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quantitative formulation by William Ferrel (1817-1891). The acceleration is known as Coriolis
acceleration (or force) or simply Coriolis and is expressed in Ferrel's law.
13.11.1. You must realize that the bubble sextant indicates the true vertical only when the instrument is
at rest or moving at a constant speed in a straight line as perceived in space. If the earth were motionless,
this straight path in space would also be a straight path over the surface of the earth; conversely, a
straight path over the motionless earth would also be a straight path in space.
13.11.2. When the aircraft is flying a path curved in space to the left, the fluid in the bubble chamber is
deflected to the right and the bubble is deflected to the left of the aircraft's path over the earth. When the
aircraft is flying a curved path in space to the right, the reverse is true.
13.11.3. In Figure 13.8, the aircraft is represented as flying on a curved path to the left. Note that in the
inset representing the bubble chamber, the heavy black bubble is indicated in its approximate position
representing the true vertical.
Figure 13.8. Error Caused by Coriolis Force.
288 AFPAM11-216 1 MARCH 2001
13.11.4. The observer always seeks to center the bubble and, on this beam shot facing to the right side of
the aircraft to observe the body, tip the sextant up. This would tilt the bubble horizon from its true
position, producing a smaller sextant reading than the true value. The smaller the height observed (Ho),
the greater the radius of the circle of equal altitude—the LOP will fall farther from the subpoint than the
true LOP. Obviously, if the erroneous LOP falls farther from the subpoint, it will fall to the left of the
true LOP and the correction to the right is valid. Corrections for Coriolis error are shown on the inside
back cover of the Air Almanac as well as in all volumes of Pub. No. 249.
13.11.5. Coriolis acceleration is directly proportional to the straight-line velocity, directly proportional
to the angular velocity of the earth, directly proportional to the sine of the latitude, and at right angles to
the direction of flight.
13.12. Rhumb Line Error. The straight Coriolis table (Figure 13.9) found in the Air Almanac or Pub.
No. 249 has a limited application. As long as a constant TH is flown, the path of the aircraft will be a
rhumb line. Because a rhumb line on the earth's surface is a curve, it is also a curved line in space. If the
aircraft is headed in a general easterly direction in the Northern Hemisphere, the apparent curve is to the
left and becomes an addition to the Coriolis error. By the same token, if headed in a westerly direction in
the Northern Hemisphere, the apparent curve is to the right, or opposite that of Coriolis force as shown
in Figure 13.10.
13.12.1. There are notable exceptions to this. When flying north or south, the aircraft is flying a great
circle and there is no rhumb line error. Also, when steering by a free running, compensated gyro, the
track approximates a great circle and eliminates rhumb line error.
Figure 13.9. Coriolis Correction.
AFPAM11-216 1 MARCH 2001 289
Figure 13.10. Coriolis and/or Rhumb Line Errors in the Northern Hemisphere.
13.12.2. At speeds under 300 knots, the error is negligible. However, at high speeds or high latitudes,
rhumb line error is appreciable. For example, at 60o N latitude with a track of 100o and a GS of 650
knots, the Coriolis correction is 15 NM right and the rhumb line correction is 10 NM right. Use the
following steps and Figure 13.11 to determine the correction for rhumb line error and Coriolis
correction:
13.12.2.1. Enter the nearest latitude on the left side. Interpolate if necessary.
13.12.2.2. Enter the nearest track across the top of the chart. Interpolate if necessary.
13.12.2.3. Choose the closest GS and extract the correction; that is, 50N, track 080o, GS 500 knots =
14.3 Right.
13.13. Groundspeed Acceleration Error. Changes in airspeed or wind velocity cause this error.
Prevent changes of airspeed through good crew coordination.
290 AFPAM11-216 1 MARCH 2001
Figure 13.11. Combined Coriolis and Rhumb Line Correction.
13.13.1. Changes in wind velocity with resultant changes in GS are more difficult to control. The change
in GS will cause the liquid to be displaced, with the subsequent shifting of the bubble creating a false
horizon. Notice in Figure 13.12 how the horizon is automatically displaced by keeping the bubble in the
center while these changes are taking place. A very simple rule applies to acceleration and deceleration
forces. If the aircraft accelerates while a celestial observation is in progress, the resultant LOP will fall
ahead of the actual position. Accelerate—Ahead. The more the LOP approaches a speed line, the greater
　
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