From notes written on paper or annotations on an electronic tablet, through heading and altitude bugs and perhaps even an audible “minimums” advisory set, you’ve briefed the approach and are prepared to fly it using every resource available in your cockpit. When the glidepath or glideslope centers or, in that increasingly rare approach without at least advisory vertical guidance, the FAF slides behind you, you adjust the airplane’s power, attitude and configuration (flaps, landing gear) to optimal for the type to follow the charted descent toward the airport.
Although flying the final part of an approach requires the greatest precision and concentration, it may be the easiest part of instrument flight. All the decisions are already made (or should be), and all you have to do is fly (or monitor as the autopilot flies). FAF inbound done right allows you to do nothing but focus on the procedure for the last five or so horizontal miles and 1500 or so vertical feet of a trip. Still, the difference between wallowing your way to the airport and precisely riding the beams to the Missed Approach Point and beyond is your ability to anticipate many variables and smoothly correct for them even before they become evident. What are the variables, and how do you control them? What turns an acceptable approach into a masterful one?
Rate and angle
The standard glidepath/slope for FAF inbound follows a 3° descent angle. What you really need to know is the rate of descent you need to fly that angle. To answer, you need two things: your current ground speed and basic geometry. The FAA publishes a rate of descent table in various places, including Advisory Circular 120-108
(figure 1). This Descent Table compares ground speed to glidepath angle and provides the necessary vertical speed. It’s designed for flying continuous descents from the FAF to Minimum Descent Altitude for approaches without vertical guidance. But it helps us fly glidepaths and glideslopes precisely as well.
Glidepath angles are published on instrument approach charts (figure 2). With the standard 3° descent, such as on the ILS 17 at Newton, Kansas, go to the Descent Table (figure 3). If your ground speed inbound from the FAF is 120 knots, it will take a sustained 637 feet per minute descent from FAF to the MAP to remain precisely on glideslope. If your ground speed is 150 knots, it requires 797 fpm. In a lighter piston airplane or bucking into a headwind with a 90-knot ground speed, you must descend at 478 fpm to keep the glideslope centered.
A standard angle implies nonstandard angles as well. Obstacles or other factors may require a steeper approach (and sometimes, a shallower one). Flying one of the non-precision approaches into Hutchinson, Kansas, a constant descent from FAF to MAP is at 3.15 degrees (figure 4). The Descent Table and a little interpolation (figure 5) tell us that at that angle it takes about 502 fpm at 90 knots, 670 fpm at 120 knots and 837 fpm at 150 knots groundspeed. On another KHUT approach, the descent angle is 3.4 degrees (figure 6). The table tells us a constant-angle descent requires 542 fpm at 90 knots groundspeed, 722 fpm at 120 knots across the ground, and 903 fpm at 150 knots groundspeed.
Call it 500-600 fpm at 90 knots, 600-700 fpm at 120 knots, and 800-900 fpm at 150 knots ground speed, and you’ll be close. In configuration, trimmed on airspeed and established inbound from the FAF, increase power slightly from your target approach power setting if your groundspeed is lower than your airspeed, and increase power slightly if your groundspeed is higher than your airspeed or the descent angle is steeper than standard. But have a target rate of descent in mind to predict whether your trend is to stay on glidepath, go high, or go low, and to control it as variables change on the way down.
Power and precision
One of those variables is power. There is roughly 1500 feet vertically between the height you cross the FAF and the altitude of the MAP. In the lower levels of the atmosphere, air pressure changes at about one inch of mercury for every 1000 feet of altitude change. That means manifold pressure in a normally aspirated piston airplane will increase about 1.5 inches in the descent FAF inbound. That’s about a 5% increase in power at a sea level airport, even more of a percentage increase at higher elevations. What does that do to precision as you fly down the glidepath?
At a constant airspeed, increasing power reduces the rate of descent. The airplane will tend to go high on glidepath. You can correct this with pitch, although that gets the airplane out of trim, destabilizing the approach, or with power. I prefer to anticipate it by making two slight throttle reductions as I fly down the approach, one about halfway to the MAP and the other about 100 feet above minimums. It doesn’t take much to make a couple of well-practiced adjustments to maintain airspeed and vertical speed so the needles stay centered and I reach the minimum altitude and the MAP at the same time.
Turboprops and jets are normally aspirated engines, too. They will develop more power as they descend down the glidepath. Like piston fliers, turbine pilots should make small power adjustments once or twice FAF inbound to avoid ending up high on glidepath at the MAP. Only pilots of turbocharged piston engines with automatic wastegate controls can set power before the FAF and forget it.
Winds of change
Compounding this, wind speed and direction usually change in the last 1500 feet above ground level. Ground speed will change, requiring an adjustment to vertical speed to keep the glidepath centered. Knowing the target rate of descent for the approach and managing power as you descend, you’ll need only small corrections if your groundspeed varies because of changing wind conditions.
