The number one record on the National Aeronautic Association’s ”Most Wanted” records is the absolute altitude in a powered airplane with an internal combustion engine. The current record is 56,046 feet and was set, incredibly, on October 28, 1938, by Mario Pezzi of Italy in a Caproni Ca 161. The record will be 75 years old this October. Wow! To break the record, one needs to achieve 57,728 feet. Any takers? There are a few airplanes around that could probably do it, with some modifications and a partial-pressure or pressure suit.
Speaking of altitude though, how do you know what that altitude really is? Altitude seems to be the one measurement we still have a hard time getting right after all these years. I was recently looking through some old “Aviation and Aeronautical Engineering” journals from the 1920s, and I ran across some articles about a controversy in how the altitude record in the 1920s had been set.
On February 20, 1920, after taking off from McCook Field in Dayton, Ohio, Major Rudolph Schroeder reached 36,118 feet of altitude in a Lepere biplane, according to a barograph he carried aboard. This new record surpassed the old record of 34,910 feet set by Roland Rolhlfs, a test pilot for the Curtiss Engineering Corporation, the previous September in a Curtiss tri-plane owned by the Navy.
The problem was, neither altitude was corrected for temperature. When the altitudes were corrected for temperature, the true altitude attained by Rolhlfs actually turned out to be 32,450 feet, and the true altitude attained by Schroeder was only 30,835 feet. What happened?
The altitude measured by a barograph is based only on the pressure of the atmosphere. As altitude goes up, the pressure goes down. However, the pressure only provides an estimate of the true altitude, which is a fixed quantity. The pressure altitude will vary significantly for a given true altitude, as temperature also affects the pressure that is measured. In general, given the same pressure for a colder temperature, the true altitude will be much lower than for a higher temperature.
It appears the FAI had all this sorted out by the time Mr. Pezzi set his record!
I started thinking about measuring altitude as a result of this and realized that even today with all our modern technology we still have issues. When pressure from a barograph (or altimeter) is corrected for temperature, there are still measurement errors in the barographs and the temperature instruments themselves. This is one of the reasons that aircraft flying at high altitudes fly at uncorrected pressure altitudes and are separated by at least 1,000 feet in altitude.
Radar altimeters are another way to measure absolute altitude, but they can also have measurement errors. In addition, a radar altimeter measures the distance between the ground and the altimeter, so unless someone has a good model of the earth they are flying over, it’s hard to use a radar altimeter to measure absolute altitude.
One might think that GPS would have solved this, but once again, GPS tends to have the greatest errors in altitude. I have seen this many times, where a GPS unit knows my horizontal coordinates to within a few feet, but is 200 feet off in altitude.
Maybe the next great invention will be to figure out once and for all how to measure altitude as precisely as we can measure time and 2-D position. Thoughts?
Reference: “Determination of Altitude Records,” Aviation and Aeronautical Engineering 8(5), 1920.