26 minutes ago, Stearmandriver said:

An altimeter is just a barometer.  It senses air pressure changes, that's all it does.  Air pressure changes with altitude, but it also changes constantly as barometric pressure changes.  To make an altimeter read correctly, you need to tell it what the current barometric pressure is.  If you give it the wrong barometric pressure, it will give you the wrong altitude. 

I suspect you've got your sim weather set to a preset instead of live weather in your shot, because it's acting like the barometric pressure is the standard 29.92.  This is the default pressure of all the presets.

You can hit B to automatically set your altimeter setting to current barometric pressure... Which is a handy way to see what it really is in the sim.

Field elevation and touchdown zone elevation (TDZE) on the chart snippet you shared are not minimums.  The minimums block is below the profile view of the chart, down at the bottom.

Apologies. I should have provided more details. At the runway in the aircraft, pressing the 'b' gave me a barometric pressure of 29.92.  Without moving the aircraft, I then checked the METAR which I downloaded from Simbrief as shown on the tablet. The barometric pressure for that was 30.09 and that gave me a different altitude (6420 ft). I am wondering why that would be different since both location and time were the same. 

The minimums I was referring to are the landing minima listed in the approach chart for the aircraft categories (A, B, C, D) at various speeds in precision and non-precision approaches. 

32 minutes ago, anavsun said:

Apologies. I should have provided more details. At the runway in the aircraft, pressing the 'b' gave me a barometric pressure of 29.92.  Without moving the aircraft, I then checked the METAR which I downloaded from Simbrief as shown on the tablet. The barometric pressure for that was 30.09 and that gave me a different altitude (6420 ft). I am wondering why that would be different since both location and time were the same. 

The minimums I was referring to are the landing minima listed in the approach chart for the aircraft categories (A, B, C, D) at various speeds in precision and non-precision approaches. 

Yep, your weather in the sim had a barometric pressure of standard - 29.92.  That's why when you pressed b, it set your altimeter setting to 29.92 and the altitude indicated correctly.  The simbrief weather shown on the tablet is real world weather - that was the actual METAR for that airport at the time.  That information is only useful in the sim if you have your sim weather set to real world weather.

8 hours ago, anavsun said:

Kinda tough to do using slew mode even when using the joystick to slew (it's impossible to do with the keyboard). I suppose I could taxi but a few inches might be a moot point considering that MSFS has the runway pointing 180 degrees when in fact it's 172 degrees with a 5 degree offset (I'm still unclear if the offset should place it at 167 or 177 degrees ie. minus or plus 5 degrees from 172 degrees).

See multiple discussions earlier in this thread that reference the runway heading as being 177 degrees.

But you can also just check this for yourself on the airport diagram. I use Navigraph charts, which require a paid subscription, but there's also the FAA airport diagram, which you can access for free through skyvector.com:

https://skyvector.com/files/tpp/2501/pdf/05416AD.PDF

And wouldn't you believe it, the runway heading charted there is 179.7 degrees, which is perfectly consistent with what you observe in the sim.

I've just gone back and rechecked, and the Navigraph charts (which come from Jeppesen) list the runway heading as 177 degrees. I see two possibilities here: a) Jeppesen introduces minor inaccuracies into charts that are made available through Navigraph, to discourage people from using them in the real world. (The charts do, after all, say that they are for flight simulation only.) Or b) This is an actual error in the Jeppesen chart. I'd be interested to hear from someone who has access to the real-world Jeppesen chart what the runway heading is that is charted there.

1 hour ago, Stearmandriver said:

Yep, your weather in the sim had a barometric pressure of standard - 29.92.  That's why when you pressed b, it set your altimeter setting to 29.92 and the altitude indicated correctly.  The simbrief weather shown on the tablet is real world weather - that was the actual METAR for that airport at the time.  That information is only useful in the sim if you have your sim weather set to real world weather.

