Instrument Insights Part 5 of 12

Precision Approaches

May 1, 1998

Riding the rails to 200 and a half

Instrument pilots generally agree: If ceilings and visibilities are heading downhill, then a letdown through the clag on a precision instrument approach is the only way to fly. Precision approaches are those with vertical guidance, and they include Instrument Landing System (ILS) and Precision Approach Radar (PAR) approach procedures. PARs involve controllers essentially talking pilots down to minimums.

PARs usually serve military bases. On a PAR approach, controllers constantly issue lateral and vertical commands to keep the pilot on the proper approach path. The terms slightly and well are used for advisories (e.g., "you're slightly left of course and well above the glideslope.") If pilots on PAR approaches receive no communications for a period longer than five seconds, then they must execute a missed approach.

Why do pilots lean toward precison approaches? Several reasons. First and foremost, precision approaches provide pilots with extremely accurate lateral as well as vertical guidance to relatively long and wide runways. They also give you information on your range from the runway, provide you with target altitudes along the approach path, and keep you well-aligned with the landing runway's extended centerline.

It's this kind of accuracy that permits descents to altitudes as low as 200 feet agl in visibilities as low as one-half mile. And when you break out of those rotten conditions, there's the greater assurance that the landing runway will be right in the center of your windshield.

Who could ask for more than a precise, stabilized, straight-in approach on a three-degree glidepath, ending with a landing on a specially prepared and approved runway? That's what you get with a properly flown ILS or PAR approach. Because ILSs are so common, we'll concentrate on them in this article.

ILS components

Like VORs, ILSs also use VHF signals to provide course information. But when you fly an ILS, you're really following two signals: a localizer, for lateral guidance; and a glideslope, for vertical guidance. Keep the localizer and glideslope needles centered as you fly the approach and you'll fly "down the pipe" to the runway.

Along the way, there will be intermediate fixes along the ILS approach path. Many times, marker beacons are used in sequence. Broadcasting on 75 MHz and sending up an elliptical, nondirectional beam, markers are great for giving you range information along the ILS.

The outer marker is usually 3.5 to seven miles from the ILS's runway threshold. It illuminates a purple light on your airplane's marker beacon display and gives a three-dash Morse code identifier that you can hear through your airplane's audio switching panel or cabin speakers.

The middle marker is usually about 3,500 feet from the threshold, causes an amber beacon light to shine, and has a dot-and-dash identifier. Frequently, activation of the middle marker nearly coincides with the decision height (DH - the altitude at which you must decide to land or not) and published minimums for the runway to which you're flying. In other words, being at the middle marker often means that you're at 200 feet agl and one-half mile from the runway threshold. If you don't see the runway when the middle marker goes off, it's probably time to execute a missed approach.

Inner markers (white light, continuous dots) are found with ILS approaches that can be flown to lower minimums, such as Category II ILSs, where decision heights can be as low as 100 to 150 feet agl. Inner markers are located right at the runway threshold.

Other fixes along the approach path can be locator outer markers (outer markers that also give out NDB signals that you can select on your airplane's ADF receiver), intersections from nearby VOR radials, or distance information from localizers that have DME capability.

But ILSs are more than just navaids. To be properly certified, there must be certain approach lights; and the runway has to be wide enough, long enough, and have lights and/or painted markers to help the pilot identify the threshold, runway centerline, touchdown zone, and touchdown zone aiming point in night or low-visibility situations.

Approach lights associated with ILSs usually include an array of flashing lead-in lights aligned with the extended runway centerline (sometimes called the "rabbit," because the flashes are rapid and sequential, and they travel toward the runway in a rabbit-like fashion), roll-alignment bars, threshold lights, a VASI or PAPI installation, and runway edge and centerline lights.

How accurate?

Every pilot is familiar with the behavior of VOR signals. To track to or from a VOR, you select a radial, intercept it, and apply heading corrections to maintain the desired course. Depending on your range from the VOR station, each dot on the course deviation indicator (CDI) represents a variable off-course lateral distance. At 30 nm from the station, a one-dot deflection of the CDI needle means that you're one mile off course; 15 miles out, one dot equals one-half mile off the intended track.

