Altimeter confusion...

[Nealius]

I am running into some problems with the altimeter being inconsistent. For starters, I have a mission starting out at Sochi-Adler with a QNH of 750mb set in the ME. Converted to in.Hg that should be 22.15 to get airfield elevation of 98ft. However when I am on the ramp and I set my altimeter to 95~100ft, it reads 29.50 or 29.51. Why does the QNH in the ME not match the actual QNH?

 

Second, I have waypoints on my flightplan set to 20,000ft MSL in the ME. Above 14,000ft I always transition to 29.92 for MSL readings (what's the TA for Georgia, anyway?). Yet when I fly at 20,000ft MSL, my waypoints are a good bit higher than I am! Why isn't MSL altitude in the ME matching actual MSL?

[Balion]

For starters, I have a mission starting out at Sochi-Adler with a QNH of 750mb set in the ME.

It's 750 mm of mercury, not millibars. 750 mm = 29.53 inches.

 

Second, I have waypoints on my flightplan set to 20,000ft MSL in the ME. Above 14,000ft I always transition to 29.92 for MSL readings (what's the TA for Georgia, anyway?). Yet when I fly at 20,000ft MSL, my waypoints are a good bit higher than I am! Why isn't MSL altitude in the ME matching actual MSL?

You don't get actual altitude with altimeter 29.92, rather you get pressure altitude or flight level. You will only get actual MSL altitude if altimeter is set to QNH.

[Nealius]

...well that's odd. Does anyone even use mm of mercury? I thought the gold standards were millibars and inches. Still, ED could have at least marked what unit they were using on that thing.

[Eddie]

Does anyone even use mm of mercury?

 

The Russians.

 

mmHG is not an appropriate unit for the A-10C, but then nor is QFE, and we're stuck with both in DCS.

 

The fact that DCS doesn't use a uniform system of units depending on the user settings and the aircraft in use has long been a source of frustration for many. Maybe one day it'll change, who knows.

[NhiTrac]

Silly question. When tower gives you the QFE reading, do I use that to adjust the altimeter using the little knob?

[ralfidude]

yup. For that airfield.

[camsr]

Yes that's what it's for. I don't know the exact details though, maybe someone does? All I know is don't adjust the QFE dial while in autopilot :smilewink:

[camsr]

yup. For that airfield.

 

So that means 0 altitude is the runway?

[GRUNT -Shrek-]

It's in the wiki..

 

Why does the QNH in the ME not match the actual QNH?

 

 

QNH is dialed in at start (as set by the mission editor or generated through current dynamic weather).

 

QFE is published by ATC.

 

Maybe you are referring to QNE?

 

Transition altitude differs, in Germany is IMHO ~4,000ft-5,000ft.

 

So when loitering, you should be in Flight Levels. But I doubt you dial in your altimeter when going 2,000ft for a gun ride.

 

Some time ago I did analysis of the mission editor and dealt with such problems: http://en.wiki.eagle.ru/wiki/Mission_Editor:_Weather_Modelling

 

 

QNH

 

Note: if you/pilots are annoyed that their altitude readouts differ esp. in high altitudes you might take a closer look at this section.'

 

This setting is crucial to altimeter settings. Pilots might be fine, as commonly experienced, as long as altimeter differences might be visually egalized. Since ATC publish QFE this also has become a common behaviour to dial in the QFE (by zeroizing the altimeter on the runway). In case of IFR weather conditions however this setting might have an amazing impact on pilot's survival rate trying to land on a different airfield or finding the correct altitude of tankers and flight group members. Again consider an offset up to 520m / 1,700ft from Beslan to Batumi.

 

Note: I want to encourage Missionbuilders to publish the QNH in mission briefing. I want to encourage pilots to follow a common rule of pressure settings.

 

 

QNH - quick overview

You might have realized that the mission editor takes QNH in 720-790. These numbers are to be taken as torr (which is, indeed mmHg, but for the matter of simplicity I keep nameing it torr) after the following overview. The A-10C known to amazingly mess around with non-SI-units takes inHg (inches of mercury) which are around 30. To add the confusion, Ka-50 dials the altimeter into values around 79, which is simply significant values of torr/mmHg (decitorr)

 

 

significance torr/mmHg hPa inHg

min. A-10C 709 945 27.91

min. ME 720 960 28.35

default press 760 1013 29.91

max. ME 790 1053 31.10

max. A-10C 792 1057 31.20

[/TABLE]

QNH - number crunching

base unit factor target unit

1hPa × 0.750061505 torr/hPa = 0.750061505torr

1hPa × 0.02952997144 inHg/hPa = 0.02952997144inHg

1inHg ×33.8639 hPa/inHg = 33.8639hPa

1inHg ×25.4000078torr/inHg = 25.4000078torr

1torr × 1.333224 hPa/torr = 1.333224hPa

1torr × 0.03937006665inHg/torr = 0.03937006665inHg

Note: most usefull might be the first factor. Assuming you get a METAR air pressure (QNH) in hPa. Multiplicate this value by 0.75 (that is ×3 and ÷4 afterwards) and you'll get a pretty good fitting approximation of QNH in torr.

 

About QNH/TA/TRL/FL

There are a lot of different regulations for setting up flight levels. I order to coordinate attacks, formation flying, air-to-air-refueling the concept of setting the altimeter to norm pressure (1013hPa/29.91mmHg) might add an interesting feature to the simulation.

