Trivia Testers : DMEs

Question: Is there a reason why you should ever shut off your aircraft’s DME if you’re not using it?

  1. You should never disable any navigational equipment, on principle (the principle being ‘if you’ve got it, use it’).
  2. Yes, though it’s considered an unlikely scenario. Unlike the passive VORs, which operate in the VHF frequencies, DMEs are UHF, and the signal can travel up to 200 miles. More importantly, it’s an active system, with the the initial signal being sent by the airborne unit, and even more critical is that each DME installation has its own ‘signature’ for the signal and return pulses (which are actually each TWO pulses separated by an amount particular to THAT installation). When the number of airborne originations reaches a certain value (coincidentally, about 200), the ground DME component gets ‘saturated’ and can go off-line. This is why you should turn it off when on the ground, or when you don’t need it, particularly when you are in a busy terminal environment.
  3. You only need DME above FL 240.
  4. If you need to conserve power, such as when your alternator goes South, the DME draws a large amount of current, and it’s always transmitting! Save your power to key that mic and to operate your most critical nav receiver.

The correct answer is number 2. Figures–it’s the most long-winded…

Subject: IAP Navigation via Euclid

Question: What’s the connection between a non-RMI DME arc procedure and an 18-sided polygon?

  1. None. If you don’t have an RMI—and most of us don’t—one way to fly the a DME arc procedure is to chase the DME readout by quickly altering course left or right as need be, whenever the DME readout changes by 0.1 mile. (It requires quite a lot of REMs, though!)
  2. Nonsense. That instills about as much serenity as working on a bomb squad—too much work! Instead, a good approximation is to fly successive arcs of a polygon inside the arc’s circle. If you can imagine inscribing an 18-sided regular polygon inside a circle, how many degrees does each chord line subtend? Lessee… 360 divided by 18: TWENTY! OK, so the sum of the interior angles of each of those 18 isosceles triangles being 180 means that each of those equal ‘base’ angles is: 80 degrees, right? Then it’s supplementary neighbors (separated by and on the other side of the chord line) is exactly 100 degrees! So, you fly up to the next radial on the plan view, and simply add (if clockwise) or subtract (if counterclockwise) 100 degrees to get the next 20 degree chord line to fly! In all DME arcs, that puts you well within the allowed tolerances. (For those math nerds out there, ‘well within the allowed tolerances’ is true because the worst deviation of a 20 degree chord line from its corresponding arc is only 1.5 percent of the radius. If you want the exact answer, just take the cosine of 10 degrees.) Anyway, this method involves MUCH less headwork!
  3. TERPS criteria require that the lead radials be exact multiples of 20 degrees apart.
  4. None. There is no requirement for lead radials (or any other) to be any set number of degrees apart.

The correct answer is again number 2. (Geez, what a windbag!)

Subject: The longest highway in the sky.

Question: When an instrument pilot files a route, he or she can shorten its description by omitting any navaids (usually VORs) that lie on the same Victor airway. For example, going from Athens, Georgia up to near Richmond, Virginia, even though one passes over several VORs (Greenwood, Charlotte, Liberty, Lawrenceville, Hopewell) and traverses hundreds of miles, you can just enter ‘AHN V454 HPW’ for the route, and the computer ‘knows’ exactly what you mean. So: going from one place to another, what is the LONGEST distance an IFR (or VFR) pilot can travel along the Victor airways (under 18,000 feet) which has the SHORTEST route description?

  1. Victor 1 from Jacksonville Florida to Boston, Mass.
  2. Victor 3 from Presque Isle Maine, wee wee wee, all the way down to Key West, Florida
  3. It’s V-16. That airway connects Los Angeles, CA with Boston, MA and is about 2,590 nautical miles long.
  4. Victor 10, from Honolulu to San Francisco.

It’s number 3 folks. Someday, I’d like to file THAT one!