Aircraft compass turns

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Aircraft compass turns are used in an aircraft when other directional instruments, such as the directional gyro or turn coordinator, have failed. To master the skill of compass turns, one must learn the errors of a compass. Most of the errors are related to the compass' construction. The compass in an aircraft can be thought of as an upside down cereal bowl balancing on a pin enclosed in a casing full of non-acetic kerosene. One point of the compass is magnetized. The polarization of the magnetic force causes the compass to be attracted to the magnetic North Pole. As the aircraft turns, the bowl tries to remain stationary due to the attraction, hence the plane is free to turn around the stationary bowl.

The first limitation imposed by a compass' construction is that the balancing bowl's pin, which is connected to a pivot point, only allows for, in most compasses, a pitch or bank of 18 degrees before the compass will touch the side of the casing. When the compass touches the side of the casing the freedom of the plane to rotate around the compass is lost and the compass becomes unreliable.

Magnetic dip is another limitation of the compass. When the aircraft is at any altitude above the earth's surface, the compass will tend to dip toward the North Pole when in the northern hemisphere, toward the South when in the southern hemisphere. At the equator this error is negligible. As an aircraft flies closer to either pole the dipping error becomes more prevalent to the point that the compass can become unreliable because its pivot point has surpassed its 18 degrees of tilt.

When in straight and level flight the effect of magnetic dip is of no concern. However when the aircraft is turned to a new heading two rules apply:

First, when on an easterly or westerly heading and the aircraft accelerates, the compass will show a false turn towards the north if in the northern hemisphere or vice versa a false turn towards the south if in the southern hemisphere. Also if the aircraft is decelerated the compass will show a false turn towards the south in the northern hemisphere and false turn towards the north in the southern hemisphere. This can further be explained by imagining the compass needle's attraction towards the pole not wanting to move. However, the aircraft's acceleration generates a force greater than that of the magnetic force. The force is neutralized when the aircraft has reached its velocity and the magnetic compass will then read the proper heading. Pilots in the northern hemisphere remember this by the acronym ANDS; accelerate north, decelerate south.

Second, when on a northerly heading and a turn towards the east or west is made the compass will lag behind the actual heading the aircraft is flying through. This lag will slowly diminish as the aircraft approaches either east or west and will be approximately correct when on an east or west heading. As a rule of thumb for light aircraft making this turn a lead roll out heading of approximately five degrees before the compass shows east or west should be used. When a turn is made from south to an east or west heading the compass will lead the actual heading the aircraft is flying through and again will diminish as the aircraft approaches either east or west. The rule of thumb for light aircraft making this turn is to roll out approximately ten degrees early.

The lead or lag roll out heading when making turns to other directions other than east or west is computed using the line of latitude and the bank angle the aircraft is using to make a standard rate turn. In order to calculate bank angle for a standard rate turn knowledge of air speed must be known. The rule of thumb using air speed requires that the last digit of the air speed be dropped then add five. For example if the air speed is 90 knots drop the zero and add five. The bank angle in this example would be (9+5=14) 14 degrees. Or if the air speed is 122 knots drop the two and add five. The bank angle in this example would be (12+5=17) 17 degrees. The line of latitude is the maximum lead or lag a compass will have.

The following explanations are for the northern hemisphere.

For example an aircraft flying at 45°N latitude making a turn to north from east or west maintaining a standard rate turn a pilot would need to roll out of the turn when the compass was 45 degrees plus one half of the bank angle before north. (From east to north at 90 knots 0+45+7=52) A pilot would begin to roll out to straight flight and on a heading of north when 52 degrees was read from the compass. (From west to north at 90 knots (360-45-7=308). A pilot would begin to roll the aircraft out of the bank at 308 degrees read from the compass to fly on a north heading. Making a turn towards south from west the pilot would have to roll the aircraft out of the turn when the compass was 45 degrees minus half the bank angle (from west to south at 90 knots 180+45-7=218, from east to south 180-45+7=142).

From the examples we see that when turning to north from east or west the bank angle used to calculate the time to roll the plane out of the turn must begin at the greatest amount of degrees or further away from north. Conversely for turns to south from east or west the bank angle is calculated to decrease the number of degrees to lead the roll out or closer to south.

Generally pilots will practice making these turns using half standard rate turns. This will decrease the bank angle so that it is half of the calculated bank angle. When turns are made at half standard rate the line of latitude will only cause the compass to have an error of half as much. So our new calculation using a half standard rate turn is as follows: (From east to north at 90 knots 0+22.5+3.5=26) the lead roll out heading read from the compass would be 26 degrees to fly on a north heading. (From west to north 360-22.5-3.5=334) The lead roll out heading read off the compass would be 334 degrees.

Turns made for other directions should be interpolated. For example a left turn made from a heading of west to south east (SE). The compass would initially show a heading that is correct as the turn gets closer to south the compass would indicate a lead heading of the greatest error, as the aircraft passes through south the error would decrease and show less of a lead. As the aircraft approaches south east the error would only lead half as much as it did when the aircraft was rolling through south. So if the turn was made using a half standard rate at 90 knots and the SE heading required to fly was 135 degrees the roll out heading would be 135-11.25+3.5=127 degrees. Hence a roll out heading read from the compass of 127 degrees would be used to actually fly the heading of 135 degrees.