Trivia Testers : All In Knots

All In Knots

When did our primary airspeed indications (that is, those used by general aviation) go from reading in miles per hour to being given instead in terms of knots?

  1. on June 26, 1946
  2. They never really did; for civil aircraft, there was never any rule absolutely requiring it, so whatever you see in whatever aircraft you happen to fly depends only upon the particular manufacturer, and the caprices of fate.
  3. in 1977
  4. For most general aviation aircraft, it was on January 3, 1991, with Amendment 23-42 to CFR Title 14, FAR Part 23.1323.

Answer: In terms of what went out the factory door, as opposed to what you might encounter on the flight line, the answer is (disappointing as such a lack of an industry agreement may be) that although aircraft manufactured after 1976 in the U.S. and Canada often do have airspeed indicators with primary markings based on indicated airspeed in knots, there is apparently no regulatory policy or guidance to which there is universal adherence. Why is this something that is disappointing? Because of the fact that, like assembly lines, things work best and get accepted, understood, and followed most widely when they’re standardized. It’s harder to hop in and out of different model years of the same Skyhawk for example, and have to remember entirely different units for best rate of climb, best glide speed, etc. And these differences are all the more significant since a knot is 15% more than is a mile per hour. So although for those with experience flying many different aircraft, and for whom choice C may have seemed like it was the best answer, in reality, it isn’t. It’s choice B. First, a little background, before we get to the facts and further explanations:

The knot of course is one nautical mile per hour, which is a bit over 15% greater than one statute mile per hour. Before ships had instruments with which to measure their progress through the water, speed was measured by heaving a rope overboard with a log attached. By measuring the length of the rope played out over time as the ship pulled away, they determined their approximate speed. But after a while, they tied regularly spaced knots, and simply counted those. Formal recognition of this unit however took awhile. The Joint Army and Navy Board on Aeronautics, approved by both the Secretaries of War and the Navy in 1919 (and before that in 1916, as the Joint Army and Navy Board on Aeronautic Cognizance), and then in 1920 shortened to just the Aeronautical Board, agreed unanimously only on June 26, 1946 that the knot and the nautical mile be adopted by the then Army Air Forces and Navy as standard aeronautical units of speed and distance, and directed that use of the terms be specified “in all future procurement of air speed indicators, charts, related equipment and future issues of applicable handbooks and technical orders.” Of course, it had been in common use long before that; this was more a formality than anything else. In the area of civil aviation though, it hasn’t been mandated. We usually see older aircraft having airspeed indicator markings for calibrated airspeed, in statute miles per hour. (And to this day, somewhere around 90% of US ultralights use airspeed indicators that read in miles per hour.) Generally, it is true that many US and Canadian aircraft manufactured after 1976 have their airspeed indicators with primary markings based on indicated airspeed, in knots. The “FARs” say, in Sections 1323(a) of Part 23 (for normal, utility, acrobatic, and commuter airplanes), Part 25 (for transport category airplanes), Part 27 (for normal category rotorcraft), and Part 29 (for transport category rotorcraft) that airspeed instruments must be “calibrated to indicate true airspeed” but it doesn’t say “knots.” In the subsequent paragraphs in those same sections, the regulations give the maximum allowed tolerances for system errors for those calibrations as being the greater of a certain calibrated airspeed percentage or a certain number of knots (three percent or five knots, for airplanes and normal category rotorcraft). But this does not constitute a binding requirement as to primary indicating units, only a reference. There is no obligation for a manufacturer to maintain strict adherence to the implication of these FAA rules, nor is there any similar Joint Aviation Authority (JAA) ruling, nor has there been any NPRM in the Federal Register. You also won’t find standards in FAA Technical Standard Order C2d on airspeed instruments (and if you pay just $59 to the kindly folks at the Society of Automotive Engineers, they’ll let you look at Aerospace Standard 8019, “Airspeed Instruments” to which this TSO defers. You also will not see anything in Advisory Circular 23.1309-1C, “Equipment, Systems, and Installations in Part 23 Airplanes.” All that said, the non-regulatory status of what you’ll see on an airspeed indicator has been confirmed for me by both Cessna’s technical services support folks in Wichita, as well as AOPA’s technical specialists, who helped me with research on this question. The bottom line seems to be, you’d better pay attention to the units as well as the numbers!

