Gas Turbines - Auxiliary Power Units and Engine Starting — Page 358, Lesson 441

I want to walk you through the APU — the Auxiliary Power Unit — and specifically how it's operated in flight and controlled from the flight deck. This is a gas turbine topic, so let's build it from the ground up. First, the big picture. The APU started as a ground-only device, but it was developed further so it could also be operated in the air. Why does that matter? Because it provides a back-up source of power to the aircraft's systems in the event of an engine failure. And that requirement has become much more important with the introduction of twin-engine aircraft now flying long-haul routes under what we call ETOPS regulations. ETOPS stands for Extended Twin Operations — those are the rules that let a twin-engine jet fly routes far from the nearest suitable diversion airport, and a reliable APU is part of that safety case. Now, the design philosophy behind the APU. Keep it simple, keep it rugged, keep it reliable. But here's the demanding part: it must be able to be started in flight at high altitudes, and continue to operate under load at even higher altitudes. Let me give you a concrete example. The L1011, the Tri-Star, can start its APU up to 25,000 feet, and it will deliver power up to 31,000 feet. So there's a difference between the altitude at which it can start and the altitude at which it can keep producing power under load. Let's move to APU control and operation. Here's a key contrast: the pilot has very little in the way of indication when starting and running the APU, compared to the aircraft's main engines. On the main engines you get a rich set of instruments; on the APU it's much sparser. What might be displayed? Indications of turbine temperature, compressor speed, and system fault indicating lights. So you're looking at temperature, speed, and fault lights. Now, extensive use is made of automatic sensors. These will shut the APU down in the event of an APU fire, system malfunctions, or operating limits being exceeded. So the protection is automatic — the sensors decide, not the pilot. Let's talk about the APU inlet. It may be of single-entry or double-entry design. It will typically have a motorized door which opens when the APU master switch is selected, and it will close automatically after a cooling period on shutdown. So the door is motorized, it opens on selection, and it doesn't slam shut immediately on shutdown — it waits for a cooling period. Here's a fundamental principle that applies to all engines using air as their working fluid: power output is reduced at higher altitudes where air density is reduced. Less dense air means less mass flow, means less power. And this is where the automatic shutdown acts as a governor device, protecting the APU against overloading. So the automatic shutdown isn't just a fire protection — it's also a load-limiting governor. Now, with modern technology, the pilot's flight deck controls for the APU are very few. They usually consist of three things. First, a power-on start switch, labelled PWR ON. Second, a normal stop switch. Third, a manual emergency shut down and fire suppression control. So you have start, normal stop, and emergency shutdown with fire suppression. There's also an external APU control panel, to facilitate shutting down the APU from somewhere other than the flight deck. That's important for ground crew safety. And that leads to the positioning question. The APU is normally positioned in a part of the airframe where its operation will not cause harm to personnel working around the aircraft while it's on the ground. This is normally the tail of the aircraft. So the tail location keeps the exhaust and intake away from people on the ramp. Let's talk about starting. The APU's turboshaft engine can easily be started by an electric starter motor powered from the aircraft's battery. So it's an electric start, battery-powered. Once started, the APU is usually allowed to stabilize in rpm and temperature before it is used to power the aircraft's systems. So you let it settle — speed and temperature — before you put load on it. Now, an important limitation: the APU may not be able to power all the aircraft's systems, but it will provide sufficient services that the aircraft can be operated safely. So it's not a full-power source; it's a sufficient-services source. Finally, when is the APU actually in operation? Normally on the ground during start and taxi of the aircraft. It's operated in the air, as I said earlier, in the event of failure of a main engine. And it is also normally selected prior to landing. So the pattern is: ground start and taxi, in-flight backup on engine failure, and pre-landing selection. Let me pull that together. The APU is a simple, rugged, reliable turboshaft engine, electrically started from the battery, monitored by just a few indications — turbine temperature, compressor speed, fault lights — and protected by automatic sensors that shut it down on fire, malfunction, or exceeded limits. Its inlet door opens on master switch selection and closes after a cooling period. Its power falls with altitude because air density falls, and the automatic shutdown acts as a governor against overloading. The pilot has just three controls: PWR ON start, normal stop, and emergency shutdown with fire suppression. And there's an external panel so ground crew can shut it down from outside the flight deck. The whole unit lives in the tail, away from personnel, and it's used on the ground for start and taxi, in the air as backup on engine failure, and selected before landing.