Rolls Royce resumed its descent today after Singapore Airlines said it was replacing engines built by the aero-engineer on three planes.The airline has decided to replace the Trent 900 engines after oil was seen on one of them at Heathrow Airport.The news is a set-back for Rolls-Royce, which rallied yesterday on the back of a $1.2bn deal to supply a Chinese airline with Trent 700 engines.The company's problems began last week, when an engine it made blew up on a Qantas flight, prompting the Australian airline to ground its whole fleet of Airbus A380 super-jumbos. The shares are down by 10% on the week.