Altimeter setting

[Hippo]

I came across this just now...

  

On 5/28/2022 at 5:20 PM, icecold951 said:

I noticed last nightF-18 carrier takeoff, I don't have to set the barometric pressure now, it comes preset. I'm not sure if this was intended, or a mistake.

 

On 5/29/2022 at 1:01 AM, Tholozor said:

Added in this patch: https://www.digitalcombatsimulator.com/en/news/changelog/openbeta/2.7.12.23362/

 

I notice that it's still set to 29.92 for missions where the aircraft starts in the air.  Is this correct?  Wouldn't it be more likely for a regional QNH to be used rather than standard?  Can anyone confirm what standard USN F-18 procedure for this is?

If I'm going to get really pedantic about it, wouldn't it also be the case that the pressure would not be correctly set in a cold and dark aircraft since it would still have the setting from the previous flight, which would likely be different?

 

EDIT: P.S. Does changing the altimeter setting affect weapons delivery?  If so, does it have to be set to QNH or standard for accurate weapons delivery?

Edited June 4, 2022 by Hippo

[Phantom711]

29.92 is the standard setting above a certain altitude. What that altitude is depends on the country you are flying in. In the US it is 18000ft if I recall correctly. In Germany it is way lower and depends on the current local QNH but is usually around 5000ft or 6000ft for example.

The reason behind that is, that flights can safely be seperated from one another without having to change their altimeter every few minutes (since aircraft travel quite a distance in a relatively short time).

Long story short: the "procedure" depends on the region you are flying in. 

 

Yes, yo are right, in a cold/dark cockoit you would usually have the altimeter setting from the last flight set. This flight might just have been as little as an hour ago, so it might still be correct though.

 

"Does changing the altimeter setting affect weapons delivery?"

Interesting question, but I guess the altitude for weapons delivery comtutation is certainly taken from GPS/INS.

[Cepheus76]

18 hours ago, Phantom711 said:

Interesting question, but I guess the altitude for weapons delivery comtutation is certainly taken from GPS/INS.

I wouldn't think so, since the so called "vertical dilution of position", i.e. the difference between the aircraft's actual altitude and the altitude derived from GPS signals can be quite significant. In any case, GPS receivers can use pressure altitudes, i.e. the altitude above the 29.92 in/ hg plane,  inputs and thus use earth itself as pseudo-satelitte to calculate a position. In other words, a pressure input is used already used for position computations, but why use a calculated altitude if a measured one is already available?

INS inputs are equally unsuitable for altitude calculations, since the platform is subject to actual and apparent drift, which means that the INS fix loses accuary over time and requires periodical position updates.

TL; DR: GPS/ INS derived altitudes are not really suitable for measuring altitudes. Also why should one use such a system if there is a altimeter-system readily available?

[Phantom711]

@Cepheus76

Your statements regarding the inaccuracies of GPS and INS are certainly correct. However, you completely ommit the fact, that the barometric altimeter is inaccurate as well. Once you switch from the local QNH to standard it is not even meant to be "correct" anymore with regards to showing an "accurate" altitude above MSL, but it is rather just a common reference for everyone flying around above a certain altitude, so that they don`t hit each other.

Question is: How inaccurate is GPS at showing elevation?

Keep in mind, that GPS accuracy is predictable and can be prebriefed for a certain time/space.

 

Now the options I see here are as follows:

1. GPS elevation inacccuracy is negligable in most cases or can be mitigated by proper preflight planning or

2. In the target area the pilot switches to the briefed local QNH of that area for accurate weapons delivery or

3. The local altimeter of the target area is put into the system seperately on some "weapon delivery" page.

 

EDIT: Removed the example since my calculations were off...

Edited June 5, 2022 by Phantom711

[Cepheus76]

If I remember correctly, vertical dilution of position with GPS can easily be off by several hundred feet. Unfortunately, I am not actively flying for the time being, so I can't check/ provide a real world example.

