Instrument Rated Pilot Ground School
The ILS Approach - BRIEFING
In this lesson you'll learn how to fly an ILS approach.
ESTIMATED TIME TO COMPLETE
Complete the Student and Private Pilot lessons before beginning this
lesson. Reading the Ground School material before starting this lesson will
help you better understand the skills being taught. You should also complete
the reading and practice in the Instrument Scan solo lesson. Completing the
VOR approach lesson is not required. Study the approach chart in this briefing
to become familiar with the approach procedure.
You'll encounter mostly cloudy conditions with some rain. Ceilings are
forecasted to be around 300 feet, so you will need to fly the aircraft
referring only to the instruments.
ABOUT THE FLIGHT
You'll start in the air and fly the ILS approach into Boeing Field. You'll
intercept the ILS, track inbound on the localizer, intercept and fly the glide
slope, and fly the approach while descending to the decision height (DH), and
land on runway 13R.
Your instructor will help you set the navigation radios and instruments.
Be careful not to chase the needles when flying the approach. The dots on
either side of the needles indicate how far you are off course or glide slope.
The needles will become increasingly sensitive as you get closer to the
KEY COMMANDS TO REMEMBER
F7 to lower flaps in 10-degree increments before landing
You'll be asked to maintain:
- Airspeed within 10 knots as assigned
- Altitude within 100 feet as assigned until established on the glide
- Headings within 10 degrees as assigned
- No full needle deflections while flying the approach
The ILS Approach - LESSON
—by Rod Machado
Are you ready to rock and roll? If you thought landings were fun, wait till
you get hooked on flying the Instrument Landing System (ILS) approach. I
talked about it a little in the overview, but we'll go into more details here,
since it's one of aviation's most challenging, yet satisfying, aerial
An ILS approach consists of a descent to a runway using both vertical and
horizontal electronic guidance. It's accomplished by following two needles
(Figure 2-1) located in the ILS display on your instrument panel.
Unlike other instrument approaches, this one takes you down to a height known
as decision height (DH). DH is approximately 200 feet above the runway
elevation, as shown in Figure 2-2.
From this not-too-lofty perch, you take a peek outside and decide if you can
see the runway well enough to land (thus the name decision height). If
unacceptable runway visibility prevents a safe landing, you apply power, climb,
and head off to someplace else with better weather. Let's take a closer look at
how the ILS approach is constructed.
The ILS consists of two electronic beams. One beam is angled outward, and one
is angled upward from the runway complex, as shown in Figure 2-3.
The outward (horizontal) beam is called the localizer. It helps align your
airplane with the runway. You track the localizer by following the needle shown
in Figure 2-3 (position A). If the needle is to the right, you go to the right;
if it is to the left, you go to the left. A needle that remains centered means
your airplane is tracking the runway centerline. Under no-wind conditions, you
need only fly the runway heading to keep the localizer needle centered. If
there's wind, you need to make small corrections to compensate for wind drift.
Sounds easy, but it does take practice to perfect this skill.
The glide slope is an electronic beam that's tilted upward at approximately a
3-degree angle (Figure 2-3). By centering the glide slope needle, shown in
Figure 2-3 (position B), you're flying an obstruction-free path down towards the
runway. How do you keep the glide slope needle centered? Fly towards it just
like a localizer needle. If the needle swings upward, then fly upward; if it
swings downward, then fly downward. The objective is to maintain the specific
rate of descent that allows the airplane to track the glide slope down to DH.
The Constant Rate Descent
For a typical ILS approach flown at 90 knots, a 500-foot-per-minute (fpm)
descent rate is required to remain on glide slope. Of course, if you fly the
approach at a faster speed, you must increase your descent rate. Glide slope
angle and wind are two factors that affect the precise descent rate required to
center a glide slope needle.
Let's suppose you want to fly a descent at a constant rate of 500 fpm at 90
knots (this is a typical profile that you'll use to fly an ILS approach). How
should you go about doing this? First, you'll do this by reducing power from its
present setting to 1,600 rpm and let the nose naturally pitch down slightly.