An ILS, on the other hand, is several times more sensitive than a VOR. And the closer you fly to the runway, the more sensitive the ILS signals become. This means that pilots attempting to track an ILS must make small corrections, lest the CDI needle slam from one side of the instrument to the other.

How small? Well, at the outer marker, a one-dot deflection on the ILS's localizer display represents approximately a 500-foot lateral off-course condition. Near the middle marker, a one-dot deflection of the localizer needle means you're about 150 feet off course. Over the runway threshold, localizer sensitivity can be measured in tens of feet - enough accuracy that "zero-zero" blind landings can be made in dire situations.

The glideslope portion of the ILS is even more sensitive. At the outer marker, each dot of glideslope deviation equals about a 50-foot excursion from the prescribed glidepath. At the middle marker, the sensitivity is an astounding eight feet per dot.

Moreover, ILS signals are unidirectional. They beam out in a narrow, fan-shaped array, aligned within 10 to 35 degrees of the runway centerline. Outside of that small arc, ILS signals are, for all practical purposes, unusable.

Traps

You tune ILSs using VOR receivers and can see course and glideslope guidance on the same basic CDI display as that used for VOR navigation. But that's where the similarity ends.

The Omni Bearing Selector (OBS) knob that you use for selecting VOR radials is useless when your VOR receiver is set to an ILS frequency. You can spin the OBS knob all you want, but it will have no effect on an ILS's needles. That's because of the unidirectional nature of the ILS signal.

In spite of this, standard procedure is to set the inbound course with the OBS anyway - as a reminder only. You'll receive usable localizer and glideslope indications only when you are within a 10- to 35-degree signal arc. So don't think that you can "select" an inbound course by twirling the OBS and watching the course needle move on target.

Another trap has to do with the glideslope. When viewed in cross-section, its signal can have lobes that may trick your receiver - and the glideslope needle - into presenting a false on-glidepath indication. This is most likely to happen if you try to intercept the glideslope from above. To prevent this, always try to intercept the glideslope from below. That is, fly at an altitude low enough to keep the glideslope above you until its needle slowly drops down from the top of the indicator's face.

Worried about inadvertently flying a false lobe? One way to confirm your position on the glideslope is to check your altitude at the final approach fix (FAF), which is usually at the outer marker on ILS approaches. You'll see a glideslope intercept altitude published on instrument approach charts near the FAF symbol. If your airplane's altitude doesn't match the intercept altitude, you're either too high or low on the real glideslope signal, or following a false lobe.

ILS signals also have back courses. When you are flying inbound on an ILS approach (on the so-called "front course"), your CDI's localizer indications will provide conventional steering commands - i.e., needle right, steer right. But fly outbound on the front course and the steering commands are reversed - i.e., needle right, fly left. Now you're experiencing the localizer's back-course behavior, and if you're to fly straight ahead - for example, as part of a full instrument approach procedure - it can be a real chore to sort out the commands and fly the airplane in the proper direction.

Remember this well: These kinds of backwards steering commands will appear any time you're flying inbound on a back course, or outbound on the front course!

Back courses are sometimes used as nonprecision localizer approaches because each ILS has a corresponding back course. It's a way of squeezing approaches into either end of a runway served by a single ILS. Sometimes back-course approaches can even have glideslope information. But that's the exception. Back courses with unsatisfactory glideslope information are always published with warnings to disregard any glideslope indications.

Flying the ILS

You'll either be given vectors to intercept an ILS or be asked to fly the full approach. A full approach involves flying outbound on the ILS's front course (remember the backwards-steering warning) for two minutes or so past the outer marker, reversing course and reintercepting the front course inbound via a procedure turn, and then finishing up by crossing the outer marker inbound and descending to minimums.

