To make those tight turns, loops, inversions, and climbs, skywriters use small, highly maneuverable, high-horsepower planes. Retired military scout planes were the norm in the early days [source: Popular Mechanics]. Now, Asbury-Oliver says crop dusters work well.
Skywriting planes carry specialized smoke-emitting systems. The machinery performs two main tasks: storing smoke-producing fluid and moving it into the exhaust.
The smoke begins as a paraffin-based mineral oil, sometimes called paraffin oil or liquid paraffin. It's the same stuff that's used in air shows. According to Cristina Domi, from TheSkyWriters.com, the fluid is approved by the U.S. Environmental Protection Agency, nontoxic and biodegradable.
The fluid sits in a reservoir near the engine, and a plane can typically carry about 30 gallons (114 liters), enough to write up to 12 letters [source: Sky-Writing.com].When the pilot decides it's time, he or she flips a switch in the cockpit, and the reservoir injects a stream of paraffin oil into the plane's exhaust system. As the fluid hits the exhaust pipes, it vaporizes, pouring from the exhaust outlets at the front and back of the plane. The streams of smoke then expand to about 75 feet (23 meters) wide [source: Hartill].
To vaporize the paraffin oil, the engine has to reach 1500 degrees Fahrenheit (816 C) [source: Library of Congress]. That's part of why skywriting planes need a lot of horsepower: In the cold of 2 miles (3.2 kilometers) up, producing that much heat takes a lot of power [source: Asbury-Oliver].
The word "PEPSI" requires 14 separate smoke triggers by the pilot [source: Olivers Flying Circus]. It's all about judgment and timing; the slightest hesitation can ruin a letter, and all is lost.
In a highly automated world, this kind of skywriting can feel like an anachronism, and maybe that's part of the appeal. The automated world has developed its own skywriting method, though. No split-second decisions. No flipping a switch. No inversions and loops. It's kind of like dot-matrix printing, but with planes.