August 1, 2008
By Thomas A. Horne
While it may be true that passengers judge a pilot’s competence by the quality of their landings, we know better. When it comes to precise flying, excellent judgment and staying ahead of the airplane, nothing is a better test of competence than flying an instrument approach. Passengers may not know all that goes into making a grade-A instrument approach, but for pilots seeing the runway approach lights breaking through the mist, it’s one of flying’s most satisfying experiences.
Volumes have been written on instrument approach procedures. Here, we don’t have the space to go into the step-by-step procedures for performing each type of approach. Instead, we’ll focus on some of the more critical aspects of the two main types of approach—precision (having vertical guidance) and non-precision (no vertical guidance)—and point out some of the traps that can snag the unwary.
Whether you’ll be flying a precision or non-precision approach, one of the very first steps is to check on the weather and give yourself a good talking-to. This means monitoring any Automatic Terminal Information Service (ATIS), Automated Weather Observation System (AWOS), or Automated Surface Observing System (ASOS) frequencies for the latest ceiling, visibility, and other information. It’s one thing to shoot an approach with a 1,000-foot ceiling and five-mile visibility, and quite another to shoot one at or near the published minimum ceiling and visibility.
The best time to get the weather is during the arrival procedure, when you’re probably not so busy tracking radials, are probably on a vector, and have enough time to listen to—and write down—the weather, approach and runway in use, altimeter setting, and communications frequencies.
Writing down the weather makes it easier for you to compare it with the decision altitude or minimum descent altitudes published on the approach plates. Besides, it’s too easy to forget the exact ceiling and visibility values when you’re making your way to any initial approach fixes. Will it be an “easy” approach, with high ceilings and visibilities, or a tense one, with a high probability of a missed approach? You should know well in advance, so this step is essential.
The next step is to review the published instrument approach procedure. There’s a whole lot of information on an instrument approach plate, and all of it is required reading. Thankfully, the information is logically presented, with a “briefing strip” across the top of the page, a plan view of the procedure; a cross-section, vertical profile view; minimums information; and missed approach instructions. Obstacle heights and minimum safe altitude information are also published.
The drill should be to go from top to bottom. The briefing strip will give you all the frequencies you’ll need, so go ahead and plug them into your avionics. Ditto the final approach course inbound. Keep the approach plate in front of you for the rest of the procedure; you’ll need to refer to it for step-down altitude, elapsed time, and minimums confirmation. Also, don’t forget to memorize the first two steps in the missed approach procedure. Doing a missed approach can be a confusing, scary procedure; it’s best to have its first parts memorized before you’re close to the ground, slow and dirty, or still in the clouds upon reaching the missed approach point.
Double-check your altimeter setting and consider whether or not to shoot a coupled approach, which lets any autopilot you may have fly the procedures soon to follow.
That’s a separate subject unto itself, but it’s worth remembering that auto-pilots can be wonderful workload-relievers. There was once a time when using an autopilot was taken to be a sign of weak piloting skills.
No more. They let you plan ahead, and can be quite precise when flown in the approach mode—sparing you work at the end of a long, tiring day.
Approaching the approach fix—be it an initial approach fix (IAF) or a final approach fix (FAF)—means carrying out the procedures laid out on the approach plate’s plan view. Of course, if you’re being vectored to the final approach course (FAC) then it’s no sweat—just comply with the vectors.
But flying a DME arc, however, can involve higher skill levels. You’ll need to know how to fly the arc manually using an HSI or VOR, or program your GPS so as to command heading changes that will make a proper arc. Either way, it means thinking way ahead of the airplane, having good situational awareness, and setting up the radios and displays prior to intercepting the arc.
If you’re flying a more or less straight-in track to the FAC, then a procedure turn probably won’t be required. Instead, a holding pattern may take the place of a procedure turn. And yes, if a holding pattern is depicted at the FAF, you are required to make one turn around it before resuming your course inbound to the runway—unless you’re on a vector, or ATC waives the requirement. Many GPS approaches use terminal arrival areas (TAAs) depicting the T-shaped approach segments leading to the FAC. Depending on the arrival sector you’re in, fly so as to intercept the perpendicular approach leg to the FAF—or have your autopilot do this.
As you cross the final approach fix, be sure to have the following items under control:
Keeping the needles centered is the only way to fly the FAC. Otherwise you risk losing obstacle clearance, situational awareness, or a good view of the runway when/if you break out of the clouds.
Small corrections are the name of the game. If you see a needle begin to move off center, then immediately correct for it. Slightly (e.g., one dot) off-center needles require less correction than ones that suddenly seem to zoom to the sides of the instrument case. Large needle deflections call for five to 10-degree corrections in the direction of needle deflection—that should be enough to stop the needle from deflecting any more. Then, as the FAC is re-intercepted, you can take out the correction and resume tracking inbound.
When flying ILSs, remember that the needles will be very sensitive, indeed. At the FAF, one dot’s worth of localizer needle deflection usually means you’re off course by about 600 feet; at the middle marker (where decision height typically occurs) it’s about 100 feet. The glideslope needle is even more sensitive.
At the FAF, being one dot low typically means you’re around 100 feet closer to the ground; at the middle marker, one dot can equal a mere eight to 10 feet!
To help tame the ILS, make no more than five-degree heading corrections near the FAF, and keep them to no more than two degrees when descending down the FAC. Another technique for mastering localizer intercepts is to use an HSI lubber line. Turn the airplane so that the lubber line is above the deflected needle, and you’ll automatically have the correct intercept angle.
Of course, full-scale localizer deflections require immediate action. The best advice is to perform a missed approach—after you turn toward the FAC, re-intercept the FAC, and simultaneously track inbound and climb to the initial missed approach altitude.
