General Principles - Climb — Page 182, Lesson 212

Right, let's get into the climb. We're looking at how outside air temperature governs the thrust an engine can actually produce, and then we'll touch on a graph that shows thrust and drag together. First, the temperature effect. For a given engine, the higher the outside air temperature — the OAT — the lower the mass air flow. 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. So EGT is exhaust gas temperature — the engine is limited by how hot the turbine can get, and that limit kicks in sooner when the air is hot and thin. Now, look at Figure 3.22. Read it from right to left. It shows thrust increasing with decreasing OAT at a given pressure altitude — but only down to an OAT of ISA + 15°C. Below ISA + 15°C, thrust remains constant. That constant value is the engine's "Flat Rated" thrust. So below that temperature, thrust is no longer limited by turbine inlet temperature; it's limited by the maximum air pressure the compressor is built to withstand. And here's the operational point: below airport OATs of ISA + 15°C, it does not matter how far the flight crew advance the throttle — the engine management computer will maintain "Flat Rated" thrust. That is the maximum certified thrust of the engine. Now, why does this matter for certification? If engines are not flat rated, and the throttles are fully advanced at OATs below ISA + 15°C, a lot more than maximum certified thrust will be delivered. That may not be immediately destructive if done occasionally, but it completely compromises the certification of the aeroplane. Here's the critical link: engine-out critical speeds — VMCG, VMCA and VMCL — are based on the yawing moment generated at maximum certified thrust. 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. So the flat rating protects the certification basis for those critical speeds. Some performance graphs incorporate the flat rated thrust to allow determination of, for instance, the Climb Limit Take-off Weight. That weight will increase with decreasing OAT, but only down to ISA + 15°C. For each pressure altitude, an OAT lower than ISA + 15°C will not give an increase in Climb Limit Take-off Weight. In the EASA Performance exam, the ISA + 15°C 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 indicate the temperature, individually for each altitude, below which the thrust will not increase with an increase in density. Now, the Region of Reverse Command. Figure 3.23 shows Thrust Available in green and Thrust Required in red, plotted against IAS. The intersection of the two curves results in unaccelerated flight at a high speed of 350 KIAS. So that's where available thrust exactly equals required thrust — steady, unaccelerated climb at that speed.