If the sim weather is based on real weather, the altitude for the minima will be based on real world weather.  Otherwise, what's the point of checking the metars. Now since that's the case, wouldn't that be a problem since the minima altitudes on the approach chart is based on standard pressure as of the date the approach chart was created and different from real weather obtained from the metar as of the date of the flight?

Put another way, in calculating the minimums, to what should the calculation of the altitude minimums be based on: 29.92 or 30.09?

Edited February 20, 20251 yr by anavsun

1 hour ago, anavsun said:

If the sim weather is based on real weather, the altitude for the minima will be based on real world weather.  Otherwise, what's the point of checking the metars.

Correct -- but all of this assumes that you've got the sim set to use "live weather", and it looks as if, in your test, you didn't. It looks very much as if you were using a weather preset. These use the standard altimeter setting of 29.92 (independent of what the real weather is), so if you're using a weather preset, that's what you should set your altimeter to.

 

1 hour ago, anavsun said:

Now since that's the case, wouldn't that be a problem since the minima altitudes on the approach chart is based on standard pressure as of the date the approach chart was created and different from real weather obtained from the metar as of the date of the flight?

I think there may be some confusion here as to how altimeters work?

Put simply, if you set your altimeter to the current local altimeter setting, it will display your actual altitude above mean sea level¹.

So if you set your altimeter to the current local altimeter setting and you then descend the aircraft to the minimum descent altitude (MDA) of 6980 feet², you will know that you are 721 feet above the touchdown zone elevation of 6259 feet. And this is what's relevant: How close are you to the ground? This, after all, is the purpose of the minimums: To ensure that, if you fly the approach, then execute a missed approach at or above minimums, you won't hit anything.

When the person who designed the approach procedure calculated the minimums, they determined how high above the ground you would need to be so that you would be guaranteed not to hit anything. When you set the current local altimeter setting, you are ensuring that the altitude displayed on your altimeter corresponds to your actual altitude above mean sea level.

For some more background on how altimeters work, I'd suggest reading up on this in the freely available PIlot's Handbook of Aeronautical Knowledge:

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak

See the material starting on page 8-3 of chapter 8:

https://www.faa.gov/sites/faa.gov/files/10_phak_ch8.pdf

----

¹ I'm purposely leaving temperature error out of the picture for the moment because it would only muddy the waters further.

² LNAV minima for categories A, B, C with the increased missed approach climb gradient

Edited February 20, 20251 yr by martinboehme

29 minutes ago, martinboehme said:

Correct -- but all of this assumes that you've got the sim set to use "live weather", and it looks as if, in your test, you didn't. It looks very much as if you were using a weather preset. These use the standard altimeter setting of 29.92 (independent of what the real weather is), so if you're using a weather preset, that's what you should set your altimeter to.

 

I think there may be some confusion here as to how altimeters work?

Put simply, if you set your altimeter to the current local altimeter setting, it will display your actual altitude above mean sea level¹.

So if you set your altimeter to the current local altimeter setting and you then descend the aircraft to the minimum descent altitude (MDA) of 6980 feet², you will know that you are 721 feet above the touchdown zone elevation of 6259 feet. And this is what's relevant: How close are you to the ground? This, after all, is the purpose of the minimums: To ensure that, if you fly the approach, then execute a missed approach at or above minimums, you won't hit anything.

When the person who designed the approach procedure calculated the minimums, they determined how high above the ground you would need to be so that you would be guaranteed not to hit anything. When you set the current local altimeter setting, you are ensuring that the altitude displayed on your altimeter corresponds to your actual altitude above mean sea level.

For some more background on how altimeters work, I'd suggest reading up on this in the freely available PIlot's Handbook of Aeronautical Knowledge:

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak

See the material starting on page 8-3 of chapter 8:

https://www.faa.gov/sites/faa.gov/files/10_phak_ch8.pdf

----

¹ I'm purposely leaving temperature error out of the picture for the moment because it would only muddy the waters further.

