Gas Turbines - Air Inlets — Page 226, Lesson 295

I want to walk you through the operational considerations for gas turbine air inlets. We've covered the design and construction of the intake, so now we're looking at how the pilot actually manages it in real-world conditions. The whole theme here is protecting that stable airflow to the compressor face. Let's start with take-off. The engine air inlet is designed to maintain a stable airflow to the compressor face. Anything that disrupts that airflow and makes it turbulent can cause the compressor to stall or surge. Now, a compressor stall or surge is a serious event — the airflow through the engine breaks down, and you can get a loss of thrust or even a flame-out. So the intake's job is to deliver smooth, even air, and we have to protect that. Here's the key limitation: the intake cannot cope with high angles of attack and still maintain a stable airflow. Think about what happens at take-off. The aircraft is pitched nose-up, so the airflow into the intake is coming at an angle. One of the most critical times is during acceleration of the engine to take-off power. That's when the engine is spooling up hard, demanding a lot of air, and the intake is being asked to deliver it under a high angle of attack. Now add a crosswind. Any crosswind can affect the airflow into the intake, and this is particularly true for those aft body mounted engines that have an 'S duct' type of intake — like the TriStar and the 727. The S duct is a curved passage that routes air from the intake to the engine, and that curvature makes the airflow more sensitive to disruption. A crosswind can create a distorted or turbulent flow entering that duct, and that can trigger a stall or surge. So how do we avoid it? The procedure defined in the operating manual must be followed. Typically, that means getting the aircraft moving forwards before smoothly increasing the power setting to the take-off value by 60 to 80 knots approximately. This is called a rolling take-off. The idea is that forward motion aligns the airflow with the intake, reducing the angle of attack and the crosswind effect, so the engine can accelerate to take-off power smoothly without stalling or surging. Next, let's talk about icing. Inlet icing can occur if conditions are conducive. The typical conditions are: ambient temperature below plus 10 degrees Celsius, visible moisture, standing water on the runway, or the RVR — that's Runway Visual Range — being less than 1000 metres. If any of these conditions exist, the pilot should activate the engine anti-icing system. That system heats the inlet surfaces to prevent ice from building up and breaking off into the compressor, which could cause damage or disrupt the airflow. Now, damage. Damage to the intake, or any roughness internally in the intake, may cause the incoming air to be turbulent and may disrupt the airflow into the compressor, causing stall or surge. So during intake inspection, be particularly careful to notice damage, uneven skin panels, surface roughness, and so on. Even a small imperfection can disturb the boundary layer and create turbulence that propagates all the way to the compressor face. Finally, foreign object ingestion. Damage to compressor blades is invariably caused by ingestion of foreign objects while the aircraft is on or close to the ground. This is why we pay particular attention to the area on the ground in front of the engine intakes prior to engine start, to ensure it is free of loose stones and other debris. A loose stone sucked into the intake at high velocity can strike the compressor blades and cause real damage. So the thread through all of this is: the intake must deliver smooth, stable air to the compressor. Take-off is the critical time because of angle of attack and crosswind, so we use a rolling take-off. Icing conditions trigger the anti-icing system. Damage and foreign objects create turbulence or physical harm, so we inspect and we clear the ground area. Each of these is a way of protecting that stable airflow and preventing compressor stall or surge.