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Using only GPS as your EFIS inputs??? Why u do dat?

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pfarber

Well-Known Member
Joined
Feb 21, 2019
Messages
1,064
Location
Dollywood
I am not picking on any unit in particular.. but I'm doing some reading and see that many DIY EFIS are not using gyros or direct sensing, but instead using GPS for AHRS.

Example is Flightview.

"The AHRS system uses GPS data to determine if, how much, and in what direction the aircraft is turning. This gives the system a rough idea of the attitude. It combines that estimate with data from a set of solid-state (electronic) accelerometers and gyroscopes which are used to fine-tune the estimate. The resulting estimate is filtered (to prevent jittery output) and sent to the displays."

My read is that the gyros are not primary, which they should be. The system appears to calculate pressure altitude, but starts from GPS altitude

"The air data computer (ADC) provides indicated airspeed, pressure altitude, and outside air temperature. These values are combined with various other factors including the QNH (aka “altimeter setting”) to calculate MSL altitude, density altitude, true airspeed, and vertical speed. "

Can anyone explain to me why GPS data is the primary source on these units? Price?

My questions:

There can/will be times when you lose GPS signals. VFR day, not an issue. VFR night?
Also, FAR 91.205 requires an Airspeed indicator and altimeter. Would not having a direct sensing altimeter, as opposed to a calculated value, satisfy the FARs? There is no testing requirement for VFR altimeters, and you could not use the altitude data for Mode C, therefore an ADSB-OUT unit like uAvionix still needs an altimeter that passes 91.411.

Anyone have any insights or can explain what I am not seeing clearly?? Or should DIY EFIS just be considered cockpit decorations?
 
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