Pressure Altitude vs. Actual Altitude

[Guest quink99]

Hello all,I am teetering on the brink of purchasing Active Sky. After having reviewed the FAQ's and searched the Active Sky forum I'm unable to find an answer to the following question which I would most appreciate help with.PRESSURE ALTITUDE is the altitude indicated on your altimeter when you maintain a flight altitude or flight level in relation to a constant barometric setting in the Kollsman window of your altimeter such as 29.92 inches. As you fly across areas of high and low pressure your ACTUAL ALTITUDE will very as the pressure gradient or slope you are flying through raises or falls.ACTUAL ALTITUDE is your actual altitude above mean sea level (MSL) and can be read in the Flight Simulator at any time by pressing SHIFT+Z.By maintaining a constant PRESSURE ALTITUDE and mathematically or with one of several flight computers such as a Jeppesen CR-2 or Dalton Mark VII comparing your PRESSURE ALTITUDE to your ACTUAL ALTITUDE it is possible, in the real world, to practice Pressure Pattern flying which allows you to determine your drift with no reference to any thing on the ground, practice single heading flight and various other functions. It was basically used on long over water flights during the era of the Flight Navigators prior to INS,IRS,or GPS.My question, then, is; Does either FS2004 or Active Sky using downloaded actual weather simulate this change in pressure over horizontal distance which would cause an aircraft's ACTUAL ALTITUDE to vary at the same time the aircraft was flying a constant PRESSURE ALTITUDE? In other words, is pressure variation simulated as are other functions such as wind, visibility, cloud cover, etc.?I regret the lengthy post but felt it necessary for those who might have little or no knowledge of Pressure Pattern flying so they could understand the question.Thanks to all,Frederick

[Damian Clark 1,085]

Hi Frederick,In a word, Yes!AS is always simulating the pressure based on the reports for particular wx stations. For stations without reports this data is computed from nearby sources. FS9 also throws some of its own interpolation into the mix to try and provide a realistic and smoothly changing pressure simulation as you fly from area to area and from one time to the next, as the METAR reports are given. FS9 and AS does this just about perfectly most of the time.. certain times oddities in data or FS9 interpolation can interfere (causing less-than-smooth changes), but it usually works very nicely.Thus, when flying a pressure altitude, let's say cruising along at FL200 (and 29.92 dialed in), your actual altitude is changing as you fly even while your PA is constant at 20,000ft. In certain extreme cases (crossing fronts and entering different air masses) you can actually see the altimeter start to drift (especially with autopilot enabled, the AP will compensate!). And your actual altitude (i.e. shift-Z) will also change.This realism is also the cause of a certain problem with ATC in FS9.... for some reason the local altimeter setting means nothing to them (even though they tell you what it is!), and they just assume ISA 29.92. ATC doesn't like when you're over 200ft from your assigned altitude, so thus when more than .2 inches difference from 29.92 (200 relative feet) and you are maintaining proper pressure altitude, ATC will yell at you (and other AI aircraft) to get back to expected (actual) altitude. Not quite realistic! As we know in the real world the controllers are fully aware of the local altimeter setting and consider this (although it's often automatically adjusted in the radar target altitude data tag).Hope that explains how it works within AS2004.5 and FS9! Let us know if you have any more questions, we're glad to help!Best,

[Guest quink99]

Damian,Thanks ever so much for the prompt and concise response to my question regarding the pressure response of Active Sky and FS2004. It was most appreciated and has assured that I'll buy Active Sky after I finish writing this reply.I shall, however, take you up on your kind offer of help if I still have any remaining questions with the following one. In the specifications section of the posting, "New B172 AS2004.5 Beta Now Available" it is stated that;"Fixed problem where areas with very few stations (no valid reporting station within 500nm range) the weather could become static and invalid

[michal 44]

I am not sure however if MSFS is sophisticated enough to simulate altitude errors due to temperature. In real world when temperatures are very cold it creates an error (even if you fully ajusted for local alt. setting) that puts you lower than you think you really are. And the reverse is true - very hot temperatures put you higher than indicated altitude. Michael J.WinXP-Home SP2,AMD64 3500+,Abit AV8,Radeon X800Pro,36GB Raptor,1GB PC3200,Audigy 2

[Damian Clark 1,085]

Hi Frederick,As you suspect, in areas were there isn't any nearby data to go on, obviously there is no way to provide consistent accuracy. AS can only set wx via stations (except for wind and vis, which are "real time" controllable) and a single global setting which is used when more than about 200nm from any station (varies). It is then up to FS9 to interpolate between such and depict pressure along your route. Thus your pressure during the majority of the route will be fairly constant at the "global" setting determined via AS (which is figured early on during departure based on departure/destination conditions at time of route processing, if enabled). So in actuality the type of flying you refer to is not very possible in low-density reporting areas. The resolution isn't there. On the other hand, in areas with enough stations (within 200nm), it should work great.Increased resolution in this department (as well as some others) is something we've been working on for quite a while now... our progress will be revealed soon! :)Best,

[Guest quink99]

Damian,Again thanks for your informative reply. Although I may not be able to use it for transoceanic pressure pattern flying for a while yet I have purchased and installed Active Sky 2004.5 and am just now beginning to learn and enjoy it. I shall keep a "weather" eye on your efforts to "...bring about increased resolution in this department...." and look forward to hearing more about it in the future.Regards,Frederick