An appreciation of localiser sensitivity combined with the following suggestions can considerably improve a pilot’s ability to execute an ILS approach to minimums. When tracking a four-degree-wide localiser, for example, at a distance of only one nautical mile from the runway threshold, when the needle is deflected one-quarter scale, the aircraft is only 141 feet from being precisely on course. To a pilot accustomed to VOR flying, a quarter-scale deflection seems like quite a bit. Between VOR stations, a return to course might require a 10-degree correction (or more) to be held for several minutes. This previous experience with the CDI has an adverse effect on the pilot because it creates the tendency to make similarly large corrections when tracking an ILS.
The same correction (10 degrees) applied to a localiser when only 141 feet off course results in such a rapid return to the centreline that overshooting the localiser is almost impossible to avoid. With respect to a localiser (and not a VOR radial), a quarter-scale deflection is not that big a deal. When 141 feet off course, the aircraft is only 41 feet from being lined up with the edge of a 200- foot-wide runway.
Putting this in proper perspective, consider how small a correction would be required when a plane is 141 feet from the extended runway centreline during a VFR, straight-in approach. Very little. The heading change would be barely noticeable. Quite obviously, the same minor correction should be made during an actual ILS approach.
MENTAL ATTITUDE
This, then, is what is meant by the need to adopt the proper mental attitude. Heading changes during an ILS approach should only be a small fraction of what is normally required to centre an equally displaced VOR needle. Most pilots who have difficulty keeping the localiser needle within reasonable limits are usually guilty of chasing the needle. They have not learned that the secret of a successful ILS approach is the result of logical, minimal, predetermined heading changes. For example, assume that a pilot is intercepting the localiser. He rolls out on the ILS heading just as the needle centres in the bull’s-eye. The published magnetic course of the ILS becomes what is called the temporary reference heading, which — in this case – shall be 095 degrees.
Under no-wind conditions and with an error-free heading indicator, this heading theoretically would lead the aircraft precisely along the localiser. Such is rarely the case, however.
Expecting some drift, the pilot pays careful attention to needle behaviour, while flying the reference heading with flawless determination. He knows that an inadvertent heading change causes the localiser needle to move and leads to the false impression that wind drift is the responsible culprit. An accurate “picture” of the wind cannot be drawn unless the reference heading is precisely maintained. The heading is 095 degrees, and the needle slowly moves left. There are two possible reasons for this: a left crosswind or an improperly set heading indicator (or a combination of both). But this pilot is sharp. Once on the localiser, he knows that to subsequently reset the HI can only interfere with his plans to execute the perfect ILS approach.
Once the HI has been synchronised with the compass, prior to localiser intercept, he will assume that all needle movement is caused either by wind drift or heading change.
As the needle moves left, the pilot rolls into a very shallow turn toward the needle. His immediate intention is not to centre the needle, but simply to stop it dead in its tracks.
After five degrees of turn, in this case, the needle stops and the pilot rolls the wings level. He precisely maintains the new heading (090 degrees) and again begins his vigil of the localiser needle. If the needle remains in its displaced position, the pilot knows that this new heading (090 degrees) is causing the aircraft to essentially “parallel” the localiser.
He knows also that whatever heading “parallels” the localiser also can be used to track the localiser when the needle is centred later.
This new heading (090 degrees) becomes the revised reference heading and quite accurately compensates (within a degree or two) for any prevailing wind and/or any discrepancy between the compass and the heading indicator. If the needle continues to move, however, it is at a much reduced rate, and the pilot can make whatever smaller correction is necessary to stop needle movement. The end result becomes the new reference heading.
Since it is his desire to centre the needle, the pilot turns further left to a heading of 085 degrees. Obligingly, the needle moves toward the bull’s-eye. As the needle centres, does this pilot have to guess at what heading shall be required to track inbound?
Of course not. He turns to the reference heading (090 degrees) and smugly observes the captured localiser. He is now ready to intercept the glideslope and continue with this, the thinking man’s approach. This two-step manoeuvre of (1) turning to stop needle movement and then (2) turning further to intercept the localiser can be accomplished by a savvy pilot in one smooth move.
He turns toward the digressing needle and simply notes the reference heading at that point during the turn when the needle comes to a halt. But the turn continues briefly and without interruption to reverse needle movement.
When the needle returns to the bull’s eye, a turn is made to the reference heading noted during the initial turn. As the descent begins, no one can be so naive as to believe that wind drift will not change. Count on it. The point is that unless a strong wind shift (or shear) exists between the ground and 1 500 feet agl, drift change will be gradual.
As the localiser needle begins to react accordingly, a pilot must similarly turn to stop the needle and establish a new reference heading, one that can be used until conditions again require a change. The idea is to fly logical headings, based on observations of needle behaviour, and not to take arbitrary, random swipes at the localiser.
As the aircraft descends on the glideslope, it also gets closer to the localiser transmitter, which further increases needle sensitivity. Although the same techniques are used when the localiser moves off centre, heading changes must be proportionately smaller.