Of course, differences between QNH and QNE can result in considerable differences between the indicated altitudes, however, I am willing to bet that there are always means to determine an accurate area pressure.

Before using a derived altitude from GPS/ INS data, it would be much better to use radar altimeter inputs to compute a weapon delivery solution- but that is just my own thought, I don't know how it is done in real life.

Edited June 5, 2022 by Cepheus76

[Spock14]

You have to understand the differnet "altitudes". 

An Altitude above MSL is usually refered as a Barometric altitude to local QNH. So because you are operating close to terrain you want to have the right QNH setting so your charts are valid etc. 

The "altitude" above ground or how it´s called in the Hornet, RADALT, is referd as Height. So this is the real distance between you and the ground. Not really useful if you operate around an airfield and the with constantly changing terrain. But it is useful for landing and minimum determination. 

At an certain altitude, which is called Transition Altitude, you switch your local QNH setting to standart QNH 2992inHg or 1013hpa. So now your altitude isn´t an altitutde anymore, it is a Flight Level. Terrain on an Flight Level is usually no factor so Flight Levels makes it easy to seperate Aircrafts enroute. Practically the readout on your altimeter would be wrong, but as long as everyone is wrong with the same offset, everyone is right again  . The Transition Altitude is dependend on the the country you are operating in. In the US it is 18.000ft. However as you are coming down from Flight Level 300, you will go through the Transition Level, were you switch back to local QNH. The Transition Level is always dependend on the local weather situation, more precisely the actual air pressure. So you want to have the Transition Level roughly 1000ft above the Transition Altitutde but imagine your local QNH is really low. So if you decrease the pressure, altitude rises and vice versa. So if you have a low local pressure you want to have the Transition Level a little higher so that if you switch down to the low pressure setting your altitude won´t be higher than your Transition Level. 

So to make a long story short everything below Transition Level should be in local QNH and everything above Transition Altitude should be standard QNH. So even though in real aircrafts the right QNH setting is hardly ever set already when you start up the plane, it is a good way in DCS to have the right setting on the ground already IMHO.

All the best spock 

[Bunny Clark]

16 hours ago, Spock14 said:

So to make a long story short everything below Transition Level should be in local QNH and everything above Transition Altitude should be standard QNH. So even though in real aircrafts the right QNH setting is hardly ever set already when you start up the plane, it is a good way in DCS to have the right setting on the ground already IMHO.

This is certainly true in commercial aviation (and I suppose technically true for GA as well, not that I've ever seen anything remotely close to 18,000 in my Cessna). But it's my understanding that military aircraft, or at least fighters, do not use a Transition Altitude and always keep their altimeters set to local QNH. 

[Swift.]

10 minutes ago, Bunny Clark said:

This is certainly true in commercial aviation (and I suppose technically true for GA as well, not that I've ever seen anything remotely close to 18,000 in my Cessna). But it's my understanding that military aircraft, or at least fighters, do not use a Transition Altitude and always keep their altimeters set to local QNH. 

I think it depends, there may well be a force QNH used in theatre or something like NTTR where you use Nellis QNH over the whole range.

[Hippo]

14 hours ago, Bunny Clark said:

This is certainly true in commercial aviation (and I suppose technically true for GA as well, not that I've ever seen anything remotely close to 18,000 in my Cessna). But it's my understanding that military aircraft, or at least fighters, do not use a Transition Altitude and always keep their altimeters set to local QNH. 

 

14 hours ago, Swift. said:

I think it depends, there may well be a force QNH used in theatre or something like NTTR where you use Nellis QNH over the whole range.

Thanks, this is actually the question I was trying to ask, sorry to everyone else if I didn't make it clear.  What do USN F-18 pilots actually do?  I find it very unlikely that they use the civil procedure outlined above, unless, I suppose, they are being separated by civil ATC from civilian aircraft.  I doubt they would be using flight levels while flying combat missions.  It would be nice to get a response from someone with real world (USN) experience.

