Question on Altitude

[Chris_Ford]

Well, you see, you have to make an assumption there, and the assumption is that you want the Pressure Altitude of the field. I thought we wanted the Pressure Altitude of the airplane. Either of us could be wrong or right, as it's not stipulated in the question.

Local altimeter setting is actually 10 feet above field elevation.

 

[ColMustard]

Well, you see, you have to make an assumption there, and the assumption is that you want the Pressure Altitude of the field. I thought we wanted the Pressure Altitude of the airplane. Either of us could be wrong or right, as it's not stipulated in the question.

Hmmmm....

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Not sure where you are getting that assumption. The local altimeter setting is the pressure at sea level in that area, so what you are finding is the pressure altitude in the area that you will be operating. I don't think there is a difference between the pressure altitude for the aircraft or the field. The performance charts want the pressure altitude that the aircraft will be operating in.

-ColM

 

[BCTAv8r]

I'm not gonna make it to the end of college...

 

[MidlifeFlyer]

The Pressure Altitude is what is indicated in the altimeter if the correct value is set in the Kollsman Window, i.e., the "altimeter setting."
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Tony, I think you accidentally mis-stated that. By definition, pressure altitude is what the altimeter indicates when 29.92 is set in the Kollsman window, not the reported altimeter setting.

 

[MidlifeFlyer]

I'm curious. What is being used to solve the problem? Calculator, E6B, abacus?

 

[MidlifeFlyer]

anyone ever seen this formula for density altitude.

DA = PA - (15 - current temp)x100

Yep, but it's a bit mis-stated. It's a =very= loose rule of thumb for calculating density altitude.

Not really mis-stated, but in most density altitude situations you care about, current temperature will be higher, so in your version, you'll always be dealing with double minuses. I think it's just easier to avoid dooble minuses where possible. But that's just stylistic.

What's wrong with the version you posted is that 15. Remember that density altitude is pressure altitude corrected for standard temperature at the pressure altitude. 15 is only standard temperature at sea level. So your formula will work for calculating density altitude at sea level, but nowhere else. So you can take out that PA and replace it with zero.

Here's the rule of thumb.

DA = PA + (current temp - standard temp at PA) x100

It's pretty close at say, about 6000' (only about 300' off). But its accuracy diminishes the higher you go and 120 starts getting closer than 100 (I use 120 where I fly). But 100 is easier.