Auto Pilots
Surfing around the web today I came across a great article about the use of autopilots. Granted that currently I am not even flying let alone have my PPL. Yet, I figure that learning about these devices will be agreat help to me in the future. As such I am including the text from this article Below. A link to the original article is here
The article below was written by my
friend and GNS430/530/480 instructor Keith Thomassen. Keith and I have
have taught the top of the line Garmin panel-mounts in a classroom environment
over the last four years. During the class Keith and I have found that
many pilots, even though very experienced don't really know how their autopilot
functions in different modes, especially the Nav. and GPS modes. With the
introduction of 2.0 software with regards to the CNX80/GNS480, the confusion
level of the autopilot increased when used with one of these GPS
Navigators. This prompted Keith to write this short article on how your
autopilot should respond when interfaced with a modern GPS Naviagator.
While the popular S-Tec System 55X is mentioned, the information is applicable
to most brands of autopilots. Be sure to consult your FMS (Flight Manual
Supplement) to see just how your autopilot should function in your
aircraft.
Autopilot Types
There are many types of autopilots. Simple units
will keep the wings level, or hold a heading, while more complex ones have NAV
and GPS Steering modes.Our discussion is restricted to those that can hold
headings and altitude, can intercept and fly a specified horizontal course (VOR
or GPS) and can track a course with a vertical slope (ILS or GPS approaches).
For understanding the autopilot, it is important to know the differences between
autopilots that are deviation-based, and those that are roll steering
capable.Or, between those receiving analog signals and those using digital (or
both) signals to direct their
actions.
One example of such a full
functioning autopilot is the S-TEC 55X shown above. It’s horizontal modes
include HDG, NAV or APR, and GPSS. Vertical modes that control pitch are
ALT, GS, and VS. This autopilot also has CWS (control wheel
steering).
Perhaps you learned about autopilots some year’s back from
tracking a VOR or LOC course. Analog signals representing deviations from the
localizer and glideslope, as seen for example on your HSI, told the autopilot
which way to turn or to climb/descend. Analog autopilots still dominate the
market, but there are now digital autopilots that don’t even look at the HSI
information to tell the servos how to bank, and digital pitch control is coming.
This overview of autopilots is prompted by the new vertical guidance GPS
approaches, and to clarify autopilot use for tracking GPS courses. These general
principles are illustrative of what autopilots do, but a word of caution is in
order. Your model or brand may not work exactly as described here, so for
individual differences be sure to rely on the manual for your unit. What follows
is specific for the S-TEC 55X.
Analog Autopilots
Analog autopilots
use analog signals from a CDI with (or without) glideslope indicator, and a
heading indicator (bug) on a directional gyro (DG) for course direction.
Alternatively, this combination of CDI and DG is found on an HSI, and hereafter
we’ll use it for discussion. The DG and CDI are shown below on the left and
center, while the HSI is shown on the right. Signals from the HSI to the
autopilot. For a VOR or LOC approach, put the autopilot in NAV mode and select
the course with the Omni- Bearing-Selector (OBS) on the HSI (yellow
pointer).
If you have a DG (no HSI), you must set the heading bug to the
desired course.
Your position is the center of the HSI, and the
split-out center portion of the yellow needle represents the selected radial
from that VOR. In the NAV or APR mode, the angular deviation between the course
and your present radial, and a right or left determination, commands a turn in a
deviation-based autopilot to reduce the error (close the split). The turn
coordinator gives input also, to limit the rate of turn.
The APR mode has
greater needle sensitivity and can be used for VOR or GPS tracking if you push
APR rather than NAV. When a localizer frequency is chosen and NAV is
selected, APR will light up as well since localizer signals are more sensitive
(less angular deviation for full scale deflection). This autopilot supports dual
mode operation, pushing NAV and HDG together. You then track the heading
set by the HDG bug until intercepting the VOR (or GPS) course selected with the
OBS, at which time the HDG light goes out and you remain in NAV mode. Again, if
you have a DG, set the HDG bug to the new course.
Several vertical tracking
modes are available with analog autopilots. Altitude hold is one, as are
VS commands to fly a given rate of climb or descent. With control wheel steering
(CWS) you can suspend roll and pitch control by pushing a CWS switch on the
yoke, and with both a roll and pitch mode activated, your autopilot will
maintain current pitch and bank values on releasing the CWS switch.
On an
ILS, which has vertical guidance, the glideslope can be tracked if it is
captured. It will be armed automatically in the 55X if you have a valid
glideslope signal (no flag), have the NAV receiver tuned to the localizer, are
in the NAV/APR/ALT modes, are sufficiently below the glideslope, and are within
half a deviation of the localizer course --- all for 10 sec. When armed,
GS is illuminated and when captured the ALT illumination goes out. If you
have to hold at the OM, you can disable GS arming by pushing APR twice while
inbound to the OM (GS will flash, ALT and DSBL come on). To re-establish, push
APR again. If above the glideslope, you can manually arm it for capture by
pushing ALT when in that mode
If you make a GPS flight plan, analog signals
representing your cross track error (XTE), and whether left or right of course,
are sent to the HSI (if you select GPS with the CDI key on the GNS 430/480/530
devices). It’s also necessary to set the OBS course on your HSI (or HDG
bug on the DG) to the DTK of the active leg, since both pieces of information
are acted on --- XTE and the difference between your OBS setting and the DTK.