With local exceptions, in the northern hemisphere, winds tend to change direction to the right in the last 1000 feet above ground due to friction with the surface and Coriolis effect. If you’re crabbing to the right to keep the course needle centered crossing the FAF, you’ll usually need to increase the crab angle as you descend. If you start the descent crabbing to the left, you may need to decrease the angle to cancel drift closer to the ground. Wind velocity also tends to decrease closer to the ground, which may increase or decrease the crab angle required.
In the thick of it
Given the choice, I’ll take an RNAV(GPS) approach over an ILS. GPS approaches are easier to fly because they are based on a grid. Inside the FAF, the beams are of constant width and height, respectively. If I have the needles centered crossing the FAF and I adjust power and heading correctly as I descend, I’ll still have the needles centered when I reach the bottom of the approach.
An ILS, on the other hand, is a pair of beams projecting from a transmitter, one for localizer, the other for glideslope. The localizer beam is between three and six degrees wide, depending on runway length and location of the ground-based transmitter, and at what angle it takes, so full-scale left-right, the beam is 700 feet wide at the runway threshold. Further up-approach, the beam is wider, so the left/right distance for a given needle indication is much wider at the FAF. You can have the localizer perfectly centered FAF inbound and cancel drift precisely and still have the needle drift left or right because what was centered further out is not centered closer in. Similarly, the glideslope beam is 1.4 degrees wide vertically, with its height broadening the further you get from the airport. Nail the groundspeed vs. glideslope angle math, and a centered needle can still move from centered to “fly up” or “fly down” as the beam narrows. Flying an ILS for precision, you not only need to master vertical speed and wind, but you must constantly correct for reduced left/right and up/down tolerances.
An RNAV (GPS) LPV glidepath is thinner than an ILS localizer. Its lateral course is designed to have a full left/right course deviation of 0.3 nm (1800 feet) at the runway threshold, more than twice the width of the ILS localizer at that point. That’s why, even in our GPS-driven world, the ILS is still the gold standard for precision approaches. Still, the beam-width part of the equation remains constant on a GPS approach, whereas an ILS does not. That makes a GPS approach easier to fly, even if it is somewhat less precise.
Sometimes, local conditions may interfere with approach path signals or otherwise cause indications that make an autopilot overcorrect. In those cases, the approach chart will advise (in the notes) that use of the autopilot is not authorized below some altitude. Figure 7 shows such a note, requiring the autopilot to be disengaged no lower than circling minimums or 270 feet above decision height on this chart for the ILS 17 at Newton, Kansas. You may use the flight director below that height, but expect it to make jerky motions close to minimums. Ignore quick changes below that height and manage pitch and heading if the FD indicates something else.
Ground-based or space-based, flying an approach precisely requires small, measured corrections. Read my article “The Rule of 10s” in the August 2017 Twin and Turbine, available at www.twinandturbine.com.
Breaking out
Nearing the bottom of your approach, it’s helpful to know where to look for the runway. Most of the time it will be almost directly in front of you when you look up. But it’s possible even on a precision approach for there to be a slight turn required to line up for landing. Instrument approach charts include an arrow on the airport view that shows the direction you’ll arrive, assuming you’re aligned on the approach and the heading you’ll be flying (figure 8). You may need to look left or right to find the runway at minimums.
Looking at the notes as you brief for an approach, you may find “VGSI and Glideslope/Glidepath not coincident.” There’s an obstacle or displaced threshold that requires a visual glideslope indicator (VGSI), such as a PAPI or VASI, to be set at a steeper angle than the electronic glidepath. If you continue to fly the approach glideslope or glidepath before decision height/altitude, you may hit a hard-to-see obstacle or risk overshooting the touchdown zone. If you see the note, you’ll anticipate the need to shallow your rate of descent after breaking out until you intercept the visual glidepath, then change it again to fly that glidepath to touchdown.
There’s a lot said about—and a lot to be said for—flying a stabilized approach. But the speed, configuration, and sometimes glide path that gets you precisely to the MAP will have to change as you to transition into a flare. Remember also that the touchdown zone is not landing on the numbers. A constant angle of descent from FAF to the runway takes you to where you begin your flare. The marked touchdown zone accounts for the extra distance you’ll travel in the flare, and is 1000 feet from the runway threshold or 1/3 the total runway length, whichever is less. This is the landing target around which obstacle clearance is designed.
Going missed
There’s a lot to flying a missed approach, but that’s a subject for another time. Suffice it to say the missed procedure is part of an instrument approach, and you need to be briefed and ready to fly it before you pass the FAF inbound, because you won’t have time to figure it out at the MAP.
I stand by my observation that FAF inbound is one of the easiest parts of instrument flight. The decisions are already made, and all you have to do is fly. Still, there’s a lot to consider and do to fly FAF inbound like an expert.
