March 1, 1999
Hands
Off!
After years of development, the
by Ron Bower
Reporting from
THESE DAYS, IT SEEMS there's an automated option for just about every flight
maneuver. What's next? Robots in the cockpit? That's
not science fiction anymore. They're already here-in the form of a new breed of
highly sophisticated autopilots.
In late December, I went to snowy
This helicopter is equipped with the PA-85T AFCS, manufactured by SFIM,
Both companies teamed up in the mid-1990s to certify a VFR autopilot for the
407 airframe. HAS began this pursuit in response to
customer requests as the
HAS Executive Vice President Jim Wagner was my host
and demonstration pilot. Wagner's commanding in-depth knowledge of the
autopilot system bears the marks of the relentless effort required to get FAA
certification. He's responsible for HAS's immaculate
maintenance and completion facility and its 100-person work force. (See R&W, June 1997, page 34.)
A real-world evaluation
Jim and I took two flights in N407AP. The first flight was for
familiarization and evaluation of system operation. I found the autopilot to be
simple to test in preflight. Once engaged, the unit is operational from takeoff
to landing. Engagement of the various modes is uncomplicated and very smooth.
When altitude hold is engaged, even in a climb, the autopilot smoothly
lowers the nose and nails the selected altitude. Both right and left turns are
equally smooth; the system uses a two-minute turn bank, which is about 15¡ in
cruise flight. The PA-85T AFCS is also precise on heading holds. Rollouts are
smooth once the system reaches the selected headings.
The force trim system holds the aircraft attitude for both pitch and bank.
The 407 maintained its attitude when I released the trim button on the cyclic.
A "coolie hat" on the pilot's cyclic grip allows for minor trim
adjustments.
With the autopilot engaged, it's easy for the pilot to "push
through" on the controls to add additional pitch or bank, without the
autopilot disengaging. In-flight smoothness is satisfactory, and is aided by
stability augmentation. This helicopter also has the optional yaw SAS that minimizes tail-wiggle due to gusting winds.
The autopilot flew vertical speed and airspeed selections accurately. The
system has its own air data device to provide information to the autopilot
computer for airspeed, vertical speed, and altitude hold.
A slightly wider panel to give more real estate for the additional equipment
replaced the standard instrument panel for the 407. This additional width did
not noticeably block pilot or copilot forward or downward visibility. The autopilot control panel, with buttons that are easy to read and
to reach, is located just in front of the pilot on the right side of the
instrument panel.
The copilot's cyclic has a multi-button grip that allows left-seat control
of the force trim and autopilot disconnect. This functional left stick is
useful for two-pilot operations or dual instruction.
The unit has the full-options installation, including a twin electronic flight
instrument system (EFIS) with radar altimeter. The modular system starts with a
basic two-axis system with force trim and includes the following: full-time
stability augmentation system (SAS); coupled modes for altitude hold; airspeed
hold; heading select (with optional HSI); and GPS navigation. The system can be
enhanced with options starting with yaw SAS, and then flight director/navigator
coupling system which will provide ILS localizer and glide slope capture and
hold.
The heart of the PA-85T AFCS is a well-proven SFIM computer and control
panel, which has been used on many Eurocopter AS-350
and AS-355 Ecureuils and the AS-332 Puma.
The trim actuators and servos for the pitch and bank control had to be
modified to match the flight control system of the
The PA-85T AFCS is a sophisticated integrated system, as shown in the
schematic diagram on page xx. Most of the basic system, which weighs 55 pounds
(24 kgs.), is located above the aft baggage
compartment. This placement means that the weight has little or no effect on
the helicopter's center of gravity. The maximum autopilot configuration, with
all the bells and whistles, adds about 100 pounds (45 kgs.)
to the 407's empty weight.
A perfect approach
A second flight the following morning was designed to shoot an ILS approach
to the municipal airport in nearby
Interception, too, was smooth and normal, though the autopilot can intercept
a course with as much as a 90¡ intercept angle. Once the localizer was
centered, it stayed locked on the localized course, precisely compensating for
a crosswind of about 15 knots.
As we intercepted the glideslope, it coupled
easily and kept the glideslope needle perfectly
centered, as we slightly reduced power to keep about 100 knots on the approach.
The entire approach was "needles centered" all the way to the runway.
Neither Jim nor I touched the controls all the way down. The PA-85T AFCS has an
auto-leveling feature that's triggered by the radar altimeter at 50 feet AGL.
