Thursday, May 12, 2016

Double Secret Environmental Conditions and Test Procedures for Airborne Equipment

RTCA produces DO-160, Environmental Conditions and Test Procedures for Airborne Equipment

RTCA DO-160G Sections


Reading the term of reference for RTCA Special Committee 135, one might surmise that DO-160 provides the basis for qualification of avionics installed on aircraft.
RTCA Special Committee 135 (SC-135) published the first version of DO-160() in February 1975. It has been periodically updated, so that it is now at revision G. DO-160() defines minimum standard environmental test conditions and standardized procedures for airborne equipment environmental tests. These tests provide a means to determine the performance of airborne equipment in environmental conditions representative of those encountered when the equipment is installed and operated on aircraft.

DO-160() is referenced in a wide range of specifications and standards. RTCA references DO- 160() in minimum operational performance standards (MOPS). Regulatory authorities throughout the world refer to DO-160() in standards such as FAA technical standard orders (TSOs), and advisory circulars (ACs). Aircraft, avionics, and equipment manufacturers refer to DO-160() in equipment specifications. Test laboratories rely on DO-160() for standard test procedures.

SC-135 has worked and will continue to work collaboratively with EUROCAE WG-14(), which has produced EUROCAE document ED-14(), which is technically identical to DO-160().
SC-135 has labored for over 40 years to produce the foundation for judging whether equipment will perform across the range of conditions that might be encountered when installed on an aircraft.  Susceptibility and emissions scrutiny have evolved particularly as radio equipment operates at higher and higher frequencies.
For an example, Ka-band carrier frequencies may achieve 30 GHz, at an EIRP over 40 dBW, and across more than 100 MHz.  While boresite emissions are directed towards a satellite, side-lobes may illuminate the entire upper fuselage and empennage. DO-160G requires susceptibility only to 18 GHz, meaning no airplane equipment is tested against Ka-band emissions.  DO-160 transmit spectral mask stops at 6 GHz, but also excludes intentional carriers anyway.  It is up to the approving authority to accept a test program that is adequate.  Airplane compatibility testing is generally added to laboratory qualification when involving intentional radiators.
Taking a closer look, for a given system, someone has to establish a basis for qualification.  DO-160 offers different categories (and versions) within each section to reflect that each aircraft installation may create different challenges.  For example, equipment installed on the outside of the fuselage is exposed to a very different environment than if installed inside the pressure vessel.  ARINC 791 Part 1 provides a tabulation of suitable categories for a Ku-band (and Ka-band) satcom installation based on DO-160, but this information is at best guidance, reflecting a snapshot in time.

Equipment suppliers must account for qualification standards when designing and testing their equipment.  The test setup a supplier might use may not be acceptable when the equipment is submitted for formal testing.  A supplier may discover that changing the test setup to satisfy the test approving authority may now cause their equipment to fail. The supplier may be forced to unexpectadly redesign the equipment in order to pass the test in the new setup.

DO-160 and ED-14 include considerable details around test setups, yet not every detail can be accounted for.  Each approving authority may express some interpretation in the test setup or test pass criteria that can create unexpected issues.

DO-160 and ED-14 are available to the public for a modest charge.  Each certification program includes submission of a qualification test plan and test report.  Each approving authority must agree to the qualification test plan before considering the test results.  The equipment used for qualification testing must be shown to conform to production drawings (to reflect production equipment, not special golden test units), the test setup must be shown to conform to the test plan, and the test results must be reviewed and accepted without any exception.  Any test result that does not pass the criteria would result in withholding qualification approval unless a waiver is agreed to in light of specific considerations around the specific issue.

The equipment supplier is faced with a whole new challenge when the approving authority includes the original airplane equipment (airplane) manufacturer (OEM), in particular for installation under a type certificate.  In these cases, the OEM levies a suite of special, proprietary test conditions to supplement or revise those expressed in DO-160 or ED-14.  Furthermore, the OEM may not accept previous test results because they did not approve the test setup or witness the testing themselves.  Equipment suppliers should generally assume they may have to repeat their qualification test program for each OEM.

The biggest issue with an OEM installation is with "double-secret" qualification test standards.  While an equipment supplier can develop to public DO-160 standards selecting the most conservative categories, they will be dismayed to discover that the OEM has created their own test criteria that supplements DO-160 in procedure, in standards, and in breadth.  These OEM test standards are considered proprietary, and only available to an equipment supplier under contract with the OEM, and usually only associated with an approved airplane installation agreement.

I have personally written test procedures, conducted test programs, and written test reports that have been adopted for retrofit and for line-fit on all Airbus and Boeing airplane production models.  Yet I cannot take these lessons learned forward, as each project must work with the information it is authorized to have access to.  I cannot advise a supplier whether their equipment will meet an OEM standard without the supplier first getting the OEM to provide the standards to the supplier.

One could ask why some OEM test standards are held proprietary, when these same parties work together to create DO-160 and ED-14.

One could ask how a retrofit project on a given airplane is held to a different standard than one does under production authority, and does that imply a different safety standard.

One could ask what cost to the industry when equipment suppliers have to redesign their equipment to meet OEM standards, while that same equipment is already approved for retrofit, or even for line fit by another OEM.   This issue can delay a line-fit delivery by more than two years from when the supplier enters service under retrofit (STC) authority, and considerable additional expense from development and managing multiple part numbers.

I yearn for when the OEMs openly share their qualification standards in a comprehensive way, if just to make their documents readily available.  Giving access to this information will ensure equipment supplier and all approving authorities are working to a common criteria.  The OEM should expect that their airplanes modified in service will operate "better", and that their line-fit programs will proceed with less disruptions.

Stay tuned,

Peter Lemme
peter @ satcom.guru

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