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June 17, 2014
By Dan Namowitz
An aircraft is transitioning from the airway to an ILS approach, airspeed 130 knots, altitude 3,000 feet, heading 060 degrees. Assigned to fly 120 degrees to intercept the localizer, the pilot rolls into a standard-rate turn and slows to 90 knots to avoid overshooting, as the intercept is being flown with a tailwind component.
Adjustments are now needed to keep the flight profile stable, but the pilot is preoccupied reviewing the approach procedure. When the instrument panel draws his eye, the pilot—who lately has been training not to overcontrol the aircraft—is shocked to see a noticeably nonstandard turn rate indicated there.
An instrument malfunction?
No time to ponder. Already the course deviation indicator is moving smartly toward the intercept. Uncertain whether the turn coordinator is trustworthy for rate information, the pilot goes with the gut and makes an adjustment that seems to stabilize things. He completes the intercept, and finishes the approach, which remains “uneventful,” as pilots say after something eventful has happened.
Instead of experiencing the satisfaction of carrying off a nice approach, the pilot feels something more akin to doubt and surprise that such a brief distraction could spoil things so quickly.
A quiz: After the pilot changed airspeed in the standard-rate turn, was it more bank or less that was needed to maintain it? Could the correct bank angle have been anticipated?
You can get up and practice the technique on your next proficiency flight—and you should. Meanwhile, this bit of basic guidance can help you visualize executing the change without hesitation or uncertainty.
"A rule of thumb for determining the standard rate turn is to divide the airspeed by ten and add 7. An aircraft with an airspeed of 90 knots takes a bank angle of 16 degrees to maintain a standard rate turn (90 divided by 10 plus 7 equals 16 degrees)," explains the Instrument Flying Handbook.
At the aircraft’s original speed of 130 knots, the bank angle for a standard-rate turn was 20 degrees (130 divided by 10 plus 7 equals 20 degrees). On decelerating, the pilot should have reduced bank four degrees, to 16 degrees.
It's a small change, but one that a tendency to overcontrol would complicate, exacerbating the startle factor already at work on the pilot after he saw an unusual attitude—a steep turn, in IFR terms—displayed on the instruments.
Dan Namowitz is an aviation writer and flight instructor.
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