The British-designed Avro RJ85 is a short-haul airplane with four jet engines. What is the purpose of four engines for such a small aircraft?
The reason it has four engines is that it was originally designed to serve remote airfields- the four engines allowed for better short field performance and redundant systems. From Flightglobal.com:
BAE's ABJ vice-president Stewart Cordner ... unusual design aspects of the 146/RJ, including the fact that the type is equipped with four engines and has an airliner-sized cabin.
"I want to bust this myth on the four engines," he says. "It's a unique aircraft design because it's got short-field performance and redundant systems, as it was originally designed to serve remote airfields.
The aircraft is also suited for operations in noise sensitive city airports- it has no thrust reversers, with large air-brakes and full width wing spoilers. These were initially designed with the idea of using the aircraft as a 'feederliner' and regional airliner.
Cordner believes the four-engined146/RJ is also more suited for operations at noise-sensitive airports ... as it is equipped with large wheel brakes and no thrust reversers. Installation of an extra battery allows the first engine to be started electrically without ground power if the auxiliary power unit is unserviceable, which is attractive to operators serving remote locations.
So, basically, the aircraft has four engines due to to its intended use- in remote airfields. From Flightglobal archive:
Perhaps the most frequently asked question about the 146 layout is, why four engines instead of just two? A cynic might reply that the 146 needs roughly 26,8001b of thrust, and that the only way to achieve this with current turbofans is to use four Avco Lycoming ALF502R-3s, But this is only a partial answer. The main advantage of four engines is in hot and high performance from short runways—conditions typically experienced by feederliners.
In any multi-engined aircraft, performance is dictated by the aircraft's behaviour after an engine has failed. It does not matter whether an air-craft has lost one of two engines on take-off, or one out of four—it must still be able to clear an obstacle at the end of the runway and achieve a safe climb rate. The BAe 146's four turbofans give it a greater reserve of power, allowing it to fly in and out of difficult airfields.
Other advantages claimed for four engines include the ability to make three-engine ferry flights without passengers, and the retention of more than half the aircraft's electrical and hydraulic power when an engine fails. Three engine ferry flights would allow an airline to fly its 146s back to base for an engine change, which is much simpler and cheaper than doing it in the field.
Actually, a variant was produced with two engines, but production stopped after only two.
Climb performance is also fantastic. it can climb safely from most airports using much less than full power which helps keep the noise down and provides high safety margins when operating near built-up areas, for example if an engine fails on take-off.
Originally the aircraft was designed with two engines. The problem was that there were not engines with enough power and small size to put it on the aircraft. The only one engine suitable was the Alf 502, but it was not power enough so they had to install 4 in stead of 2. Also they had to redesign the aircraft because in the original model the engines were located on the rear part of the fuselage( like a MD87 or CRJ)