Insights: The Funnel

On occasion I fly with a pilot who seems oblivious to deflection of the OBS (omni-bearing selector) needle as he or she approaches a VOR station. He knows the needle should be centered, yet he doesn?t make a correction. This can be suicidal during an instrument approach.

Specific deficiencies cause this kind of mistake ? poor radio navigation training, the inability to fly a constant heading, and no practical understanding of tracking procedures or VOR transmission characteristics. A pilot with this kind of deficiency cannot use ?the funnel? for its intended purpose ? precise station passage. The funnel is the narrowing of the radial, caused by increased VOR receiver sensitivity as you near the station.

It may not be fair to describe radio navigation training as poor, because some pilots are trained in benign weather conditions. They have experience in light winds only, and the terms ?heading? and ?course? become synonymous. These conditions make it imperative that a flight training device be used for initial instrument training so students become habituated to proper tracking procedures. Flight simulation allows for adverse wind conditions on every flight.

Tracking a course ? LOC, GPS, VOR, or NDB ? is a basic navigation task that requires practice and pilot discipline. It?s not a secondary task that the instructor discusses briefly during en route training. It?s a stand-alone task the pilot must master, with crosswinds, during initial training.

The standard tracking procedure follows a two-step correction rule. You put in something that obviously will work (20 degrees normally, 40 degrees for extreme winds, and 10 degrees when tracking the localizer), and when you?re back on course, you remove half the correction. In normal wind conditions, you may need two more heading changes to get your wind correction angle correct within five degrees. You can then ?chase the needle? by making small, one- or two-degree heading changes. Nothing more.

Did I say one or two degrees? Am I nuts?

No, I?m dead serious. And this illustrates a critical, course-tracking problem ? the ability to fly a constant heading. If you can?t do this, you may be able to keep the OBS needle centered, but believe me, you?re spending way too much time looking at it. You?re not managing your cockpit properly, and I?ll bet that you?re not happy with the ILS approaches you fly because tracking a localizer requires perfect heading control.

The funnel illustrates why heading adjustments are necessary for precise VOR station passage. These adjustments correct for VOR transmission characteristics: as distance to the station decreases, radials narrow, and receiver sensitivity increases.

The figure, at left, represents just one VOR radial, so if you approach the funnel on any of the five track lines, A through E, the OBS needle will be centered. Track C will keep the needle centered until you?re over the VOR, at which point the needle oscillates quickly as the TO-FROM flag snaps to the FROM indication.

The other tracks result in needle movement when you reach the bottom of the funnel ? right if on A or B, left if on D or E. When this occurs, you should turn toward the needle, but when it starts to move toward the center, you must turn back immediately to your previous heading. You should turn no more than 10 degrees when you?re close to track C, 20 degrees when the situation is extreme.

Here?s a trick that simplifies funnel turns. If you roll into and immediately roll out of a standard-rate turn you should find you have turned six degrees. If this doesn?t work for you, your rate of roll is incorrect. I hold the bank angle for a second or two when I need a larger heading change, and I do just the opposite when the OBS needle starts to re-center. In other words, I make funnel turns without looking at the heading indicator, and like all of instrument flying?s finer points, practice makes perfect.