# Ground ISA

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Can someone either point me in the right direction or explain how ground ISA in-sim effects things? Both the plane and the weather, what does a large or small ISA actually do in-sim?

Thanks!

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Not 100%  sure what you are asking.  Are you referring to entering the sea level ISA  of 29.92 into your altimeter rather than the actual barometric pressure on the day ?

Generally for low level VFR you use actual pressure so your altimeter reads correctly but for higher altitude and in particular IFR in the jetways you set standard ISA so your altimeter reads the same as other traffic in the area.

If your talking a Cessna doing circuits the usual practice is set the altimeter to read correctly for the known runway height ASL and not worry about what the barometric pressure reads unless the number is so crazy the altimeter is obviously faulty..

If on the other hand you are asking what difference does barometric pressure make to aircraft performance you need to read up on density altitude and pressure altitude.

Edited by Glenn Fitzpatrick
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ISA can't be large or small, - it's fixed, because it's a standard. It's the standard conditions of temperature and pressure that pilot's use to calculate things like what length of runway they need for take off. For example, on a very hot day of 30 degrees C, they'll need a longer runway than they'd need at the standard temperature (which is 15 degrees C at sea level). The standard pressure is 1013.25hPa (29.92 inches of mercury) at sea level, so on a day with low pressure, say 1000hPa, the plane will also need a longer runway because it's the equivalent of a higher altitude. Aircraft performance charts are based on ISA and are used to calculate the aircraft's expected performance according to how much local conditions of temp pressure and humidity) differ from the standard.

As Glenn mentions above, ISA pressure is used above transition altitude (say 10,000') so that all pilots have the same altimeter reading at the same actual altitude. Otherwise, if 2 pilots were converging, one from a hot place and one from a cold place, even though their altimeters showed different altitudes, they might be at the same height above the ground and on collision course. Switching to ISA pressure at high altitudes is therefore safer, and coming back down from high altitudes, they switch back.

This video explains it pretty well if you can understand the presenter's accent:

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