercury, theactual sea level pressure varies hour to hour, and place to place.To enable the “zero” reference to be correctly set for sea level atany pressure within a range of 28.0 to 31.0 inches of mercury,altimeters incorporate a controllable device and subscale.Whether a pilot inadvertently sets an incorrect pressure on thealtimeter subscale or sets the correct pressure for one area andthen, without altering the setting, flies to an area where thepressure differs, the result is the same – the “zero” referenceto the altimeter will not be where it should be but will be“displaced” by an amount proportional to 1 000 feet indicatedaltitude per 1 inch of mercury that the subscale setting is inerror. As pressure decreases with altitude, a subscale settingthat is higher than it should be will “start” the altimeter at alower level, therefore, A TOO HIGH SUBSCALE SETTINGMEANS A TOO HIGH ALTIMETER READING, that is theaircraft would be at a level lower than the altimeter indicates;A TOO LOW SUBSCALE SETTING MEANS A TOO LOWALTIMETER READING, that is the aircraft would be at alevel higher than the altimeter indicates. As the first instanceis the more dangerous, an example follows:
A pilot at Airport A, 500 feet ASL, sets the altimeter to theairport’s altimeter setting of 29.80 inches of mercury prior todeparture for Airport B, 1 000 feet ASL, some 400 NM away.A flight altitude of 6 000 feet is selected for the westbound![](file:///page411image49400)
flight so as to clear a 4 800-foot mountain ridge lying acrosstrack about 40 NM from B. The pilot does not change thealtimeter subscale reading until he makes radio contactwith B when 25 NM out and receives an altimeter setting of29.20 inches of mercury. Ignoring other possible errors (seebelow), when the aircraft crossed the mountain ridge theactual ground clearance was only 600 feet, not 1 200 feetas expected by the pilot. This illustrates the importance ofhaving the altimeter setting of the nearest airport along theroute set on the instrument.
1.5.4 Non-Standard Temperatures-
(a) The only time that an altimeter will indicate the “true”altitude of an aircraft at all levels is when ICAO StandardAtmosphere conditions exist.
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(b) When the current altimeter setting of an airport is set onthe subscale of an altimeter, the only time a pilot can becertain that the altimeter indicates the “true” altitude iswhen the aircraft is on the ground at that airport.
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When 29.92 inches of mercury is set on the subscale of analtimeter within the Standard Pressure Setting Region, thealtimeter will indicate “true” altitude if ICAO StandardAtmosphere conditions exist or if the aircraft is flying atthat particular level for which 29.92 inches of mercurywould be the altimeter setting.
In general, it can be assumed that the altitude indication ofan altimeter is always in error due to temperature when anaircraft is in flight.
The amount of error will be approximately 4% of the indicatedaltitude for every 11°C that the average temperature of the aircolumn between the aircraft and the “ground” differs fromthe average temperature of the Standard Atmosphere for thesame air column. In practice, the average temperature of theair column is not known and “true” altitude is arrived at fromknowledge of the outside air temperature (OAT) at flightlevel and use of a computer. The “true” altitude found by thismethod will be reasonably accurate when the actual lapse rateis, or is near, that of the Standard Atmosphere, i.e., 2°C per1 000 feet. During the winter when “strong” inversions in thelower levels are likely and altimeters “habitually” over-read,in any situation where ground separation is marginal, a pilotwould be well advised to increase the altimeter error foundusing flight level temperature by 50%. Consider the aircraftin the above example; assume that the OAT at flight level inthe vicinity of the mountain ridge was -20°C; what was thelikely “true” altitude of the aircraft over the mountain ridge?
To calculate “true” altitude using a computer, the pressurealtitude is required. In this case, the altimeter indicates6 000 feet with 29.80 inches of mercury set on the subscale,therefore, if the pilot altered the subscale to 29.92 inches ofmercury momentarily, the pilot would read a pressure altitudeof 6 120 feet. Although the indicated altitude is 6 000 feet,if the altimeter setting of the nearest airport (B) was set, theTC AIM
indicated altitude would be 5 400 feet. With 29.20 inches ofmercury set on the altimeter subscale if the aircraft was on theground at B, the altimeter would indicate the “true” altitudeof 1 000 feet; assuming no pressure difference, it can betaken that the altimeter set to 29.20 inches of mercury wouldindicate the 1 000-foot level at the mountain with no error dueto temperature, therefore temperature error will occur onlybetween the 1 000-foot level and the 5 400-foot level, i.e.,4 400 feet of airspace.
(a) Set pressure altitude, 6 120 feet, against OAT, -20°C, inthe appropriate computer window.
(b) Opposite 4 400 feet (44) on the inner scale read 4 020 feet(40.2) on the outer scale.
Add the 1 000 feet previously deducted as being errorlessand find the “true” altitude of 4 020 feet + 1 000 feet =5 020 feet ASL. The margin of safety is now just over200 feet, but this does not take into account variableswhich may prevail as outlined immediately above and dueto mountain effect as explained below.
