Problem Traced To TAWS Sensor

Avidyne said late last week that following multiple reports from R9 owners about their Enhanced Ground Proximity Warning System (EGPWS) not working, its engineers determined that there is a problem with the GPS receiver's transmission of the Figure of Merit (FOM) to the TAWS sensor.  During the investigation, it was also discovered that this incorrect high FOM value will cause LNAV+V approaches to not be enabled.  The company says there is no problem with the GPS position data utilized within R9, and that they are working on a minor software release to fix this issue which should be released within the next two weeks. Avidyne says this software release is only needed for those who have Release 9 and the EGPWS, and that it is recommended immediately for operators who rely heavily on the TAWS. For other users, the company says Release 9.2, which includes the R9 DFC100 software, will also contain this fix.

A Service Information Letter (SIL) from Avidyne advises owners and operators of Avidyne Entegra Release 9-equipped aircraft that they may lose terrain awareness provided by their TAWS system. This only applies to TAWS systems which rely on GPS data from the R9 Integrated Flight Display (IFD). European, Asian, and African customer are not affected. This SIL also advises owners and operators of Avidyne Entegra Release 9-equipped aircraft that they may lose the advisory vertical deviation indication (VDI) in LNAV+V approaches.

The problem was found to be due to increasing positional uncertainty of a drifting WAAS satellite. While the influence of this satellite on the GPS position solution is minimal, its positional uncertainty affects R9’s Figure of Merit calculations, causing the FOM to exceed a maximum limit. This in turn causes R9 to invalidate the FOM output to external TAWS devices, causing the TAWS to cease monitoring. A high FOM value will also invalidate the advisory VDI indication in a LNAV+V approach, reverting the approach to a LNAV only.

Avidyne stresses that the GPS position integrity is not compromised in any way. Navigational procedures and approaches do not need to be altered. Also, when established on a precision approach (LNAV/VNAV, LP or LPV), the faulty satellite is excluded from all calculations and FOM will return to a normal  range, restoring TAWS for the duration of the approach.

Avidyne says that customers who notice this issue can contact the Avidyne Entegra Release 9 Hotline at 1-877-MyAVIR9 (1-877-692-8479).

FMI: www.avidyne.com