Looking at something and it says -1300 FT/10C above STD

+700 FT/10C Below STD

 

Am i using ISA at that altitude or ISA DEV for the above?

 

So at FL360 its -56 ISA and the OAT is -46 would i take +700 FT for my calculation?

Looking at something and it says -1300 FT/10C above STD

+700 FT/10C Below STD

 

Am i using ISA at that altitude or ISA DEV for the above?

 

So at FL360 its -56 ISA and the OAT is -46 would i take +700 FT for my calculation?

What are you trying to calculate? It would be helpful to know what you are using the results of your calculation for.

 

Temperature changes the density altitude primarily, which will indeed be higher than pressure altitude at temperatures above ISA, and lower at temperatures below ISA.

 

Density altitude affects aircraft performance, and is of immediate concern when calculating takeoff power and distance requirements on hot days at higher altitude airports.

 

TRUE altitude is also affected by temperature. Assuming that the pressure is standard (29.92 in/hg / 1013 hp), and your altimeter is set to STD, your actual MSL altitude will be lower than what your altimeter reads when the air is colder. This mainly affects aircraft which have a simple altimeter directly connected to the outside atmosphere with a relatively short static pressure line and have no ADC to correct the altitude readout.

 

True altitude of course is also affected by the current air pressure - whether it is above or below STD. Even a simple altimeter will automatically correct for that when flying below the transition altitude, as long as you have set the correct local barometric pressure.

 

At the flight levels, in a high-performance aircraft, the Air Data Computer automatically corrects your altitude and speed readouts for the current TAT (which varies with your speed).

 

But, to answer your question, yes, if you are trying to calculate true altitude, you will generally add when temps are above ISA, and subtract when it is colder than ISA.

 

Air temps will also affect your indicated airspeed due to changes in the air density, which is why ADC-equipped aircraft use Mach numbers for speed control at higher altitudes. The Mach number is automatically corrected for temperature.

In short look at post 12

 

http://forum.aerosoft.com/index.php?/topic/80454-short-trip-cruise-altitude-problems-still-not-correct/

Looking at something and it says -1300 FT/10C above STD

+700 FT/10C Below STD

 

Am i using ISA at that altitude or ISA DEV for the above?

 

So at FL360 its -56 ISA and the OAT is -46 would i take +700 FT for my calculation?

 

The delta between -56 (ISA or "standard") and -46 is +10, which would mean you're looking at -1300 FT.

 

To be honest, though, this isn't something you'd really be messing with as a pilot in tubeliners. This is more of a dispatcher thing.

 

...and a PFPX question at that.

If you have a OAT of 8c on the ground would that make temp below ISA 15c ?

If you have a OAT of 8c on the ground would that make temp below ISA 15c ?

 

Provided "ground" is at sea level, yes. It would be ISA-7.

If you have a OAT of 8c on the ground would that make temp below ISA 15c ?

Vernon

 

It depends on the elevation of the ground. 15C is at sea level. At 2000 ft. it is 11C. 5000 ft. is 5C. So it depends on how high the ground is.

Yeh sorry at Ground level. That would make you 7c colder than ISA ?

 

 

Let’s consider a flight in the following conditions: Altitude = 33,000 feet

Actual Temperature = -41oC

The standard temperature at 33,000 feet is : ISA = 15 - 2 x 33 = -51oC, whereas the actual temperature is -41oC, i.e. 10oC above the standard.

 

Conclusion: The flight is operated in ISA+10 conditions

 

Does that mean i'm 10c warmer than ISA ?

Yeh sorry at Ground level. That would make you 7c colder than ISA ?

 

 

Let’s consider a flight in the following conditions: Altitude = 33,000 feet

Actual Temperature = -41oC

The standard temperature at 33,000 feet is : ISA = 15 - 2 x 33 = -51oC, whereas the actual temperature is -41oC, i.e. 10oC above the standard.

 

Conclusion: The flight is operated in ISA+10 conditions

 

Does that mean i'm 10c warmer than ISA ?

 

Might be helpful to step back and try to understand how I've gotten to my two earlier observations:

 

Take ISA. What makes you get from ISA to observed? If you to ISA to get to observed, you're warmer than ISA. If you subtract from it to get to observed, you're colder than ISA.

 

KDCA 291752Z 34013KT 10SM SCT038 OVC250 21/09 A2970 RMK AO2 SLP057 T02060089 10228 20167 58004 (DCA might as well be sea level)

 

ISA is 15

Observed is 21

 

You add 6 to get from ISA to observed. ISA DEV is +6, or you're 6 C warmer than ISA.

Ok i undestand that. But those calculations i pasted are correct ?

Ok i undestand that. But those calculations i pasted are correct ?

 

2C per thousand feet is the approximation, so yes. There are probably charts out there if you're looking for more accuracy (adiabatic lapse rate varies, so the 2 degrees isn't a perfect approximation).

ISA Table on page 6 of this link - http://home.anadolu.edu.tr/~mcavcar/common/ISAweb.pdf

What are you trying to calculate? It would be helpful to know what you are using the results of your calculation for.

 

Temperature changes the density altitude primarily, which will indeed be higher than pressure altitude at temperatures above ISA, and lower at temperatures below ISA.

 

Density altitude affects aircraft performance, and is of immediate concern when calculating takeoff power and distance requirements on hot days at higher altitude airports.

 

TRUE altitude is also affected by temperature. Assuming that the pressure is standard (29.92 in/hg / 1013 hp), and your altimeter is set to STD, your actual MSL altitude will be lower than what your altimeter reads when the air is colder. This mainly affects aircraft which have a simple altimeter directly connected to the outside atmosphere with a relatively short static pressure line and have no ADC to correct the altitude readout.

 

True altitude of course is also affected by the current air pressure - whether it is above or below STD. Even a simple altimeter will automatically correct for that when flying below the transition altitude, as long as you have set the correct local barometric pressure.

 

At the flight levels, in a high-performance aircraft, the Air Data Computer automatically corrects your altitude and speed readouts for the current TAT (which varies with your speed).

 

But, to answer your question, yes, if you are trying to calculate true altitude, you will generally add when temps are above ISA, and subtract when it is colder than ISA.

 

Air temps will also affect your indicated airspeed due to changes in the air density, which is why ADC-equipped aircraft use Mach numbers for speed control at higher altitudes. The Mach number is automatically corrected for temperature.

 

It's been a while since I cracked the books, and this is maybe semantics, but would it be more accurate to say True Altitude is corrected for temperature or pressure or elevation from another altitude? True Altitude isn't really affected by anything other than going up or down in relation to MSL.

 

I'll get some more coffee and re-mull in a bit.

Been reading and wouldn't - True altitude = Indicated altitude + 28 x (QNH [hPa] - 1013)

Vernon, only a suggestion......you enjoy going into the needy greedy. Have you considered taking a private certificate ground school course ?