Power Bus Testing
Most electrical and electronic products deployed on aircraft must meet strict requirements with respect to power bus anomalies, such as brown outs, surges, sags, interruptions and transients. To verify compliance, commercial aircraft manufacturers publish standards that vendors and subcontractors must satisfy. For other aircraft, either military MIL-STD-704 or the commercial RTCA/DO-160 test standard is applied.
A significant problem for electrical/electronic equipment suppliers is developing, testing and validating the AC power bus test regimen for the unit under test.
Product qualification usually involves the use of programmable AC/DC power sources that support the voltage, current and frequency range called for in these test standards, which typically require the application of specific waveform types and output sequences.
A significant problem for electrical/electronic equipment suppliers is developing, testing and validating the AC power bus test regimen for the unit under test (UUT), which can be very time consuming and costly. A specific product model or variations thereof could be used on several different types of aircraft; that requires qualification under multiple test scenarios. This would include cases where a new airframe calls for some of the same avionics as an older model, but testing must simulate a new power bus. To further complicate matters, the power source/simulator manufacturer may also need to upgrade its products to meet new airframe requirements.
Because of these complications, any organization performing compliance tests must carefully consider both the performance capabilities of the test equipment that simulates the power bus, and the qualification process the equipment has been through. Generally, these compliance test issues can impact any or all of the following organizations:
➤ Aerospace system development labs
➤ Airframe manufacturers
➤ Aircraft electrical/electronic equipment makers
➤ Airline backshops doing overhauls and return-to-service testing
➤ Military depot repair and service shops
Fortunately, the full-blown qualification process may not be required for articles that are derivatives of existing equipment. For example, an FAA Advisory Circular, AC No. 21-16G dated 6/22/2011, modifies the requirements of RTCA/DO-160 test standards to recognize the realities of equipment operation under actual environmental conditions and evolving needs of the aviation community. In particular, it pertains to applicants using RTCA/DO-160 to seek Technical Standard Order authorization for airborne equipment. Item 6e of AC 21-16G states:
“When a new application is based on the design of an existing approved article, the applicant may ask to use environmental test data from the existing article’s environmental qualification, based on similarity between the two articles. This request must be fully supported with a detailed similarity assessment comparing the changes from the earlier approved article to the article in the new application. The aircraft certification office (ACO) may accept the data if the similarity assessment clearly shows that the design changes will not adversely affect the environmental qualification.”
Essentially, the same considerations apply to the power source used to simulate environmental conditions associated with the aircraft’s AC power bus. In this case, the similarity assessment must consider two major design areas of the power source: hardware and firmware. For example, when AMETEK Programmable Power upgraded its California Instruments SW Series AC/DC power source for compliance testing on newer aircraft, it was not necessary to re-qualify every aspect of the new CSW Series. Where its performance would clearly be identical to the earlier design, it was simply a case of documenting this to reduce testing and verification that would otherwise have been repeated from the SW qualification.
On the other hand, there were hardware and software changes that required new qualification testing. For instance, the CSW combines an AC/DC power source with a high-performance power analyzer and arbitrary waveform generator. This makes it capable of complex testing at a lower cost than traditional test systems requiring multiple instruments, such as digital multimeters, power harmonics analyzers and current shunts or clamps. In addition, a new CPU and firmware meant that code compatibility could not be guaranteed by a similarity assessment.
Therefore, new qualification tests were documented, and a number of units supplied to users for backward compatibility verification when running actual Test Program Sets on legacy systems. This made it possible to provide a more sophisticated GUI and several ready-to-run test routines per published standards for both old and new aircraft, such as RTCA/DO-160, IEC61000-4-11, IEC61000-4-13, Mil-Std-704F, Airbus A350 (ABD0100.1.8.1), Airbus AMD24C, Boeing B787-0147 and Watt-Hour Meter Measurements.
William Ruff is vice president, marketing, for AMETEK Programmable Power in San Diego. He can be reached by email at [email protected].