Dealing with glideslope deflections takes a gentle hand. If you see you’re dropping below the glideslope, consider arresting your descent instead of chasing the needles; you’ll soon recapture the needle. If you’re climbing above the glideslope, pitch down two degrees or so—maybe three or five degrees if you’re two dots high—and reduce power slightly to help recapture the needle. It’s all about continuous, gentle, small (the closer to the runway, the smaller) corrections—unless you peg the glideslope on the low side.
This, obviously, also dictates performing the missed approach procedure if inside the FAF.
Precision approaches such as ILSs, and quasi-precision approaches such as GPS-WAAS LNAV/VNAV and LPV approaches are nice, but only non-precision localizer, VOR, GPS, RNAV, and even NDB approaches serve many airports. These present special problems. Perhaps the biggest challenge is what we can call the “slam-dunk” profile. This issue crops up right at the missed approach point (MAP), when you’re still 400 or 500 feet above the airport elevation, and right over the runway threshold.
Many non-precision approaches have profiles like this. The problem—and it’s a big one—is that it can be extremely difficult to descend rapidly to a suitable touchdown point on the runway without building excessive airspeed.
Imagine it: The weather is at or just above minimums, you’ve done a great job flying the approach, you’re elapsed time is up, you’re at the MAP, and there it is.... The end of the runway, just beyond the nose of the airplane. Naturally, you’re tempted to make the landing, but a normal descent angle would put you off the end of the runway. So you chop the power, push the nose over, and aim for the first third of the runway.
In the process, you’ve picked up airspeed. So when you arrive at the runway, you have plenty of airspeed to bleed off in the flare. So much that you run off the end of the runway in a desperate, failed attempt to stop in time. It’s happened many, many times.
The antidote is to establish a decision point that lets you descend to the runway at a normal rate. This point should obviously be sooner than the MAP. How much sooner? That depends on your groundspeed, which in turn depends on the winds aloft. A rough rule of thumb might be to subtract a minute from the MAP time.
This should allow you enough time to safely land out of a non-precision approach with an MAP at the runway threshold. The only drawback here is that if the weather is at minimums you may not see the runway environment this “early” in the approach.
Another good practice is to make a prompt descent to the minimum descent altitude (MDA) immediately after passing the FAF inbound. Hopefully, this gives you the benefit of flying visually beneath any low cloud layers, and seeing the runway. Gradual descents to the MAP can set you up for the slam-dunk by keeping you in the clouds all the way to the runway threshold.
Some non-precision approach procedures acknowledge the slam-dunk problem. These establish a visual descent point (VDP), which is designed to permit a normal descent rate to the runway threshold. The VDP is most often defined by a distance to the runway threshold, and is most often between one and four miles away. That’s plenty of distance for a sane, controlled final descent and landing at a safe airspeed.
What if the surface winds—or construction crews—don’t favor the instrument approach you’re flying, and the best course is to land on a runway oriented in another direction? The answer here is to perform any authorized circle-to-land maneuvers. This involves higher minimums, and increased risk. Some airlines don’t even allow their crews to fly circling approaches. That’s because you have to remain very near the airport, and within prescribed boundaries, while you make relatively steep turns low to the ground—and perhaps, close to terrain or obstacles. While circle-to-land procedures have their place, it’s a complicated topic that deserves separate treatment. For now, let’s agree to try to avoid them when possible—unless you’re completely familiar with the airport environment, and completely comfortable circling to land. At night, that goes double.
At the end of every instrument approach in weather at or near minimums, there comes a challenge. It’s the transition to visual flying, and it can be a stressful time because you’ve just spent the last leg of the flight with your eyes glued to the gauges. Suddenly, you break out of the murk and have to make your way to the runway visually. This transition can be jarring, especially if you’re peering through fog or mist.
Why have some pilots come to grief at this critical time? Visual illusions play a big part, particularly at night. If the runway is narrow, you may get the impression that you’re higher than you really are. This can tempt you into making a premature descent, resulting in a crash into terrain or obstacles, or otherwise landing short of the runway. Wider runways have the opposite effect, and make you feel as though you’re too low. In this case, you risk levelling off high, landing hard, or overshooting the runway. “Black hole” approaches—those made at night over unlighted or featureless terrain—can create the same, too-high illusion, and also lead you to risk an early descent.
Then again, the runway may not be where you think it is. This can happen during approaches with strong crosswinds, when you’re crabbing down the final approach course. You break out of the clouds, look straight ahead, and—nothing! In truth, the runway may be 10 or 20 degrees off your nose, so be sure to scan.
Yet another nighttime illusion can happen when lights on the ground trick you into thinking you’re too high—and that “runway” you’re fixating on may really be a string of highway lights.
Extending flaps and retrimming right after breakout can cause still other problems. In extreme cases, pitching moments caused by extending flaps can cause you to balloon back into the clag, triggering a missed approach. Retrimming can take your attention away from the landing—which is mere seconds away.
For these reasons, many experienced pilots stabilize their approaches from the FAF to the runway.
The idea is to set up your power and configuration (flaps, landing gear) well in advance, so you can fly a constant-airspeed, constant-rate descent. This leaves you free to scan your instruments and track the final approach course. When breakout occurs, the idea is to continue the approach without making any other than the usual, flaring and wind-correction inputs you’d make for any landing.
If, as the saying goes, “good approaches make good landings,” then you’ve got a fair shot at finishing your actual-IFR flight with a memorably smooth landing.
But for confirmation, you can always ask your passengers.
E-mail the author at firstname.lastname@example.org.
AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.
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