Then, operation is no different than tracking a VOR course in the horizontal
plane. Analog signals from the HSI to the autopilot command turns to reduce the
deviation.
With the recently certified v2.0 software for the GNS480, it
sends signals to the glideslope indicator so you can see if you are tracking the
“GPS glideslope” associated with LPV and LNAV/VNAV GPS approaches. It also sends
a “discrete” signal to arm autopilots, equivalent to arming the ILS glideslope.
So, with the 55X for example you’ll fly these GPS approaches just like an
ILS.Put the autopilot in the NAV/APR/ALT modes when a GPS approach with vertical
guidance is selected, then sit back and watch it capture the GPS glideslope and
fly it right to the runway! There are around 1800 vertical GPS approaches at
this moment so GPS receivers with WAAS and certified software are now opening up
very exciting new opportunities for precise approaches at airports without an
ILS.
At the moment you can’t legally couple your S-Tec (or any other
autopilot) to the HSI for this operation because it is not described in
their manual and approved by the FAA. Whether or not autopilot
manufactures amend their FMS (Flight Manual Supplements) to fly GPS approaches
with vertical guidance remains to be seen.
Digital
Autopilots
There are some new kids on the block, and they don’t play by
the old rules. Digital autopilots receive digital signals directly from
your GPS navigator over ARINC-429 standard (two-way) digital links. Analog
information from the GPS to the HSI is still there for you to monitor, but the
digital link drives the autopilot. When you make a GPS flight plan the active
leg is defined and your position relative to it is known. So if off course, the
GPS unit can process that information to compute a smooth closure track and
command a specific bank angle at all times.
GPS
Navigator
Digital Autopilot
ARINC-429 digital signals
Roll steering
commands
Analog Autopilot
HSI or CDI and DG
GPSS mode GPS-V
(soon)
HDG, NAV, APR, ALT, GS
Analog signals
(Pitch steering commands
coming)
The required bank angle computed by your GPS is
called a roll steering command, and a digital autopilot can accept those
commands directly from your GPS navigator. The autopilot then directs the servos
in the wing to accomplish them. This is known as GPS Steering, or
GPSS. On an S-TEC 55X autopilot, for example, you engage the GPSS mode by
pushing the NAV button twice. The analog HSI signals are used for your
information, but not for autopilot control in this mode.
To summarize, you
have a choice of the analog mode if you push NAV once, and the digital mode if
you push NAV twice. In the NAV mode you operate through your HSI, and need
to set the OBS pointer (or HDG bug on a DG) to the DTK of each leg. In the
GPSS mode the HSI is ignored for control, and the autopilot relies on the GPS
computed roll steering commands (on a 429 digital link) to send bank commands to
its roll servos.
On some installations, you may not need to switch your GPS
unit to the GPS mode (depends on how it was wired). But if you stay in the VOR
mode, the HSI information comes from your active VOR, while your autopilot is
looking at GPS course and position data. For that reason, some
installations do not allow GPSS operation unless you switch to GPS with the CDI
button on your GPS navigator.
Some autopilots have a switch to put you into
the GPSS mode from the HDG mode, and back again. In that circumstance,
when in GPSS mode, pushing the NAV button has no affect on autopilot
control. It does send analog signals to the HSI, for you to monitor, but
you have to go back to HDG with that GPSS switch before you can engage the NAV
mode.
Dual mode operation with HDG and GPSS is available in the S-TEC
55X. With this operation, you pick your intercept heading with the HDG
bug, and the course is intercepted by the GPSS mode when within the criteria for
that intercept.
GPS-Vertical
There are several digital
autopilot manufacturers working on the vertical equivalent of GPSS, known as
GPS-V. None are certified as of this writing, but with WAAS approved receivers
already certified in the Chelton Flight Logic EFIS and the GNS480, it is only a
matter of time before your autopilot could be fully digital.The new Chelton
AP-3C autopilot below now accepts GPS-V, but the GPS receivers are not yet
certified to send these signals on the 429 data link. The Garmin GNS480 group is
working on providing digital signals from their unit. For the experimental
aircraft market, TruTrak has nearly completed their Sorcerer unit with GPS-V,
and since it will not be certified it can be used for experimental aircraft as
soon as they complete the hardware and software and bring it to
market.
Digital autopilots are the brave new frontier. Capability available
to us soon, together with WAAS receivers, will afford new and exciting
opportunities for assistance in flying our aircraft on GPS approaches.
posted by Marcus at 2:37 PM
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