You might also want to take a look at en.wikipedia.org/wiki/Flight_level

Suggestions:

 

1. QNH is published in the briefing. Pilots use QNH all the time and need to know the field elevation of runways in use.
   
2. QNH is published and used by the pilots. TA/TRL is set to a simple value e.g. 5,000ft (as in Germany)
   

Edited January 20, 2014 by GRUNT -=Shrek=-

[howie87]

It's a setting for your altimeter that'll make your dial point at zero when you're on the ground on that particular airfield. Other than that, its bad practice to use QFE. It's basicly putting an incorrect setting in your altimeter that's absolutely pointless. Hardly anywhere in the world QFE is used, except supposedly in Russia, but I can't confirm that.

 

Funnily enough I flew on QFE this Saturday right here in the UK. I was doing circuits, which QFE is perfect for. It's easy to see at a glance how far above the runway you are while you're in the pattern. QFE, QNH and QNE all have their place.

[Tucano_uy]

 

The fact that DCS doesn't use a uniform system of units depending on the user settings and the aircraft in use has long been a source of frustration for many. Maybe one day it'll change, who knows.

 

At least, the unit used should be displayed ALWAYS by the figure, in the ME, in the external views, etc.

 

EDIT: To be fair most units are there, my gripe has always been with the QNH and the aircraft speeds (what speed is that in the ME and on the outside views? IAS, TAS, GS? probably TAS)

Edited January 20, 2014 by Tucano_uy

[Derbysieger]

Outside view in the A-10C is groundspeed in knots. You can easily check with the TAD

[Nealius]

ME is TAS, from what I can tell.

[Eagle7907]

It's a setting for your altimeter that'll make your dial point at zero when you're on the ground on that particular airfield. Other than that, its bad practice to use QFE. It's basicly putting an incorrect setting in your altimeter that's absolutely pointless. Hardly anywhere in the world QFE is used, except supposedly in Russia, but I can't confirm that.

 

Actually, IRL American Airlines does this. Not sure if it's for certain airports specifically for special go-around and departure procedures in the event of engine failure? But that's the only occasion I can think of. AE or now Envoy (stupid name) does not do this but our dispatchers sometimes sends data to us from AA. Why? I dunno. I fly the plane.

[Glamdring]

Actually, IRL American Airlines does this. Not sure if it's for certain airports specifically for special go-around and departure procedures in the event of engine failure? But that's the only occasion I can think of. AE or now Envoy (stupid name) does not do this but our dispatchers sometimes sends data to us from AA. Why? I dunno. I fly the plane.

 

I doubt that AA would use QFE when everybody else around them is using QNH.

[MTFDarkEagle]

I doubt that AA would use QFE when everybody else around them is using QNH.

This.

 

Everyone has to be on the same altimeter setting. Could you imagine the possible chaos?

Airport elevation can be found on the charts. Ground clearance (if an issue) can be found on the charts.

 

Eagle: what makes you say that AA uses QFE? It just does not make sense, at all.

[howie87]

I wish DCS had ATIS...

[howie87]

Well, actually it's more a choice of pilot discretion. I knew I wouldn't be leaving the circuit and the terrain around EGNX is pretty flat, so I went with QFE.

[Tucano_uy]

I wish DCS had ATIS...

 

+1

 

DCS ATC needs sooo much more love...

[Flamin_Squirrel]

Yes, some airfields in the UK use QFE. That doesn't mean it's wrong. It's bad practice and insults the intelligence of pilots.

 

I only encountered a QFE once in a UK simulator with an UK instructor. I spoke with Vsq buddy who flies in the UK, he's never encountered QFE. So I thought the UK finally stopped using it. So yeah, it's a rare thing. And there's an obvious reason it's rare.

 

It's not rare at all. Flying VFR I've always been issued QFE when I'm in or joining the circuit. It makes flying the circuit easier as you simply fly the circuit at 1,000ft. You'll be issued QNH before departure if you're leaving the airfield however.

 

You'll also be issued QNH when transiting MATZ (military aerodrome traffic zone).

[Eagle7907]

This.

 

Everyone has to be on the same altimeter setting. Could you imagine the possible chaos?

Airport elevation can be found on the charts. Ground clearance (if an issue) can be found on the charts.

 

Eagle: what makes you say that AA uses QFE? It just does not make sense, at all.

 

 

Because I work for AE, I fly Embraers. I have seen the ACARS print out. AE does not use it, but there are times when dispatchers forget and it slips through to us. From what I understand the Pilot flying would use QFE and Pilot monitoring would use QNH with transponder 2 used.

 

It can be done.

[Eagle7907]

And don't get me wrong. I'm not saying QFE is better. That's what the RA is for. I think ATC should be advertising QNH.

[Nealius]

The only time I've seen QFE in GA here in the US is with gliders, and the pilots there told me that they use QFE when flying locally, QNH when flying cross-country.

 

Don't you need QNH to calculate density altitude anyway?

[xxJohnxx]

It's not rare at all. Flying VFR I've always been issued QFE when I'm in or joining the circuit. It makes flying the circuit easier as you simply fly the circuit at 1,000ft. You'll be issued QNH before departure if you're leaving the airfield however.

 

You'll also be issued QNH when transiting MATZ (military aerodrome traffic zone).

 

On the ohter hand, over here in Austria/Germany/Switzerland you will always get QNH. The only one using QFE at some airports are the gliders.

[Eagle7907]

The only time I've seen QFE in GA here in the US is with gliders, and the pilots there told me that they use QFE when flying locally, QNH when flying cross-country.

 

Don't you need QNH to calculate density altitude anyway?

 

 

Correct. QNH and temperature.