Trivia Testers : Getting Rusty

Question: What is the best way to combat rust in an aircraft engine?

  1. Fly every chance you get.
  2. During any period of inactivity longer than a week, run the engine for a minute or two every three or four days.
  3. Top off your tanks to avoid condensation of water in the fuel, which contributes to engine corrosion.
  4. Change the oil frequently.
  5. A and D

Answer: E. Flying as often as possible is one part of it. As your engine sits idly by, warm moist air in the crankcase cools down at night and the moisture in the air can form water droplets on internal engine parts. Over time this results in accumulations of water in the oil, which can cause rusting of cylinder barrels, connecting rods and other internal parts. Whenever an engine reaches its normal operating temperature, the water boils off. But just running it up for a few minutes and then shutting it down is likely to increase the amount of moisture in the crankcase. Flying for at least 20 minutes allows accumulated moisture to boil off. The second way is by regular oil changes to flush out contaminants. A rule of thumb is to change the oil every 25 hours if your engine doesn’t have an oil filter and every 50 hours if it does. If you fly infrequently, changing the oil at least every four months is a good idea.

Subject: That Last Turn Is a Doozy…

Question: A fixed wing aircraft pilot, flying alone, enters a fairly wide canyon one morning — let’s say it is a mile and a half wide — in his Cherokee at 100 kts, keeping at a comfortable distance (a half mile) from the right side of the canyon. He then executes a 30 degree banked turn to the left and reverses direction, clearing the cliff at the other side by an equal margin. Later in the afternoon, he brings three friends along, and repeats the maneuver. Wisely knowing that stall speed goes up with increased weight (or wing loading), he plays it safe and adds an extra 15 knots of airspeed to maintain the same wide margin above stall speed. (He’s an engineer, and he knows that about matches the square root of the ratio of present to earlier gross weight.) Wanting to also compensate for increased stall speed by reducing his bank angle, as well as giving his right side passengers a better view with less discomfort, he turns with a more gentle 20 degree bank. What is wrong with this?

  1. The true airspeed will also go up in the less dense afternoon air, and his safety margin against getting boxed in (and then boxed up) may vanish.
  2. Weight actually has nothing at all to do with turn radius, but speeding up increases it. Turn radius varies with the square of velocity, so that 15% speed increase really means about a 32% increase in turn radius. That, plus sloppy technique could erode that half-mile safety zone down to 2000 feet or even less.
  3. Turn radius varies inversely with the tangent of bank angle. That 10 degrees less bank actually means over a 50% increase in turn radius, so the safety margin might be 1500 feet or less, especially in the less dense afternoon air.
  4. He’s an engineer, but maybe not a very thorough one. Turn radius depends on both speed and bank angle. Going 15 knots faster and banking 10 degrees less actually results in over twice the turn radius. Since a 100 knot airplane in a 30 degree bank needs almost 3100 feet to turn around under ideal conditions, that half mile safety zone, especially in the warmer air, is now well under 1000 feet.

Answer: D. That would be quite a pucker factor for those passengers as he came around towards the other side of the canyon. The buzzards are waiting…

Subject: Attitude Readjustment Hour

Question: In most general aviation airplanes, the standard ‘six pack’ configuration of instruments includes an attitude indicator and a heading indicator, both of which are vacuum powered. Most of us know that every so often, we need to manually readjust our heading indicator against the magnetic compass to correct for precession. Yet when you think about it, aside from an occasional pre-takeoff adjustment, we hardly ever need to adjust the attitude indicator in flight. Why is that so, and in what circumstances might we want to think about doing so?

  1. Attitude indicator gyros would indeed precess just like our heading indicator, if they spun at the same speed, but because they spin over ten times faster, their ‘rigidity in space’ is much more persistent. Unless there is turbulence that is severe or extreme, they are required by CFR 14 Part 23.1303 not to precess more than one degree per hour. This is more than adequate for all but ocean-crossing flights.
  2. Attitude and heading gyros both precess at the same rate, but attitude indicators actually have two gyros spinning in opposite directions, and each precesses such that the opposite movements cancel out. This does not immunize them against the effects of turbulence, which introduces nutation (a ‘nodding’ of spin vectors in addition to precession) and can result in accumulated errors.
  3. The vertically oriented axis of rotation in attitude gyros allows a means of automatic correction with the addition of a pendulum which senses the local gravity vector. This either opens air blast vanes on an air-driven gyro or powers electric torquer motors to return the spin axis back to vertical when it precesses.
  4. The best answer, as usual, is the most simple and elegant one: the spin axis of an attitude indicator is locked in the vertical and thus does not precess.

Answer: C. There really is such a design, and it works. However, even though the system is not disoriented by coordinated turns, very shallow-bank turns, or flying out of trim for long periods (due to sloppy flying or improperly rigged controls) can introduce errors.