Gas Turbines - Auxiliary Power Units and Engine Starting — Page 358, Lesson 443

Let’s start with the Ram Air Turbine, because it’s a neat piece of emergency hardware that sits alongside the APU. Some aircraft, in addition to their APU, are fitted with a Ram Air Turbine — usually shortened to RAT. Its whole job is to provide power to aircraft systems in an emergency situation. So think of it as a backup power source that only really matters when things have gone wrong. The RAT is built around a turbine wheel. That wheel is driven by airflow caused by the aircraft’s forward speed — that’s the “ram air” part. The name comes from the fact that the air is being rammed into the turbine by the aircraft’s motion through the atmosphere. Now, there are two ways the turbine can be arranged. It can be internally mounted inside the aircraft, with the ram air directed onto it through a control valve. Alternatively, the turbine can be extended out into the airflow — so it physically deploys into the airstream. Either way, the design is normally fail safe. That means if power is lost on the aircraft, the RAT will automatically be selected to run. You don’t have to remember to deploy it; the system does it for you. The turbine drives a gearbox. And fitted to that gearbox can be either a generator or a hydraulic pump. Those two devices are what actually do the work — the generator powers essential electrical supplies, and the hydraulic pump powers flying controls, in an emergency. So the RAT gives you the energy to keep the critical systems alive when the main power has gone. Now let’s move to the heart of this section: the requirements of a starting system. To start a gas turbine engine, there are three basic requirements. First, the compressor/turbine assembly must be rotated. Why? To get air into the combustion chambers. The engine can’t start if there’s no airflow being drawn through. Second, fuel must be provided in the combustion chambers. So you need the fuel supply flowing in. Third, ignition must also be provided in the combustion chambers, to start the air/fuel mixture burning. So you need a spark, essentially, to light that mixture. Those are the three basics: rotation, fuel, and ignition. But there are two extra requirements beyond those basics, and these are important for real-world operation. The first extra is the necessity to motor over the engine with no igniters operating. This is sometimes called a “blow out” or a “motoring over cycle.” Let me explain what that means. Motoring over means rotating the engine without ignition. When would you need that? It usually occurs when there has been a failure to start. There are two specific cases. One is called a “wet start” — that’s when fuel has been introduced but the engine didn’t light, so you have unburned fuel sitting in the engine. You dry it out by motoring it over, which blows the fuel through. The other case is a “hot start” — that’s when the engine temperature has gone too high during a start attempt. You cool it down by motoring it over. So the motoring over cycle serves two purposes: drying out after a wet start, and cooling down after a hot start. The second extra requirement is the need for the igniters to be operated independent of the start cycle. So the ignition system has to be controllable on its own, separate from the automatic start sequence. That gives you flexibility in managing the start. Now, the starter motor itself. There are several methods of obtaining engine rotation upon engine start. On modern civil aircraft, the two most common methods of rotating the HP compressor — that’s the high-pressure compressor — are the air starter motor and the electric starter motor. And here’s a key operational point: any starter system will have a “duty cycle.” That’s the time limit that the starter is allowed to be energized. And after that time, it may have to be followed by a cooling down period before you can re-energize it. So you can’t just keep cranking the starter indefinitely — there’s a limit, and you may need to let it cool before trying again. Let me show you what a free turbine turboshaft APU looks like, because that’s the type of APU we’re talking about here. And here’s what the pilot sees on the APU indications panel. Finally, note where the APU is positioned — it’s normally placed in a part of the airframe where its operation will not cause harm. So to recap: the RAT is the emergency power source driven by ram air, with a fail-safe auto-deploy. The starting system needs rotation, fuel, and ignition — plus the ability to motor over without igniters for wet and hot starts, and independent igniter control. And the starter itself, whether air or electric, has a duty cycle you must respect.