Jump to content

Archived

This topic is now archived and is closed to further replies.

JustinWheat

Altitude

Recommended Posts

Ok I need a little help. I don't have the latest beta version downloaded so forgive me there. When I'm flying the pmdg 737ngx and I'm at my cruise altitude say fl380 on vatspy or any monitoring program for vatsim my altitude is saying about 5000 feet below my set altitude in the airplane. Yes I have the baro pressure set standard at 29.92 at that altitude. Any suggestions as to why my altitude readings would be different??


Justin Wheat

Share this post


Link to post
Share on other sites

Have you tried to turn off weather engine and set "fair weather" in FSX?


[color=#a9a9a9][size=1][size=4][img]http://forum.avsim.net/public/style_images/flags/rs.png[/img][/size] Lj. Prodanovic[/size][/color]

Share this post


Link to post
Share on other sites

This has nothing to do with the LWE or any other engine. A Flight Level is not an altitude since it is based on the standard pressure setting and not the actual QNH. You are comparing chalk with cheese.

 

To answer this question even though it has absolutely nothing to do with the weather (apart from the setting of the surface pressure QNH) I would need to know what your actual local QHN was. You could see this for yourself by examining your local weather report. As a rule of thumb there is approximately 30 feet difference per millibar between FL and Altitude AMSL based on the difference between the standard pressure FL setting 1013.2 mb or 29.92 inches Hg.

 

So nothing to do with the weather engine. Flight Levels are always different from Altitudes unless the local QNH is 1013.25 mb or 29.92 ins.

 

Stephen

 

N.B.

 

If you get any obviously strange readings then you should check the downloaded METARs and check to see if there are any erroneous METAR reports. The current software does validate the reported surface pressures and discard any that are not within the normal pressure setting range. Cannot remember of hand what version or beta included these validation checks. Discovered and corrected after an erroneous METAR was actually encountered and reported by a user.

 

If you download and install the latest beta (soon to be released as version 3.00) you will know for sure you have those validation checks included. The Announcements topic in our SimForums will show when this mod was included.

Share this post


Link to post
Share on other sites

So nothing to do with the weather engine. Flight Levels are always difference from Altitudes unless the local QNH is 1013.25 mb or 29.92 ins.

But not for 5000ft

 

What's the calculated pressure at FL380 by ISA? And how big deviation you need for altitude difference of 5000ft?


[color=#a9a9a9][size=1][size=4][img]http://forum.avsim.net/public/style_images/flags/rs.png[/img][/size] Lj. Prodanovic[/size][/color]

Share this post


Link to post
Share on other sites

There is no need for calculated pressure. The difference between the two is based on either the actual surface pressure (QNH) for Altitudes, or the standard setting of 1013.25 mb for all Flight Levels (used above the specified transition level usually 18000 feet in the USA and 6000 feet elsewhere).

 

But also see my P.S. to the last post and check your METAR statements for a possible offending erroneous QNH report.

 

If you upgrade to the latest beta then you will certainly include the validation checks where any obviously erroneous QNH is replaced by the average QNH reported within the region.

 

Stephen

 

5000 feet is a difference of about 166 mb between QNH and the standard setting of 1013.2.

 

Stephen

 

Altitudes are useful for avoiding terrain. Flight levels are useful for avoiding other aircraft.

 

But do check your METAR statements if you suspect a rogue report.

 

Stephen

Share this post


Link to post
Share on other sites

To make 5000ft difference between standard pressure altitude and calculated one, you need ~166hpa difference (all that if we assume pressure in function of altitude is flat line, but in reality, it's not), that's just too much. That's why weather engine with erroneous reading can make a difference.


[color=#a9a9a9][size=1][size=4][img]http://forum.avsim.net/public/style_images/flags/rs.png[/img][/size] Lj. Prodanovic[/size][/color]

Share this post


Link to post
Share on other sites

Also note, the pressure lapse rate does vary with height.

 

So I think at the surface and at low levels you can use approximately 28 feet per mb, at 10000 feet though the actual rate is nearer 36 feet per mb, and at 18000 feet the rate is approximately 48 feet per mb.

 

Hence a difference of 5000 feet between Altitude and FL requires much less than 166 mb difference. I will let you work it out. You will most probably find you were flying in just a normal winter low pressure zone.

 

Stephen

 

The weather engine does NOT set the ambient pressures, those are calculated by FSX. But these have nothing to do with your Altitude or Flight Level.

 

Altitude is based on actual surface pressure. Flight Levels adopt a standard pressure setting reference so that every pilot is working from the same song sheet. Neither care what the ambient pressure is which as I said is NOT set or controlled by ANY weather engine.

 

Stephen

 

Obviously, the pressure difference is FAR less than 166 mb. The 28 feet per mb is only relevant at low altitudes which is where I mostly fly in real life that's why I use the 30 feet per mb rule of thumb. I will let you work out the actual difference and convince yourself that nothing more than a low or high pressure zone can result in considerable difference between Alt and FL at high altitudes or flight levels.

 

Stephen

 

Just check your downloaded METARs. Erroneous QNH reports have been seen before, and these have already been validated and weeded out. Just download and install the latest beta if you wish.

