Delegate Newsletter Volume 6, Number 1

The DO-160 Environmental String Explained!

Commonly referred to, but seldom understood? That is probably an accurate description for the term "DO-160 Environmental String" that is quoted in relation to the environmental qualification of equipment. 

DO-160 is the RTCA document for the Standard describing "Environmental Conditions and Test Procedures for Airborne Equipment", the latest revision being DO-160D. The DO-160 environmental string refers to the "string" of letters and numbers that are identified on the nameplate of avionics or other electronic equipment, or in the Environmental Qualification Form or Installation Manual. An example of a string would be: DO-160C Env. Cat. [A2F2]WBA[MN]XXXXXXABABAATAKXX. To the uninitiated this looks like a lot of mumbo-jumbo, but it is very important to an installer, delegate or certification engineer.

So, why is it important?. The answer lies in the certification basis of the aircraft in which the equipment is to be installed. For example, AWM/FAR 25.1301 states: "Each item of installed equipment must - (a) Be of a kind and design appropriate to its intended function;" Also, AWM/FAR 25.1309 states: "(a) The equipment, systems, and installations whose functioning is required by this subchapter, must be designed to ensure they perform their intended functions under any foreseeable operating condition." So, how can compliance be demonstrated to either or both of these requirements? For this one must look to the equipment manufacturer. 

For an article that has an FAA Technical Standard Order Authorization (TSO), the TSO requires the manufacturer to test the equipment to verify that it will meet the TSO performance standard over the range of environmental conditions that it will experience when installed in the aircraft. FAA AC 21-16D calls attention to the use of DO-160D for TSO authorizations. The range of environmental conditions is defined by Sections and Categories in DO-160. Each Section defines a particular environmental condition, which range from the "shake and bake" conditions such as temperature and altitude, vibration and humidity, to the "electro-magnetic effects" conditions such as radio-frequency susceptibility and emissions, and lightning induced transients. Then, within each Section there are Categories for levels of severity of the test conditions. So, the "string" consists of an identification of each Section and its Category, e.g. [A2F2] in the above DO-160C string is the first environmental Section, which is Section 4, Temperature and Altitude, and indicates Categories A2 and F2 have been met. If you refer to DO-160C you will see specific operating and survival temperatures have been specified for these Categories, and also altitude conditions related to the pressurized or non-pressurized location of the equipment, and maximum operating altitude of the aircraft. Following through the string, each letter identifies the Section/Category. In some cases there will an X identification, which indicates that this test was not performed, and hence the equipment is not qualified to that Section. In the DO-160C string example above, the sequence of XXXXXX in the middle of the string identifies that the equipment was not tested to Sections 6 through 11; Explosion Proofness, Waterproofness, Fluids Susceptibility, Sand and Dust, Fungus Resistance, and Salt Spray. So what, you might say? Well, if this equipment is to be installed in an aft equipment bay, outside the pressure vessel, with fuel lines and an APU above it (i.e. in the "hell-hole"), it should probably have been qualified for fluids susceptibility and explosion proofness. Details of the environmental string nomenclature can be found in Appendix A of DO-160( ). 

In a review of the DO-160 string for acceptability relative to a particular installation of the equipment, the installer, delegate and certification engineer should evaluate the basis of certification of the aircraft and the revision of DO-160( ) claimed for qualification of the equipment. Equipment with, for example a DO-160A qualification may not be acceptable for installation in an aircraft with a recent basis of certification, where equipment was qualified to DO-160D. In such a case, the engineers will have to look at each string Section to determine if the Category is acceptable to qualify the equipment for that particular aircraft. Even if the equipment has a DO-160D string, the Categories still must be evaluated for acceptability to the particular installation.

As an example, if the vibration Category is identified as DO-160D [SBM] this equipment should not be installed in a rotorcraft, as [SBM] is a fixed-wing qualification. Rotorcraft installation would require an additional Category such as [UF] for vibration. Also, for avionics installations in modern, digital aircraft, the RF categories for Sections 20 and 21 are very important, and must be matched to the criticality of the equipment functions and installation location of the equipment. One final item to look for is that the string may not tell the complete story, and the engineer should always look at the Environmental Qualification Form for any specific conditions associated with the identified Categories.

The final word is that just because an article has a DO-160 "string" identified for its qualification, this does not mean that it is automatically compliant with the applicable installation standards, e.g. 25.1301(a) and 25.1309(d). In each case, close scrutiny must be made of the environmental Sections and Categories to ensure they are appropriate to the operational and environmental conditions of the aircraft and installed equipment location.

 

Date Modified: 2009-02-24

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