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F. E. 'Fred' Potts |
This article originally appeared in IFR MAGAZINE and is reprinted here by permission of Belvoir Publications.
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| About the Author ... |
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Fred Potts is the author of the award-winning book F.E. Pott's
Guide to Bush Flying — Concepts and Techniques for the Pro.
I heartily recommend this book to all pilots, even if you only fly spam cans
on paved runways! It'll change the way you fly.
Check out Fred's remarkable web site at
http://www.fepco.com/.
Parts of Fred's book are online on his site, as are some of his truly
exceptional collection of Alaska photographs.
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"They're nothing but a crutch," my friend Saul said over the phone
from Los Angeles.
"Well, you know how I am," I replied. "I need all
the help I can get."
It was good that he couldn't see the smile on my face. Or my wife's
rolling her eyes to heaven.
"Sure," Saul said sourly. "Help. You didn't seem
to need any help in Alaska. And you flew a lot of IFR there. When
you weren't landing on those glaciers and gravel bars."
"Yeah," I replied. "But now that I'm in Tucson..."
And I was no longer kidding, for here the need for an FD/AP system
was obvious. With all the traffic, ARSAs and TCAs and snitch patches
and enforcement lunacy, the Lower 48 is not the tranquil wilderness
I am accustomed to flying in.
"Okay," Saul sighed, giving up on me. "I'll be
in Tucson on Thursday. If your plane's out of the shop on schedule
you can give me a demo flight. But I still think you're making
a mistake."
"Well, we'll find out shortly," I said. And we agreed
to meet at my home strip.
The "crutch" concept seems to be widespread, and perhaps
has more than an element of truth to it. But just as any tool
can be misused, so too can Flight Director/Autopilot systems.
Yet to the pro, the FD/AP is an exquisite tool that, by taking
some of the low-level "grunt work" of flying (much as
a computer does for the accountant), allows the pilot to pay closer
attention to certain details that lately are becoming more important
to flight safety. These details include position awareness, subtleties
of navigation, and, when in VMC, an enhanced ability to keep a
sharp scan for traffic while still adhering precisely to altitude
and heading restrictions. This is especially important in TCAs
and ARSAs.
And, sitting on top of this high workload when operating single-pilot
IFR, is the fatigue factor. Anything that can help lower fatigue
at the end of a long hard flight, when a critical approach to
minimums is in the offing, is worthwhile.
While flight directors and autopilots have a certain family resemblance
in how they operate and are operated, in this article we will
be taking a look at the King KFC 150 FD/AP as installed in a Turbo
Cessna 206. This unit, which is currently state-of-the-art for
light aircraft, is showing up in most new singles, as well as
twins, and is very popular for retrofit. The tips and suggestions
given for operating this unit can, in most cases, be applied to
older, and less capable, FD/AP systems. We will look at how pros
operate these systems during takeoff, climb, level off, holding,
and descent, and how best to use them for an ILS approach.
The KFC 150 is a two-axis autopilot combined with a flight director
computer that, as King points out in the manual, "calculates
the appropriate pitch and roll attitudes required to intercept
and maintain headings, courses, approach paths, pitch attitudes
and altitudes."
These commands are shown to the pilot through the use of an inverted-V
command bar in the artificial horizon. The KFC 150 also uses a
slaved HSI for course and heading selection; and has a control-wheel
insert that provides control for autopilot disconnect, manual
electric trim, and CWS (control wheel steering).The computer,
mounted in the radio rack, has a set of mode annunciator lights
and buttons for controlling the various modes.
With this setup, you have the option to hand-fly the airplane
in the normal manner, hand-fly it using the FD command bar, or
let Uncle Otto do all the flying for you. So let's go out to the
airplane and put it through its paces on a typical ILS approach.
Before takeoff
Prior to takeoff, it's necessary to run a check of the FD/AP's
computer, servos, and trim interrupt functions to make sure they
work correctly, and that the servos, in case of an emergency (usually
a runaway trim condition), can be overpowered manually.
These test sequences, which vary somewhat from model to model,
can be found in the POH and should be added to the airplane's
checklist.