 

Stephen

 

The latest software validates the reported QNH (surface pressure) which must lie in the normal altimeter adjustment range which is 850mb to 1050mb.

 

Stephen

Share this post


Link to post
Share on other sites

So, you calculated? I just experimented with some parameters, in all cases I still get too much pressure difference, even with higher lapse rate.


[color=#a9a9a9][size=1][size=4][img]http://forum.avsim.net/public/style_images/flags/rs.png[/img][/size] Lj. Prodanovic[/size][/color]

Share this post


Link to post
Share on other sites

I haven't calculated anything, I believe the actual difference is actually logarithmic. I have just tried to explain the difference in the meaning between Altitude and Flight Level and describe why they are both used and needed (terrain avoidance as opposed to collision avoidance). I recommend you check your METARs to check for any erroneous QNH reports, and if concerned just install the latest beta which validates all reported pressures.

 

I admit 5000 feet seems a lot but guidelines do give conditions were there is 3000 feet difference when converting min safe altitudes to flight levels, and that is usually done at lower altitudes (low enough to be concerned about the terrain).

 

The easiest solution would be to note what your actual local or even area QNH was and see if this explains the difference at your FL. but it would also be simple to check the METARs in the OpusWeather.txt file.

 

The LWE does not calculate anything other than the average valid QNH to ensure rogue QNH settings are not injected into the sim. The surface pressures are also smoothed between adjacent weather cells, on a resolution of 16km or 10 miles, to eliminated obvious erroneous spikes in the surface pressure.

 

Stephen

Share this post


Link to post
Share on other sites

Ok thanks for your replies. I have one more issue/question. What is up with the sudden altitude increases/decreases during flight +/- 2000ft?? It seems to happen when the weather is updated. I have the weather being updated every 10 minutes. Is that to often?? I can't seem to have a smooth decent into arrival destination during my flights. I will be desceding per the star and all of the sudden I will go from 8000ft to 5000ft in the blink of an eye. I have been using the default settings for opus. Any help with this issue would be greatly appreciated.


Justin Wheat

Share this post


Link to post
Share on other sites

again, it's too much, in reality you have very smooth gradient of 1hpa change. 3000ft altitude change at that low altitude is ~100hpa change (if we assume gradient of 27-30ft per 1hpa). Something is configured wrong at your system.


[color=#a9a9a9][size=1][size=4][img]http://forum.avsim.net/public/style_images/flags/rs.png[/img][/size] Lj. Prodanovic[/size][/color]

Share this post


Link to post
Share on other sites

How do you think something is configured wrong with my system? So installing the program with default settings still makes the system be misconfigured? I don't understand. The program installed correctly and I read over the getting started guide. Im pretty optomistic it has nothing to do with my system. Im just looking for a few suggestions as to why this occurs. Maybe back the updating off to every 60 min?? 


Justin Wheat

Share this post


Link to post
Share on other sites

Following common sense logic, 60min update should make things even worse, because if in 10min interval QNH can change for 100hpa, what do you think how much hpa change you can expect in 60min interval?


[color=#a9a9a9][size=1][size=4][img]http://forum.avsim.net/public/style_images/flags/rs.png[/img][/size] Lj. Prodanovic[/size][/color]

Share this post


Link to post
Share on other sites

Pressure smoothing is included in the LWE but you must not enable the LWE once already flying or after using a different weather theme or weather engine otherwise you will be prone to large changes in the surface pressures.

 

Make sure you only have one source of weather on your system as some of the rapid changes you are seeing have not been seen before. Also make sure you enable the LWE before flight, specify your Destination and Cruise Altitude and update your weather and check your Upper, Lower, and local weather reports before takeoff.

 

The LWE will only allow a 1mb change in QNH between adjacent 16km (10 mile) weather cells, all pressures set in accordance with the METARs. It also stabilises the pressure changes between updates. If you have enabled Maximum Stabilisation then full (static) pressure stabilisation will also be enabled, the recovery can be in stages but never more than a 4mb change at any one time.

 

Make sure you are using the latest Beta, reporting problems with an earlier version is futile since that could pre-date the QNH validation checks in the reported METARs.

 

I suggest you do the above also set the default update rates. Set the Adjust Settings Automatically and Adjust Sim Friendly GRIB options, and if necessary the Maximise Stabilisation option. Always make sure you specify your Dest and Cruise Alt before flight and let the LWE take care of the rest. Do not change or adopt a new weather theme in flight or anything like that and monitor the Opus Weather Reports, the Local report will tell you what the actual QNH is, the Destination report will tell you what it is at your Destination when in range.

 

Stephen

Share this post


Link to post
Share on other sites

Thanks everyone for the help and the more then explanational responses. I have everything running smooth as silk. Keep up the outstanding work.


Justin Wheat

Share this post


Link to post
Share on other sites

  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
  • Donation Goals

    AVSIM's 2020 Fundraising Goal

    Donate to our annual general fundraising goal. This donation keeps our doors open and providing you service 24 x 7 x 365. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. We reset this goal every new year for the following year's goal.


    18%
    $4,525.00 of $25,000.00 Donate Now
×
×
  • Create New...