Aircraft
Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
SW-10-43
August 5, 2010
This is information only. Recommendations
aren’t mandatory.
Introduction
This Special Airworthiness Information Bulletin advises you of an
airworthiness concern regarding potential electromagnetic interferences
with required aviation radios caused by non-aviation transmitters
installed in rotorcraft. At this time, this airworthiness concern is
not an unsafe condition that would warrant airworthiness directive (AD)
action under Title 14 of the Code of Federal Aviation Regulations (14
CFR) part 39.
Background
Various government agencies, emergency medical services (EMS),
electronic news gathering (ENG), and other special service operators
have a valid need for non-aviation, multi-band mission radios to be
installed in rotorcraft. These non-aviation radios may be capable of
transmitting directly on radio frequencies reserved for aviation. In
all instances, these non-aviation radios are capable of generating
harmonics while transmitting that creates interference with and
adversely affects the operation of required communication, navigation,
and surveillance equipment in the rotorcraft. This interference may
cause the loss of a function or introduce misleading information to the
flight crew. 14 CFR 29.1431 addresses interference to required radios
on all Part 29, as well as Part 27 IFR rotorcraft. 14 CFR 27/29.1309
addresses required systems functioning under any foreseeable operating
condition for VFR normal category rotorcraft. Hence the interference
caused by non-aviation radios to required systems is a foreseeable
operating condition and needs to be addressed in all installations.
While these interference indications typically recover quickly after
the stop of transmission, interference with required systems is
non-compliance to the applicable rule(s). Many existing installations
of non-aviation radios in rotorcraft are likely to have such an
interference issue. The following adverse harmonic interference affects
have been observed:
- Missed voice communications with air traffic control when the
non-aviation radio is transmitting on a frequency with a harmonic that
coincides with the ATC communication frequency (e.g., the non-aviation
radio is transmitting on 60 MHz which prevents the reception of ATC
communication at 120 MHz through the VHF communication transceiver).
- An unexpected deviation in roll during coupled ILS flight when the
ILS localizer is tuned to a frequency that is close to a harmonic of
the non-aviation radio transmission.
- VOR navigation mode temporarily reverts to dead reckoning during a
non-aviation radio transmission when the non-aviation radio is
transmitting on a frequency with a harmonic that coincides with the VOR
frequency.
- The autopilot uncouples from the ILS during a non-aviation radio
transmission when the non-aviation radio is transmitting on a frequency
with a harmonic that coincides with the ILS frequency.
- GPS satellite signal reception degradation when the non-aviation
radio is transmitting on frequency with a harmonic that coincides with
the GPS frequency.
- An erroneous VOR and ILS flight director needle indication of up to two dots during non-aviation radio transmission.
Recommendations
To Aircraft Certification Offices (ACO) on Supplemental Type
Certificates (STC) and Flight Standards on Field Approvals (FS may
choose to request ACO engineering assistance on these field approvals)
for new installations of non-aviation transmitters, we recommend the
following:
- To minimize the risk of radio frequency (RF) interference,
determining of the optimum separation between the non-aviation radio
transmitting antenna and the ILS, VOR, VHF communication, GPS, etc.
receiving antennas needs to consider the frequencies and wavelengths of
the involved radios. Additional RF filtering (e.g., notch, low pass) in
the RF cables or in the non-aviation transmitting radio may also be
used to further attenuate the harmonic emissions.
- A comprehensive harmonic EMI test is recommended to validate that
non-aviation radio transmit harmonics do not adversely affect the
operation of the required communication, navigation, and surveillance
equipment. This harmonic interference testing is in addition to the
electromagnetic compatibility tests described in Appendix C of
RTCA/DO-313, which demonstrate that the radio does not have other
adverse affects.
-It is further recommended that an issue paper be initiated by the ACO
engineer to document the steps taken to show compliance to the
interference rules as part of the Supplemental Type Certificate data
package.
-Address and submit for acceptance by the Aircraft Evaluation Group,
the instructions for continued airworthiness (ICAs) so that maintenance
of system integrity can be maintained.
Note: Installation of non-aviation radios capable of
transmitting directly on aviation frequencies, which are not FCC Part
87 type accepted (typically military radios), should not be approved
under civil aviation regulations. These radios are for public use only
and their installation will need to be approved under other than civil
aviation regulations.
To operators, crew and maintenance personnel with existing approved
installations of non-aviation transmitters the following steps are
strongly recommended.
- The crew needs to be attentive to and aware of interference issues
while transmitting with non-aviation radios. Appropriate crew
communication and procedural adjustments to mitigate operational issues
is the first line of defense.
- When interference issues are identified, the crew should make a
record of the frequencies and systems affected and make a concerted
effort with flight operations, maintenance, and certification entities
to incorporate the same RF interference elimination procedures used for
new installations (as described above).
- Until the interference issues are resolved, incorporate procedures
and limitations to preclude the transmission on non-aviation radios
during critical phases of flight such as approach, departure and during
IFR operations.