sentences:  Trans-Canada Air Lines (TCA), the Royal Canadian Air Force (RCAF), Canadian Pacific Airlines (CPA) and British Overseas Airways Corporation (BOAC) were the principal operators of the "North Star", with the CPA examples known as the "Canadair Four" and BOAC examples known as the "Argonaut".The RCAF North Stars were unpressurized and were used on a variety of transport duties.Like other North Stars, they were notorious for the high level of cabin noise caused by the Merlin engines, as unlike the radials of the DC-4, the exhaust from the individual cylinders is not collected and exhausted via a single outlet, but instead exits the separate individual ejector-exhaust stubs in high-pressure bursts.