General Principles - Climb — Page 182, Lesson 210

Let’s start with the core idea of this section: how turbojet thrust behaves with speed, with density altitude, and with temperature. I’ll walk you through each one, because these three variations are the foundation of climb performance. First, speed. For a given engine rpm and a given operating altitude, the thrust of a turbojet is essentially constant with indicated airspeed, or IAS. Look at Figure 3.20 — the thrust available line is flat across the speed range. That’s a defining trait of the turbojet: unlike a propeller engine, where thrust falls off as speed increases, the turbojet keeps producing roughly the same thrust regardless of how fast you’re flying. So, unless we’re talking about the take-off run specifically, we can draw Thrust Available from a turbojet as a straight horizontal line on all our performance charts. That simplification is used throughout the rest of the climb work. Now, density altitude. A turbojet engine is un-supercharged — there’s no compressor stage forcing extra air in beyond the normal compression. That means the engine’s thrust depends directly on the air flowing through it. As Density Altitude increases, which means lower air density, the mass flow through the engine decreases, and Thrust Available decreases. So at high density altitude, you get less thrust. This matters in two situations: during the climb itself, and also when operating from airfields at high Pressure Altitude and/or high Outside Air Temperature — OAT. Let me clarify Pressure Altitude, because it’s a precise term. You determine it on the ground by setting 1013 hPa on the altimeter subscale. If the altimeter then reads 1000 ft on the ground with 1013 set, the Pressure Altitude is 1000 ft — regardless of the actual height of the airfield above sea level. The aeroplane experiences the air pressure that corresponds to 1000 ft in the International Standard Atmosphere. So Pressure Altitude is a pressure-based height, not a geometric height. Figure 3.21 shows this effect: at high density altitude, the Thrust Available line sits lower than at low density altitude. Finally, temperature — and this is where the engine’s limits come in. Generally, the thrust of any turbojet is restricted by the maximum temperature the turbine blades can withstand. The more heat-resistant the blade material, and the more efficient the blade cooling, the higher the maximum turbine inlet temperature can be — and therefore the greater the thrust the engine can safely develop. So the turbine blade temperature limit is the real ceiling on thrust. Now look at Figure 3.22, which plots Thrust Available against Outside Air Temperature, OAT, at a given Pressure Altitude. There’s a "kink" in the curve. At lower OAT, the thrust is "flat rated" — meaning the engine is deliberately limited to a constant, flat thrust value even though the cooler air could allow more. At higher OAT, the thrust becomes "EGT limited" — that’s Exhaust Gas Temperature limited. Here the turbine temperature limit is the binding constraint, so as OAT rises, the thrust falls off. The ISA +15°C point is marked on the chart as a reference — that’s the International Standard Atmosphere temperature plus 15 degrees, a common hot-day reference. So the kink is the transition: below it, flat rated; above it, EGT limited. Let me tie it together. Thrust Available from a turbojet is constant with IAS, decreases with increasing density altitude because of reduced mass flow, and is capped by turbine temperature limits — flat rated at low OAT, EGT limited at high OAT. Those three behaviours are what you’ll carry into every climb performance calculation.