Dear Bill Peacock,
The metric system is a coherent group of seven basic units of measure, from which all needed derivations are readily made. The universal conversion factor is ten. It is the system routinely used in science, and in the daily lives of people everywhere - except in one country, the United States.
Measurement units in the United States are an odd-ball collection of items from ancient English history. Units of length are the nautical mile, the statute mile, the fathom, the yard, the rod, the foot and the inch. (For harmonization with the metric system, the length of the inch was recalibrated a few years ago to be exactly 2.54 cm.) Conversion factors between them are irregular, and include 5,280, 3, and 12. Schoolchildren are burdened with 231 cubic inches in a gallon, and 62.4 pounds being the weight of a cubic foot of water. The temperature difference between freezing and boiling is divided into 180 degrees, instead of a rational100 as in the metric system. (Miseducating schoolchildren in the United States is also an important issue.)
Ever since president Thomas Jefferson's call in 1803 for officialization of the metric system in the United States, its compelling logic has been urged through the years. Reason has not been able to overcome Congressional inertia, however. The United States perseveres in its shoddy measurements. One consequence of the lethargy is the loss of a $325 million probe to Mars recently due to measurement confusion.
If non-metric units wounded only the United States, the sane outer world could ignore the situation. But much of aviation development occurred in the United States, and so aviation inherited its queer measurements. Aviation instruments give altitudes in feet, air pressure as inches of mercury, and temperatures in Fahrenheit. After WWII the United States was triumphant. Its bizarre measurement units then contaminated aviation. Unsurprisingly, the overpowered International Civil Aviation Organization in 1951 incorporated the English language and American measuring units into its regulations.
Today world aviation remains hamstrung with the inches, nautical miles, statute miles, feet, inches, etc. There is no justification for prolonging non-metric units in aviation except for the comfort of doing nothing, clinging to the status quo. But that status quo keeps measurement confusion in the cockpits of the world. Metric units and American units appear side by side on some instruments. Some countries, like Russia, rebel and altitudes are given in meters.
Pilots and controllers in the world elsewhere than the United States are attuned to the metric system. Their mental estimates of quantities are based upon numbers related to it. The ICAO, cockpit instruments, and air traffic radio instructions force them to twist their minds to an arrangement for American comfort. They must mentally translate feet to meters, or miles to kilometers, often in a flash. Not only is this morally unfair, but the effect can be to distract the attention or distort the judgement of pilots during critical landing manoevers. Thus the risk of accidents is made greater than it would be with metric measurements universally used.
The United States should take the lead toward an orderly transition, both in the FAA and ICAO. A date should be set, say 5 years from now, when all aviation instruments, maps, instruction manuals, government regulations, and other documents will appear with none other than metric units. Such modernization is a necessary step for better flight safety.
In order to reassure the ICAO that the transition to metric measurements has broad support, ministers of aviation from the other 188 countries of the United Nations should express themselves to the it. Responsible citizens of those countries should contact their representatives and aviation ministers, and demand that they insist on the metric system in worldwide aviation. An aroused world public will be necessary to overcome governmental inertia, I fear.
Kent Jones, 773-271-8673
5048 N. Marine, D6