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 airspeed must be known. The rule of thumb using airspeed requires that the last digit of the airspeed be dropped then add five. For example, if the airspeed is 90 knots, the bank angle would be (9+5=) 14 degrees. For 122 knots, it would be (12+5=) 17 degrees. The line of latitude is the maximum lead or lag a compass will have.