Gas Turbines - Auxiliary Power Units and Engine Starting — Page 363, Lesson 451

I want to walk you through the engine start sequence now, and I want you to think of this as the moment the whole powerplant comes alive. The two instruments that demand your absolute attention during a start are the EGT gauge — that's exhaust gas temperature — and the HP compressor rotational speed gauge, which we call N2. These two tell you whether the start is proceeding safely. You'll also be monitoring fuel flow, LP rotation which is N1, duct pressure, and the start valve warning light if your aircraft has one. Let me give you the sequence in order, because the order is everything. When you select start, the starter motor is powered. At this point fuel and ignition are not yet supplied. The compressor begins to accelerate under the influence of the starter motor, and it starts forcing air through the combustion chambers. So you're just spinning the compressor and moving air — no fire yet. When the compressor has achieved the rpm stated for that engine, you activate fuel and ignition by selecting the switch, and you hold that switch until the start is successful. Now here's the term you'll hear constantly: light up. Light up is indicated by an increase in EGT, and it must occur within a specified time — typically 20 seconds. The initial rise is quite sharp, because there's an excess of fuel in the combustion chamber. Once that excess fuel is burnt off, the rise steadies. Now the gas being produced in the combustion chambers adds impetus to the turbine blades, which eases the task of the starter motor, and the engine continues to accelerate. Next, the Fuel Control Unit — the FCU — progressively increases the fuel flow as the compressor accelerates towards idle. This means the air/fuel ratio becomes biased towards being very rich, and the evidence of this is the second steep rise in EGT. So you see two steep rises: the first at light up, the second from the FCU enriching the mixture. Continued acceleration brings the compressor to self-sustaining speed. That's the speed at which the engine can accelerate without the help of the starter motor. But here's a critical point: the starter motor is not de-selected at this point. It's kept supplying power until the engine has accelerated a little more. That gives the engine a better chance of smoothly reaching idle rpm. Self-sustaining speed is approximately 30% N2 — that's the high pressure compressor speed. The starter motor and igniters may be cancelled automatically by a speed switch in the N2 gauge. As the engine continues towards this point, the EGT peaks. This peak is caused by the airflow reaching the value appropriate to the idle fuel flow. When that happens, the temperature drops from its highest value down to idle EGT. When the engine has stabilized at ground idle, you can release the fuel and ignition switch and carry out the after start checks. Idle rpm is approximately 60% N2 and 25% N1. And one important note: these indications will be observed during a normal start regardless of the type of starter motor used. Let me show you this graphically — Figure 24.6 illustrates the RPM/EGT starting relationship, so you can see how EGT and HP rpm should react during a normal start. So the whole story is: spin the compressor, light up within 20 seconds, watch EGT rise sharply twice, pass through self-sustaining speed at about 30% N2, let the starter carry you a bit further, then EGT peaks and settles to idle, and you stabilize at about 60% N2 and 25% N1. That's your normal start.