Towards safer fuel systems

Safety risks in aircraft fuel systems may be relatively low, but still deserve serious consideration. Sparks occurring in fuel tanks are the biggest hazard in this area, as illustrated by the disastrous explosion aboard TWA 800 in 1996.

The risk of sparks may increase due to various factors. This includes the application of composites with unknown electrical characteristics, the growing use of alternative fuels with unknown properties, the development of More Electric Aircraft (MEA), and the extreme weather conditions in which aircraft have to fly.

The European Framework Programme SAfer FUEL system (SAFUEL) was established to curtail this risk and eventually reduce it by 80%. The programme’s participating aircraft manufacturers, aeronautical institutes and suppliers tackle these problems on many fronts simultaneously. Together with various international partners, NLR plays an important role in this programme by testing new sensors that accurately register the level of fuel in aircraft tanks and oxygen concentrations above. At the end of 2014, NLR’s test array was ready to start testing for SAFUEL, which runs on into 2016.

The SAFUEL participants share a single major objective: to prevent explosions. That too is the aim of NLR’s contribution. The sensors used to measure fuel levels and oxygen concentration make use of optical fibre, a material that, unlike electric cabling, cannot generate sparks.

The key benefit this sensor offers is that it is less likely to generate a spark than traditional fuel gauges. When it comes to oxygen concentrations another element comes into play: once this exceeds a certain threshold value, resulting in an increased spark risk, the pilot has the option of pumping Nitrogen Enriched Air into the fuel tanks, thus reducing the oxygen concentration and explosion risk.

NLR is integrating the sensor elements – supplied by an Italian knowledge institute and a Portuguese university – in a test array that closely simulates the actual fuel system. The 'tank' in the test array can revolve on two axles, allowing the sensors and reference sensors to be exposed to aircraft manoeuvres in a simulated flight profile.

The stipulated Technology Readiness Levels (TRLs) for the sensors are not yet high enough for research to immediately result in market-ready products. Thanks to NLR’s contribution, however, the SAFUEL programme is a big step closer to technologies that reduce spark risk and explosion hazard.

SAFUEL images