TAILORING OF SPECIFICATIONS FOR RANDOM VIBRATION TESTING OF MILITARY AIRBORNE EQUIPMENTS FROM MEASUREMENT

Bipin Kumar Das, Prakash Kumar

Abstract: Every Military Airborne Equipment needs to be subjected to Environmental Conditions for its Qualification and Acceptance. As a guideline, the standard environmental conditions are generally taken from MIL- STD 810 which is followed as a sacrosanct document for all environmental testing followed in US Department of Defence. Random Vibration Testing is also one particular environmental testing activity and the levels are defined in MIL- STD 810. It also gives the spectrum depending on the zone where the equipment is installed, for which the equipment is to be qualified. Generally the specification is arrived based on the measurements and analysis performed and recorded over the period of years in different aircraft at different zones. The MILSTD also suggests taking the test levels as a guideline only if the measured results are not available and if plausible the measurement may be undertaken and test levels may be tailored. It was observed that generally the test levels suggested in the MIL- STD are relatively stringent resulting in over-testing and in turn over-design of the equipment. In this work, a process is arrived by which the specifications could be tailored from the measured data and ensure that the equipment is tested optimally. The process involves in-flight vibration measurement in all phase of flight (taxi, takeoff, climb, cruise, descent and landing) through the accelerometers and the data acquisition system and software. The auto-power spectral density directly obtained from the software is extracted and the maximum of all phases is considered as the measured vibration spectrum. From the measured acceleration power spectral density spectrum, the Vibration Response Spectrum is obtained for the frequency range of 20 Hz to 2000 Hz. The enveloping of the vibration response spectra is carried out iteratively through trial and error with minimal break points and constant slope lines to find the input PSD spectrum what is proposed for acceptance testing. Further, the qualification testing is done by adding additional 6 dB margin to the proposed acceptance testing levels. As a case study, the process is applied for an LRU installed in Airborne Early Warning and Control Aircraft which is in advanced stage of development.

Keywords: Random vibration, Vibration Response Spectra, Power Spectral Density (PSD)

DOI: https://doi.org/10.15623/ijret.2015.0412057