Gas Turbines - Auxiliary Power Units and Engine Starting — Page 363, Lesson 456

We're now into the starting malfunctions and the shutdown behaviour of the gas turbine. Let's take them one at a time, because each one has a very specific set of indications and a very specific drill. First, the wet start. The name sounds odd, but it's the classic failure-to-start. The indications are precise: the EGT — that's the Exhaust Gas Temperature — does not rise, and the engine rpm stabilises at the maximum that the starter motor can achieve. So the starter is spinning the compressor, but combustion isn't happening, so the engine never accelerates on its own. Here's the danger. Starting malfunctions on gas turbines are rare, so they always come as a surprise. That means a long period can pass before you realise it's a wet start. During that whole time, fuel is being pumped into the engine, so the engine becomes saturated with fuel. You can confirm this on the fuel flowmeter indication. Now the hazard: if that accumulated fuel ignites, it will produce a very large jet of flame from the exhaust system. That phenomenon is called torching. So before you attempt a second start, you must carry out a "motoring over" or "blow out" cycle. And here's the key preparation: when you diagnose the wet start, do not terminate the start cycle. Instead, close the HP fuel — that's the High Pressure fuel cock — and close the ignition switch, and let the starter continue to turn the compressor for a specified time. That blows the fuel out of the engine before you try again. Now the second malfunction, the hung start. The indications here are different. The EGT is higher than would be expected for the rpm at which the engine has stabilised, and that stabilised rpm is lower than self-sustaining speed — the speed at which the engine can keep accelerating on its own without starter assistance. So the engine is running, but it's stuck below that critical speed. Now, importantly, this high EGT is not greater than the limit. But maintaining the engine in this state will do it no good at all, and could do it a great deal of harm. So the action is to close the HP cock and investigate the problem. The usual answer is that there isn't sufficient airflow through the engine to support efficient combustion — for example, a contaminated compressor. Because of that poor airflow, the gases from the combustion chambers won't have enough power to assist the starter motor in accelerating the engine beyond self-sustaining speed. So once the starter motor cycle finishes, the engine rpm remains stable below the figure that would enable it to accelerate away to idle speed. Finally, let's look at what happens when you shut down a healthy engine. Engine Rundown Time, also called Spooldown Time, is the time taken for the engine to stop after the HP fuel cock is closed. And this is a habit you should build: take a mental note of the rundown time of each engine and compare them. That comparison gives you advance warning of an engine malfunction — if one engine is taking noticeably longer or shorter to spool down, something is wrong before it becomes a serious problem. So to summarise the three: wet start is no EGT rise and rpm stuck at starter speed, with fuel saturation and the torching risk — you blow it out. Hung start is high EGT below self-sustaining speed — you close the HP cock and investigate airflow. And rundown time is your daily health check on each engine.