In-depth lessons learned: Review of an avionics thermal analysis project

We present the results of an in-depth review of the thermal analysis of a complex, high performance, commercial air transport, avionics product. The initial analysis did not correlate well with the subsequent test results for one key scenario. The product in question is the data processing element of a cockpit display system for a new, large commercial aircraft. The product is forced-air cooled; designed to meet Aeronautical Radio, Incorporated (ARINC) Standard number 600. It comprises a top-level chassis or Line Replaceable Unit (LRU,) that dissipates approximately 100W with several subsidiary LRUs or modules inserted into it. The system had a requirement to operate for 180 minutes after the loss of the aircraft supplied cooling air; termed a Loss of Cooling or LoC scenario. Given the team's long years of experience in analysis of this sort, it came as a surprise when the up-front computational fluid dynamics (CFD) analysis results failed in a non-conservative way to predict the performance of the system under these conditions. This lessons-learned project uncovered a number of facets of the original analysis work that go beyond a simply flawed analysis approach. Several of the usual assumptions for this type of CFD modeling proved to be inadequate and/or incorrect. As a side benefit of this effort, a procedure for quickly and reliably correlating a large complex thermal model to measured thermal data was developed and refined. The results presented here are applied on and will improve the results of all follow up development projects.