Gas Turbines - Auxiliary Power Units and Engine Starting — Page 358, Lesson 446

I want to walk you through the air starter motor, which is the most popular engine starting system in use today. Let's start with why it's so popular: it's light, it's simple to use, and it's very economical because it runs on low pressure air. The motor itself is fastened to the accessory gearbox of the engine — that's the gearbox that drives all the engine's accessories. Now, the air to drive this starter has to come from somewhere. There are three sources, in order of preference. First is the aircraft's own APU, the Auxiliary Power Unit. Second is the Ground Power Unit, which is external equipment at the airport. Third is a cross-bleed start, where you use bleed air from an engine that's already running to start another one. Here's the sequence of what happens. Air from one of these sources is fed through an electrically controlled start valve to the air inlet. That air rotates the turbine rotor, and then it's exhausted. The turbine turns a reduction gear, and that reduction gear rotates the engine drive shaft through a sprag clutch ratchet. So the sprag clutch is the mechanical link that lets the starter turn the engine. Now, ignition may be selected automatically at the same time as engine start, or it may be selected in conjunction with the introduction of fuel. A few moments after the engine starts rotating, the fuel HP cock is opened — that's the high pressure fuel cock. A few moments after that, the engine should "light up," meaning the fuel ignites. You know it's lit because you see an increase in EGT, exhaust gas temperature, and the engine accelerates more rapidly. At a predetermined engine speed — and this is important — a speed greater than self-sustaining, the start valve closes. Self-sustaining speed is the speed at which the engine can keep itself running without the starter's help. Once the valve closes, the sprag clutch automatically disengages as the engine accelerates up to idling speed, and the starter motor stops rotating. Why does the sprag clutch disengage? Because it's designed to prevent the starter motor from being driven by the engine after start. If the engine drove the starter, the starter would spin at a speed high enough to break it up due to centrifugal force. That's the danger — the starter would literally fly apart. There's also a flyweight cutout switch that may be included. This switch shuts off the starting air supply by removing the electricity that energizes the starter air valve. So it automatically terminates the engine start cycle when the engine reaches a speed slightly above self-sustaining speed. That's a backup safety device to make sure the starter doesn't get overdriven. So to summarize the whole cycle: air comes in through the start valve, spins the turbine, the reduction gear and sprag clutch turn the engine, fuel and ignition light it up, EGT rises, and at the right speed the start valve closes and the sprag clutch disengages — with the flyweight cutout as a safety net. That's the complete air starter motor system.