Aero-Tips!

A good pilot is always learning -- how many times have you heard this old standard throughout your flying career? There is no truer statement in all of flying (well, with the possible exception of "there are no old, bold pilots.")

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Aero-Tips 08.24.06

Here's the secret to flying a smooth, accurate instrument approach: the Course Deviation Needle (CDI) and glideslope indicator are not the focus of your scan.

The Needles

Flying a precision or nonprecision approach, you follow a course along the ground as displayed on your navigation head, Horizontal Situation Indicator (HSI) or glass-cockpit equivalent. In almost all current technologies this display consists of a CDI needle that travels (or swings) left and right of center. Keep the needle centered and you're on the displayed course. If the needle is to the left you need to go left, if it's to the right you need to go right (unless your indicator is a configuration that results in "reverse sensing").

Similarly, a precision approach (by definition, one that includes vertical guidance) includes a glideslope indicator with a needle or pointers. If the needle/pointer is centered you're on the glideslope, if it's above center you're low and if it's below center you're high.

The CDI and glideslope needles, then, tell us our current position relative to the approach. But they do not tell us whether or how long we're going to stay that way. Simply "flying toward the needles" to keep them centered as they move condemns the pilot to a high-workload, passenger-sickening approach as he/she chases them back and forth. In addition to current position information, a safe and smooth approach requires we know and use performance trend as well.

Trend Information

The CDI may be centered and show us on the courseline, but we need to know if our trend is to remain on the course. This requires we know the aircraft heading.  If heading is aligned with the selected course, plus or minus wind correction, then not only does a centered needle mean you are on track, but it also means you'll keep the needle centered as you progress. If the needle is to the left but your heading is still the published inbound course, you'll only get farther from the courseline as you get closer to the airport and the course width narrows (for ground-transmitted course data). To re-intercept you'll need to change heading slightly toward the needle (the amount of heading change is dependent on the amount of needle displacement and your distance from the signal transmitter)…you need to establish a trend back toward the course. When the needle re-centers, change heading back to inbound (with wind correction as needed) for a trend to remain on course.

A centered glideslope needle means you're currently on the glidepath between intercept and the Final Approach Fix (FAF). But will you stay on glidepath, or will you end up high or low? To answer you need to know the vertical speed. At most lightplane approach groundspeeds it takes between 500 and 600 foot per minute descent to remain on the typical three-degree glideslope. If the needle is centered but your rate of descent is only 300 fpm, you'll soon be above glideslope. Lower the pitch or reduce power slightly to increase rate of descent. If you're already above glideslope, increase vertical speed to, say, 700-800 fpm to reacquire glideslope, and when the needle centers re-set pitch and power for a 500-600 fpm descent. If you're below glideslope raise the nose/increase power to fly level until the needle again centers, then re-establish your normal pitch and power to remain on glideslope. Throughout, you're using vertical speed to determine your trend to remain on or return to glideslope.

Smooth, low-workload instrument approaches come not from chasing needles, but from flying basic attitude flight-heading and vertical speed-while scanning the CDI and glideslope to gauge the effectiveness of attitude instrument flying. Watch the flight instruments, scan the needles. This is backward from the way most of us are taught to fly approaches (watch the needles, scan the instruments).

Aero-tip of the day: Evaluate position and trend to fly smooth, low-workload approaches.

FMI: Aero-Tips