Many variations of aircraft engine starting have been used since the Wright brothers made their first powered flight in 1903. The methods used have been designed for weight saving, simplicity of operation and reliability. Early piston engines were started by hand, with geared hand starting, electrical and cartridge-operated systems for larger engines being developed between the wars. Gas turbine aircraft engines such as turbojets, turboshafts and turbofans often use air/pneumatic starting, with the use of bleed airfrom built in auxiliary power units (APUs) or external air compressors now seen as a common starting method. Often only one engine needs be started using the APU (or remote compressor). After the first engine is started using APU bleed air, cross-bleed air from the running engine can be used to start the remaining engine(s). Piston engines[edit] Hand starting/propeller swinging[edit] A 1918 sketch of ground crew receiving instruction on hand starting Hand starting of aircraft piston engines by swinging the propeller is the oldest and simplest method, the absence of any onboard starting system giving an appreciable weight saving. Positioning of the propeller relative to the crankshaft is arranged such that the engine pistons pass through top dead centre during the swinging stroke. As the ignition system is normally arranged to produce sparks before top dead centre there is a risk of the engine kicking back during hand starting, to avoid this problem one of the two magnetos used in a typical aero engine ignition system is fitted with an 'impulse coupling', this spring-loaded device delays the spark until top dead centre and also increases the rotational speed of the magneto to produce a stronger spark. When the engine fires the impulse coupling no longer operates and the second magneto is switched on.[1] As aero engines grew bigger in capacity (during the interwar period), single-person propeller swinging became physically difficult, ground crew personnel would join hands and pull together as a team or use a canvas sock fitted over one propeller blade, the sock having a length of rope attached to the propeller tip end.[2][3] Note that this is different from the manual "turning over" of radial piston engine, which is done to release oil that has become trapped in the lower cylinders prior to starting, to avoid engine damage. The two appear similar, but while hand starting involves a sharp, strong "yank" on the prop to start the engine, turning over is simply done by turning the prop through a certain set amount. Accidents have occurred during lone pilot hand starting, high throttle settings, brakes not applied or wheel chocks not being used, all resulting in aircraft moving off without the pilot at the controls. [4] "Turning the engine" with the ignition and switches accidentally left "on" can also cause injury, as the engine can start unexpectedly when a spark plug fires. If the switch is not in start position, the spark will occur before the piston hits top dead center, which can force the propeller to violently kick back. Hucks starter[edit] Main article: Hucks starter The Shuttleworth Collection's working Hucks Starter positioned with their Bristol F.2 Fighter The Hucks starter (invented by Bentfield Hucks during WWI) is a mechanical replacement for the ground crew. Based on a vehicle chassis the device uses a clutch driven shaft to turn the propeller, disengaging as the engine starts. A Hucks starter is used regularly at theShuttleworth Collection for starting period aircraft.[3] Pull cord[edit] Main article: Recoil start Self-sustaining motor gliders (often known as 'turbos') are fitted with small two-stroke engines with no starting system, for ground testing a cord is wrapped around the propeller boss and pulled rapidly in conjunction with operating decompressor valves. These engines are started in flight by operating the decompressor and increasing airspeed to windmill the propeller. Early variants of the Slingsby Falkemotor glider use a cockpit mounted pull start system.[5] Electric starter[edit] Main article: Starter (engine) Aircraft began to be installed with electrical systems around 1930, powered by a battery and small wind-driven generator the systems were initially not powerful enough to drive starter motors. Introduction of engine-driven generators solved the problem.[6] Introduction of electric starter motors for aero engines increased convenience at the expense of extra weight and complexity. They were a necessity for flying boats with high mounted inaccessible engines. Powered by an onboard battery, ground electrical supply or both, the starter is operated by a key or switch in the cockpit, the key system often combining switching of the magnetos.[6][7] In cold ambient conditions the friction caused by thick engine oil causes a high load on the starting system, another problem is the reluctance of the fuel to vaporise and combust at low temperatures. Oil dilution systems were developed (mixing fuel with the engine oil),[8]engine pre- heaters were used (including lighting fires under the aircraft) and a Ki-Gass priming pump system was used to assist starting of British engines.