Either way, you'll want to know how to configure your airplane for an ILS descent once you've crossed the outer marker with the needles centered. For fixed-gear singles, a power setting of 1,700 rpm will usually give you a reasonable airspeed and rate of descent. For light twins and single-engine retractables, power settings of 16 to 17 in Hg and 2,400 rpm - with the landing gear extended - ought to do the job.

Flying the ILS requires a rapid scan rate and a steady hand on the controls. The closer you fly to the runway, the tinier your corrections must be. If you're to keep the ILS needles centered all the way down the pipe, make heading corrections of no more than five degrees at the outer marker, narrowing to no more than two degrees or so at the middle marker. As for glideslope corrections, don't chase them, either. If slightly high, briefly lower the nose to increase your descent rate and recapture the glideslope. If slightly low, arrest your descent until you fly back to a needle-centered condition.

If you make corrections any wilder than that, you could find yourself with ILS needles at full deflection - a bad situation that calls for a mandatory missed approach.

Practicing in VFR conditions

Avoid throwing around phrases like "I'm procedure turn inbound to Runway Two-Three," or "I'm at ROWDY [intersection] inbound." Those with instrument ratings may know that you're about six miles northeast of the runway at 3,000 feet, but VFR-only pilots in the pattern won't have a clue. It's better to simply state your position and altitude with respect to the airport.

That goes double for when ceilings and visibilities are high enough for low-altitude VFR flight, but an undercast requires that instrument pilots make a letdown and approach on the gauges. You want to avoid any surprises when breaking out of the overcast near the airport - such as airplanes turning base leg in front of you.

Bottom line

With practice your skill in flying ILS approaches can become quite impressive. That's when you'll come to appreciate their precision. When asked which approach you'd prefer, you'll find yourself telling approach control, "I'll take the ILS."

Tip Sheet

Turn, time, tune, talk - a memory aid for use during procedure turns. It reminds you to time your flight once entering a procedure turn, turn around and fly a reciprocal heading, tune in the inbound course (or, in the case of a nonprecision approach, the VOR radial or NDB bearing) on the OBS, and talk to ATC, reporting that you've completed the procedure turn inbound.

ILS descent rate - To know the descent rate needed to stay on the glideslope, divide your approach airspeed by half and add a zero. This will give you a good ballpark three-degree glidepath. Let's say your approach speed is 100 knots. Half of 100 is 50. Add a zero and you've got 500, or a 500 fpm descent rate.

Localizer intercept angle - This should be no more than a 20- or 30-degree angle. Pilots should question ATC about intercept angles greater than this, because they would involve steeper turns - something you want to avoid when beginning an ILS.

Effect of siting factors - Most ILS approaches have 200-and-a-half minimums, but some are higher than that. It all depends on factors such as nearby obstacles, runway length, terrain, and other local conditions.

Configuration control - With complex airplanes, a good practice is to approach the FAF with the first increment of flaps extended (to help slow the airplane), then lower the landing gear at glideslope intercept. The resultant drag will help to begin the descent down the final approach course.

Taming the back course - If you're flying with a horizontal situation indicator (HSI), there's a trick you can use to always be sure that the CDI needle gives correct steering commands. When flying a localizer outbound on the front course or inbound on the back course, rotate the HSI's course arrow 180 degrees from the desired course. By "flying out the tail" like this, you won't have backwards-steering commands. Just make sure that when flying the ILS front course, the point of the HSI's course arrow is pointing to the desired inbound course.

Lexicon

Procedure turn - a means of reversing course while on a full instrument approach. It usually involves turning 45 degrees off course, flying straight for a minute, performing a 180-degree turn, then flying straight once more until reintercepting the final approach course.

Feeder fix - a fix that defines the starting point of an instrument approach.

Decision height (DH) - the height at which a decision must be made either to continue an ILS approach to a landing or to execute a missed approach.

Height above touchdown (HAT) - the decision height above the highest runway elevation in the touchdown zone of the runway.

Final approach fix (FAF) - the point of an instrument approach procedure where the final approach segment begins.


E-mail the author at tom.horne@aopa.org.