May Days
True or False: In the United States, there are actually more “mayday” calls in the month of May than any other.
Do-It-Yourselfers
The percentage of single-engine piston home-built aircraft in the general aviation fleet is now roughly
- one tenth of one percent
- three percent
- fifteen percent
- twenty-five percent
This Message Was Brought To You By the Letter “E”
Pilots rightfully have something of a vested interest in that benchmark of assessing their visual acuity: namely, the eye chart. Why is it that the letter E is usually the topmost letter?
- This is a trick question; it actually isn’t.
- Because it has the greatest number of lines for the space it occupies, it is actually the hardest of all upper case letters to differentiate. (The fact that it gets top billing is simply its “handicap”.)
- The reason is that Dr. Hermann Snellen found that the letter “E” shares several qualities with the greatest number of other letters (e.g., B, F, P, etc.)
- The letter E appears atop most eye charts only because it is the most frequently used letter of the English alphabet.
- It’s because it most closely mirrors the attributes for testing vision (a series of parallel lines) that had been used by the predecessor of the famous Dutch professor of ophthalmology, Dr. Snellen, also the originator of what we now see (with varying degrees of success) as the modern eye chart.
The Answers…
May Days
Answer: False. What a silly idea!
Do-It-Yourselfers
Answer: It’s not quite one out of four (though that may be true someday) but one out of about six is actually what we have at this point, early in the 21st century. According to the EAA, there are now about 25,000 amateur-built aircraft. In fact, there have been several years during which the number of amateur-built airplanes exceeded the number that were factory-built.
This Message Was Brought To You By the Letter “E”
Answer: The answer is E (appropriately enough), because the letter E (as upper case), with its three horizontal lines and white spaces between them, forces an observer to distinguish between white and black, and is perfect for measuring how small an image a person’s eye can perceive and still identify the characteristics of that image. (Some letters are actually more difficult than others, such as the relatively easy letter L, with its open spaces.) It also most closely resembled the formulas for visual acuity that were based on parallel lines, which were developed by the Director of the Netherlands Hospital for Eye patients, Dr. Frans Donders, who was, in the mid-nineteenth century, the world’s leading authority on optics. He based his fairly complex geometric algorithms quantifying how the eye resolves and differentiates between what it sees on this letter. His successor Dr. Hermann Snellen actually first used the letter “A” atop his eye chart in 1862, but soon switched to the letter “E”. Actually there are only a limited number of letters (perhaps ten) on Snellen charts, in order to minimize the range of difficulty, and the letter E in fact does not always get top billing. The well known Snellen quotients relate to a person’s ability to identify letters of a certain size at a specified distance. They give no information as to whether or not any meaning is obtained from them, how much effort is needed to see them clearly, or whether or not both eyes are used, as opposed to each eye individually. In the Snellen fraction 20/20, the first number represents the test distance (20 feet). The second number represents the distance that the average eye can see the letters on a certain line of the eye chart. So, 20/20 means that the eye(s) being tested can read a certain size letter when it is 20 feet away, as well as a having what is considered normal vision. (The standard definition of normal visual acuity, or 20/20 vision, is the ability to resolve a spatial pattern separated by a visual angle of one minute of arc, or 1/60 of a degree. In most people who have normal abilities for spatial resolution, their limit is derived from the fact that each degree of whatever they see is projected across 288 micrometers of their retinas by their eye’s lenses. In most people, within this 288 micrometers dimension, there are 120 color sensing or central vision cone cells. Thus, if more than 120 alternating white and black lines are crowded side-by-side in a single degree of viewing space, they will appear as a single gray mass to the human eye. Some folks like Ted Williams or Chuck Yeager, in addition to their eyes having had nearly optimal optics, were just blessed with more.) By the way, in case you’re wondering, getting the top line right at your next flight physical isn’t much to brag about. (That’s 20/200 vision.) But if you get the bottom line, well, that is. That represents 20-10 vision. (And you don’t have to be Chuck Yeager, either. A little-known standard dictates that credit be given if you succeed in identifying only a majority of the letters on a given line.)
