After some deep thoughts on these three important crossing-the-Rockies factors, I’ve decided to hold off on my coast-to-coast trip – at least until this (abnormally hot) summer is over. As my dad pointed out when I told him my decision the other day, cross-country planning is about two things: where to fly, and when to fly.

Those of you who are not pilots have likely never even heard of density altitude, but it affects reciprocating engines of all kinds. Air density is a major factor in engine performance, which is directly related to how dense the air is: the more air available to an engine, the more horsepower it will produce. On a given day, air pressure decreases with altitude, reducing power output, but higher temperatures and humid air reduce air density further. This effective altitude, as a result of temperature and humidity, is what we call Density Altitude; the engine simply performs like it is much higher than it actually is. A car crossing a mountain pass on a hot day will see a slight drop in power, but to an airplane thousands of feet in the air the effects will be far more obvious.

Climb and takeoff performance suffers accordingly. Yesterday’s high in Missoula was 90° F, and late last week it reached 104° F; these numbers have been doubling its absolute altitude of 3,205 feet to a density altitude of at least 6,500 feet. Our Yankee, which will clear an obstacle in 1,440 feet on a standard day (59° F) at sea level, would require at least 3,600 feet on a humid 100° day in Missoula. Turbochargers help combat density altitude performance losses by increasing the air density at the intake… unfortunately, the Yankee is not turbocharged.

American Aviation’s AA-1A manual is a mere 41 pages long, compared to the 150 or 200 page manuals found in modern Pipers and Cessnas, but it has enough data to let me make an informed decision about cross-country flights. On a ‘standard’ day, the air temperature at 9,000 feet would be below freezing, and the Yankee would still be able to climb at 350 feet per minute. Last week’s temperatures at 9,000 averaged around 60° F; climb performance in the AA-1, if it could even get there to begin with, would be awful. It’s a great plane for cruising around, but the stubby wings and O-235 engine can’t handle the hot, humid, and high conditions the rockies produce. I’ll make the big trip later, and occupy myself with less ambitious trips up and down the West coast.