DENSITY ALTITUDE, Who Cares?
Remember when we went through our private pilot training and the instructor first mentioned density altitude? Most of us had never heard of the term before. I sure don’t remember it being in any of my math or physics courses in high school or college.
Even my instructor didn’t spend much time on the subject in primary training. I remember, “When it is hot and humid, the air is less dense.” Really? What does that mean? Well, my instructor told me that when, “The density altitude is high, like 5000 feet, the airplane ‘thinks’ it is at 5000 feet.” My first thought was, how does an airplane “think?” Then he went on to say that it is pressure altitude corrected for non-standard temperature, more confusion…
Obviously an airplane really does not think. It is a mechanical device and it is subject to the laws of physics, and we learn a lot about that in primary training. My instructor went on to give a mathematical formula for computing density altitude as my eyes focused on a cloud in the sky and I wondered if they know anything about density altitude? I didn’t ask. The answer would have taken too long.
There might have been a question on your Private Pilot Knowledge test (we called that the “written” in those days.) If there was one, you would need the formula to solve for the answer.
In these modern times, we hardly need to solve the problem. The ATIS, AWOS, or ASOS will probably give us the Density Altitude at that airport, no math required. At some airports when the temperature is high, the ATIS will give that number in Celsius, with the further comment: “Check density altitude.” Maybe it is a good idea for us to know how to do that. Remember, it is the pilot’s responsibility to have all available information for the safety of any flight.
Again, who cares? Getting back to that “thinking” airplane, it is a good idea that WE care, if we are flying that day at that airport. As I said, the airplane really does not think, but it does react to the environment it is in.
As an example, say we are at an airport near sea level on a hot and humid day. The AWOS reports that the Density Altitude is 3000 feet. Since the air molecules are farther apart than on a “Standard Day,” it will take a longer take off roll to become airborne, and your climb will be slower. Where do I find that information? In the Performance Section of my Pilots Operating Handbook (POH).
Here is another example: We are at Flagstaff (KFLG) where we are already at 7000 feet above sea level just standing on the ramp. On a hot and humid summer day, the density altitude could well be over 10,000 feet! Now check those performance charts and see what your Cessna 172 will do at 10,000 feet. Take off roll will be longer, climb will be slower, and what is the service ceiling for that tired and old Cessna 172? Remember that the numbers in the POH were put there when the plane was new from the factory, and performance may have deteriorated since then.
You stopped for fuel at KFLG with your wife, your nine year old daughter on board, and some bags in the back for that vacation trip you are taking. NOW is the time for you to care about density altitude. Your passengers are too valuable to risk not knowing how your airplane will react to the DA at that airport.
For another example, on a recent trip I landed at Las Vegas, New Mexico, for fuel. LVS starts off at 6877 feet above sea level. The ASOS reported DA at 9700 feet. After an uneventful landing and taxi to the self service fuel, the engine stopped before I was ready. The reason? The mixture was too rich. I knew right away that I had not leaned the engine for that altitude. I paid much more attention to the DA before departing that airport, and I leaned the engine properly. I sure did not want an engine to stop just after takeoff!
It is not only take off and climb you need to think about, but landing distance and landing performance as well.
Some pilots think that a turbocharged airplane relieves them from thinking about DA. While turbocharging an engine will allow it to develop full horse power up to a critical altitude, it does nothing to change the molecules flowing over the wings, or for the prop to bite into. Turbocharging does a lot for us overall, but it does not release us from understanding the nature of our environment and the effects it has on our non-thinking airplane.
Here in Arizona most of our airports are well above sea level, and we do get some hot days as well as humidity during monsoon time. It is not always a dry heat. If you have lived in Arizona and flown here for more than a year, you probably noticed the difference in your aircraft performance from summer to winter. On a cool winter day, the plane will get you off the ground quickly, and climb much better than on that 105 F day in the summer at that same airport.
So, who cares? We all should. Want to know more about density altitude or any other important subject in aviation? Come to a safety seminar sponsored by your ARIZONA PILOTS ASSOCIATION.
Check the website for seminars near you, and don’t come alone, bring your Wingman!