Flying The ILS Approach

Most, but certainly not all, ILS approaches will be vectored approaches, rather than full approaches. An Approach – or occasionally Center – radar controller will vector you to the final approach course. (See Vectors to Final). As in most approaches, the difficult part of the approach is in the preparation, the situational awareness (what am I doing, what is being done to me, and what is going to happen next?), and positional awareness (where am I, where am I supposed to be, and where am I supposed to be next?). Knowing that the controller should vector you to the final approach course from either side at an angle of approximately thirty degrees to the course, you should visualize your position as soon as you begin receiving vectors and visualize how you will most likely be directed to the approach course. Prior to receiving that last intercept vector, you should have all of your preparation completed – I use the approach checklist – you should use whatever systematic method works for you, but it should include weather awareness for the terminal area, wind conditions, runway awareness including the approach and runway lighting, checking all of the instruments – DG, etc. – setting up the communication and navigation radios, tuning and identifying all navaids, briefing the approach, noting the minimums, noting the missed approach instructions, whether as published or amended, and setting up the airplane for landing – boost pump, cowl flaps, approach flaps if employed, fuel checks, and so on. All of these issues should be settled prior to that last intercept vector. If vectored on a classic downwind, base, and intercept pattern, I will endeavor to complete my landing checklist preparations no later than the base leg, beginning and perhaps finishing on downwind.

Decide at what point you wish to slow to approach speed. If you are being vectored close to the outer marker, you may want to slow to approach speed on the base or intercept vector. If put on the localizer at some distance from the outer marker (more than a couple of miles) you may want to remain at cruise until on the localizer because you will have time to slow down. You may want to fly the approach at cruise, which is certainly feasible in a PA28. Once given the intercept vector and the approach clearance, concentrate on intercepting the localizer. Do not be distracted and fly through the localizer. Use anything available to maintain positional awareness – GPS, ADF, a VOR on the field or elsewhere, etc.

Once you see the localizer needle come alive (move off the peg and into its active range), use good intercept technique to place yourself on the localizer. Here is where many pilots make the mistake of focusing initially on centering the localizer needle. If the needle is captured, within one or two dots of the center, concentrate first on determining the necessary wind correction. Do not turn to a heading you believe will center the localizer needle. And do not turn to the approach course heading unless you know the wind to be calm or a direct head or tailwind. You know that heading will not keep you on the localizer. Turn first to the heading that you believe will correct for the crosswind and achieve flight parallel to the localizer – as indicated by a stationary localizer needle.

How do we determine this heading? One knot of crosswind will require one degree of crab for every 60 knots of speed. For example, ten knots of crosswind will require 6.6 degrees of crab into the wind at 90 knots; 20 knots of crosswind will require 13.3 degrees of crab at 90 knots. From your knowledge of the winds aloft, estimate the crosswind and therefore the required crab angle and establish that crab angle immediately. Do we actually do this arithmetic while flying the approach – probably not – but this rule of thumb should allow us to quickly estimate a reasonable crab angle and from there we work empirically. To keep it simple, bias immediately into the wind, five degrees if you have no better guess. Stabilize heading, airspeed, and altitude and observe the localizer needle after a few moments. Do not fret if it is not exactly centered. We are interested first in stabilizing its motion. If it is moving, make another correction of a few degrees on the heading and stabilize again. Note the localizer needle again. Once you have found a heading that stops its movement, you have found the proper wind correction – your reference heading. Note that heading – with the heading bug, if you wish. Now turn and intercept the localizer with whatever heading is necessary to center the needle. That should not be difficult. Once it is about to center – turn back to your previously determined reference heading. Any fool can turn toward the localizer needle and make it center. The more difficult task is determining what heading will correct for the wind and keep it centered once you are there.

Track the localizer with small heading changes, never chasing the localizer needle but instead making adjustments to your reference heading to stop any motion, noting the heading, recentering the needle, then turning to the new reference heading. Typically, since the wind usually diminishes closer to the ground, you will find yourself beginning with a relatively large wind correction and, as you descend on the glideslope, working your way slowly back toward the approach course as the crosswind diminishes – typically but certainly not always.

Endeavor to make small corrections on the localizer – inside the outer marker refrain from corrections or heading changes of more than five degrees. Bear in mind that, near minimums, a heading change of 2 degrees will have the same effect as a turn of 6 degrees at the outer marker. If necessary, you do what you have to do to recapture the localizer, but if you find yourself having to make heading changes of more than five degrees inside the marker you are at risk of an unstabilized approach. Be cautious.

