While we have an ongoing eruption with unknown and varying levels of ash concentrations through out European airspace, how can we resume normal operations? A few isolated test flights by KL/LH can not give a true indication of the impact of cummulative effects on aircraft and engines operating day in day out, 8 sectors a day into varying level of ash concentrations. Are we for example, going to boroscope every engine after each day of flying? No we couldn't.
As suggested in an earlier post, it might be that we need to conduct a series of test flights across Europe in a research context and evaluate the cumulative effects and establish just what is the risk. Exposure time x hazard = % risk. When we consider thousands of flights per day someone somewhere will find the most hazardous area and suffer as a result. We don't want that.