A two-degree heading change near minimums, for example, has about the same effect on needle movement as a six- degree “bite” when near the outer marker. As the aircraft approaches decision height (DH), it becomes increasingly more important to fly a specified heading and to not chase the needle.
The most urgent requirement is that the needles not be in motion, because this indicates cross-tracking and is usually more responsible for missed approaches than arriving at minimums with slightly offset, yet motionless needles. If the localiser is slightly left or right (and motionless), it is better to accept being a few feet off course than to risk initiating a crosstrack correction that could result in a larger needle displacement in the opposite direction.
In other words, do not be so precise that a slight needle deflection cannot be tolerated (unless you can make exacting one- or two-degree turns or are below the glideslope). The obsession to exactly centre the needles can blow an approach. (This applies, of course, to Category I approaches only; the lower minimums associated with Category II and III approaches do require substantially more precision and equipment.) The glideslope is another breed of cat, similar to the localiser but even more sensitive. It has an effective width of only 1,4 degrees. In other words, a vertical deviation of only 0,7 degree fully deflects the horizontal needle.
When an aircraft is two nautical miles from the runway touchdown zone, for example, a needle deflected half-scale indicates that the aircraft is only 74 feet above or below the glideslope. When only half a mile from the touchdown zone, the same needle deflection translates to only a 19-foot deviation from perfection.
To put it another way, the glideslope is 14 times as sensitive as a VOR needle and three times as sensitive as the localiser at equal distances from the station transmitters. Think about it this way: When tracking a glideslope one mile from the touchdown zone, the needle has the same sensitivity as when tracking a VOR radial when only 0,07 nautical mile from the VOR transmitter (if that were possible). Such sensitivity requires thinking about the controls (or perhaps breathing on them) more than it does moving them. Tracking the glideslope also requires the proper mental attitude. Instead of requiring a reference heading (as does the localiser), the glideslope demands a reference sink rate.
The vertical-speed indicator (VSI) is often ignored, but it is the magic key required to unlock the airport when ceiling and visibility conspire against you. Prior to glideslope intercept, determine from the approach plate the recommended sink rate required to slide down the glideslope at the groundspeed anticipated during the approach. For example, a four-degree glideslope (the steepest is usually 3,9 degrees) requires a 709 fpm sink rate when groundspeed is 100 knots. Usually, however, a pilot can predict the required sink rate without referring to a chart. Since most glideslopes are on the order of 2,25 to three degrees, this handy rule of thumb can be used: “Cut the approach groundspeed (knots) in half and add a zero.”
When using a three-degree glideslope with a groundspeed of 80 knots, for example, sink rate should be approximately 400 fpm. As the glideslope is intercepted, immediately establish and attempt to maintain the recommended sink rate. If this is done correctly and if groundspeed remains constant, the glideslope needle will require no further attention. But this happens only in textbooks; the glideslope undoubtedly will move off centre.
Quite obviously, variations in sink rate are required to arrest a displaced glideslope needle, but it is the method and amount of correction that require emphasis.
What I am about to say is certain to raise eyebrows and attract scowls from the purists, but the best and easiest way to recapture a displaced glideslope needle is to simply apply the appropriate elevator pressure without regard to airspeed and power. Allow airspeed to vary (within reason) and to hell with power adjustments. Why complicate the issue by trying to rub your tummy and the top of your head simultaneously? Simply nudge the yoke and adjust sink rate slightly. Do, however, keep a ready hand on the throttle in case airspeed starts to get out of hand. Unless a wind shear is present, however, airspeed usually takes care of itself rather nicely. The required sink-rate adjustment rarely exceeds 200 fpm. So, if a 500 fpm sink rate is being used and the glideslope needle begins to rise, change the sink rate to 300 fpm and watch needle behaviour. Usually, it will return toward the bull’seye, at which time the original 500-fpm sink rate (or slightly less) should be resumed.
If the needle stops or only slows a little, then reduce sink rate an additional 100 fpm. Very little change in sink rate is usually all that is necessary to recapture the glideslope. Just tickle the yoke; don’t horse around with it. Unless the glideslope needle is fully deflected upward, do not reduce the sink rate to zero. Such an abrupt change requires subsequent abruptness (and sloppy technique) to prevent the needle from dropping rapidly toward the bottom of the instrument. Unless wind conditions change dramatically and unless an aircraft is dangerously below the glideslope, varying sink rate by more than 200 fpm is rarely necessary.
To appreciate the finesse required to do this property, concentrate on varying sink rate by increments of 100 or 200 fpm during a visual, straight-in approach. Learn how little control movement is required. Observe also — during a visual approach — how little the elevator is used to remain in the slot. It is no different when the aircraft is engulfed in cloud and the glideslope is being used for descent. So you see, flying the cross-pointers is sort of like sex. Each requires the proper mental attitude, a soft touch, and the ability to put it all together (meaning the localiser, the glideslope, and the ILS bull’seye, of course).
