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Topic: The Reason Of Calculating Pressure Altitude
Username: mawingho
Posted 2012-09-06 07:26:14 and read 6039 times.

ISA MSL is 1013hPa.
Pressure altitue is vertical distance above 1013hPa pressure level, at about 1 hPa per 30 feet gain in height in the lower levels of the atmosphere.
We also have QNH and the local(aerodrome) QNH.

To determine the pressure altitude on the surface of an airfield which has an elevation of 1,875 ft if the QNH is 1,020 hPa.

Different between ISA MSL pressure and QNH = 1013 - 1020 = -7hPa
Different in pressure altitude = -7 x 30ft / hPa = -210 ft
Aerodrome elevation = 1,875
Therefore pressure altitude = 1875 - 210 = 1,665 ft.

Since we already know the altitude of aerodrome, why don't we just set the altimeter to 0 when we are on the ground of the aerodrome and if we fly within the local area.

When we need to fly to different areas, and above the certain altitude (e.g., 10,000 ft), then we just set the altimeter (subscale) to use the standard atmosphere which is 1013hPa.

I have read the page
https://www.brisbanehotairballooning.com.au/faqs/exam-help/133-pressure-density-height.html

even it does explain the reason of calculating it here "We can determine pressure height by either: reading the altimeter with 1013 set in the sub-scale; or by using the difference between QNH and 1013 to convert altitude to pressure height. If actual sea level pressure differs from the standard atmosphere of 1013 hPa, then a simple diagram will help us with any calculations of pressure height. We convert altitude to pressure height by allowing 30 feet for each 1 hPa pressure difference."

but when I look at the picture in EXAMPLE 1, and we got
Elevation = 670 feet
QNH = 1020 hPa
Pressure Height = 670 - 7 x 30 = 460 feet

Why we need to know the Pressure Height between MSL and the aerodrome level which is (460 ft), in that case, should I set the altimeter to 1020 QNH in the subscale?

I don't know how this useful to me.

[Edited 2012-09-06 07:30:52]

Topic: RE: The Reason Of Calculating Pressure Altitude
Username: tdscanuck
Posted 2012-09-06 07:52:46 and read 6011 times.

Quoting mawingho (Thread starter):
Since we already know the altitude of aerodrome, why don't we just set the altimeter to 0 when we are on the ground of the aerodrome and if we fly within the local area.

That's QFE. It's used some places but it's not terribly common. On most airliners you've got radio altitude anyway, which is close to QFE except in very hilly terrain.

The reason to not use altitude above ground level (AGL), most of the time, is that your airplane's performance has everything to do with pressure altitude and nothing to do with AGL. 0' AGL in Denver is wildly different than 0' AGL in San Francisco.

Some additional information here:
QNE, QNH, QFE (by Tigermoth Nov 25 2005 in Tech Ops)

Tom.

Topic: RE: The Reason Of Calculating Pressure Altitude
Username: Pihero
Posted 2012-09-06 11:08:23 and read 5943 times.

Quoting mawingho (Thread starter):
Since we already know the altitude of aerodrome, why don't we just set the altimeter to 0 when we are on the ground of the aerodrome and if we fly within the local area.

As T6om said, that's the QFE.
The problem is, the higher the aerodrome elevation, the lower the setting and this leads to inherent system errors and potential mis-setting errors., either through computing the QFE value ( and you would need a standard altimetry computer - can't use the 1 hPa ~30 ft gross equivalent ) or mis'setting the instrument
For instance the elevation of 1875 ft, will it be equivalent to 60 hPa... 65 ... what would be the effect of temperature ?

It is quite interesting that your link portrays the (in-) famous Kollsman two-needled / two indexed QNE altimeter. : the indexes give the pressure altitude (Note that they indicate zero when 1013 hPa ( then mb) is set)... So if one dials the airfield altitude on the index, it will show zero on the ground.... That's jolly fine... except when someone mistakes the small index with the bigger one... BOOM !

Quoting mawingho (Thread starter):
Why we need to know the Pressure Height between MSL and the aerodrome level

One has to be very precise when dealing with altimetry.
1/- An altimeter only gives the height of the column of air between the reference surface ( here called "altimeter setting " ) and the position of said altimeter... That value is highly inaccurate away from the position it was computed at.
2/- This value is called altitude if the ref surface is the QNH, pressure altitude if referred to 1013.2 hPa, and QFE if referred to the pressure at the airfield reference point.
That's what one needs to know... plus how to compute actual altitude when flying over high terrain, especially in cold conditions.
Just remember that an altimeter is mainly used to give a common reference to airplanes evoluting in the same airspace so that they could be separated from one another and a reference over terrain at short distances from a runway.

To give more importance to an altimeter is asking for big trouble.

