As a pilot living in the midwest I always look forward to when the weather warms up and the snow gives way to warm sunny days. But the warm summer air brings with it some unique weather problems for aircraft. Thunderstorms are arguably the most dramatic weather hazard with hail, severe winds, and lightning. That being said however there are other hazards that are just as dangerous and easier to over look. The big one in my opinion is heat itself. The hot air can drastically reduce aircraft performance. Density altitude is just as important to check before flying as it is to check to make sure that you have enough fuel to complete your journey. This becomes especially important when operating from airports in the mountains that are already at a high elevation. What makes density altitude so hazardous? The fact that you can’t see anything outside that would indicate dangerous conditions. Glancing at the thermometer can give a clue that the day may have problems with density altitude, however it isn’t as “in your face” as seeing a towering cumulonimbus cloud spitting lightning out into the sky. But what is density altitude and why is it important? Well, density altitude is pressure altitude corrected for non-standard tempertures. Hotter air is not as dense as cold air, the result is less air molecules in a given space. This reduces the efficency of an airfoil causing propellers and wings to produce less lift. For example, Winterset airport (3Y3) has a field elevation of 1,110 feet above sea level. So on a standard ISA day, even though the airplane is on the ground, aerodynamically speaking the aircraft will perform as if it was flying at 1,110 feet. This fact isn’t much to worry about because the service ceiling of a Cessna 172 is 13,000 feet, however with density altitude this raises that 1,110. If we had a density altitude of 6,000 feet here then the aircraft would perform like it was flying at 6,000 feet even though it is on the ground. This leads to increased runway distances for takeoff and landing. This can also lead to a deficiency in rate of climb, or worst case, a failure to become airborn. So how do we know what the density altitude is at a given airfield? Well, if theres a weather reporting station at your airport then it will be included in the METAR. If there is an AWOS, ATIS, or ASOS at the field then it will also be included in all of those. But say your at an airfield like Winterset where there isn’t weather on the field or an AWOS, what then? Well if you know the temperature outside and the pressure altitude (set altimeter to 29.92. altitude indicated is pressure altitude) then the e6b flight computer can solve that problem. You can also check an airport nearby if they have weather reporting on the field. Once you know the temperature and density altitude then consult you aircraft’s pilot’s operating handbook and calculate your takeoff and landing data for the days conditions. All aircraft accidents caused by weather are preventable. As always, flight safety starts with a thorough preflight. Take a look at the first video showcasing a density altitude accident on a hot day in the mountains.Thankfully everyone on board survived. The second video is an instructional video on how to figure out density altitude on your e6b flight computer.
Until next time, fly safe everyone.
Sincerly, Michael Cornelius
If you liked what you saw here and wanted to discuss it in more detail or have a suggestion for a topic you would like me to cover then feel free to contact me via email or phone.
Manager of flight operations/comercial pilot at Winterset Aviation Services Inc.