Then, you'll adjust the pitch as necessary to maintain a descent rate of 500 fpm
and adjust the power to maintain 90 knots of airspeed. Yes, this is a reversal
of the control functions we used in a previous lesson. Using the controls in
this manner allows you to maintain precise control of the descent rate required
for an ILS approach.
Here's how the sequence should look.
- 1. Adjust power to maintain 90 knots in level flight.
A speed of 90 knots requires a pitch attitude of approximately 6 degrees
nose-up pitch in level flight.
- 2. Reduce power to 1,600 rpm, let the nose pitch forward naturally, and
adjust the pitch to maintain a 500 fpm descent rate.
This requires approximately 3 degrees nose-up pitch on the attitude
- 3. Trim to maintain the attitude for this descent rate.
- 4. Make small adjustments in power to maintain 90 knots.
(Airplanes have inertia, so it may take a few seconds to change speed when
moving the throttle. Be patient).
Believe it or not, this is precisely what you'll do when intercepting the
glide slope. Since glide slopes are normally intercepted from below, you'll fly
level at 90 knots until the needle lowers to a center position in the ILS
display (Figure 2-4).
Once centered, you'll reduce power to approximately 1,600 rpm, adjust the
pitch, and trim the airplane for a 500 fpm descent rate, maintaining 90 knots.
Assuming you're in perfect harmony with the universe, the airplane will remain
on glide slope all the way to DH. But you know how easy it is to get a kink in
your chakra, so you can't count on your karma being perfect. Therefore, you'll
need to make slight variations in descent rate to keep the glide slope needle
centered. Let's examine this.
Let's assume you're above the glide slope and must increase your descent rate
to capture it. If you want to change the descent rate from 500 to 700 fpm,
you'll need to place the airplane in a 3-degree nose-down pitch attitude, as
shown in Figure 2-5.
You'll need to reduce power to keep the airspeed at 90 knots. The secret to
maintaining a specific rate is not to chase the VSI needle. Simply place the
airplane at the precise attitude on the AI, and then make small pressure changes
on the joystick to adjust the rate of descent.
Let's assume you've captured the glide slope and want to change the descent
rate back to 500 fpm. Do so by increasing the pitch to 3 degrees nose-up and
increasing power to approximately 1600 rpm.
Now assume you're below the glide slope and must decrease your descent rate
to capture it. Change the descent rate from 500 to 300 fpm by placing the nose
in a level pitch attitude, as shown in Figure 2-6.
Increase the power to approximately 1,700 rpm to maintain 90 knots.
Remember, don't chase the VSI needle. Make pitch changes on the AI, followed
by small pressure adjustments on the joystick to fine-tune the VSI's indication.
Radial-Scanning Primary Instruments
ILS approaches are not the place to catch a little shut-eye. Following the
ILS needles to decision height is a demanding task. That's why you never leave
Step 2 of the three-step instrument scan. In other words, you spend almost all
of your time radial-scanning the primary instruments for a constant rate
descent. Figure 2-7 shows the primary instruments for an ILS approach.
Figure 2-7 Primary Instruments for an ILS Approach.
Airspeed, primary for power;
Directional Gyro, primary for bank; Vertical Speed, primary for pitch.
The VSI is primary for pitch; the HI is primary for bank, and the AI is
primary for power. These instruments are radial-scanned along with the ILS
display (you don't, however, need to radial-scan the airspeed indicator that
often). Therefore, these three instruments are continuously radial-scanned when
flying an ILS, with other instruments occasionally included. Things are far too
busy to perform the monitor scan found in the final step of the three-step scan.
Additionally, not all glide slopes are created equal; some are angled
differently than others. Therefore, they may require different descent rates
based on the airplane with which they are flown. Figure 2-8 shows the descent
rates versus different groundspeeds required to fly various glide slopes based
on this approach.
At 90 knots, for this 3-degree glide slope, a 485 fpm descent rate should
keep you right on target.
Now it's your turn. If you're having trouble tracking the localizer, look at
the runway ahead of you and visually align yourself with it. Observe how easy it
is to fly a constant heading when looking at an actual runway. Why is it easier?