Topic: RE: The Reason Of Calculating Pressure Altitude
Username: rwessel
Posted 2012-09-06 12:02:07 and read 5921 times.

One place it's fairly common to use QFE is in local glider flying, where the actual distance to the ground is the primary datum of interest.

Topic: RE: The Reason Of Calculating Pressure Altitude
Username: Glom
Posted 2012-09-07 05:52:51 and read 5793 times.

We use QFE in the UK, but I think it's really a bad habit that we need to shed. We use it when staying in the circuit and also when arriving at an aerodrome. The problem is that we are conditioned to understand 0 ft as the reading when we land. What happens if we forget to set it? There have also been controlled airspace infringements due to an aircraft departing from the circuit and forgetting to reset QNH. If we just got used to the idea of always using QNH and we understand we needed to always take into account aerodrome elevation, then it would mitigate many of these mistakes.

Topic: RE: The Reason Of Calculating Pressure Altitude
Username: CosmicCruiser
Posted 2012-09-07 07:32:36 and read 5763 times.

Of all my flying into and out of UK I've never seen nor heard QFE settings given either by ATIS or the tower. Granted I'm not doing circuits as a general aviation pilot but I've never seen it referenced in the AOI either. Interesting. I've only flown into one airport that uses QFE and that's Almaty Kz. Going thru the Trans Lvl and seeing the altimeter jump over 2000' ft is dis-concerning at the least and can be darn right confusing. Since most data is based on a normal std of sea lvl I see no real reason for QFE at all with the possible exception of pattern work as Glom said but then just make your pattern alt reference SL. As some else noted any reference to arpt or touchdown zone is easily shown on the RA. Granted the RA can give you some interesting readouts in hilly terrain but at least it's accurate. A baro alt. set to QFE would reference nothing except HAT.

Topic: RE: The Reason Of Calculating Pressure Altitude
Username: Pihero
Posted 2012-09-07 08:18:21 and read 5751 times.

Quoting CosmicCruiser (Reply 5):
I've only flown into one airport that uses QFE and that's Almaty Kz.

It used to be a feature of the ex- soviet union... It is still the procedure in Russia, Ukraine...etc...
In Moscow, below transition level you're given an altitude ( QNH ) then when you're turning into final, a QFE setting and a height. One really needs sharp altimetry discipline.
In France, QFE setting had only been discontinued in the mid-seventies, IIRC;

Topic: RE: The Reason Of Calculating Pressure Altitude
Username: bueb0g
Posted 2012-09-07 10:39:31 and read 5704 times.

Quoting tdscanuck (Reply 1):
That's QFE. It's used some places but it's not terribly common. On most airliners you've got radio altitude anyway, which is close to QFE except in very hilly terrain.

Yup, not common on airliners but we use it a lot with GA flying over here.

Quoting CosmicCruiser (Reply 5):
Of all my flying into and out of UK I've never seen nor heard QFE settings given either by ATIS or the tower. Granted I'm not doing circuits as a general aviation pilot but I've never seen it referenced in the AOI either.

Yeah, it's a GA thing. Generally if you're in a circuit it's 1000 ft on the QFE, and often if you're overflying an airfield, or flying through a MATZ, the controller will give you a QFE and a height to stick to while you pass the zone, and give you back the area QNH once you're clear.

Topic: RE: The Reason Of Calculating Pressure Altitude
Username: CosmicCruiser
Posted 2012-09-07 16:20:03 and read 5668 times.

Quoting Pihero (Reply 6):
you're given an altitude ( QNH ) then when you're turning into final, a QFE setting and a height.

In ALA we took the QFE alit. setting and made the conversion from a chart to QNH. They also were using meters. Last time in there (thankfully it was last one) they were feet above the Trans Lvl and meters below. weird.

Topic: RE: The Reason Of Calculating Pressure Altitude
Username: Fabo
Posted 2012-09-07 16:20:08 and read 5668 times.

Quoting CosmicCruiser (Reply 5):

Having the 0 on runway level does allow for simpler procedure bulding, standardisation, and makes for an easier mind picture building. Terminal procedures in Soviet and ex-Soviet aviation were largely bulit around the pattern concept, thus allowing for a rather standard mental image of where the pilot is, relative to the airport, and how high he should be. Makes for simpler flying into new aiports (when not in mountainous terrain) and for simpler non-precision approach flying.

It does make sense for flying in terminal area, but I agree that it makes for more trouble than it is worth, especially nowadays with RNAV, RNP, VNAV and whatnot.

Topic: RE: The Reason Of Calculating Pressure Altitude
Username: CosmicCruiser
Posted 2012-09-07 19:51:56 and read 5638 times.

that's what RA are for.  

Topic: RE: The Reason Of Calculating Pressure Altitude
Username: zeke
Posted 2012-09-07 20:32:19 and read 5631 times.

