G3000 interfered GPS signal

[DCTunltd]

Hi,

I had some nasty adventures recently on CJ3+. We've lost GPS1 & 2 signal and because of that we've lost AP, YD and both attitude indicators while at FL400 over Mother Russia.
I sent AReS data to Cessna and they said this is normal, AHRS will behave this way without GPS signal. Really??

About week ago had problems around LCLK, during one approach to LCLK both GPS lost signal so G3000 went into DR mode. All happened over LCA VOR. Once turning final signal was regained and it turned out that map shifted around 3NM. So DR mode did 3NM mistake during 2-3 minutes. Shouldn't it inhibit DR while on approach or in Terminal?

Have you got similar issues on Gx000 equiped aircraft?

Best regards

[zloi]

I flew one week ago on CJ3 (standard Proline avionic) over Russia from south to the northern part and we lost GPS as well (regular thing in Russia). The FMS couldn't restore GPS signal at all, so next 3 hours we were flying in DME/DME or VOR/DME navigation mode. But we didn't have any issue with AP, YD or anyhting. I wouldn't say AHRS should fail loosing a GPS signal - very strange behavior to me.

[Down in front]

Same on the Garmin 3000 in the Phenom 300 (Tel Aviv is a common location for this issue).

Aircraft craps itself and dumps a lot of systems - the FMS only uses GPS for position referencing and the only time it uses ground based aids is during an approach. So AHRS and consequently AP / YD go.

[733driver]

I think this is a very interesting subject. Some modern biz jets rely exclusively or almost exclusively on GPS for FMS position but also other key systems. I do vaguely remember an incident a few years ago on a Phenom 300 where the loss of GPS did indeed lead to the failure of other major systems such as AP/YD and possibly even AHRS. I remember thinking at the time: "that's not good..." Maybe a current Phenom 300 driver or one with better memory than I have can chime in.

I have had similar experiences to the OP on a Citation using G5000. Within moments of losing both GPS we had gone from normal ops to DR. DME/DME did not provide an acceptable FMS position. It doesn't sound like a big deal but a lot of stuff was affected: No synthetic vision, incorrect aircraft position on map. Incorrect wx radar picture with reference to the map, GS wrong, wind info wrong. Distance and time to go wrong, fuel at destination wrong, displayed MSA unreliable. So a lot of the info we rely on to form our SA on a day to day basis was no longer available or unreliable. And of course these things happen mostly in places where there is either no radar (such as Cyprus) or where the work load is already quite high (such as Moscow).

I will be interested to read the experience of others.

[Blenk]

As others have already posted, not an unknown issue on the Phenom. I can't post a link but search PPRuNe for "Phenom 300 & GPS" and you will find some interesting reading under Tech Log including the FAA notice regarding what happens in the Phenom when GPS is not available.

I thought I read somewhere that the issues have since been solved, but couldn't find anything searching again just now.

[DCTunltd]

I thought G5000 planes are equipped with IRS/INS. With range of let's say Sovereign+ and flying charter in Europe you will fly through GPS interfered areas without doubt. Big question mark is how will your Garmin handle it that day and if it's still reliable on approach.

[Beacon_Outbound]

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.