 

Edited June 7, 2022 by Hippo

[Razor18]

Not that it matters too much, but in game "automatic" altimeter setting seems to be always 0.01 mmHg lower on the gauge as said on the frequency. For instance Mother said for Case II altimeter is 29.89, when I looked on the gauge, it was showing 29.88.

Just sayin'

[=475FG= Dawger]

QNH is the local pressure corrected to sea level. What you see on the altimeter is your altitude above sea level

QNE is pressure referenced to the International Standard Atmosphere. What you see on the altimeter is termed “Flight Level”

QFE is uncorrected local pressure. The altimeter indicates Height above the reporting station. 
 

All aircraft in the IFR structure above the transition level/altitude will be using QNE. 

[Razor18]

I understand that, but I wasn't talking about the altitude on the gauge in my post. All I was saying is, when Mother says altimeter pressure should be set to 29.89, the program still sets altimeter to 29.88 (automatically).

If we want to talk about altitude, when you set local QFE on any airfield as given in the briefing, or anywhere else, the needle will be a bit off from 0 altitude. You need to set pressure to 0.01 inHg higher (or lower, don't remember correctly) that the QFE value is given, if you want to see altitude needle to point exactly to 0.

So in short, pressure value is always off 0.01 inHg comparing value given vs value shown. No matter if its QNH, QFE or QNE. Hope this helps.

[Phantom711]

Am 6.6.2022 um 23:49 schrieb Bunny Clark:

This is certainly true in commercial aviation (and I suppose technically true for GA as well, not that I've ever seen anything remotely close to 18,000 in my Cessna). But it's my understanding that military aircraft, or at least fighters, do not use a Transition Altitude and always keep their altimeters set to local QNH. 

Sorry, but your „understanding“ is wrong then…

If the military operates aircraft (no matter which type) in civilian airspace, they will adhere to the standard civilian rules and procedures (for the most part). Otherwise deconfliction would be an impossible task for ATC. There might be exceptions to that laid out in some local procedures.

Even in a conflict scenario/wartime, your mission might take off in a peacetime airspace (i.e. Operation Allied Force, the Kosovo Operation, where many missions startet from Italy). And even if not, using solely local QNH is very impractical as soon as you want to cover larger distances, since then you would have to change the QNH setting every few minutes.

[Spock14]

vor 10 Stunden schrieb Phantom711:

Sorry, but your „understanding“ is wrong then…

If the military operates aircraft (no matter which type) in civilian airspace, they will adhere to the standard civilian rules and procedures (for the most part). Otherwise deconfliction would be an impossible task for ATC. There might be exceptions to that laid out in some local procedures.

Even in a conflict scenario/wartime, your mission might take off in a peacetime airspace (i.e. Operation Allied Force, the Kosovo Operation, where many missions startet from Italy). And even if not, using solely local QNH is very impractical as soon as you want to cover larger distances, since then you would have to change the QNH setting every few minutes.

Now I can give a correct answer to that.
Yes I agree, so for normal Admin flights especially in civilan airspace normal civilian procedures are used. On ranges local altimeter setting is used. However operating from the boat even if they are supposed to use TA/TL they sticked to local QNH most of the time from mother.

 

 

Am 7.6.2022 um 22:14 schrieb Razor18:

Not that it matters too much, but in game "automatic" altimeter setting seems to be always 0.01 mmHg lower on the gauge as said on the frequency. For instance Mother said for Case II altimeter is 29.89, when I looked on the gauge, it was showing 29.88.

Just sayin'

I have seen this too, but gives a little sense of realism sometimes altimeter settings change by a little

[Ahmed]

If it is always .01 mmHG lower it is not sense of realism but a bug. While real life METAR or ATIS may give you an outdated altimeter setting, ATC do give you a current reading. It would be a different thing if your altimeter is slightly miscalibrated internally (tolerances irl for this are surprisingly high).