It will also be necessary to set the airplane's avionics and HSI
for the initial portion of the flight, which in this case will
be a departure from Tucson to intercept the TUS R-256 radial to
DRIBB, followed by the Ryan runway 6 ILS.
The best way to do this is to set Nav 1 to the Ryan LOC, with
the HSI course pointer set on the inbound localizer course(058
degrees); set Nav 2 to the TUS VOR, with 256 in the OBS; and the
ADF to RBJ NDB. The DME should be set to Nav 2.
On the HSI, set the HDG bug to runway heading, which, since you
will be departing Tucson's runway 11L (TUS VOR is off the end
of this runway), will be 123 degrees. These settings will allow
you to takeoff, intercept the TUS 256 degree radial, fly it to
DRIBB, enter the holding pattern, hold, then fly the ILS to DH,
all without resetting the navs.
Takeoff and Initial Climb
With the FD/AP system off, make a normal takeoff, then clean the
airplane up and go to best-rate-of-climb airspeed and power settings.
At this point, Uncle Otto can be turned on, and this is best done
by first pressing the HDG mode button, which will also engage
the FD, causing its command bar to appear in the artificial horizon.
The command bar will command wings level, and whatever pitch the
airplane happens to be flying at the moment.
Here the first airborne crosscheck of the system is made, and
this is done by matching the logic of the FD command bar with
what the airplane is actually doing as shown by the AH and the
performance instruments (ASI, VSI, DG, etc.).
Then the HDG bug is checked to see that it matches the current
heading the airplane is flying. If it does not match, set it either
to that heading, or the one you wish to fly. Finally, the AP ENG
button is pressed. That will engage the autopilot, and the airplane
will fly the FD commands.
Intercepting a Radial
About this time, tower will usually instruct "resume normal
navigation, contact departure." This, of course, means to
intercept the TUS 256 degree radial, and to do this you will use
the HDG bug.
Just turn it to the intercept angle you wish Uncle Otto to use,
and the airplane will make a nice smooth standard-rate turn onto
that heading. If your airplane has Nav 2 interfaced with the FD/AP
system, you can press the NAV button and Uncle Otto will smoothly
intercept and fly the radial.
If you don't have Nav 2 interfaced, you will have to track the
radial manually, using the HDG bug.
This is no big deal, since you will be using the exact same instrument
scan flying a FD/AP system as you would were you hand-flying the
airplane—with one exception. With the FD engaged, you will want
to monitor its commands to make sure that its logic remains correct
(in other words, that the command bar moves in the correct direction
when you adjust the HDG bug, and maintains the correct pitch attitude).
Leveling Off
There are four ways to level off at our assigned altitude. The
most elegant is to have King's optional altitude select and vertical
speed selector. One just presets the desired altitude and rate,
and Uncle Otto does the rest.
But as this feature costs about $6,000, and most of us can find
better places for the money, I will deal only with the manual
methods here.
The method the pro will usually choose is the CWS (control wheel
steering) method. This button, on the yoke, temporarily disengages
the AP, in effect telling the FD to "standby for new pitch
and heading instructions." Here is how it works with the
Turbo Cessna 206:
Climbing at best rate, which is better than 1,000 feet per minute,
you will have 88 knots and 10 degrees of pitch up.
Fifty feet below your target altitude, press the CWS button and
manually lower the nose to 0 degrees pitch as you intercept your
altitude. You will then release the CWS button, and once again
Uncle Otto will take over. All that remains now is to press the
ALT hold button, set in cruise power settings, close the cowl
flaps, and retrim the rudder (if the rudder is out of trim, the
FD/AP will not track heading accurately).
An alternate method, often used by those pilots who take a laid-back
attitude toward their flying, is to use the vertical trim control
rocker switch, which readjusts the pitch servo. Here you will,
at approximately 500 feet below your target altitude, start to
lower the airplane's nose by easing the rocker switch forward.
Using a light touch, continue to adjust pitch attitude as required
until your target altitude is reached. Once on target, press ALT
hold, and rig for cruise.
