DAMN IT. I JUST DONT GET QFE 2 QNH SETTINGS in F-5E - Page 7 - ED Forums
 


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Old 02-14-2018, 09:28 AM   #61
Cake
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Originally Posted by Raptor9 View Post
Prior to takeoff, or even en-route, just reference the F10 map and look at the airfield info for your destination. Whatever you see in the field elevation, that is the altitude you will see on your barometric altimeter upon touchdown. If you want a meaningful decision altitude on an instrument approach on when you should execute a missed approach if you don't see the runway, simply add 200 feet to the field elevation. When you see this value on your altimeter on the approach, execute a missed approach and try again, or proceed to your alternate airfield.

What I'm trying to explain to you is NOT to use the QFE. It's not a common practice in a lot of ICAO countries, and is never used (as far as I've experienced) in FAA airspace. I know DCS World uses it, but DCS has been using pretty much the same ATC system for almost 10 years. And it is not accurate at all to real-world ATC, it's at best a placeholder system.



Some aircraft aren't even designed to be used in this way. The reason the standard barometric altimeter used in the F-5 (and a lot of other western aircraft) can't go that low on the pressure setting knob is because they weren't DESIGNED to, because western flight procedures don't use it. But DCS started being based around Russian regions and Russian aircraft, which do use QFE. And unfortunately the ATC system has never been updated to account for western maps like Nevada or western flight procedures.
Ive never seen QFE used in North America.

When you get the altitude using F10, what is it, true, indicated, pressure, or denisty altitude?

Without ever paying attention, I was kind of thinking it must be true altitude. If this is the case, referencing it to set the altimeter is a bad idea at altitude, because it is not safe to assume standard temperature and you will get errors magnified down low where accuracy is even more critical.

Using normal settings on the altimeter, if it’s colder than standard, pressure levels are more tightly packed, so your true altitude will be less than indicated altitude. This is why approaches sometimes require cold weather correction.

I’m with you on most of this, and Especially on your recomendation not to use QFE. My preference would be to stick with the western convention for local altimeter and pressure altitude afte climbing above the transition level.
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Old 02-14-2018, 09:45 AM   #62
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Originally Posted by bbrz View Post
When flying into Moscow ATC always issues the QFE. After receiving the QFE during the descend you ask for the QNH. After that you set the QNH as usual and use the conversion table that's provided.

Temperature correction is only applied for low temp.
Thanks for that... So, when you say ATC, are you talking the tower, approach, or what? Just curious because if you have two airfields at different elevation nearby, and aircraft all using QFE going into each are talking to the same controller, this could be a great source of confusion and could lead to worse. If we’re just talking about traffic for one airport, fine, but otherwise it’s a bad idea all around.

It’s also confusing to think some airplanes will be using QNH and others QFE. More potential for misunderstandings.
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Old 02-14-2018, 04:12 PM   #63
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Originally Posted by Cake View Post
Thanks for that... So, when you say ATC, are you talking the tower, approach, or what? Just curious because if you have two airfields at different elevation nearby, and aircraft all using QFE going into each are talking to the same controller, this could be a great source of confusion and could lead to worse. If we’re just talking about traffic for one airport, fine, but otherwise it’s a bad idea all around.

It’s also confusing to think some airplanes will be using QNH and others QFE. More potential for misunderstandings.
This situation is not unique. SoCal approach works dozens of airports and local altimeter settings can vary by quite a bit despite the relatively close proximity. As an example, it would not be uncommon for one aircraft to be given a QNH of 29.78 at say, Victorville, while another aircraft on the same frequency might be given a QNH of 29.99 for their approach into Ontario. The ATC in DCS is not really set up for something like this which is why I don’t use the dynamic weather option and just set the QNH given in the briefing.
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Old 02-14-2018, 05:28 PM   #64
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Originally Posted by Midnightzulu View Post
This situation is not unique. SoCal approach works dozens of airports and local altimeter settings can vary by quite a bit despite the relatively close proximity. As an example, it would not be uncommon for one aircraft to be given a QNH of 29.78 at say, Victorville, while another aircraft on the same frequency might be given a QNH of 29.99 for their approach into Ontario. The ATC in DCS is not really set up for something like this which is why I don’t use the dynamic weather option and just set the QNH given in the briefing.
I get where you are going with this Midnight but I think this is apples to oranges. I won't disagree that there can be differences between the altimeter settings at the fields SoCal TRACON covers (even for a single controller). But lets do some math. 29.78 (1008.5mb) to 29.99 (1015.6). Granted, I admit this is a large discrepancy (basically 200 feet) between indicated altitudes of airplanes that might only be 34.2 nm (ONT to VCV, 63 km) apart.

However, consider that according to Ahrens' "Meteorology Today" 8th edition a storm in 1998 flipped over trucks and ripped the roofs off houses with a pressure gradient of only 32 mb per 500 km (1 mb per 16 km). Google pressure gradient force and you will find a set of slides from the University of California Irvine that associates tornadoes and hurricanes with pressure gradients of 1 mb per 6 km. Your example has a pressure gradient of 7.1 mb per 63 km, or 1 mb per 9 km. So your example requires something WELL beyond a normal Santa Ana wind and closer to a hurricane to be valid. (1/16 for 90+ knot winds, 1/9 for your example, 1/6 for a hurricane).