Quoting mawingho (Thread starter):
We also have QNH and the local(aerodrome) QNH.

When you are QNH, and local(aerodrome) QNH, you really mean area QNH and local QNH. Area QNH is just the lowest QNH observed in across the area. We use area QNH as that is a common datum in that area where aircraft can maintain a altitude reference to that datum, and following the normal rules, have vertical separation. If one aircraft is flying off the standard pressure, and another off local or area QNH, it is possible that they could have a loss of vertical separation.

Quoting mawingho (Thread starter):
Since we already know the altitude of aerodrome, why don't we just set the altimeter to 0 when we are on the ground of the aerodrome and if we fly within the local area.

At non controlled aerodromes it is perfectly acceptable to set the airfield elevation, the altimeter should be accurate enough to be close enough to area QNH. If moving away from the aerodrome you should use the area QNH from a Wx service or ATC.

Quoting mawingho (Thread starter):

When we need to fly to different areas, and above the certain altitude (e.g., 10,000 ft), then we just set the altimeter (subscale) to use the standard atmosphere which is 1013hPa.

1013 is the "standard" setting, depending on where you flying the world, the "transition" from local/area QNH to "standard" can range from say 4000 ft to 18,000 ft. The aircraft transponder however always works on 1013, the ATC displays calibrate the altitude readout with area QNH.

When flying on QNH we are flying "Altitudes", when flying with "standard" above transition, you are flying 'Flight Levels", these are common datums all aircraft are flying with, it gives vertical separation.

Quoting mawingho (Thread starter):

Why we need to know the Pressure Height between MSL and the aerodrome level which is (460 ft), in that case, should I set the altimeter to 1020 QNH in the subscale?

The main reason for calculating pressure heights is that is a common input used on aircraft performance charts, normally the charts have a pressure height and temperature input, which is then is working with density height. Yes you would use 1020 on the sub-scale, and you should read close to the airfield elevation on your altimeter.

Quoting Pihero (Reply 6):

It used to be a feature of the ex- soviet union... It is still the procedure in Russia, Ukraine...etc...
In Moscow, below transition level you're given an altitude ( QNH ) then when you're turning into final, a QFE setting and a height. One really needs sharp altimetry discipline.

One of the main reasons it is used there is for cold Wx operations. Altimeter errors are worst when it is very cold, and it is the wrong way, i.e. it over reads, 200 ft on the altimeter could actually be 150 ft. Using QFE changes the theoretical column of air being looked at instead of being from sea level past airport elevation to the aircraft, to runway elevation to aircraft, the errors are not as great. In Russia normally they have a different QFE for each runway.

Topic: RE: The Reason Of Calculating Pressure Altitude
Username: Glom
Posted 2012-09-08 01:45:46 and read 5591 times.

Quoting CosmicCruiser (Reply 5):
Of all my flying into and out of UK I've never seen nor heard QFE settings given either by ATIS or the tower.

I should have been clearer. I meant specifically us l'il guys in a SEP planes flying VFR. The big boys won't use it. Even when we get to doing IMC ratings* we use QNH, which is a big example of what's wrong with teaching the use of QFE. If I, as an IMC rated pilot, encounter bad weather, enter IMC and divert somewhere for an instrument approach, I'll fly the approach with QNH on my altimeter, break cloud and proceed to land. Except I'll start getting confused that my altimeter is showing a few hundred feet more than expected because I'm forgetting that it's showing QNH still.

Also, the military are pretty bad for using it.

* IMC rating is a UK rating that might be characterised as the poor man's instrument rating. It's confers some limited IFR privileges on the holder essentially to allow him to continue a flight that was planned VFR but has since suffered from unexpected deterioration of the weather.

Quoting Fabo (Reply 9):
Having the 0 on runway level does allow for simpler procedure bulding, standardisation, and makes for an easier mind picture building.

Yes but that applies to many things. A typical cat I decision height of 200 ft is a height, but anyone flying it will need to incorporate that into his altitude since he'll be flying on the QNH.

Topic: RE: The Reason Of Calculating Pressure Altitude
Username: Pihero
Posted 2012-09-08 06:06:58 and read 5564 times.

Quoting Glom (Reply 12):
A typical cat I decision height of 200 ft is a height, but anyone flying it will need to incorporate that into his altitude since he'll be flying on the QNH.

Don't need to for the Decision Altitude or the OCA will be shown on the plates and so will be the glide paths altitudes.

Topic: RE: The Reason Of Calculating Pressure Altitude
Username: CosmicCruiser
Posted 2012-09-08 09:56:44 and read 5519 times.

Quoting Pihero (Reply 13):
Don't need to for the Decision Altitude or the OCA will be shown on the plates and so will be the glide paths altitudes.

All in QNH, right Pihero!


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