Because you get pitch, bank, and alignment information in one
"over-the-nose" picture. When you can't look outside, it takes a
trained instrument scan to acquire the same information from three different
instruments: the AI, the HI, and the ILS display, respectively.
A Few Important Secrets
Now you have the basic idea about how ILS approaches are flown. So here's
what the pros know: First, the most important instruments to radial-scan are the
HI and the VSI. It's not necessary to radial-scan the airspeed indicator or the
ILS display every time. In fact, you might limit your radial-scan of the
airspeed indicator to, perhaps, once for every 10 radial-scans of HI and VSI.
You can also reduce your radial-scan of the ILS display to once every three
scans of the HI and VSI. Of course, you want to take in the altimeter,
tachometer, and other assorted instruments on occasion, as time permits. Once
you've found a heading and a descent rate that allows you to track the ILS, you
must fly those values precisely until you have a reason to change them. And I do
mean precisely: Good instrument pilots can hold a heading to a single degree and
a descent rate to within plus or minus 25 fpm. Honest! But it does take a lot of
In turbulence, it's easy to have your heading and VSI indication bouncing all
around. In these situations, it's best to fly averages. Do this by relying more
on the AI for pitch and bank control. Find the pitch that allows for the
approximate desired descent rate. Fly this pitch and keep the wings level on the
Additionally, it's sometimes necessary to make small, but jerky, motions on
the joystick when flying a simulator. Unlike the actual airplane, you can't
sense a change in pressure on the flight controls. This prevents you from
anticipating a change in attitude. Furthermore, airplanes have rudders, which
help fine-tune the airplane's directional control. You may not have rudders
available with your simulator hardware. In that case, small, jerky motions on
the joystick are sometimes necessary to keep the airplane at precise attitudes.
If you do have rudder pedals or a rudder joystick, keep your motions nice and
Wind Correction on the Localizer
I recall the first time I told my dad that I needed some space as a teenager.
He locked me out of the house and said, "Now you've got all the space you
need." At that precise moment, I understood the power of feedback. Feedback
changed my behavior, as I know it will change yours, especially in reference to
flying the localizer.
When you first begin flying the ILS, head the airplane in the direction of
the localizer. In the case of Oakland, the localizer direction is 294 degrees.
Fly 294 degrees, and watch the needle's movement. You want feedback in the form
of localizer needle movement. In particular, you want to know which way and how
much the needle moves as you hold 294 degrees.
The movement of the localizer needle tells you two things: wind direction and
wind speed (determined by how fast the needle moves). Once the needle moves from
its center position (use a one-dot horizontal deflection), recenter it using a
5- to 10-degree intercept angle (IA). The smaller the intercept angle, the less
likely it is that you'll overcorrect. Of course, if you use a 10-degree
intercept angle and the needle doesn't move back to center or moves farther from
center, then a larger intercept angle is necessary. You also know that you'll
need at least a 10-degree wind-correction angle once you're reestablished on the
Once the localizer needle is centered, apply a small correction for wind. Try
a 1-, 5-, or 10-degree wind-correction angle (WCA) based on your best estimate
of the winds. With the WCA established, watch the localizer needle. If it
returns to center, you know that the WCA is an angle between the WCA and the
For instance, upon intercepting the localizer at Oakland, you fly 294
degrees. In a few seconds, the localizer needle begins moving to the left. You
fly a heading 10 degrees to the left of 294 degrees, or an IA of 284 degrees, to
reintercept the needle. When the needle recenters, you apply a 5-degree WCA to
the left of 294 degrees (289 degrees). If this WCA works, the needle will stay
centered. If not, repeat the process using smaller heading changes to recenter
the needle. This technique is called bracketing, and it's the technique all
professional pilots use (with slight modification) to center VOR and localizer
Practicing this technique is sure to save you embarrassment during later
flights. The last thing you want is the localizer needle banging against the
instrument case. That's when the passengers start asking those annoying little
questions like, "Hey, what's that clicking noise? You got your blinker on,
Bud? Is that a time bomb, or what?"
Now, click the Fly This Lesson Now link to practice what you just
learned about ILS approaches. You'll have fun—trust me!
THIS LESSON IS AVAILABLE IN THE ACTIVE FLIGHT