So I’m flying my Black Square King Air over the central US right now. Target altitude was/is 31,000.  Dialed in the ALTITUDE SET at 31,000 and the aircraft is currently level at what appears to be 31,000 except for the fact my GPS and my ForeFligt tracking report me at 28,000.

I’ve checked for failures and made sure pitot heat is active (both) but the 3,000 discrepancy remains.

Any ideas and Merry Christmas to all!

-B

PS. Altimeter is set and checked at 29.92

Edited December 23, 20223 yr by btacon

GPS Altitude vs Barometric Altitude

We often have questions about the difference between the reported barometric altitude and GPS altitude. These are two very different parameters, one based on atmospheric pressure and one based on a geometrical calculation. So there can be differences between the two parameters.  

GPS altitude is calculated through a satellite fix that determines the height above mean sea level, it is not related to atmospheric pressure or RADAR altitude, it is based on a geometric calculation of GPS satellites.

The barometric altimeter calculates pressure altitude based upon the atmospheric pressure setting applied to the aircraft’s altimeter (QNH/QFE).
 

The two calculations are therefore based upon fundamentally different parameters:

GPS altitude a geometrical calculation.

Barometric altitude a pressure-related calculation.

So there can be differences when GPS altitude and pressure altitude are compared.  

This is not an error. Foreflight shows the value of the sim variable TRUE ALTITUDE, which is the aircraft’s actual geometric height above sea level. Back in sim update 5 and 6, Asobo redesigned the atmospheric emulation of MSFS to accurately include the effects of non-standard pressure and temperature on altitude. If the actual temperature and pressure at high altitude are both lower than normal (ISA) then TRUE altitude can be much lower than indicated (pressure) altitude. 

I just looked at the ADSB data from a Gulfstream 5 flying over my location in the US Northeast at a pressure altitude (Mode C) of 43.000 feet. His true (GPS) altitude shows as 41,100 feet.

45 minutes ago, JRBarrett said:

This is not an error. Foreflight shows the value of the sim variable TRUE ALTITUDE, which is the aircraft’s actual geometric height above sea level. Back in sim update 5 and 6, Asobo redesigned the atmospheric emulation of MSFS to accurately include the effects of non-standard pressure and temperature on altitude. If the actual temperature and pressure at high altitude are both lower than normal (ISA) then TRUE altitude can be much lower than indicated (pressure) altitude. 

I just looked at the ADSB data from a Gulfstream 5 flying over my location in the US Northeast at a pressure altitude (Mode C) of 43.000 feet. His true (GPS) altitude shows as 41,100 feet.

Thanks JD. Way cool and aren’t the Laws of Nature so cool, most certainly  consistent.

-B

52 minutes ago, JRBarrett said:

I just looked at the ADSB data from a Gulfstream 5 flying over my location in the US Northeast at a pressure altitude (Mode C) of 43.000 feet. His true (GPS) altitude shows as 41,100 feet.

What is causing this is nearly record or historical high pressures associated with this current storm.  Look at Dallas @ 30.80 in.  And the upper great plains where some pressure readings are even greater.  I did a lot of flight weather breifings in my day and do not recall presenting one with pressures much higher than 30.50 in.

17 minutes ago, fppilot said:

What is causing this is nearly record or historical high pressures associated with this current storm.

If this was the only effect in play, then the actual altitude would be _greater_ than the barometric altitude (referenced to the standard altimeter setting). Conversely, low pressure causes the actual altitude to be lower than the barometric altitude, hence the mnemonic "high to low, watch out below".

The effect that is causing the actual altitude to be lower than the barometric altitude, despite the high pressure, is the temperatures that are significantly below ISA this time of year. This is also why cold temperature corrections need to be made to the altitudes on approaches to certain airports if the temperature is below a given specified value.

I think its the steep gradient to very low low pressure in the East that is significant in this weather system

An aircraft’s pressure altitude (referenced to 29.92) can vary greatly in terms of true height above sea level. 

Upper air charts used by meteorologists depict the MSL height (in meters) of a standard pressure surface. For instance, this is today’s 250 millibar chart, which shows how high the 250MB level is at any given location.

https://weather.uwyo.edu/cgi-bin/uamap?REGION=naconf&OUTPUT=gif&TYPE=obs&TYPE=an&LEVEL=250&date=2022-12-23&hour=12

Over the Canadian Arctic, the 250MB surface is 9,600  meters which is 31,469 feet. 

Over central Mexico, the 250MB surface is at 10,600 meters or 34,776 feet. Quite a difference!

Under ISA (standard atmosphere) conditions the height of the 250 millibar surface is 10,363 meters or exactly 34,000 feet.

 

Edited December 23, 20223 yr by JRBarrett