The fourth method could be considered the bump-on-a-log approach.
Here you would just wait until the airplane reached your target
altitude; then, as you shot through it you would press the ALT
hold button. Unfortunately, this will cause an overshoot of approximately
150 to 200 feet, and under some conditions will be enough to activate
an ATC snitch program.
Of these methods, pilots who stay in the loop with a good instrument
scan (and everyone should) seem to prefer using the CWS method.
Pilots who use their FD/AP systems as a crutch use the bump-on-a-log
method. The vertical trim method is kind of half-way between these
two extremes.
Slowing for the Approach
Instrument flying is attitude flying, and attitude flying is doing
it by the numbers. It is an old principle that "attitude
plus power equals performance." Using a FD/AP system does
not change this, and in fact these systems cannot be flown properly
unless they are flown by the numbers.
A good example of this principle is slowing to the holding/approach
configuration. With the Turbo Cessna 206, the target airspeed
will be level flight at 100 knots using 10 degrees of flaps. Three
or four miles from DRIBB, reduce the power to 22 in. Then, very
slowly, feed the flaps in, using small increments so the autopilot,
which has to keep up with you through slow trim adjustments, will
not fall behind causing the airplane to balloon off altitude.
Then retrim the rudder.
If your airspeed is off slightly, a small power adjustment will
bring it on target. This speed you will maintain all the way to
DH, unless requested otherwise by ATC.
Flying the Hold
Going around and around in a race track pattern burning expensive
fuel is the bane of instrument flying, and while there is nothing
that will make holding pleasant (the fix off LAX named LIMBO being
a perfect example), the FD/AP goes a long way towards making holding
patterns less of a pain. Using it, all we have to do is enter
the thing, then time our legs and make the necessary adjustments
for the current wind conditions while we await our EFC time.
To do this at DRIBB, you will simply turn the HDG bug to a 305
degree heading to make a parallel entry. The rest is by the book,
except that Uncle Otto does the grunt work while you keep track
of where you are.
However, it should be noted that there are two ways to make turns
using the heading mode: The first method (and the one most used)
is to just turn the HDG bug to the desired heading, and, with
feet flat on the floor, let Otto make a standard-rate turn for
you. The problem is that the ball of the turn coordinator ends
up half out of its cage, which is sloppy flying. This is a 'crutch'
technique.
The second method is to feed in a bit of rudder with your turn,
keeping the ball centered, and this is one of those subtle touches
that separate the pros from the Sunday pilots.
Approach Descent
From your cruise/holding altitude of 6,000 feet (in this example),
you will have to descend to the published GS intercept altitude.
Once again, it should be done by the numbers. In the 206, turn
the ALT hold off, then reduce power to 15 inches. Then using CWS,
lower the nose to -2.5 degrees, and retrim the rudder.
This will maintain knots and produce a 500 foot per minute descent.
As you have 1,000 feet to lose, descent will take two minutes.
Note that these numbers will be different for different airplanes,
as are the procedures. The numbers here serve as a procedural
example.
Fifty feet from your target altitude, raise the nose (using CWS)
to 0 degrees as you intercept the target, then press ALT to maintain
it. After that, reset the power to your level approach setting,
and retrim the rudder.
A word of caution: FD/AP systems use elevator for altitude control,
and power for airspeed control. Therefore, neglecting to increase
power after activating the ALT hold will cause the nose to raise
as the FD/AP attempts to maintain the assigned altitude, which
will cause the airspeed to drop. This can lead very quickly to
a stall.
Localizer Intercept
There are two ways to intercept a localizer using the KFC 150.
The easiest way is to use its any-angle intercept abilities by
setting the HSI course pointer on the LOC course and the HDG bug
to the desired intercept angle, then pressing the APR button.
Once this is done, the APR annunciator light will start to blink,
telling you the APR mode is armed.
On LOC intercept, the HDG light will go out, indicating the heading
mode has been disengaged, and the blinking APR light will turn
steady, indicating that the FD/AP has locked onto the LOC.