[9] Aircraft fitted with variable-pitch propellers or constant speed propellers are started in fine pitch to reduce air loads and current in the starter motor circuit.[citation needed] Many light aircraft are fitted with a 'starter engaged' warning light in the cockpit, a mandatory airworthiness requirement to guard against the risks of starter motor failure.[10] Coffman starter[edit] Main article: Coffman engine starter The Coffman starter was an explosive cartridge operated device, the burning gases either operating directly in the cylinders to rotate the engine or operating through a geared drive. First introduced on the Junkers Jumo 205 diesel engine in 1936 the Coffman starter was not widely used by civil operators due to the expense of the cartridges.[11] Pneumatic starter[edit] In 1920 Roy Fedden designed a piston engine gas starting system, this was in use on the Bristol Jupiter engine by 1922.[3] A system used in early Rolls-Royce Kestrel engines ducted high- pressure air from a ground unit through a camshaft driven distributor to the cylinders via non- return valves, the system had disadvantages which were overcome by conversion to electric starting.[12] In-flight starting[edit] In the event of needing to restart a piston engine in the air the electrical starter motor can be used, this is normal procedure for motor gliders that have been soaring with the engine turned off. During aerobatics with earlier aircraft types it was not uncommon for the engine to cut during manoeuvres due to carburettor design, with no starter system engines could be restarted by diving the aircraft to increase airspeed, 'windmilling' the propeller.[13] Inertia starter[edit] An aero engine inertia starter uses a pre-rotated flywheel to transfer kinetic energy to the crankshaft, normally through reduction gears and a clutch to prevent over-torque conditions. Three variations have been used, hand driven, electrically driven and a combination of both. When the flywheel is fully energised either a manual cable is pulled or asolenoid is used to engage the starter.[14] A Supermarine Spitfire at readiness with a trolley accumulator connected Aircraft began to be installed with electrical systems around 1930, powered by a battery and small wind-driven generator the systems were initially not powerful enough to drive starter motors. Introduction of engine-driven generators solved the problem.[6] Introduction of electric starter motors for aero engines increased convenience at the expense of extra weight and complexity. They were a necessity for flying boats with high mounted inaccessible engines. Powered by an onboard battery, ground electrical supply or both, the starter is operated by a key or switch in the cockpit, the key system often combining switching of the magnetos.[6][7] In cold ambient conditions the friction caused by thick engine oil causes a high load on the starting system, another problem is the reluctance of the fuel to vaporise and combust at low temperatures. Oil dilution systems were developed (mixing fuel with the engine oil),[8]engine pre- heaters were used (including lighting fires under the aircraft) and a Ki-Gass priming pump system was used to assist starting of British engines.[9] Aircraft fitted with variable-pitch propellers or constant speed propellers are started in fine pitch to reduce air loads and current in the starter motor circuit.[citation needed] Many light aircraft are fitted with a 'starter engaged' warning light in the cockpit, a mandatory airworthiness requirement to guard against the risks of starter motor failure.[10] Gas turbine engines[edit] Starting of a gas turbine engine requires rotation of the compressor to a speed that provides sufficient pressurised air to the combustion chambers. The starting system has to overcome inertia of the compressor and friction loads, the system remains in operation after combustion starts and is disengaged once the engine has reached self-idling speed.[15][16] Electric starter[edit] Main article: Starter (engine) Two types of electrical starter motor can be used, direct-cranking (to disengage as internal combustion engines) and starter-generator system (permanently engaged).[17] Hydraulic starter[edit] Small gas turbine engines, particularly turboshaft engines used in helicopters and cruise missile turbojets can be started by a geared hydraulic motor using oil pressure from a ground supply.[18] Air-start[edit] Main article: Air-start system Cutaway view of an air-start motor of a General Electric J79 turbojet With air-start systems gas turbine engine compressor spools are rotated by the action of a large volume of compressed air acting directly on the compressor blades or driving the engine through a small, geared turbine motor.