General Principles - Climb — Page 188, Lesson 218

Let’s start with the core idea that drives the whole climb performance story: air density. I want you to think of the engine as a machine that breathes. The amount of thrust it can produce depends directly on the mass of air flowing into it. That mass flow is governed by air density — the mass of air per unit volume. So when air density drops, the mass flow into the engine drops, and that reduces what we call Thrust Available. Thrust Available is simply the maximum thrust the engine can deliver under those conditions. Now, climb performance doesn’t depend on thrust alone — it depends on Excess Thrust, which is the thrust available minus the thrust required to maintain level flight. If Thrust Available falls because air density falls, then Excess Thrust falls too. And since Excess Thrust is what powers the climb, the ability to climb decreases with decreasing air density. That’s the fundamental chain: less density → less mass flow → less Thrust Available → less Excess Thrust → poorer climb. Now, how do we handle air density in practice? We can’t measure it directly, so performance graphs present it through two components: Temperature and Pressure Altitude. Let me define Pressure Altitude precisely, because it’s a term you’ll use constantly. Pressure Altitude is the reading on the altimeter when 1013 hPa is set on the subscale. The subscale is the little window on the altimeter where you set the barometric pressure. So if you set 1013 hPa — which is the standard sea-level pressure — whatever altitude the altimeter shows is your Pressure Altitude. Any variation in atmospheric pressure or temperature will change air density, and therefore change Excess Thrust. So both of those variables feed into the climb picture. On the relevant aircraft performance graphs, you’ll see a horizontal axis for temperature, and a series of sloping guide lines for Pressure Altitude. You find the temperature on the horizontal axis, you find your Pressure Altitude line, and where those two intersect gives you the compensation for Density Altitude. That intersection is how the graph builds in the combined effect of temperature and pressure on air density. Now let’s talk about Density Altitude itself, because the official definition can be confusing. The formal definition is: “A high Density Altitude is one that represents a higher altitude in the International Standard Atmosphere.” That’s technically correct but not very helpful until you understand what it means. So let me walk you through a concrete example, because that’s the clearest way to see it. Imagine you’re on an airfield physically at sea level — the waves are lapping at the end of the runway. But due to the weather, the air pressure is low, lower than the standard sea-level pressure of 1013 hPa. With 1013 hPa set on the altimeter subscale, the altimeter reads 1000 ft. Why does it read 1000 ft? Because the actual air pressure is the same as the pressure at 1000 ft in the International Standard Atmosphere. So you are at a high Pressure Altitude — even though you’re physically at sea level, the pressure says you’re at 1000 ft. Now, reduced air pressure gives reduced air density — remember, density is mass per unit volume, and lower pressure means fewer air molecules packed into the same space. But air density is also affected by temperature. In the International Standard Atmosphere, the standard temperature at 1000 ft is 13°C. That comes from the temperature lapse rate of 2°C per 1000 ft, starting from 15°C at sea level — so 15 minus 2 gives you 13. But in our example, the actual Outside Air Temperature is measured at 25°C. So the actual temperature is 12°C higher than standard — 25 minus 13 is 12. We call that ISA+12. ISA stands for International Standard Atmosphere, so ISA+12 means 12 degrees warmer than standard. Now, to get the Density Altitude, you can use either a table or a circular slide rule. In this case, the Density Altitude comes out to approximately 2400 ft. So here’s the key point: although the aircraft is physically at sea level, the air density is the same as that at approximately 2400 ft in the International Standard Atmosphere. That’s what Density Altitude means — it’s the altitude in the standard atmosphere that has the same air density as your actual conditions. It’s a way of expressing how the air “feels” to the aircraft, regardless of where you physically are. What are the consequences? A turbojet engine would theoretically generate less thrust, because the air is less dense. And the True Airspeed — the TAS — would need to be higher for a given Indicated Airspeed — the IAS. That’s because the airspeed indicator measures dynamic pressure, and with less dense air you need more true speed to produce the same indicated reading. Finally, let’s tie this back to the climb. For take-off and initial climb from the same airfield, any increase in pressure altitude or air temperature due to local weather will reduce Excess Thrust, and therefore reduce the ability to climb or accelerate. And this is in addition to the more obvious decrease in air density that happens during a climb to high altitude. So you have two effects stacking up: the local weather effect at the airfield, and the natural density decrease as you climb higher. Both reduce Excess Thrust, and both hurt your climb performance. That’s the complete picture of how air density governs the climb.