However, coming out of the holding pattern at Ryan proves such
a sharp intercept angle that if you used the above method, you
would overshoot the localizer, almost full scale. From there,
the FD/AP will go through considerable hunting and seeking until
it once again settles on a stable heading. As you would not fly
that crudely were you flying the airplane manually (unless you
were following the FD cues), why should you let Otto get away
with it? A better way is to simply manually intercept the LOC
using the HDG bug, then press the APR button when on course.
GS Intercept
As you approach the glideslope, the GS pointers on your HSI will
come alive, and start down the scale. When their bottom edge reaches
the center line of the deviation scale, reduce power to the glideslope
setting that will maintain 100 knots. On the 206, that's 14 in.
to 15 in.
At this point, the ALT annunciator light will go out, the GS light
will come on (showing GS capture), and the command bar will command
nose down. The nose will drop, and you will start down the hill.
As the FD/AP system uses elevator to maintain glideslope, you
will have to adjust power to maintain your desired speed. If you
have a strong headwind, you will need a bit more power than your
standard setting to maintain 100 knots. If you have a tailwind,
you will need a touch less. But once set, you should not have
to touch your power setting until reaching DH.
Naturally, as you always must when making pitch or power changes,
you will have to retrim the rudder once again so the ball is in
the center of its cage and the FD/AP is tracking correctly. Note
FD/AP systems are designed to intercept the GS from low; however,
the KFC 150 will make the intercept from above if you fly it onto
the GS using the CWS control—a useful feature for those rare
occasions when a bad vector puts you on the localizer a bit too
high for a normal glideslope intercept.
Runway in Sight
The way I like to fly a FD/AP system on an ILS approach is to
let Uncle Otto take it to about 400 feet above DH, then use the
red autopilot disconnect/trim interrupt button on the control
wheel to disengage the autopilot so I can fly the rest of the
approach manually. Also, at this point I like to set the prop
to maximum RPM and mixture to full rich, in anticipation of a
missed approach. Most other pilots I know prefer to leave the
FD/AP coupled until DH, then take it from there.
Naturally, once you have the runway environment in sight, you
will make a normal landing; but in low IMC, it's good practice
to follow the LOC/GS all the way to touchdown. That will prevent
an optical illusion from grabbing you; and if an unexpected go-around
suddenly became necessary you would still be on the published
approach.
Unfortunately, not all our autopilots handle missed-approach the
same way. In my Turbo Cessna 206, this is how the missed approach
is executed. First, go to best-rate-of-climb power and airspeed
(pitch). Power, as always, even in a missed approach, should be
applied smoothly and precisely. Then, use the electric trim to
remove elevator pressure as the 10 degrees of flaps are retracted.
Finally, retrim the rudder. At the appropriate point, reengage
the FD/AP system by pressing the HDG button (making sure the HDG
bug was where you wanted it), then the AP ENG button.
And how do you determine the appropriate point to reengage the
FD/AP system? Well, that's easy. As soon as you have a positive
rate of climb and the airplane is nicely cleaned up and safely
on course, Uncle Otto can be trusted with it. However, close to
the ground it is wise to hand-fly the airplane. If a problem were
to develop, such as runaway trim, you'll have more time and altitude
to deal with it if you're well above DH.
Some FD/AP systems have a "go-around" button that will
command a certain nose-up pitch angle and wings level; with this
feature you can simply add power, push the button, go to NAV mode,
and let Uncle Otto fly you out of trouble. My personal thinking
is that close to the ground on a go-around, I want to be flying
manually; this is perhaps King's thinking as well, for while it
would have been an easy to add a go-around button to the KFC 150,
they didn't bother to do so.
"Interesting," Saul said, looking out the windshield
at the scattered clouds around us. "The FD/AP system doesn't
really lighten your workload on approach, it just seems to transfer
it."
"Yes," I agreed. "Instead of constantly having
to concentrate on basic control functions, it permits us to focus
more of our attention on the subtleties of procedure, navigation,
and cross-checking. In other words, one is able to take a more
methodical approach to their instrument flying."