² LNAV minima for categories A, B, C with the increased missed approach climb gradient

Thanks. Appreciate the info. I'll visit the links.

1 hour ago, anavsun said:

If the sim weather is based on real weather,

The sim weather is only based on real weather if you explicitly set it to be.  You can also choose for it to be a weather preset, or you can manually define every setting to be exactly what you want.  The sim weather can literally be anything.  If you want to use real world weather sources for reference, you'll want to make sure your sim weather is set to "live weather".  Otherwise, you can look in the weather menu or use the in-sim Metar lookup to get a report of in sim weather.

 

1 hour ago, anavsun said:

Now since that's the case, wouldn't that be a problem since the minima altitudes on the approach chart is based on standard pressure as of the date the approach chart was created and different from real weather obtained from the metar as of the date of the flight?

All altitudes on the approach chart are altitudes above mean sea level, as indicated by a properly calibrated altimeter.  These altitudes do not change, unless the airport experiences significant geologic uplift or subsidence 😉.

Martin is right, you seem to not be completely understanding altimeters.  When you set the current barometric pressure (or "altimeter setting") in your altimeter, you've calibrated it.  You can fly into that airport every day for a year, and you'll likely set a different altimeter setting every day, but as long as you set it, the altimeter will read the same altitude on the runway.  Every day.

1 hour ago, anavsun said:

Put another way, in calculating the minimums,

Hold up right there 😉.  You do not "calculate" minimums, any more than you "calculate" an offset course.  The numbers are printed right on the chart, you just read them and use them.  Remember, you're simulating being a dumb pilot - math in public during a stressful situation is not our strong suit 😉.  Don't overcomplicate it.  You're a category C aircraft in your 737-800.  The altitude listed as your minimum descent altitude for LNAV mins is 6980.  When your altimeter reads that, you are 721ft above the runway and it's time to decide if you're landing.  See the runway?  Look out the window and land on it.  Don't see the runway?  Go around.

Edit to avoid confusion for folks who are more up to speed on this: yes, sometimes you do calculate minimums if you're flying a CDFA profile instead of a "dive and drive" at MDA, and everyone at an airline is doing a CDFA these days, and this is one of those times.  But let's not even go there yet.  😁

Edited February 20, 20251 yr by Stearmandriver

39 minutes ago, Stearmandriver said:

Don't see the runway?  Go around.

Edit to avoid confusion for folks who are more up to speed on this: yes, sometimes you do

Also consider diverting to the alternate you chose in the flight plan!

To OP you DID chose an alternative and are aware of your fuel remaining right? That's when the real sim fun starts! I wouldn't imagine that as anything approaching 'fun' irl. 

I'm trying to mentally draw a flowchart (see picture below). 🙂

I want to confirm something. I'm assuming that the data input for the metar shown on the tablet below directly comes from the flight plan generated in the Simbrief website. Is this correct? (it's showing a barometric pressure of 30.20). 

@Stearmandriver unfortunately I used the wrong word.  I shouldn't have used the word 'calculate'. I didn't mean to mathematically calculate. Rather, I meant to reference the figures in determining the MDA in the minimums section.

@martinboehme Haven't read the links yet. But will... I've also been using 'live' weather all along since I started this post.  Here's the thing though... I don't know if it makes a difference... I don't subscribe to Navigraph so my AIRAC is outdated. Does this make a difference in the context of this discussion and my questions?

Basic question: why are there two different barometric pressure values (30.20 vs 29.92)??

35 minutes ago, anavsun said:

Basic question: why are there two different barometric pressure values (30.20 vs 29.92)??

Now we're getting somewhere.

The "Data Connection" section isn't where you turn on live weather. This is just where you allow the sim to download live weather data in principle.

You actually turn on live weather in the weather menu:

It looks like this isn't turned on for you, which is why you're getting the standard altimeter setting of 29.92.

