Microlighters

“Safety: reflect on it or it may reflect on you.”
DENSITY ALTITUDE
Looking through accident reports from the past several years for a particular time of year can reveal some interesting and instructive information. We hear lots of stories about the dangers of ice and snow and freezing in winter, but what are some of the hazards of this time of year that have led pilots to come to grief? One of the items that appear in too many accident reports as a contributing factor is Density Altitude. Pulling the threads of accidents leads us to some important safety considerations for flying in the warm, summer months. In most of the accidents the pilots were qualified for the intended flight and there appeared to be no mechanical problems with any of the aircraft prior to their contact with the ground. All of the aircraft were at or approaching their gross weight and temperatures were above standard, in the 20 to 30 degree Celsius range
In ground school, we generally discuss, in one form or another, the concept of the “4 H’s”: high, heavy, hot and humid, also known as altitude, aircraft weight, outside air temperature and humidity level. Each of these is a factor affecting aircraft performance. Maneuvering an aircraft increases G loading, this increases wing loading and stall speed, just to put icing on the cake. Our “ideal” scenario for light aircraft operation would be low altitude, a light load and a cold, dry day. In most of the DA accidents, at least three of the four H’s were on the wrong side of pretty, and the pilots were maneuvering just prior to the stall and subsequent crash. One of the key windows we have for predicting reduction in aircraft performance is to understand the concept of density altitude. We have all learned how to calculate density altitude in ground school and probably retained that information right through the written exam. Understanding how the concept relates to aircraft performance and making the needed allowances for the effect it has might just save your life. In simple terms, density altitude is pressure altitude corrected for temperature. It gives us what I like to think of as the aircraft’s experiential altitude: the altitude at which the aircraft “thinks” its flying. If we want to be a bit more specific, we can determine the density altitude ourselves and work from there. Using our E-6B Flight Computer, we look in the window for True Airspeed & Density Altitude, set the air temperature over the pressure altitude and read the density altitude.
While humidity is not nearly as significant a factor as temperature and pressure or weight, it does have adverse effects on aircraft performance. Given two masses of air with the same temperature, the moist air mass will always be less dense. Water vapour in the air affects both engine power output, with piston engines, and the amount of lift an airfoil can produce. High levels of humidity, in effect, increase the density altitude by decreasing the ambient air density. What all these calculations demonstrate for us is that heat, altitude, weight and humidity can seriously degrade aircraft performance. In high density altitude conditions, our take-off roll will be extended, our rate of climb will be reduced and our aircraft’s ability to maneuver will be impaired. As pilots responsible for ensuring safe flight, what we need to know and understand about these factors is the specific ways they will affect the aircraft we are intending to fly.
A hot day does not cause an accident. Neither does operating an aircraft at gross weight or in a light drizzle or taking-off from an aerodrome with a high density altitude. However, coupling these factors with a serious underestimation of their effects on performance and a pilot who thinks he or she can maneuver an aircraft in the same manner and expect the same performance as at sea level on a normal day can set the stage for a tragic outcome.
Just as we know visibility, turbulence and icing are factors that must be considered before commencing flight, so too are heat, altitude, weight and humidity. Knowing what to expect from an aircraft under any particular flight conditions allows us to make the decisions that will result in a safe, enjoyable flight

Marc G wrote: As I understand it - this why fixed wings don't suffer from density altitude as badly as rotor wings.

Am I correct? I have never done any rotor wing theory courses, so just trying to figure it out. Any comments? Corrections?

... Just thinking about it...