International Standard Atmosphere

VooDoo_daVinci

Grabbed the latest greatest nightly build and it looks great runs great. It has a weather setting called International Standard Atmosphere. Any idea what this is? Doesn't seem to do anything.

TIA!

VooDoo

stuartc

I have no idea

Avionyx

It's basically a "level playing field" for the weather.

ISA is what is considered to be the baseline for any calibration or unless otherwise known. It's also the level that Flight Levels are generally based off.

In FGUK we don't do much consideration of the weather when it comes to instrumentation on flight night, partly because a lot of the participants aren't interested in that but also because of variations between machines.

ISA assumes that the atmosphere is:

• Pressure of 1013.2 millibar - Pressure is taken to fall at about 1 millibar per 30 feet in the lower atmosphere (up to about 5,000 feet).
• Temperature of +15 °C - Temperature falls at a rate of 2 °C per 1,000 feet until the tropopause is reached at 36,000 feet above which the temperature is assumed to be constant at -57 °C. (The precise numbers are 1.98 °C, -56.5 °C and 36,090 feet)
• Density of 1,225 gm/m3.

All stuff that any pilot of any type of aircraft (certainly in the UK/Europe and probably the rest of the world) is expected to be able to reel off for the various exams.

You'll note that all altimiters are set at 1013 milibars (or 2992in/Hg for the Americans) as standard. Plus once you get above the "transition" level, which is either set quite universally in an area, or specifically if there's a reason to do so.

As an example in the UK and most of Europe as soon as you go over 6,000ft altitude you'll switch to "Standard" which is 1013 / 2992 (in most of the USA it's 18,000ft IIRC)

At THAT point you start using Flight Levels rather than altitude. Your altitude can't be in feet when flying IFR above transition because you simply don't know what the exact pressure is, so you use 1013 / 2992 and suddenly 6,500ft is FL065, or 33,000ft is FL330

It only works because EVERYONE is using the same setting. You know if you are flying at FL150 and someone else is flying at FL170 coming the other way you're going to have clearance. If you were flying at ALTITUDE of 15,000ft and they were flying at an altitude of 17,000ft then given the discrepancy for settings you'd have no idea how close you might pass (or if you would at all without hitting them)

When you approach an airfield you'll get the "altimiter" (in the US) or the QNH / QFE (most of the rest of the world) and that'll be the pressure setting you need to use so you have the right altitude. QNH will give you the altitude above sea level, QFE will be actually above the airfield. So on QNH an airfield that is 200ft above sea level would show an altimiter reading of 200ft when you land, if they gave you a QFE setting the altimeter would read closer to 0ft - if that makes sense?

QFE is only useful when you don't need to take into consideration any terrain around the airfield that might need to be avoided or used as a reference, which is why QNH is typically used because it allows you to look at your charts and see the airfield and surrounding area and know that the altitudes on the chart are what you can use to decide your own altitude.

I imagine ISA is useful in FG for calibration or removing any environmental factors when doing tests or, probably just as useful, for multiplayer flights as it'd be the only time it'd set everything the same for everyone.

Hopefully that wasn't too boring and answered the question?