During your training for your PPL, you read or learned about density altitude, but do you really understand and respect it? It’s hard to truly understand the poor performance without ever experiencing it. Now, it is possible to get a simulated density altitude experience with your instructor at sea level by doing a partial power takeoff. However, it is a bit difference to really be in a low density altitude scenario. I don’t suggest experiencing either simulated or actual high density altitude flying without an instructor or experience pilot on board. But wait, before you go out trying to experience it, let talk a little bit about what exactly is Density Altitude is and how do we plan for it.
Defining Density Altitude
Density altitude is pressure altitude corrected for nonstandard temperature. It's the altitude at which the airplane "feels" like it’s flying. The sole purpose of knowing density altitude is to be able to determine the aircraft performance. The density of the air decreases with altitude. The higher the density altitude, the more your performance will decrease. Regardless of the actual altitude at which an aircraft is flying, it will perform as though it is operating at the existing density altitude—which can be thousands of feet higher than the altitude indicated on the altimeter. When looking at the performance numbers of an aircraft in the POH, you must understand that they are based off standard atmospheric conditions (15 degrees C and 29.92) at sea level. When conditions are standard, pressure altitude and density altitude are the same. You will need to do performance calculations (which you will also find in the POH) with the current atmospheric pressure and temperatures.
Contributing Factors of Density Altitude
As we discussed previously, the 3 contributing factors in changing density altitude are your height, temperature, and the outside humidity.
Height
I’m sure we can all agree that at higher altitudes, the air becomes less dense. The higher you go in altitude, the less atmospheric pressure there will be, which causes the air molecules to spread further apart and be less dense. When taking off at an airport with 5,000 ft elevation, the air will be less dense than that of an airport at sea level. Flying around in Florida, where almost everything is at sea level, you don’t see too much of a difference flying to different airports. However, flying out in the western part of the country, airports can vary a few thousand feet in elevation and you will begin to see a difference in aircraft performance. A good example is taking off from the Phoenix area where elevation can be around 1,000 and heading to Flagstaff where the airport elevation is about 5,000ft. You will have better aircraft performance when you are taking off out of Phoenix than when you are taking off out of Flagstaff.
Heat
As the air temperature warms, the air molecules will gain more energy and begin to move further apart, causing the air to become less dense. As the air density is reduced, the density altitude is raised. With that being said, that means that the hotter it gets, the higher the density altitude will be at that location, which will then cause a decrease in aircraft performance. If you find yourself flying in and out of airports with higher density altitudes, it is best to try and takeoff early morning or late evening when the temperatures are lower. Your aircraft will have its worst performance mid-day when temperatures are at their highest.
Humidity
The more humid the air, the more water molecules there are that that will displace air molecules. Humidity is a contributing factor in density altitude, however, it is very negligible. For the sake of understanding this concept, we will still discuss how (even if very minutely) it effects density altitude. Humidity mostly relates to engine power and relates only a little bit to aerodynamic efficiency. Whereas, altitude and temperature, relate to both engine power and aerodynamic efficiency. As the humidity goes up, the air pressure for a given volume of air goes down. High humidity reduces available oxygen (air) molecules for combustion, which leads to enriched mixture and eventually, reduced power.
Understanding Density Altitude
So, based off of what we just discussed, we can conclude that the worst case scenario would be if you were at an airport at a high elevation above sea level, on a very hot and humid day. All of these factors combined, would cause the density of the air to be much lower than that of a standard day at sea level. As the air density is reduced, the density altitude is raised. I’m going to try and break this down into a more deeper explanation in a different way. Think of a standard day at sea level. The airport field elevation is 0ft MSL and the atmospheric pressure is 29.92 with an outside temperature of 15 degrees celsius. We can agree that when you climb in altitude, the air will become less dense. So think of that in reverse, as the air becomes less dense, its as if the density is that of a higher altitude and aircrafts performance will be reduced at the higher altitudes. You’ll notice when you take off and keep a certain airspeed, the VSI will show less and less climb rate as you get higher in altitude. So now if we go back to the factors effecting air density. On a hot day, or at an airport at higher field elevation, the air density will be decreased. When air density decreases, density altitude goes up, because it is as if the aircraft os operating at higher altitudes with lower pressure. Therefore, the aircraft will have a decreased performance.
Aircraft Performance
Density altitude directly affects an aircraft’s performance. A wing’s lift depends on the mass of air molecules it displaces every second. At higher density altitudes, the air is less dense, and there are fewer air molecules available to provide the needed lift. A propeller’s thrust is dependent on the mass of air molecules it can hurl backward, and similarly, the engine’s power output depends on the amount of air available for combustion. Combined, these result in longer takeoff and landing rolls, decreased climb performance, and a reduction—often significant—in engine performance. I suggest watching the following video to see what happens when you don’t respect density altitude.
Planning for High Density Altitude
We know now know that when density altitude is increased, aircraft performance will decrease. So, what sort of things should we be paying attention to when calculating aircraft performance?
Increase in takeoff distance
Decrease in climb rate
Increase in TAS
Increased ground roll
During your preflight, you need to take density altitude into effect and do a proper weight and balance as well as performance calculations. Remember, this is an FAR requirement. Look in the aircraft POH for the performance charts in order to calculate things like takeoff distance, climb rate, TAS, and landing distance. You could be taking off at an airport at 1,000MSL, but have a density altitude of 5,000ft. You’ll find that
You should always fly indicated airspeed no matter what our density altitude is. Yes, your groundspeed will be higher, but that doesn’t mean you need to fly a slower indicated airspeed. You will land at the slowest airspeed, which will be a faster groundspeed, also adding to a longer ground roll than normal.
Here are a few good rules of thumb:
A normally aspirated aircraft engine will lose approximately 3.5 percent of its horsepower for every 1,000-foot increase in density altitude. So at a density altitude of 7,000 feet, 25 percent of engine power has vanished.
To determine density altitude at a given pressure altitude, add 600 feet to the existing pressure altitude for every 10 degrees Fahrenheit above standard temperature for that altitude. (At her 5,000-foot airport, standard temperature is 41.5 degrees Fahrenheit.)
If high humidity does exist it is a good rule of thumb to add 10 percent to your computed takeoff distance and anticipate a reduced climb rate.
Reach at least 70% or your rotation speed by the runways midpoint. So if your rotation speed is 70knots, you should abort your takeoff if you don’t have at least 49knots by the time you’re halfway down the runway.