Is there a need for a hammer made to the specifications of A-A-1292B, when one from a hardware store is adequate? Is it necessary to get screwdriver bits made to MIL-B-9946 to install a screw tested to MIL-STD-1312 or MIL-STD-1716? Does your office need a chair as shown in Figure 1 from A-A-3180? Just don’t use the previous standard A-A-693, or you may end up with Rust Orange fabric.
Buying a hammer from the hardware store or a chair from the furniture store is faster, much less expensive, and “may be” entirely adequate. However, what if the need is for a device on an aircraft? For example, electronic flight bags (EFBs) in their earliest forms were basic laptop computers that pilots brought with them. It seems that FedEx pilots are given credit there. But is there a need for those laptops to comply with DO-160, MIL-STD 461, or any other military or aerospace standard of that nature? One can argue that passengers use their laptops on aircraft all the time, and the likelihood that any of them are designed to meet DO-160 Category M is rather remote.
In the early 1990s, laptop computers were tested to FCC and VDE standards, predating the CE mandates of 1996—no immunity (read: susceptibility) testing was performed at that time. Emission testing was limited to 1 GHz. I do not indicate that any issues were observed; however, there are many stories of interference caused by laptop computers and other electronics that were used onboard commercial aircraft by passengers.[i]
A direct comparison of various radiated emissions standards can be misleading[ii], as seen in Figure 2. Although the frequency ranges may vary significantly, the differences in test levels do not seem great at first. However, note that the test distances used for commercial testing are typically 3 or 10 meters, while commercial aerospace and military standards are often 1-meter antenna distances.[iii] Commercial testing is typically done outdoors (open area test site) or in a full scan height anechoic chamber. The antenna is moved vertically from 1 to 4 meters in an attempt to get the direct radiated path in phase with the signal reflected off the ground plane. The equipment under test is placed on a non-conductive bench. The power cord is routed vertically to a power source, not to a LISN for radiated testing. The unit is rotated to obtain emissions from all sides. And so forth.
Some of these differences make testing more stringent (antenna scan heights, rotation of the unit), while others may be less stringent. Examples include how the emissions are measured. Peak detection is used for aerospace and military measurements, and an absolute maximum is captured over slowly scanned frequency ranges. Commercial testing uses quasi-peak and average detection, which can provide some relaxation for time-varying signals.
Recognizing the issues with COTS, MIL-STD 461G states in Appendix A paragraph 4.2.4.1, “The use of commercial items presents a dilemma between the need for EMI control with appropriate design measures implemented and the desire to take advantage of existing designs which may exhibit undesirable EMI characteristics.” They go on to clarify that, “The contractor retains responsibility for complying with EMI requirements regardless of the contractor’s choice of (COTS) items”, unless the procuring activity specifies the part.
Some contractors wishing to use commercial electronics may desire to perform some level of MIL-STD 461 or DO-160 testing on the system or subsystem. I was involved in a few systems that were built with commercial equipment. My clients chose to use CE-marked equipment designed for the industrial market. The equipment was intended to pass 10 V/m radiated immunity testing and a similar level of conducted immunity. Shielding and filtering were provided to improve conducted immunity and emission performance. Testing was performed to Navy Below Deck (10 V/m) conducted and radiated susceptibility levels. It was impressive to see how well the equipment performed. One meter, which was not industrial rated for EMC, was discovered during testing and was replaced by another that was. The system was then found to be fully compliant.
The difficulty with using COTS equipment is for environments where emissions must be carefully controlled, for example, to Category H or Category P of DO-160, where notches in the basic limit line may be 20 dB deep or more. These limits can be difficult to meet even for equipment designed for the aircraft. Also, FCC standards do not have any immunity (susceptibility) requirements, so unless the COTS equipment has been CE marked, it may experience some degradation of performance. Such equipment should not be used for any critical function without redundancy and backup systems in place.
So, remember that if you try to use a tablet computer as an EFB on a helicopter, and it flies through an area where Section 20 Category L is important to meet (i.e., fields as high as 7200 V/m), your tablet may become damaged by that very intense field. Of course, it might be useful as a hammer afterward, but I doubt it would meet the requirements of A-A-1292B.
Endnotes
[i] Keith Armstrong has supplied several articles to In Compliance titled “Banana Skins”. Over 10% deal with aircraft, and many of those due to COTS equipment on aircraft.
[ii] DO-160G Section 21 Category L is copyright by the RTCA used with permission.
[iii] Several arguments and issues can arise concerning the use of antennas at lower frequencies at short distances being in the near field, however this is beyond the scope of this discussion.
