Soft wing impactor for testing of aviation masts

Abstract

Due to the imminent danger of colliding into aviation aids located with close proximity to runways or taxiways, ICAO was in the early 80 s specifically assigned to develop specifications for frangible lighting structures. As a result, the ICAO Frangible Aids Study Group (FASG) was established in 1981 to subsequently propose design specifications and crash test procedures regarding frangibility of aviation aids and supporting masts. Two specific types of impactors were used for testing and development of aviation lighting structures. These two types were rigid and soft impactors. However, during multiple tests conducted the last decades, the rigid impactor was soon discovered to generate initial peak forces far above the limit of 45kN stated by ICAO FASG. It was also impossible to analyse structural damage since National Aerospace Laboratory (NLR), Netherlands, stated that the pass/fail criteria considering frangibility of an ALS must be based on damage applied to the wing. Damage to the skin was accepted, but damage to supporting wing-structures like the front spar was unacceptable. The rigid impactor is still used based on arguments such as inexpensiveness of production and the simple and reusable construction. However, FASG soon stated that only soft-impactors allowed for damage identification and was therefore the correct choice during testing of aviation masts. In this master project, the assigned students have been tasked to document, benchmark and qualify a standard Soft-Wing-Impactor (SWI) based on the proposed design by NLR. In total, four soft wing impactors are built and tested by conducting quasi-static compressions tests. The tests are conducted in accordance with the test procedure specified by NLR and supervisor Rølvåg. The results from testing are compared to criteria specified by ICAO. The main concern considering the validation of a SWI was that supporting aviation structures must not impose peak forces and energy to an aircraft wing higher than 45kN and 55kJ. Test results are also compared to a virtual test carried out by Rølvåg, and to physical compression tests carried out by Wiggenraad et.al. These are used as references in our discussions regarding the test results.

Tensile and shear tests of the aluminum 2024-T3 and rivets were also conducted to verify mechanical properties. The results and results from the physical compression tests are intended for further calibration of the virtual model in Abaqus.

The results proved that the SWI s were sensitive to large shear-stress in the transition between the skin and the tip of the nose-ribs. This happened to all SWI s since the test procedure specified to place the intruder in between the two centre nose-ribs.

Test 1 and 2 yielded low peak forces due to rivets with low shear and tensile strength. The force reached 43 and 37kN before the main-spar was detached from the supporting ribstructure. Based on these results, test 3 and 4 were substituted with stronger rivets which yielded sufficient shear and tensile strength based on tests carried out on the rivets. Test 3 and 4 resulted in a peak force of 47 and 55kN. According to the ICAO limit of 45kN, these were considered sufficient. Test 4 was the most reliable and sustainable impactor and can be used in future tests of aviation masts based on the margin of 10kN.