"Well," Saul said, "that takes care of the 'crutch'
theory; though, of course, many pilots do use it that way. But
haven't we just scratched the surface today? Seems all we did
was cover the basics—surely there is more to it than what you've
had to show me so far."
"There is," I admitted. "I especially would like
to show you a few of the more advanced techniques, such as those
used to vary speed on an ILS. With the Cessna 206, we can begin
our approach at 140 knots; then, while actually on the ILS, slow
to 80 knots just prior to reaching DH—all without deviating from
the LOC or GS. But that's for another day."
Understanding FD Displays
When first confronted by a flight director, a pilot who has never
seen one is sure to be utterly confused by what the display is
really showing. The overwhelming impulse (and a perfectly logical
one) is to just disable the thing and fly raw data.
Given the state of the training fleet these days, this reaction
is understandable. By the time most instrument students are ready
for the checkride, few will have seen a flight director, let alone
flown with one. Rare indeed is the FBO rental with even an HSI;
a flight director is luxury beyond all expectation.
To make sense of the FD display, it helps to think of the flight
director as what it really is: a very simple analog computer.
True, the computing circuitry may be digital, but the logic that
drives most general aviation flight directors is analog, meaning
that it works by comparing inputs and resolving them to achieve
a certain flight path.
Functional schematics of FD/APs look like fuel system charts,
with various lines from the tanks and to the engines converging
on single valve. For this analogy, the valve is the FD computer
and the lines to it furnish inputs: pitch and roll information
from the gyros; (or pressure sensors, in some cases) heading data
from the bug; nav data from the avionics. The computer compares
these inputs with what the pilot has asked the flight director
to do then figures out a flight path that will get the airplane
where the pilot wants it. If the autopilot is engaged, part of
the computer's output instructs the pitch and roll servos to fly
the commanded flight path. Whether the AP is engaged or not, however,
the computer's output moves the command bars to give the pilot
steering cues—both pitch and roll—to achieve what the pilot
has asked for. That, in a nutshell, is what the FD display does:
it provides in a single display information from several instruments,
thus considerably reducing the pilot's need to scan and integrate
raw data and navigate at the same time. Various flight director
displays have evolved over the years but most are variations of
two basic designs: the single-cue, which is the most popular,
and the two-cue. One of the earliest single-cue types and one
that's still found in older light twins, is the Bendix DH-841.
This display has a pair of command dots (sometimes called "bouncing
yellow balls) that just touch the wings of the attitude gyro's
miniature airplane. To command pitch, the dots move together vertically;
to command roll, they move differentially. By keeping the wings
aligned with the dots, the pilot satisfies the FD commands.
The modern single-cue FDs—King's KI-256, Century's 2000 and S-Tec's
60-series, to name a few—have a pair of v-bar command bars and
a delta-winged miniature airplane. As with the dots on the Bendix,
the bars move together for pitch and differentially for roll.
The pilot satisfies FD commands by flying the delta so it's upper
edge is flush with the bottom of the v-bars.
Less common but still available are two-cue displays that have
a horizontal bar for pitch and a vertical bar for roll. Flying
toward the bars and keeping them centered will satisfy the commands.
However, many a pilot has been baffled by what the two-cue display
is actually showing, especially on an ILS. It's only natural to
assume that the vertical bar is the localizer, the horizontal
the glideslope. So, if both bars are centered, the airplane must
been on the localizer and glideslope, right?
Not necessarily. Centered two-cue FD bars tell you nothing about
where the airplane actually is relative to localizer and glideslope.
All they do is confirm that you've satisfied the commands and
that at your present bank and pitch, you'll eventually get to
the localizer and slope. The only way to judge your actual relationship
to the course (and your arrival there) is to monitor raw data;
the HSI's course deviation indicator and the glideslope flags.
This is true of single-cue displays as well but they're less likely
to create confusion because there's no similarity to LOC/GS needles.
Although the FD does reduce the scan load, it doesn't entirely
relieve you of having to look at the other instruments. As a matter
of habit, you should always cross check what the FD is telling
you against raw data, whether you're hand flying the FD or having
the autopilot do a coupled approach.
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