Aircraft Compass Turns - Standard-rate Compass Turns

Standard-rate Compass Turns

Standard rate turn is a standardized rate at which the aircraft will make a 360 degree turn in two minutes (120 seconds). Standard rate turn is indicated on turn coordinator or turn-slip indicator.

All turns during flights under instrument rules shall be made at standard turn rate, but no more than 30 degrees of bank. In case of vacuum-driven instruments failure (i.e. directional gyro, attitude indicator) the rollout to new heading is timed: let's say the aircraft is flying 060 degrees heading and it needs to fly new heading 360. The turn will be 60 degrees. Since the standard rate turn is 360 degrees in 120 seconds, the plane will need 20-second standard rate turn to the left.

In case of electrical instrument failure, which include turn coordinator or turn-slip indicator, the following formula will help to determine turn bank at which the turn will be made at standard rate: 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=142, from east to south 180+45-7=218).

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.