The external fund manager backed by Berkshire Hathaway's Charlie Munger, Li Lu, makes no bones about it when he says 'The biggest investment risk is not the volatility of prices, but whether you will suffer a permanent loss of capital.' So it seems the smart money knows that debt - which is usually involved in bankruptcies - is a very important factor, when you assess how risky a company is. As with many other companiesSingapore Airlines Limited(SGX:C6L) makes use of debt. But should shareholders be worried about its use of debt?Generally speaking, debt only becomes a real problem when a company can't easily pay it off, either by raising capital or with its own cash flow. If things get really bad, the lenders can take control of the business. However, a more usual (but still expensive) sit