Summer Flying -- Hot & Humid

For those of you who have been flying this summer you have no doubt noticed that your trusty steed does not leap off the ground and climb with the same authority that it did in the spring and fall (winter performance is particularly spectacularly).  Why is that?

Air Density

Airplanes like people need air.  Their engines need to combine air with fuel to create power.  The propeller needs air to convert the engine's power to propulsion.  The wings need air to create lift.  The thicker the air the better the aircraft performs because there are more molecules to work with (oxygen in particular).  The same is true for us.  This summer my family and I spent some time in the mountains in Colorado and Wyoming.  Going from our home at 900 feet to our mountain campgrounds averaging 7000-9000 feet required some getting use to.  It was much harder to breath and we got winded much more quickly than we did at home.

So what affects air density.  As you may have guessed based on my mountain camping example above altitude is a big factor.  Other factors affecting air density are heat and humidity.  Let's look at each of these.

Altitude

Simply stated, the higher we go the less dense the air is.  I think everyone knows this.  The air at sea level has all the air above it pushing down on it creating pressure.  In fact, this air typically creates enough pressure to raise mercury in a tube 29 to 30+ inches.  But as you go higher the amount of air pushing down gets less and less.  That means the number of molecules are getting farther and farther apart (less dense).

Temperature

Temperature also affects the density of all substances.  With the exception of water (which actually expands when it freezes) all other matter gets denser when it gets colder and less dense when heat is applied (even water gets denser as it gets colder but because of its crystalline properties it does expand when it turns into a solid).  Air is no different.  In cold air the molecules move slower and get closer to each other.  Apply heat and those molecules start flying around and spreading out.

Humidity

While the effects of altitude and temperature are intuitive and easy to understand, it is not obvious why adding water to air makes air less dense.  After all, water is pretty heavy stuff.  So what is going on here?  As it turns out moist air is less dense than dry air.  That's why moist air rises until the moisture condenses out and forms those nice fluffy clouds that tell us pilots the air is bumpy and unstable.  While water is heavy steam is not.  Steam rises because it is less dense than air.  Performance charts don't include humidity in their calculations because of the three it has the least impact but when combined with the other two it does affect overall performance.

Why do we care?

When I fly out of my home airport with a runway of 7000' I don't care.  I'm at 900 feet MSL (mean altitude above sea level), so even at gross weight on a hot and humid day I have plenty of room to take off, and in my aircraft I'm still going to climb out at 700 feet/minute or better.  But 25 miles to the north is a grass strip that is surrounded by trees and obstacles on three sides.  At that field, if I am at gross on a hot, humid day I always take off in the one direction without obstacles.  Yesterday was one of those very hot, humid days and had I taken off in the wrong direction I think I would have still cleared the trees, BUT ONLY BARELY. 

For those of you who fly in mountainous terrain you know that altitude, heat, and humidity can create real problems when you are trying to climb out of that canyon or cross that ridge line.

The bottom line is this -- you need to be aware of everything that affects the flying characteristics of your aircraft if you are going to aviate safely.  And depending on what, where, and when you are flying that includes altitude, temperature, and humidity.

References: FAA's Pilot's Handbook of Aeronautical Knowledge (source of pictures) and Rod Machado's Private Pilot Handbook.