My LZ is at 300′ Above Seal Level(ASL). My density altitude last night was 2800′. I’ll explain the real world consequences in just a bit.
First what is density altitude? The simplest way for me to say is: It’s a measurement of how dense the air “feels” compared to what altitude you are really at.
Think about climbing a mountain. As you go up the air becomes thinner, harder to breath. For aircraft this means your wing has less air to “bite”, less lift is created as a result. And just like it harder for you to breath, it’s a tiny bit harder for your engine to breath.
The equation to figure this out takes into account temperature, barometric pressure, and humidity. In hot, humid conditions your density altitude will be higher. Colder temps give us denser air and a lower density altitude.
Flying from home I’ve gotten into the habit of checking winds aloft(ryancarlton.com) and the radar but not tuning into the AWOS(Automated Weather Observation System) at the airport. Mistake.
The Setup: It’s 1700, winds 3-8, and it is hot here in TN with 99.9% humidity(possible small exaggeration). I’m flying with my bulky XC bag, luckily with most of the weight removed. I’ve got 400′ of runway in front of me. With this wind I usually need about 100′.
The Launch: Wing inflates well and within a few steps I have it “locked in”, go full throttle and commit to launch. Pretty quick I notice a lack of lift. I can tell by the sound my engine is spinning up nicely, but the glider just isn’t wanting to fly today. No cravats or other problems so I push on. Finally at about 250′ I start to lift off, barely. I have to tiptoe for another 50′-75′ while applying more than normal brake pressure before I finally break contact with the grass.
The Climb out: Roughly 1500′ beyond my runway there are trees and power lines. Most days I would clear them by 100′ or more easily. But I was not climbing at all. I was “stuck” 20′ up still holding brake pressure at full power. Finally about 500′ after takeoff I started to get some lift but no way was I going to clear the obstacles. I released my left brake pressure and let the glider go into the gentlest bank that would allow me to turn away from the trees, and got away with it.
A friend called a few minutes later from the airport to ask about the flying conditions. That’s when it dawned on me what the likely culprit was and I asked him to check. In the future I will for sure keep a closer eye on the Density Altitude before attempting a launch.