Originally Posted by
DCTunltd
Hi,
I sent AReS data to Cessna and they said this is normal, AHRS will behave this way without GPS signal. Really??
Best regards
The following is correct for the Phenom at least but since it is a similar setup on other G3000 equipped aircraft, I imagine it is applicable...
The primary input for aircraft positioning data is GPS & the GPS sensors provide information for obvious sources such as the moving map etc. Less well known is that GPS is also used behind the scenes for other items.
One of those items are the AHRS units. AHRS is made up of two parts - the Garmin Reference System (GRS) and the Garmin Magnetometer Unit (GMU). From the manual "The GMU provides magnetic information to the GRS. Its power supply is provided by the GRS. On the ground the GMU may be affected by magnetic sources or structures" (which is why there is often a Heading Miscompare between the two AHRS when parked next to large metal hangars)... Now the key point "The GRS uses a combination of internal sensors and external input data to determine airplane heading and attitude. External sources include in addition to the magnetometer, one air data computer and two GPS" "In normal operation AHRS relies on GPS and magnetic field measurements supplied by the magnetometer. If either of these is unavailable or invalid, the AHRS uses air data information for attitude determination"
What this means is that in normal operation, the magnetometers readings are effectively updated and corrected by GPS to keep them both aligned. When GPS is jammed or missing, magnetometer data is still available, however GPS is not hence the correction element cannot take place. On the Phenom this shows as an AHRS 1&2 FAULT CAS message but the AHRS will still function at this stage. (however moving map unavailable / TAWS unavailable etc due to loss of GPS)
Checking the QRH for AHRS 1&2 FAULT instructs the crew to slow down to MAX 240kts / 0.63M. What this does is reduces the 'drift' on the magnetometers. If the drift becomes too large, AHRS FAULT becomes AHRS 1&2 FAIL which also loses attitude / heading information plus the autopilot, ventral rudder and yaw damper along with the other items you mention in your original post.
For the Phenom at least there is no IRS / INS or any means of using DME/DME etc to provide position information automatically. You can still use NAV sources in the normal way.
Hope this helps.