Let me start with the definition — Page 136, Lesson 171

I want to walk you through the concept of density altitude, because it's the single most important number for understanding engine performance on a hot day. Let me start with the definition. Density altitude is the altitude in the standard atmosphere that corresponds to the prevailing pressure and temperature. In other words, it's the height at which the air density you're actually experiencing would be found in the standard atmosphere. It's a convenient parameter when we talk about engine performance figures, because engines don't care about your elevation on a map — they care about how much air they can actually breathe. Now, how do we get it? There are three methods. The first is an airspeed correction chart. The second is a navigational computer — that's the flight computer you'll use in training. The third is an approximate method, and I want you to remember this rule: add to the pressure altitude 118 feet for every degree Celsius that the actual temperature exceeds the standard temperature. Let me walk you through the example. Suppose an aerodrome has an elevation of 5500 feet, with a temperature of ISA plus 30, and a QNH of 1013 millibars. ISA plus 30 means the actual temperature is 30 degrees Celsius above the standard atmosphere value. Standard temperature at 5500 feet would be about +4°C, so the actual temperature is +34°C. Here's the key relationship: higher temperature means lower density. That lower density would be found at a level higher than 5500 feet in the standard atmosphere. So we calculate the density altitude as 30 × 118 = 3540 feet higher than the pressure altitude. The density altitude — the altitude with which engine performance is associated — would therefore be about 9040 feet. You can check this on the flight computer by setting pressure altitude, 5500 feet, against temperature, +34°C, in the Airspeed window, and reading off Density Altitude, about 9000 feet, in its own window. Now let me connect this to why it matters. The power output of an engine depends basically on the weight of mixture which can be burnt in the cylinders in a given time. And the weight of mixture drawn into each cylinder on the induction stroke depends on the temperature and pressure of the mixture in the induction manifold. So when density altitude goes up, the air is less dense, less mixture weight enters the cylinders, and the engine produces less power. That's the whole reason we care about density altitude — it directly ties atmospheric conditions to engine performance. Now, that brings us to superchargers and turbochargers. These are devices designed to overcome exactly this problem — to force more air into the induction manifold, raising the pressure and therefore the weight of mixture available for combustion. That's the natural bridge into our next topic.