To answer some of your other questions:

38 minutes ago, anavsun said:

I want to confirm something. I'm assuming that the data input for the metar shown on the tablet below directly comes from the flight plan generated in the Simbrief website. Is this correct? (it's showing a barometric pressure of 30.20).

This isn't coming from the Simbrief flight plan, but from the actual, real-world METAR that is current for KTVL right now. It will only match what you see in the simulator if you have live weather turned on.

39 minutes ago, anavsun said:

@martinboehme Haven't read the links yet. But will... I've also been using 'live' weather all along since I started this post.  Here's the thing though... I don't know if it makes a difference... I don't subscribe to Navigraph so my AIRAC is outdated. Does this make a difference in the context of this discussion and my questions?

No, this is irrelevant for live weather.

45 minutes ago, martinboehme said:

Now we're getting somewhere.

The "Data Connection" section isn't where you turn on live weather. This is just where you allow the sim to download live weather data in principle.

You actually turn on live weather in the weather menu:

Agree. We're getting somewhere. Progress. Another valuable lesson learned. Thank you!

BUT, apparently this doesn't solve the discrepancy apparently even though live weather is turned on.  Pressing 'b' still results in a barometric pressure of 29.92.  And live weather in MSFS is 30.20, the same one as in Simbrief. One step forward, half a step back. 😵

I'm certain live weather is on because now there's snow in the scenery. What am I missing still??

E

EDIT: I re-started MSFS after activating Live Weather.

Edited February 20, 20251 yr by anavsun

8 minutes ago, anavsun said:

BUT, apparently this doesn't solve the discrepancy apparently even though live weather is turned on.  Pressing 'b' still results in a barometric pressure of 29.92.  And live weather in MSFS is 30.20, the same one as in Simbrief. One step forward, half a step back. 😵

I'm certain live weather is on because now there's snow in the scenery. What am I missing still??

I think the only thing that you're missing is that, apparently, the "b" shortcut doesn't work in the PMDG 737.

Your altitude in the PFD is now reading only about 6000 feet, which is considerably lower than the KTVL field elevation. This is a pretty clear sign that the barometric pressure in the sim is actually 30.20! Try manually setting your altimeter setting to 30.20, and the altitude in the PFD should be identical or close to the KTVL field elevation.

Edit: BTW, thanks a lot for providing the screenshots. These help a lot in diagnosing what is actually going on.

Edited February 20, 20251 yr by martinboehme

12 minutes ago, martinboehme said:

I think the only thing that you're missing is that, apparently, the "b" shortcut doesn't work in the PMDG 737.

Your altitude in the PFD is now reading only about 6000 feet, which is considerably lower than the KTVL field elevation. This is a pretty clear sign that the barometric pressure in the sim is actually 30.20! Try manually setting your altimeter setting to 30.20, and the altitude in the PFD should be identical or close to the KTVL field elevation.

Edit: BTW, thanks a lot for providing the screenshots. These help a lot in diagnosing what is actually going on.

Good catch!  As you suggested I set the altimeter to 30.20 and the elevation came out to 6260 ft. which is the TDZE elevation in the chart.  It appears that the "b" shortcut doesn't work in the PMDG 737. I wonder if that issue is isolated to me or to something else. It's a problem for another day.  

I have to take a break.  I now know about live weather and what the problem appears to be. I can work around it at least temporarily. Happy to provide the screen shots. As the saying goes, a picture is worth a thousand words. I really appreciate your help. Thank you!
 

1 minute ago, anavsun said:

I have to take a break.  I now know about live weather and what the problem appears to be. I can work around it at least temporarily. Happy to provide the screen shots. As the saying goes, a picture is worth a thousand words. I really appreciate your help. Thank you!

No problem, glad we got it sorted out!

Huh that's strange; the b shortcut does work for me in the PMDG.  Maybe just a general keybinding issue?  Sometimes the sim will randomly delete a keybind. 

Either way, glad you guys got the main point about live weather sorted!  👍