Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
CE-09-32
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 3M™ Dual Lock™ Reclosable Fastener SJ3552 (Type
170) engaged to 3M™ Dual Lock™ 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 3M™ Dual Lock™
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 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@faa.gov.