Airbus lifted the lid on one of the aircraft industry's worst-kept secrets on Thursday, revealing the scale of average discounts in the cut-throat global jet market. For years, planemakers such as Airbus and U.S. rival Boeing have grabbed headlines by announcing orders at catalogue prices that are far above the real market value of jets. But on Thursday, Airbus issued new figures suggesting that the average aircraft discount is close to 50 percent. That is because the planemaker was forced to reveal the net value of its huge order backlog at market prices under new IFRS 15 accounting rules. The order book is valued at 460 billion euros ($519 billion), it said with its annual results on Thursday. That compares with 997 billion announced at list prices a year ago. Although the manufacturer has taken some new orders since then, it has also delivered aircraft, meaning that the two sets of figures are broadly comparable, according to analysts. Also read|\u00a0Oyo stake sold: China\u2019s largest ride-hailing company Didi Chuxing puts in $100 million Market sources say the rule of thumb is that aircraft sell for about half their advertised catalogue price. But the price can vary according to factors such as engine performance. The catalogue prices are not completely redundant - they are used to calculate adjustments for inflation when planes are delivered years after they were ordered: a key source of additional income for planemakers on top of the published deals.