[Swift.]

I dont have the brain power right now to figure it out, but is it perhaps taking the 'QFE' to the decks altitude, rather than QNH? Some 60ft difference

[Phantom711]

Am 9.6.2022 um 01:28 schrieb Spock14:

Now I can give a correct answer to that.
Yes I agree, so for normal Admin flights especially in civilan airspace normal civilian procedures are used. On ranges local altimeter setting is used. However operating from the boat even if they are supposed to use TA/TL they sticked to local QNH most of the time from mother.
 

I have zero real life experience with carrier ops, so could you elaborate further on that, please?!

 

Do they ALWAYS do it as you describe it? Or only when they are in the middle of the atlantic where there are no other assets involved, let alone civilian ATC...?

[Razor18]

It's not just on carrier, but also on land runways, so the difference is not just "deck height".

[Spock14]

Am 10.6.2022 um 17:32 schrieb Phantom711:

I have zero real life experience with carrier ops, so could you elaborate further on that, please?!

 

Do they ALWAYS do it as you describe it? Or only when they are in the middle of the atlantic where there are no other assets involved, let alone civilian ATC...?

So I guess the usual answer is, it depends. If they have a short flight to AO the just stick to local Altimeter setting evene though qoute "they are supposed to change at TA/TL". If they are flying for hours in civil airspace I guess ATC would mock them that they are on the wrong altitude

[Frederf]

Tactical flying is typically QNH. Most Tactical aircraft usually capture altimeter on takeoff and can smartly filter out QNE ops later. All ship ops reference the radalt so they'd never calibrate baro to the deck.

[AngelAtTheTomb]

Between this thread, previous discussions about just where the aircraft is getting its altitude reference for SA/RDR ATTK pages, and the editor still giving all aircraft a magically autoadjusted ISA atmosphere altimeter so they're all flying at different altitudes than you, it's clear that DCS needs to get some altimeter issues sorted.

 

Here's another one. I've been noticing a difference between my HUD altitude and my standy altimeter indication (with HUD altitude set to BARO, obviously). I was up at 42,000 feet on the HUD yesterday and the standby altimeter was showing 41,700. Why would these two be different? Doesn't the HUD get its altitude information from the CADC, which gets it FROM the altimeter? Is this a bug, or a feature?

[Frederf]

The HUD is from ADC (CADC is F-16) and ADC gets its Kollsman setting from standby altimeter but not the altitude. Basically ADC calculates altitude from the air data probes, standby Kollsman, temperature, Mach, etc. and calculates a number to send to HUD. Standby is purely mechanical so it shows an old school value not subject to all the adjustments of the ADC and that's why it differs.

[AngelAtTheTomb]

Interesting. All my real-world experience is civil aviation, where keeping altimeters consistent between different aircraft is probably more important than knowing you are EXACTLY this many feet from the ground. Civil airliners have fancy CADCs/ADIRUs/etc., but they still display the "old school" altitude value so you don't have one aircraft thinking it's at 5,000 feet because it uses an old pneumatic altimeter and another wandering into it because its advanced, computed, corrected altimeter at 6,000 feet uses different corrections. The 300 foot difference I noted in the F18 is not nothing, especially with RVSM over the oceans.

 

It makes me question again how exactly military aircraft ensure they're on a Flight Level. Follow the HUD or the standby?

[Frederf]

Often in nav (not AA or AG) the altitude reference will be more old school compared to attack modes. I don't know if the F-18 does that or not.

Maybe F-18s don't do RVSM. Also you can usually look up the raw pressure alt data in a display from the air data computer somehow directly even if it's not the value on the HUD.

Heck I wouldn't be surprised if Airbuses did all those adc corrections too.

[=475FG= Dawger]

At 41,000 feet, the ADC altitude differed from the baro standby about 700 feet in everything I flew. If you lose one ADC you have to leave RVSM airspace unless you have three on board, all certified.

Domestic aircraft generally only have two.