Gas Turbines - Auxiliary Power Units and Engine Starting — Page 363, Lesson 453

Let’s pick this up right where the start sequence leaves off. We’ve got the engine turning, fuel and ignition on, and we’re watching the EGT and the HP compressor speed. Now I want to walk you through what happens when a start doesn’t go cleanly, starting with the blowout cycle. Here’s the scenario. You’ve just made an attempt to start the engine, and it failed. That means fuel was sprayed into the combustion chamber, but the engine never actually lit up. That unburnt fuel is sitting in there, and if you just try to start again, you risk something we call “torching.” Torching is when that pooled fuel ignites suddenly and violently. To prevent it, the fuel has to be allowed to drain away or evaporate — to be blown out — before you make another start attempt. So the starting circuit is built with a facility that lets you run the starter motor without using fuel or ignition. That’s the blowout, or motoring over, cycle. You’re just cranking the compressor with the starter, no fuel, no spark, and the airflow flushes the unburnt fuel out of the engine. Now, here’s a critical number to remember. In most modern turbofan engines, the air turbine starter motor has a duty cycle of 3 to 5 minutes. That’s the maximum continuous running time the starter is designed for. So if the engine fails to light up within the specified time limit, you select the fuel and ignition switch off, but you let the starter keep turning the compressor to blow out the unburnt fuel. Then you can make a second start attempt — but that second attempt, plus the blowout time, must all stay within that 3 to 5 minute duty cycle of the starter. Exceed it, and you risk damaging the starter motor itself. Now let’s move to a different situation — in-flight starting. Suppose the engine flames out while you’re airborne. You don’t necessarily need the starter motor at all. Because the aircraft is moving through the air, the airflow itself is spinning the compressor — that’s the windmill effect. So you can achieve what we call an airborne windmill air start. You simply activate the fuel and ignition without operating the starter motor. The evidence that the relight attempt has been successful comes from the EGT and the rpm gauges. If either the EGT or the rpm shows a rise in value, that tells you a light up has occurred — the engine has caught. Now, starting malfunctions. I’ve said it before, but it bears repeating: the two instruments that demand the most attention during engine start are the EGT gauge and the HP compressor rotational speed gauge. And here’s a practical piece of advice — it’s prudent to keep your hand on the engine fuel and ignition switch during the entire start cycle, until the parameters indicate they’ve stabilized. You’ll see why in a moment. That brings us to the hot start. This is the big one. A hot start is when the EGT runs away toward its limit. And here’s the tricky part — it’s really only possible to determine that a hot start is happening by comparing its indications to those of a normal start. The EGT can initially rise just as it would in a normal start. The rapid acceleration toward the EGT limit only becomes apparent a few seconds into the start. So you don’t see it coming immediately. In many cases, the only chance you have of stopping the temperature limit from being exceeded is the ability to switch off that engine’s fuel and ignition switch as quickly as possible. If you wait for instructions, or you stop to discuss the indications, you will almost certainly cost yourself or your company the price of a new engine. That’s why you keep your finger on that fuel and ignition switch. And here’s the hard rule: if the EGT exceeds the limit by even one single degree, the engine is to be considered unserviceable. One degree. That’s it. The engine is written off for service. So what causes a hot start? The reasons lie almost entirely in having too much fuel and not enough air to cool the gases through the turbine. Let me break that down. The turbine is being hit by hot combustion gases. If there’s too much fuel, the gases are hotter. If there’s not enough airflow, there’s less cool air to mix in and bring the temperature down. Either way, the turbine temperature climbs past its limit. Now, what can cause that imbalance? A variety of reasons. The throttles might not be set to idle during the preflight check, or they might get knocked away from the idle position. That would feed too much fuel. Alternatively, the engine might not be rotating fast enough — maybe a partial seizure because of ice. That reduces the airflow. And here’s a very common fault, and it’s most likely to be caused by a tailwind during the second start of the day. The residual heat in the engine from the first run adds to the problem. So you get a tailwind, which reduces the airflow through the engine, plus residual heat already in the engine from the previous start, and that combination pushes the EGT over the limit. So to tie it all together: the blowout cycle clears unburnt fuel after a failed start, within the starter’s 3 to 5 minute duty cycle. In-flight, you can do a windmill air start without the starter. And the hot start is the danger — too much fuel, not enough air, EGT runs away, and one degree over the limit means the engine is unserviceable. Keep your hand on that fuel and ignition switch until the parameters stabilize.