Jesse wrote:
It gives the GPS depency topic another kick. We all are quite depended on this technology, and shows that a complete GPS of all GPS units onboard can be a real issue.
I don’t know but it feels like a cheap way out. Not that it’s entirely bad. The ability to calculate all kinds of things from vertical and lateral position data apparently is quite tempting to system designers.
In the Phenom 300, a loss of GPS signal is degrading the AHRS ability to error-check itself. This may lead to a loss of AP and YD in certain flight regimes with the current software. Due to the swept wing, flying without the YD at higher speeds may lead to a dutch roll.
A lot of aircraft have a requirement for Yaw Damper to be operative if they are above a certain level. For example a standard Beech Kingair 200 needs yaw damper above FL170. In theory if you are above that level and the YD goes inop you should descend.
I think that’s what this Notam is saying.
Not sure about an emergency descent though!
RoryDixon wrote:
In the Phenom 300, a loss of GPS signal is degrading the AHRS ability to error-check itself. This may lead to a loss of AP and YD in certain flight regimes with the current software. Due to the swept wing, flying without the YD at higher speeds may lead to a dutch roll.
Stupid I didn’t think of that, its the same AHRS which feeds the yaw damper. Thanks for explaining! Why it’s only this aircraft that is listed? Maybe it is more sensitive then others. The same issue would apply to other swept wing aircraft with glass cockpit as well.
Neil wrote:
A lot of aircraft have a requirement for Yaw Damper to be operative if they are above a certain level. For example a standard Beech Kingair 200 needs yaw damper above FL170. In theory if you are above that level and the YD goes inop you should descend.
Sure, only on a King Air 200 (old ones, that I work once in a while) have an analog cockpit, these don’t have GPS feedback to their gyros. So these are immune.
Stephan_Schwab wrote:
I don’t know but it feels like a cheap way out.
I don’t understand?
Stephan_Schwab wrote:
The ability to calculate all kinds of things from vertical and lateral position data apparently is quite tempting to system designers.
It is, it can generate a lot of usefull warnings and system checks. GPS is very reliable, I would say by far the most reliable avionics system. It is important as pilot to know the system can fail though, when it’s fails is often the aircraft part. If you are aware of that, and on the impact on your aircraft your fine.
In this case there seem to be some temporary external factor.
I still don’t get why the AHRS fails with GPS.
It makes sense that it fails when you lose both GPS and (either of) airspeed/heading, but GPS alone??
@RoryDixon, you write about “certain flight regimes” – what else has to happen in addition to GPS failure for the AHRS to throw in the towel?
dnj wrote:
The G1000 obviously needs GPS to function “fully”, but it does not need GPS for attitude indication provided that it still has airdata/magnetometer – it needs one of the two available in addition to the AHRS.
Correct, though it goes in degrated mode.
Cobalt wrote:
I still don’t get why the AHRS fails with GPS.It makes sense that it fails when you lose both GPS and (either of) airspeed/heading, but GPS alone??
Like with the G1000 the system likely goes into a degrated mode. It seems AHRS depend systems are switched off.
Cobalt wrote:
I still don’t get why the AHRS fails with GPS.
It doesn’t fail, but the answer probably lies in the fact that the AHRS needs to achieve a certain reliability / certainty of it’s attitude solution. For that it uses a variety of inputs including a magnetometer for heading, and the GPS. If you’ve ever flown with a PFD you would have seen the “Cross-check Attitude” warnings or “Check Attitude”. It still indicates, but with degraded accuracy. Similar with a loss of GNSS input, it won’t have failed, but it loses a level of internal cross-checking. I’m going a bit out on a limb here, but I can see how this would be the case.
If the instrument if certified for IFR flight, it would need to achieve a high level of intergrity monitoring and thus without a GNSS input it will not be able to achieve this.
Cobalt wrote:
what else has to happen in addition to GPS failure for the AHRS to throw in the towel?
I did some googling on this and it turns out there is a lot of work being done in the UAV business, where GPS is indeed used to sense yaw.
I guess GPS is a lot cheaper than a yaw gyro…
Peter wrote:
where GPS is indeed used to sense yaw.
How is that possible? A GPS gives you position, not attitude.
