bakerocb
Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA
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 Posted: Fri May 29, 2009 4:26 pm Post subject: Under-Glareshield GPS/WAAS Antenna |
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5/29/2009
Hello Fellow Listers, A while back there was a thread initiated by a
question on where to best mount a GPS WAAS antenna.
Below is a copy of an FAA SPECIAL AIRWORTHINESS INFORMATION BULLETIN that
relates to tlhis subject. A couple of comments:
1) 3MT Dual LockT is 3M's version of a Velcro like product.
http://www.andybaird.com/travels/gertie/superlock.htm
2) My GNS 430W antenna mounted on an aluminum shelf under the fiberglass
fuselage shell forward of the windshield on my KIS TR-1 has given excellent
service. The antenna is mounted with machine screws.
'OC' Says: "The best investment we can make is the effort to gather and
understand knowledge."
===========================================================
1
FAA
Aircraft Certification Service
SPECIAL AIRWORTHINESS
INFORMATION BULLETIN
SUBJ: Navigation: Cirrus Aircraft Under-Glareshield GPS/WAAS Antenna SAIB:
CE-09-32
Mount Date: May 29, 2009
This is information only. Recommendations aren't mandatory.
Introduction
This Special Airworthiness Information Bulletin is being issued to alert
owners and installers to an
airworthiness concern regarding under-glareshield mounted antennas that may
cause loss of Global
Positioning System/Wide-Area Augmentation System (GPS/WAAS) navigation
service on Cirrus
Design Corporation (Cirrus) Models SR20 and SR22 airplanes. This Special
Airworthiness
Information Bulletin applies to all Cirrus aircraft with GPS/WAAS antennas
installed using the
original under-glareshield mounting location when replacing the non-WAAS GPS
antenna.
The airworthiness concern is not an unsafe condition that would warrant
airworthiness directive (AD)
action under Title 14 of the Code of Federal Regulations (14 CFR) Part 39.
Background
Cirrus originally incorporated an internal mounting location under the
aircraft glareshield for non-
WAAS GPS antennas on the Models SR20 and SR22 airplanes as part of the
aircraft type design.
These antennas were secured using the 3MT Dual LockT Reclosable Fastener
SJ3552 (Type 170)
engaged to 3MT Dual LockT Reclosable Fastener SJ3551 (Type 400). This method
to secure the
antennas was evaluated during the type certification process and found
satisfactory.
To upgrade the non-WAAS GPS equipment to GPS/WAAS capability requires new
antennas that
may be installed in the same location using the same fastener method as the
non-WAAS antennas.
There are two potential issues for GPS/WAAS service associated with the
under-glareshield
mounting location and fastener method: 1) robustness of the antenna mounting
method; and 2)
Localizer Performance with Vertical Guidance (LPV) service interruption due
to signal loss from
airframe shadowing. Both issues affect service availability and may
ultimately result in loss of
GPS/WAAS navigation capability. Loss of GPS/WAAS navigation capability
requires the operator to
rely on other on-board navigation equipment that may not have area
navigation or LPV capability.
Antenna Mounting Robustness
Two service difficulty reports (S8Y2008F00000 and 2007FA0000554) indicate
the possibility for
antennas secured using the 3MT Dual LockT fastener system to detach, which
would result in loss
of navigation. A detached antenna will not have an adequate field of view to
receive satellite signals
resulting in loss of GPS/WAAS navigation.
Airframe Shadowing
Cirrus airplanes use composite construction, which, unlike metal aircraft,
is essentially transparent to
radio signals. There is always concern that metal components such as
engines, instrument panels, or
embedded metal mesh for lightening protection can affect GPS/WAAS signal
reception for internally
mounted antennas. Antenna location, even on composite aircraft, can have an
affect on GPS/WAAS
2
signal availability due to signal shadowing and/or attenuation effects.
Shadowing and attenuation are
more critical during LPV operations due to the more stringent requirements
receivers must meet.
The current receiver and antenna performance standards use a conservative
estimate for intra-system
noise environment and antenna gain patterns that provides significant signal
margin. However, the
margin available today will gradually decrease as new systems, signals, and
satellites come on-line
over the next decade. This decrease could potentially cause decreased LPV
availability or loss of
GPS/WAAS navigation on Cirrus aircraft with antennas mounted under the
glareshield.
Recommendations
We recommend that owners and operators of Cirrus Models SR20 and SR22
airplanes using the
under-glareshield location for GPS/WAAS antennas relocate the antennas to
upper fuselage external
locations following Cirrus Optional Service Bulletins SB 2X-34-23 R1 (for
aircraft without a primary
flight display) or SB 2X-34-24 R1 (for aircraft with a primary flight
display). Relocating the
antennas according to the Optional Service Bulletins provides a more robust
antenna mounting; and
assures maximum GPS/WAAS signal reception for optimum service availability
when performing
LPV instrument approaches.
To obtain copies of Cirrus Optional Service Bulletins SB 2X-34-23 R1 or SB
2X-34-24 R1, contact
Cirrus Design Corporation, 4515 Taylor Circle Duluth, MN 5581-1548 or visit
their website at
www.cirruslink.com/mycirrus/servicepubs.aspx.
For Further Information Contact
Kevin J. Bridges, General Engineer, AIR-130; phone: (202) 385-4627; fax:
(202) 385-4651; e-mail:
kevin.bridges(at)faa.gov.
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