Okay, so we know a knot is a nautical mile per hour, but then, what is a nautical mile? Although it has long been a useful fact that each geographic minute of latitude is equivalent to about one nautical mile, it is not precisely correct. As has already been mentioned in earlier Trivia questions, the Earth isn’t a perfect sphere, but is instead flatter at the poles (by about 23 out of some 6880 nmi of diameter). The result is that a minute of arc is about 6046 feet at the equator and 6109 feet at the poles, the current international value being exactly 1852 meters (or 6076.1155 feet). The meter itself was originally defined in terms of the Earth, as one 10 millionth of the distance from the North Pole to the Equator (at the meridian of Paris). The original measurement is now known to be incorrect, but it is now nonetheless enshrined in platinum rods and sanctified in terms of cesium wavelengths.

Flying On Mars
Off in the far azure infinity of man’s spacefaring destiny, if he has one, aircraft may someday soar through the Martian skies. Due to the thin air however, they won’t be Skyhawks. They will need a high aspect ratio (like 30-to-1) and “flying wing” proportions more like Helios, NASA’s high-atmospheric solar vehicle that has already nibbled at nearly 100,000 feet (which is just about what the Martian atmosphere is like at its surface, at least in terms of density). With a weight about equal to a Cessna 152, but with a nearly 250-foot wingspan, the thin air allows Helios somewhere around a 140 knot ground speed at altitude (though its takeoff and landing speeds are more akin to those of a bicycle). Just for fun though, if someone could pull some kind of a trillionare “jackass” stunt and fly a rocket-powered 172 on Mars–it would need to be rocket powered, since any air-breathing engine would probably suffocate–how fast would it need to go?

  1. Due to the fact that the gravity on Mars is only about one-third that of earth, takeoff and landing speeds would only be about twice what they are here on earth.
  2. Indicated airspeeds would be exactly the same, but you’d need titanium tires, because your takeoff speed would be somewhere around 500 knots
  3. supersonic
  4. impossibly fast: somewhere between 2300 and 2900 knots, depending on the Martian season

Answer: B. In a way, lift is fairly intuitive, and in a way, it’s not. Lift is a function of the shape of whatever airfoil you use of course, its angle of attack, velocity, as well as the air density and viscosity, and some of these properties are just lumped together in something called the coefficient of lift. The “lift equation” says that lift is equal to this lift coefficient, multiplied by the density of the air times half of the square of the velocity, in turn multiplied by the wing area. (The density times one-half of the velocity squared is sometimes called the dynamic pressure.) But since indicated airspeed is proportional to the square root of air density, and the Martian air is about one percent as thick as it is here on earth’s surface, the fact that the wings would need to “see” the same indicated airspeed would mean that the true airspeed would have to be the square root of one one-hundredth, or about ten times greater! Since the speed of sound in a gas is mostly a function of temperature–it also depends on the specific heat of the gas–and the average temperature on Mars is, very roughly, about minus 55 degrees C, sound would travel about as fast there as it does up in the stratosphere here on earth, which is actually only about 15% slower than at sea level… somewhere around 575 knots! That means that you’d be just about supersonic, yes. But…there is one thing that would be in your favor: gravity. The gravity on Mars is about one-third that of earth, which would decrease takeoff speeds to a bit below Mach 1. Don’t expect to do turns about a point though, at least not one that is nearby, because at those speeds and with the same mass, your inertia will take you many miles before you even started turning. And landings would be pretty hairy, because with one-third of the weight (and one third of the traction) you would need a l-o-n-g runway to slow down.

The Atmospheric Sponge
On an average day (averaged over the entire year, that is) about how much water is in the atmosphere, just over the United States?

  1. forty million gallons
  2. forty billion gallons
  3. forty trillion gallons
  4. forty quadrillion gallons

Answer: C. Including water droplets, ice, snow, and water vapor, averaged over the seasons, there are approximately 40,000,000,000,000 gallons of water poised over our heads. (That’s forty trillion, using the definition of trillion as being ten to the twelfth power.) The quantity is more than that in the summer, when the air can hold more moisture, and less in the winter, but that’s the average. Incidentally, on average also, about 10 percent of this water falls to the ground as precipitation.