Feel free to use the rudders to make those small heading changes. Slight rudder pressure will yaw the airplane a few degrees more accurately than you can bank and turn three or four degrees – and the yaw will induce a slight bank anyway. Just try to be smooth about it.

Resist the temptation to overcontrol and overcorrect. Try to visualize the runway ahead and realize that if you can stop the motion of the localizer needle, even if it isn’t exactly centered, and keep it motionless you will arrive at the runway threshold in position to land. (A motionless localizer needle as you approach the runway indicates a constant angular offset, which will result in a steadily lessening linear offset, which will result in steady progress toward the runway centerline.)

Glideslope intercept will occur at the position indicated on the profile view of the approach chart by the lightning bolt – if you are on the localizer level at the altitude indicated on the profile view. ATC may, however, have you on the localizer farther out, in which case, you may encounter the glideslope at a higher altitude. If you have been cleared for the approach, you may descend on the GS when encountered. You should, in all cases, intercept the glideslope from below it, so you should see the GS needle descend from the top of the indicator.

As the glideslope indicator needle comes “alive,” which is to say, starts descending from the top of the indicator, check to see that the glideslope warning flag is off, which confirms a valid navigation signal from the glideslope transmitter.

Techniques for tracking the glideslope vary, but the object is always to fly a stabilized approach, with a smooth, consistent descent. In a retractable gear airplane, deploying the gear will often establish a descent consistent with the glideslope. In a fixed-gear airplane, a power reduction is usually employed to establish a descent on the glideslope. In the Warrior, we usually stabilize trimmed at ninety knots level on the localizer, with a power setting of approximately 2150 RPM. To initiate a descent, we reduce power to approximately 1800 RPM for a descent of approximately 500 FPM at 90 knots. You may also choose to fly the approach at cruise speed, in which case you would strive to stabilize at cruise speed level on the localizer and initiate a descent by reducing power to approximately 2150 RPMs. The faster speed will require less wind correction, a greater descent rate, and a longer runway, but may be more stable and may blend better with faster traffic.

The exact vertical rate of descent required is a function of groundspeed. The faster the groundspeed, the greater the required descent rate. The slower the groundspeed, the slower the required descent rate. How do we determine the groundspeed? If we have DME on the localizer, we simply check the groundspeed readout on the DME for our exact groundspeed. If we do not have DME on the localizer, we infer groundspeed from other information. We might have DME on a VOR located on the field. If not, we make an educated guess as to groundspeed. The groundspeed will almost always be something less than our airspeed, since we will usually be flying the approach into the wind. The only exception might be an airport with an ILS into only one runway where we might choose to fly an ILS downwind to obtain the lower minimums and then either circle to land or, in some circumstances, land downwind. Normally, however, groundspeed will be something less than the indicated airspeed – airspeed minus the headwind component of the wind aloft.

Once we determine, or estimate, our groundspeed, how do we determine the required vertical descent rate? The rule-of-thumb formula is five times the groundspeed. A groundspeed of 100 knots requires a vertical descent rate of 500 feet per minute; 90 knots requires 450 FPM, 80 requires 400 FPM, 120 requires 600 FPM, etc. You can see that simply dividing by two and adding a zero will compute the required descent rate.

Once we know the approximate necessary descent rate, how do we achieve it? Pilots debate whether to use pitch or power to track the glideslope. In reality, a stabilized descent requires an understanding and management of both. Try first to establish the target descent rate by reducing power and maintaining the airspeed. Endeavor to trim for level flight at your intended approach speed as you establish on the localizer course. As the glideslope indicator centers, reduce power, let the airplane pitch down, manage the pitch to maintain the previous airspeed, and adjust the power to obtain the target descent rate. A frequently-quoted rule of thumb holds that 100 rpm or 1 inch of manifold pressure equates to a descent rate of 100 feet per minute. By this rule, a 500 fpm descent rate would require a power reduction of 500 rpm or 5 inches of manifold pressure. I generally find those numbers to be excessive. In a Warrior, approximately 2150 rpm will establish level flight at an approach speed of 90 knots and a power reduction to approximately 1800 will achieve a 500 fpm descent rate. In a retractable-gear aircraft, deploying the gear at glideslope intercept will usually achieve an appropriate descent reate with no change of power. Once those pitch and power settings are established and stabilized, track the glideslope with slight pitch adjustments, accepting the associated slight airspeed deviations. If the airspeed varies substantially – more than plus or minus ten knots – you may have to adjust the power as well to keep it within bounds. This will occur in rough or turbulent air. If you deviate, for whatever reason, substantially from the glideslope, you may have to change the power setting to obtain a new descent rate without varying the airspeed excessively.

