When we talk about performance, in simple terms it is how efficiently you can carry out an activity. But when it comes to the performance of an aircraft, it is related to its engines. Now I’ll not go into the depth of functioning of engines, but lets understand in brief. The engines suck in air, which is mixed with fuel. The fuel air mixture is compressed and combusted. Basically a mini explosion of this mixture takes place inside the engine, and the exhaust that comes out acts as the thrust, because of which the aircraft moves forward.
So what do we understand from this, air and fuel are two necessary elements that are necessary for performance. Fuel is in the control of the pilot, but air is in the control of nature. Now lets revise the concept of density. Density is mass per unit volume. It is essentially how tightly matter is packed together. But with respect to the engine, what are we really interested in, it’s the density of air. Let’s look at the factors affecting density of air and in turn the performance of the aircraft.
Air is nothing but a mixture of gas molecules. So imagine each molecule of air as a ball. Now let’s take two cylinders and fill it with 10 air molecules each. Let’s heat up one cylinder and leave the other cylinder as it is. Now which cylinder will have the higher density of air, the cylinder at normal temperature or the one that is cooler will have higher density of air. The reason behind this is, as you heat up air, the molecules get excited and start moving vigorously, and they get dispersed. Now recollect the definition of density again and you will get it clearly.
So, as temperature increases, air density decreases
The concentration of water vapour present in the air is the humidity. If you take two containers of equal volume and fill them with different gases like below, both containers will always contain equal number of molecules under constant temperature and pressure. But what has changed? It’s the density.
The formula of density being mass/volume and as we know that different gases have different mass, with different gases the density will also change. Now lets look back at the boxes in the previous page and consider that one has dry air and the other one has humid air, which one is heavier?? The answer is dry air. The density of water vapour is at sea level is 0.760 kg/ and that of dry air is 1.225 kg/, which means water vapour only weighs 5/8 of that of dry air. Hence,
As humidity increases the density of air decreases.
We know that as altitude increases, the force of gravity decreases and we also know that the gravity is responsible for the atmosphere to stick to earth.
As altitude increases, density decreases.
But here let us consider temperature and pressure as well. First, lets consider atmospheric pressure which can also be defined as the force exerted by Earth’s atmosphere against a given surface. As altitude increases, we know that the air density keeps reducing, and hence
Atmospheric pressure reduces with altitude.
In case of temperature, i would like to introduce you to the term lapse rate. The Lapse Rate is the rate at which temperature changes with height in the Atmosphere. If the lapse rate is positive, the temperature decreases with height (as it usually does); conversely if negative, the temperature increases with height (an inversion).
The standard lapse rate is 3 degree Celsius per thousand feet, which means with increase in every 1000 feet, the temperature will reduce by 3°C. Why does this happen?? We know that as altitude increases, the atmosphere gets thinner which means there are lesser molecules of air for the sun to heat up. Hence,
As altitude increases, temperature decreases.
So finally what we understand is:
As altitude increases, air density, atmospheric pressure and temperature decreases.
But as we have read before, as temperature decreases , air density should increase right?? Yes, but the effect of pressure is much more dominating in this case.
There’s a famous acronym called 3H of performance, namely HOT HUMID HIGH. So, if it’s hot humid and high, you should reconsider your performance standards by referring to your aircraft charts and also reconsider the load you are planning to carry. Hot, humid and high is a condition that is detrimental for aircraft performance. So, fly safe guys. Happy Landings.