Many years ago a friend of mine was instructing at night in a C150 over the Peak District, talking to Manchester who were reporting negligable surface wind. They noticed a considerable and increasing rate of descent which could not be arrested even with full power.
My friend took control, reduced speed right back to minimum, and they impacted the lee side of Kinder Scout. The aeroplane was wrecked but they survived with minor injuries. They had been caught in strong sink downwind of the hill, despite the few knots of surface wind at Manchester.