As of version 5, altitude data from MSFS to Foreflght is not accurate. Mine seems to be off by 1000 feet, ForeFlight showing high. Using Flight Events as my connect socket.

Also exhibits wild barometer data. Hope it’s resolved in version 6, but I doubt it. Too many things for them to fix.

Planes generally flown - Cessna Longitude, TBM930

4 hours ago, homemonster said:

As of version 5, altitude data from MSFS to Foreflght is not accurate. Mine seems to be off by 1000 feet, ForeFlight showing high. Using Flight Events as my connect socket.

Also exhibits wild barometer data. Hope it’s resolved in version 6, but I doubt it. Too many things for them to fix.

Planes generally flown - Cessna Longitude, TBM930

Foreflight displays GPS altitude when used in a real airplane, and that is true altitude above sea level. Foreflight is actually working correctly with MSFS since SU5 because most MSFS-to-Foreflight connector programs display the sim variable for “PLANE ALTITUDE” which is the equivalent of GPS altitude i.e. TRUE altitude MSL.

I use XMapsy as the connector program myself, but most connectors like Flight Events work the same way.

At high altitude, when the air temperature is higher than ISA (which is almost always the case at this time of year), GPS altitude can be as much as 2000 feet higher than indicated altitude (on the aircraft altimeter).

I have seen this myself when using the iPhone version of Foreflight on airline flights. I will get the filed flight plan for the flight from FlightAware and import it into Foreflight. I always sit in a window seat, and can usually get enough GPS signal through the window to track the aircraft’s position on the Foreflight map. I did an airline flight recently from DTW to OMA with a filed altitude of FL340. The captain confirmed that when making an announcement on the PA to give our estimated time enroute. On Foreflight, GPS altitude was showing 35,700 feet, which was our true MSL altitude. (Air temperature was about 12 degrees higher than ISA that day).

The recent change Asobo made to how pressure altitude is calculated in the sim  now causes the “true” altitude variable to generally (and correctly) show higher than indicated altitude when air temperatures are above ISA standard. It would normally show lower than indicated altitude when air temperatures are colder than ISA. (This applies when flying above 18,000 feet with the altimeter set to 29.92). Actual sea level pressure will affect the difference between altimeter altitude and what Foreflight displays as well.

TL:DR the altitude discrepancy you see when using Foreflight with MSFS since SU5 released is not a bug. It is actually correct behavior, and you would get exactly the same result if using Foreflight in a real airplane flying at high altitude.

Now, the sudden jumps in pressure and 500-foot altitude shifts you sometimes see in MSFS is a bug. It does not happen to me on every flight, but sometimes it happens several times on a given flight while in cruise. It is either caused by glitches in the MeteoBlue weather model in certain locations, or (more likely) problems with how the model pressure data is being converted for use in the sim. It could also be related to the use of real time METAR data for airport surface weather in Live Weather.

Edited September 4, 20214 yr by JRBarrett

Ok, thanks Jim. I only brought this up since this was not the case with FSX. FSX and ForeFlight both agreed to the same altitude, but I now understand what they’re doing in MSFS. I am aware of the bug concerning altitude swings, and look forward to a resolve.