1.5.5 Standard Pressure Region
When flying within this region, the altimeter must be reset,momentarily, to the altimeter setting of the nearest airportalong the route to obtain indicated altitude, or indicatedaltitude calculated from the altimeter setting, and the stepsgiven above followed, or, when over large expanses of wateror barren lands where there are no airports, the forecastmean sea level pressure for the time and place must be usedto get indicated altitude. In the other instance, “airport” levelwould be zero, therefore subtraction and addition of airportelevation would not be done. The “true” altitude determinedin such a case would be “true” only if the forecast pressureused approximates the actual sea level pressure. (If sea levelpressure is not known and pressure altitude is used also asindicated altitude, the resultant “true” altitude will be the“true” altitude above the 29.92 level, wherever it may be inrelation to actual mean sea level).
1.5.6 Effect of Mountains
Winds which are deflected around large single mountainpeaks or through the valleys of mountain ranges tend toincrease speed which results in a local decrease in pressure(Bernoulli’s Principle). A pressure altimeter within such anairflow would be subject to an increased error in altitudeindication by reason of this decrease in pressure. This errorwill be present until the airflow returns to “normal” speedsome distance away from the mountain or mountain range.
Winds blowing over a mountain range at speeds in excessof about 50 KT and in a direction perpendicular (within30°) to the main axis of the mountain range often create thephenomena known as “Mountain” or “Standing Wave”. Theeffect of a mountain wave often extends as far as 100 NMdownwind of the mountains and to altitudes many times higher than the mountain elevation. Although most likelyto occur in the vicinity of high mountain ranges such as theRockies, mountain waves have occurred in the Appalachians,elevation about 4 500 feet ASL (the height of the ridge ofour example).
Aware and the Air Command Weather Manual (TP 9352E)cover the mountain wave phenomena in some detail; however,aspects directly affecting aircraft “altitude” follow.
1.5.7 Downdraft and Turbulence
Downdrafts are most severe near a mountain and at aboutthe same height as the top of the summit. These downdraftsmay reach an intensity of about 83 ft. per second (5 000 ft. perminute) to the lee of high mountain ranges, such as theRockies. Although mountain waves often generate severeturbulence, at times flight through waves may be remarkably“smooth” even when the intensity of downdrafts and updraftsis considerable. As these smooth conditions may occur atnight, or when an overcast exists, or when no distinctive cloudhas formed, the danger to aircraft is enhanced by the lack ofwarning of the unusual flight conditions.
Consider the circumstances of an aircraft flying parallel to amountain ridge on the downwind side and entering a smoothdowndraft. Although the aircraft starts descending becauseof the downdraft, as a result of the local drop in pressureassociated with the wave, both the rate of climb indicatorand the altimeter will not indicate a descent until the aircraftactually descends through a layer equal to the altimeter errorcaused by the mountain wave, and, in fact, both instrumentsmay actually indicate a “climb” for part of this descent;thus the fact that the aircraft is in a downdraft may not berecognized until after the aircraft passes through the originalflight pressure level which, in the downdraft, is closer to theground than previous to entering the wave.
1.5.8 Pressure Drop
The “drop” in pressure associated with the increase inwind speeds extends throughout the mountain wave, thatis downwind and to “heights” well above the mountains.Isolating the altimeter error caused solely by the mountainwave from error caused by non-standard temperatures wouldbe of little value to a pilot. Of main importance is that thecombination of mountain waves and non-standard temperaturemay result IN AN ALTIMETER OVERREADING BY ASMUCH AS 3 000 FT. If the aircraft in our example had beenflying upwind on a windy day, the actual ground separationon passing over the crest of the ridge may well have been verysmall.
1.5.9 Abnormally High Altimeter Settings
Cold dry air masses can produce barometric pressures inexcess of 31.00 in. of mercury. Because barometric readingsof 31.00 in. of mercury or higher rarely occur, most standardaltimeters do not permit setting of barometric pressures aboveApril 7, 2011
that level and are not calibrated to indicate accurate aircraftaltitude above 31.00 in. of mercury. As a result, most aircraftaltimeters cannot be set to provide accurate altitude readoutsto the pilot in these situations.
When aircraft operate in areas where the altimeter setting isin excess of 31.00 in. of mercury and the aircraft altimetercannot be set above 31.00 in. of mercury, the true altitude ofthe aircraft will be HIGHER than the indicated altitude.
Procedures for conducting flight operations in areas ofabnormally high altimeter settings are detailed in RAC 12.12.After reading this … i think you will understand that you are doing something wrong.
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In RL, When tower gives you the current local altimeter setting, it is up to date b/c they (the tower) has a barometer readout right there. It is the most accurate. The only time you make certain manual corrections is when the temperature is very cold (below 0 degrees).
The creator of the altimeter (cant remember his name atm) once said something that made a lot of sense… “It is only when you understand that this instrument (the altimeter) is a device for measuring pressure, and not altitude, that you will truley understand its capabilities and limitations…”