General Principles - Climb — Page 182, Lesson 212

Let's pick up with the climb phase of performance. I want to walk you through a critical concept that governs how much thrust an engine can actually deliver, and it all starts with temperature. For a given engine, the higher the outside air temperature, or OAT, the lower the mass air flow through the engine. Lower mass air flow means lower fuel flow before the maximum turbine inlet temperature is reached. And consequently, the lower the thrust the engine is able to develop. This is known as EGT limited thrust. Let me unpack that. EGT is exhaust gas temperature, and it's the limiting factor here. The engine can only burn so much fuel before the turbine gets too hot. When it's hot outside, there's less dense air coming in, so you hit that temperature limit sooner, and you can't produce as much thrust. Now, here's where it gets interesting. I want you to look at Figure 3.22, and I want you to read it from right to left. It shows thrust increasing as OAT decreases, at a given pressure altitude. But here's the catch: it only increases down to an OAT of ISA plus 15 degrees Celsius. Below that temperature, thrust remains constant. This is the engine's "Flat Rated" thrust. Let me explain why. At OATs below ISA plus 15 degrees Celsius, thrust is no longer limited by turbine inlet temperature. Instead, it's limited by the maximum air pressure the compressor is built to withstand. So the engine management computer steps in. Below airport OATs of ISA plus 15 degrees Celsius, it doesn't matter how far the flight crew advance the throttle. The engine management computer will maintain "Flat Rated" thrust. This is the maximum certified thrust of the engine. Now, from a performance point of view, this matters enormously. If engines are not flat rated, and the throttles are fully advanced at OATs below ISA plus 15 degrees Celsius, a lot more than maximum certified thrust will be delivered. While this may not be immediately destructive to the engine if done occasionally, it completely compromises the certification of the aeroplane. Here's why that's so serious. Engine-out critical speeds — VMCG, VMCA, and VMCL — are based on the yawing moment generated at maximum certified thrust. Let me expand those acronyms. VMCG is the minimum control speed on the ground. VMCA is the minimum control speed in the air. VMCL is the minimum control speed for landing. If significantly more thrust is produced during one-engine-out flight with the IAS at the recommended minimum, directional control of the aeroplane will be lost. In plain terms, the asymmetric thrust from one engine running at excessive power would overcome the rudder's ability to keep the aircraft straight. Some performance graphs incorporate the flat rated thrust of the engine to allow determination of, for instance, the Climb Limit Take-off Weight. This is the maximum weight at which the aircraft can meet the required climb gradient. Climb Limit Take-off Weight will increase with decreasing OAT, but only down to ISA plus 15 degrees Celsius. For each pressure altitude, an OAT lower than ISA plus 15 degrees Celsius will not give an increase in Climb Limit Take-off Weight. The thrust simply won't go any higher, so the weight limit won't either. In the EASA Performance exam, the ISA plus 15 degrees Celsius temperature for each pressure altitude is referred to as the "kink" in the pressure altitude lines of CAP 698, Figures 4.4, 4.5, and 4.29. The kinks in the pressure altitude lines indicate the temperature, individually for each altitude, below which the thrust will not increase with an increase in density. That's the exact definition you need to remember. Now let me move to the Region of Reverse Command. I want you to look at Figure 3.23. Thrust and drag on the same graph will show the result of many variables. The figure shows Thrust Available in green and Thrust Required in red. The intersection of the two curves will result in unaccelerated flight at a high speed of 350 KIAS. KIAS is knots indicated airspeed. So when the thrust available equals the thrust required, the aircraft is in equilibrium — no acceleration — and that equilibrium point here is at 350 knots indicated airspeed. That's the core of what I want you to take away. The flat rating concept explains why thrust plateaus at ISA plus 15, and the thrust-required versus thrust-available graph shows you where the aircraft will settle into steady, unaccelerated flight.