As you track the glideslope, be alert to a failure of the glideslope transmitter or indicator. If the glideslope needle remains immobile, particular when centered, you are either flying a perfect track of the glideslope – or the glideslope indicator has failed. For most of us, the latter is far more likely than the former. Some indicators will center and remain fixed when they fail, which can be a fatal deceit. If the GS needle is immobile and centered, you may want to level off, or reduce the descent rate momentarily and look for a momentary upward deviation of the GS needle.

You should make several checks of the GS as you proceed on the approach. As the needle comes alive, check the warning flag to make certain it is off. If the needle becomes immobile, be suspicious. You should also note the crossing altitude. The profile view of the ILS will always indicate a crossing altitude, usually at the outer marker or FAF. This is the MSL altitude at which you should find yourself, and which your altimeter should read, when you pass this identifiable fix, if you are on the glideslope. So, as you pass this fix on the approach, note the altimeter reading and compare it to the crossing height on the chart; this will give you confirmation of your descent guidance.

Typically, the groundspeed will increase as you descend lower because the winds will reduce in intensity and thus the headwind component will diminish – and the groundspeed will increase to something closer to the airspeed. In this case, the usual case, you will need to increase the vertical descent rate somewhat as you descend toward decision height. This occurs for the same reason that you will often find yourself having to turn the airplane somewhat back toward the final approach course as the winds diminish on your descent.

This increasing groundspeed often causes us to rise above the glideslope. As the groundspeed increases, the initial descent rate becomes insufficient to maintain the glideslope. Our natural inclination – quite properly – is to pitch down slightly to increase our descent rate. Unfortunately, this reduced angle of attack not only increases our descent rate but our groundspeed – requiring an even greater descent rate. We can easily find ourselves chasing a moving target – an ever-increasing required descent rate. The remedy is to control the airspeed as we increase the descent rate. Beyond five or ten knots, we must decrease the power to prevent the airspeed and thus the groundspeed and thus the necessary descent rate from continuing to increase as we chase the glideslope downward.

As with the localizer, stability is the secret to a successful ILS approach. When correcting toward the localizer or glideslope, we prefer to see the needle moving toward the center of the instrument very slowly. Rapid movement indicates an excessive intercept angle and the likelihood that we will travel through the localizer or glideslope. Even at decision height, a stable needle deflected quarter-scale is preferable to a centered needle that is still moving.

At decision height, you will either execute the missed approach, either as published or as amended by ATC, or you will land. To descend below DH, you must have the required visibility, the runway environment in sight, and be in a position to make a normal descent and landing. We do not normally fly the approach with flaps, although that is certainly an option. If flaps are not deployed on the approach, you simply deploy them on descent from decision height. At DH, reduce power to approximately 1500 RPMs, maintain the same pitch, and allow the airplane to slow to landing speed, approximately 65-70 knots. If you fly the approach at cruise speed, be careful to let the airplane slow to below flap extension speed before deploying flaps – a power reduction to idle may slow the airplane more quickly but will change the pitch force. If you maintain the approach attitude as you add flaps, the flaps will slow you to touchdown speed. You will float some distance, but the typical ILS approaches a runway of considerable length.

If you do not have the landing minimums when reaching decision height, you may descend to one hundred feet below DH if you have the approach lights in sight, but this is a specialized maneuver that requires more precision and a better understanding of approach lighting systems than most of us possess. Think seriously before availing yourself of this privilege.

If you do not have the landing minimums in sight and are not descending to one hundred feet below DH as per the above, you must execute the missed approach promptly at DH. Your momentum will carry you slightly below DH, which is acceptable, but you should initiate the missed approach promptly, crisply, and decisively to avoid excessive descent below DH. One of the advantages of flying the approach at 90 knots is the fact that the application of full power will initiate an effective climb at something near Vy or best rate of climb without adjusting the trim of the airplane. In any case, apply full power and pitch to your Vy attitude to obtain an effective climb. Climb as per your missed approach instructions, retract flaps if you deployed them for the approach, and concentrate on flying the airplane. Do not worry about anything else until you have the airplane stabilized and trimmed in a climb. Once under control, notify ATC of your missed approach and inform ATC of your intentions thereafter – another approach or flight to your alternate or to another airport of your choice.