Mon, Oct 04, 2010
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).
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