General Principles - Climb — Page 188, Lesson 218

Let’s start with the big picture. When we talk about climb performance, we’re really talking about Excess Thrust — the thrust left over after you’ve overcome drag. That excess is what actually pulls the aeroplane upward. And the single biggest thing that eats into that excess is air density. Here’s the chain of cause and effect I want you to hold onto. Air density affects the mass flow of air into the engine. More air going in means more thrust available. So when air density decreases, the mass flow into the engine drops, Thrust Available drops, and therefore Excess Thrust drops. And since Excess Thrust is what gives you climb capability, the ability to climb decreases with decreasing air density. That’s the core relationship of this whole section. Now, how do we actually get air density onto a performance graph? We can’t measure density directly, so we break it into two components: Temperature and Pressure Altitude. And I want to be precise about Pressure Altitude, because it’s a specific term. Pressure Altitude is the reading on the altimeter when 1013 hPa is set on the subscale. So you set your altimeter subscale to the standard setting of 1013 hectopascals, and whatever the altimeter reads — that’s your Pressure Altitude. It’s a way of expressing the actual atmospheric pressure in terms of an altitude in the International Standard Atmosphere. So on a performance graph, you’ll typically see a horizontal axis for temperature, and then a series of sloping guide lines, each one representing a different Pressure Altitude. You find your temperature on the horizontal axis, you find your Pressure Altitude line, and where they intersect gives you the compensation for Density Altitude. That intersection is how the graph builds in the density effect. Now let’s talk about Density Altitude itself, because the official definition can be a bit circular. The formal definition is: “A high Density Altitude is one that represents a higher altitude in the International Standard Atmosphere.” That’s true, but it’s not very helpful until you understand what’s going on. So let me walk you through the worked example in the text, because it makes it concrete. Imagine you’re on an airfield that is physically at sea level — the waves are lapping at the end of the runway. But the weather is such that the actual air pressure is lower than the standard sea level pressure of 1013 hPa. You set 1013 on the altimeter subscale, and the altimeter reads 1000 ft. Why does it read 1000 ft? Because the actual air pressure you’re in 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. Now, reduced air pressure gives reduced air density — density is mass per unit volume, and lower pressure means fewer air molecules packed into the same space. But 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 — standard sea level temperature is 15°C, so 15 minus 2 gives you 13°C at 1000 ft. 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 equals 12. And that’s what we call ISA+12. That’s the notation you’ll see on performance charts: ISA plus or minus a number of degrees, meaning how far the actual temperature deviates from the International Standard Atmosphere at that altitude. Now, to get the Density Altitude, you use either a table or a circular slide rule — the kind of flight computer you’ll use in training. In this case, with pressure altitude 1000 ft and ISA+12, the Density Altitude comes out to approximately 2400 ft. So even though the aircraft is physically at sea level, the air density is the same as it would be at approximately 2400 ft in the International Standard Atmosphere. That’s the whole point of Density Altitude — it’s the altitude in the standard atmosphere that has the same air density as your actual conditions. Why does this matter for the engine? A turbojet engine would theoretically generate less thrust at that high Density Altitude, because there’s less air mass flowing through it. And here’s a subtle but important point: the TAS — True Airspeed — would need to be higher for a given IAS, Indicated Airspeed. Because at lower density, the air is thinner, so to get the same dynamic pressure — the same indicated airspeed — you need to be moving faster through the air. Now let’s tie this back to the operational picture. For take-off and initial climb from the same airfield, any increase in pressure altitude or air temperature due to local meteorological conditions will reduce Excess Thrust, and therefore reduce the ability to climb or accelerate. And this is on top of the more obvious effect — that as you climb to high altitude, air density naturally decreases anyway. So you have two effects stacking: the local weather effect at the airfield, and the altitude effect as you climb. Let me show you the geometry of this on the performance graph. The figure shows how temperature and pressure altitude lines intersect to give you the density altitude compensation. So the takeaway for you as a pilot: climb performance is governed by Excess Thrust, Excess Thrust is governed by air density, and air density is governed by pressure and temperature. Hot days and low pressure — high Density Altitude — mean less climb capability. That’s the whole story of this section.