Conversely, take Centennial airport in Denver and Denver International. They are 19 nm apart and have a field elevation difference of 451 feet. If Denver approach used QFE, aircraft taking off 19 nm apart would have a difference of 451 feet between their indicated altitudes at the moment of takeoff, every single time. Then mix in all the VFR traffic in the area (just like in SoCal) and imagine trying to deconflict that as a controller. Of course, if it is like Bbrz says and a single controller rarely works multiple fields, this isn't a problem, but in your example this isn't the case.

Maybe I am missing something here or I have some math or meteorology wrong (please point it out if I do!) but I think Cake has it correct here. Sure there can be some slight variations with QNH across a sector but NOTHING like QFE would create.
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Old 02-14-2018, 08:33 PM   #65
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Originally Posted by tom_19d View Post
Maybe I am missing something here or I have some math or meteorology wrong (please point it out if I do!) but I think Cake has it correct here. Sure there can be some slight variations with QNH across a sector but NOTHING like QFE would create.
Yes, thanks. This is exactly my point. Not a problem below the flight levels using QNH, but QFE is a problem, and I'm sure that's why they don't do it in the states.

Another example of two nearby airports is Boise (KBOI) and McCall ID (KMYL). It's just a short 80 nm hop between Boise (2871.4 ft. / 875.2 m) and MCall (5024.2 ft. / 1531 m). Salt Lake City Center handles MYL and BOI, but BOI is also handled by Big Sky Approach. Using QNH, the setting betwwen the two will most likely differ, but not usually enough to cause a problem

Let's say we use QFE. If two jets are flying between the two airports in opposite directions they are likely less than 10 minutes apart. Say the aircraft leaving BOI is using BOI QFE and indicating 10,000 ft. Assuming ISA, true altitude would be about 12,900 ft. An aircraft leaving MYL using QFE and indicating 8,000 ft would be at approximately the same true altitude. See the problem?

Also, MSA is probably something like 11,000ft MSL in between them.

Talk about confusing. Now imagine a lost comms situation, aircraft to aircraft communication, or inexperienced pilots using the wrong settings for an approach, or a diversion after a missed approach. I think the QFE system is WAY less safe. I understand it is used in places, but just to have the field indicate zero?????

As far as DCS is concerned, I think we should be getting the way it is done in the local we are flying. I also think think there should be options to:

Respond to the radio checkin with
(a) the altimeter given or
(b) a QNH request if appropriate.

Furthermore, I think the act of responding through the DCS comms menu should also set the aircraft altimeter(s) accordingly.
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Old 02-16-2018, 11:00 PM   #66
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Originally Posted by tom_19d View Post
I get where you are going with this Midnight but I think this is apples to oranges. I won't disagree that there can be differences between the altimeter settings at the fields SoCal TRACON covers (even for a single controller). But lets do some math. 29.78 (1008.5mb) to 29.99 (1015.6). Granted, I admit this is a large discrepancy (basically 200 feet) between indicated altitudes of airplanes that might only be 34.2 nm (ONT to VCV, 63 km) apart.

However, consider that according to Ahrens' "Meteorology Today" 8th edition a storm in 1998 flipped over trucks and ripped the roofs off houses with a pressure gradient of only 32 mb per 500 km (1 mb per 16 km). Google pressure gradient force and you will find a set of slides from the University of California Irvine that associates tornadoes and hurricanes with pressure gradients of 1 mb per 6 km. Your example has a pressure gradient of 7.1 mb per 63 km, or 1 mb per 9 km. So your example requires something WELL beyond a normal Santa Ana wind and closer to a hurricane to be valid. (1/16 for 90+ knot winds, 1/9 for your example, 1/6 for a hurricane).

Conversely, take Centennial airport in Denver and Denver International. They are 19 nm apart and have a field elevation difference of 451 feet. If Denver approach used QFE, aircraft taking off 19 nm apart would have a difference of 451 feet between their indicated altitudes at the moment of takeoff, every single time. Then mix in all the VFR traffic in the area (just like in SoCal) and imagine trying to deconflict that as a controller. Of course, if it is like Bbrz says and a single controller rarely works multiple fields, this isn't a problem, but in your example this isn't the case.

Maybe I am missing something here or I have some math or meteorology wrong (please point it out if I do!) but I think Cake has it correct here. Sure there can be some slight variations with QNH across a sector but NOTHING like QFE would create.
Holy cow that's some detailed analysis! My only point was that the ATC system in DCS is wholly unsuited to detailed (or even realistic) altimetry, especially for "Western" style flying. I will admit, my example might be a bit excessive, but remember there is a mountain range between ONT and VCV, so some pretty significant differences can, and do often exist. I really wish ED would make the sim ATC capable of reporting QNH, but this is a combat oriented sim and the nuances of altimetry are not what its really about. Setting the weather as static and using the briefing QNH works fine for my purposes.
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Old 02-17-2018, 02:58 AM   #67
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Not a personal attack on you MidnightZulu... but I really dislike people making the excuse that this is an combat sim so its okay.

Everyone sets their altimeter (well I can't really say it happens everywhere; grass strips in the boonies where a guy might not care, but in the major areas like LA where I learned) Its so basic its taken for granted... that is until you come to DCS. Its not a good excuse... its within the very fabric of flying in the real world and it should be in this sim.

On my very first flight lesson I was introduced to dealing with the radios and setting the altimeter. I learned out of KLGB so had to do the full listen in on ATIS (set altimeter), then contact clearance delivery, ground, and finally tower to get airborne... all on my first flight.

I can't say my experience is universal but I bet its close.

ETA: I mean having to contact ATIS and then set altimeter, and use radios in some fashion when starting.

Last edited by aileron; 02-17-2018 at 03:18 AM.
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