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Trivia Teasers

1) How long have directions been expressed in degrees clockwise from North?
2) How fast would you need to go in order for at least part of your Skyhawk to exceed Mach One?
3) When may a pilot descend below the glide slope on a visual (or even an instrument approach) and land on the displaced threshold?
How long have directions been expressed in degrees clockwise from North?

A) only since 1947
B) since World War I
C) since the mid-1700s
D) only since the Peloponnesian Wars!

Answer: First off, I offer my apologies to the late Cary Grant, who starred in the movie along with Eva Marie Saint and the late James Mason, and the late Alfred Hitchcock, who made the 1959 adventure thriller, and the late (or almost late) anybody else who had anything to do with it.

Historically there have been only a few ways by which direction was expressed in modern times, although all of them involved the compass rose. In marine navigation since the 1300s in the western world, there were the traditional cardinal 'points of the compass' (incidentally of which there are at least 32), which was the oldest system. In between, say North and West was Northwest, thus bisecting the four main 'winds' or points of the compass rose into eight 45-degree slices. Then, further dividing these eight points, in between Northwest and West for example was West Northwest (WNW), and along with NNW, NNE, ENE, ESE, SSE, SSW, and WSW, the compass was thus further separated into 16 221/2 degree wedges. Then, in between those and their less complicated neighbors lay (taking WNW again as an example) names like Northwest by West and West by North, which gave the 32 points of the compass, each 111/4 degrees apart. Even that wasn't enough for the more precise steering that became possible with steam-powered vessels, and then half and quarter points were added, further dividing up the compass rose into 128 skinny wedges, each one only 213/16 degrees wide. (They had names like 'Northwest by West, 3/4 West'. Clearly, that wouldn't work in today's National Airspace System, now would it?) The second system had the compass rose graduated in 360 divisions, but numbered from both North and South, increasing to 90 degrees at both East and West. It was the third system, just coming into use by various nations only in the late 19th century, which made the most sense. It involved the same 360 divisions, but counted them clockwise from North. The United States and Great Britain were not among those nations, by the way. Only in 1916 did it drop the compass points (as other than additional decorative information) from its charts. The correct answer is B.

There actually is not any such point of the compass as 'North by Northwest'. They should have spent a little time looking at a Boy Scout handbook, or perhaps hired a consultant. There's Northwest by West (which equals 303 degrees, 45 minutes), and there's Northwest by North (326 degrees, 15 minutes). There is a 'NNW' between due North and northwest, but there is no intermediate 'by' for any of the 16 'higher level' points of the compass (those appearing in bold type, below). Here are all 32 of the most well known compass points. See for yourself! 

Conversion of Compass Points to Degrees

NORTH

N by E11° 15'

NNE22° 30'

NE by N33° 45'

NE45°

NE by E56° 15'

ENE67° 30'

E by N78° 45'

EAST90°

E by S101° 15'

ESE112° 30'

SE by E123° 45'

SE135°

SE by S146° 15'

SSE157° 30'

S by E168° 45'

SOUTH180°

S by W 191° 15'

SSW202° 30'

SW. by S213° 45'

SW225°

SW by W236° 15'

WSW247° 30'

W by S258° 45'

WEST270°

W by N281° 15'

WNW292° 30'

NW by W303° 45'

NW315°

NW by N326° 15'

NNW337° 30'

N by W348° 45'

NORTH360°



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How fast would you need to go in order for at least part of your Skyhawk to exceed Mach One?

A) That's easy: 662 knots, at sea level, on a standard day.
B) Approximately ninety five knots past Vne. (You'd never get even that far, however.)
C) There are actually parts of almost any light piston airplane that are already exceeding the speed of sound in level cruise flight.
D) a bit over 400 knots

Answer: You may have already guessed which part was the most likely one to go supersonic: your propeller. Taking a C172 of average vintage, say a 1981 172P, it might have a 75-inch prop and a maximum allowed engine speed of 2700 rpm. Translated into a tip velocity at the start of a short field takeoff (say while the brakes are held and it isn't yet moving), that would be about 884 feet per second, or around 523 knots. (That's already fast enough to make quite a racket!) To calculate the forward speed needed for the resultant velocity vector to equal Mach 1 involves simply involves a bit of trigonometry. Assuming a standard temperature of 15 degrees Centigrade, at which the speed of sound is about 662 knots, the necessary forward speed works out to about 683 feet per second…or about 405 knots (choice D). Since the speed of sound in air is dependent almost entirely on temperature, and because it goes down about one and one-seventh knot for each Centigrade degree decrease in temperature…I guess you'd want to pick a cold day. Taking the silliness level down a peg though (or maybe that should be up a peg), in reality the air accelerated over the propeller airfoil, because it travels at a speed considerably greater than its speed of flight, would reach supersonic speeds well before it did. The so-called critical Mach number for an airfoil is often much less than one, such as 0.7. (In the case of this propeller, that would mean you would create sonic booms on the run-up pad, because its tip speed is already close to four-fifths of the speed of sound.)

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When may a pilot descend below the glide slope on a visual (or even an instrument approach) and land on the displaced threshold?

A) anytime normal bracketing maneuvers are performed for the purpose of remaining on the glide slope
B) any time the approach is flown in daylight visual conditions
C) only in an emergency
D) only at certain airports at which the normal regulations have been waived by the FAA Air Traffic Division in order to accommodate aircraft requiring a greater runway length, and only under certain conditions (namely, those in choice B)

Answer. First of all, CFR Title 14, Part 91.129(e)(2) says that for large or turbine-powered airplanes approaching to land on a runway served by an instrument landing system, they shall fly at an altitude at or above the glide slope between the outer marker or point of glide slope interception and the middle marker. For the rest of us, 91.129(e)(3) says that an airplane approaching to land on a runway served by a visual approach slope indicator shall maintain an altitude at or above the glide slope until a lower altitude is necessary for a safe landing. At the end of sub-paragraphs (e)(2) and (e)(3) it says basically what's in choice A. But that won't hold the lawyers back, if you prang something 'behind the lines'. And the conditions in choice B are nowhere near sufficient to justify such a thing. (I don't know about you, but every flight instructor that I've ever flown with has pretty much branded into my forehead the fact that one is not supposed to land on a displaced runway threshold.) Choice C is of course always a thumbs-up. (Got fire? Well heck, then you can land on the closest taxiway, if you think it will help!) If you picked it, give yourself half a point for basic common sense. But you don't get the whole goody bag, because the most correct response would be if you had choices C and D. Yes, D. First, a displaced threshold is similar to a runway, but is designed as a safety net for pilots to use in their calculations for the amount of room needed for a takeoff or landing. It is generally available for takeoffs in either direction, or for landings from the opposite direction. It is not designed for use specifically for a landing, but rather provides extra 'rolling' room. So what gives? Well, there are actually airports at which these rules have been waived. Where might you see this? The northeast U.S. Airport/Facility Directory has a Special Notice page for Boston-Logan International Airport's runway 4R stating that, during daylight hours and under VFR meteorological conditions, the full 10,005 feet of the runway (and not just the measly 8,840 feet past the landing threshold) is available to even large and turbine-powered aircraft. (It does also caution operators to be vigilant for vessels traversing the Boston Inner Harbor Channel, so watch out!)

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