As we crossed over the numbers above the runway threshold, the autopilot
sensed our altitude above the ground and adjusted pitch aft to hold the 50-foot
height as it guided us right down the runway centerline. No power change was
needed with our 100-knot approach, though a slower approach speed might need
some additional collective pitch.
About mid-field, we punched the go-around button, which is conveniently
located on the left side of the collective control head. The autopilot then
maneuvered the helicopter into a "wing's level" 70-knot climb for a
go-around. We established, automatically and immediately, a positive rate of
climb. This was a textbook example of an ILS approach.
Shooting a perfect ILS instrument approach in a VFR-only helicopter equipped
with an FAA-certified autopilot begs the question of why. The answer is
multi-faceted. First, the ILS capture capability is already in the autopilot
computer from an earlier design. Second, it beautifully demonstrates the
precise flight control capabilities of the autopilot better than en route
flying. Third, the autopilot has practical benefits as a tool of last resort in
the event the pilot inadvertently flies into instrument meteorological
conditions (IMC).
As a Certified Flight Instructor with a helicopter instrument rating, I have
found that instrument training makes better VFR pilots. The 407 with the PA-85T
AFCS would be a great instrument-training platform.
Wagner reports that HAS has
installed nine systems in the first year of certification with no significant
operational problems. With about 350 new
Orders for the HAS autopilot STCs are steady; the
backlog includes six installations for Air Methods in
According to Chuck Hallett of HAS's
Overall, my two flights with the HAS PA-85T AFCS indicate that the system
works as advertised. Certifying an autopilot is a major accomplishment for HAS.
The results speak for themselves.
Contributing editor Ron Bower has logged more than
8,000 flight hours in both fixed-wing aircraft and helicopters. A former UH-1B
Huey gunship pilot in
Defining the Autopilot
The following glossary will help you better understand autopilot controls
and their capabilities.
Two-axis: Control in pitch (nose up and down) and bank (right and left
turns).
Three-axis: Control in pitch, bank, and yaw (nose movement right and left).
Force trim: An electronic clutch system that maintains the cyclic in
whatever position the pilot moves it. Normally there is a button on the cyclic
grip, which the pilot presses to release the force trim. When
the pilot releases the button, the cyclic stays in position. Sometimes a
cone shaped "coolie hat" on the top of the cyclic grip will allow
fine tuning of pitch or bank.
Stability Augmentation System: A system of dampening (minimizing) externally
induced control movements. Normally, SAS systems use potentiometers that
quickly detect the externally induced movement, allowing the system to apply
opposite countermovements. This produces a noticeable
increase in stability and therefore smoothness. SAS's
effectiveness can best be demonstrated in a stable hover while watching the
frequency and distance the pilot must move the cyclic to hold the helicopter
over a fixed position. If you turn off SAS, the movements of the cyclic by the
pilot greatly increase, causing obvious changes in helicopter attitude
movement.
Coupled mode: The coupling of the autopilot to other data
inputs for guidance. An example is nav
coupling to a GPS or VOR, allowing the autopilot to track exactly on course. A
coupled ILS approach uses data from the localizer and glideslope
to fly the aircraft precisely down the approach path. Other coupling modes
include altitude hold, heading, airspeed hold, or vertical speed hold.
AFCSs: Past and Present
AFCSs are still pretty rare in the helicopter
industry. By some estimates, no more than 4% of civil helicopters worldwide are
so equipped. Though no exact figure is available, it's clear that aftermarket
installers, helicopter operators and airframe manufacturers have only scratched
the surface of a potentially lucrative market.
Because this market has yet to be tapped, most rotorcraft pilots have never
flown a helicopter equipped with an autopilot. Part of the
reason for the failure of the technology to catch on, besides the obvious one
of cost, is a cultural taboo against hands-off flying. Training that
says, "fly the aircraft, don't let it fly you" and "never take
your hand off the cyclic" is ingrained in pilots early on. The idea of
flying "hands off" seems sacrilegious.
An autopilot will fly exactly as it's directed, precisely holding a given
altitude, heading, or navigational track. Some will hold a given airspeed or
vertical speed in a climb or descent, and some keep the "ball in the
center" with yaw control.
An autopilot can do all of these flying tasks more precisely than a human
pilot. However, we're still a long way from the day when even an enhanced
autopilot will replace human "wetware" in the cockpit. Flying is
largely a matter of exercising good judgment, and that critical job still
resides in the pilot's seat.
VFR versus IFR
Civilian helicopter autopilots typically have been derivatives of airplane
autopilot systems. One of the more common early autopilots to make the
transition from airplane to helicopter was the Collins 841. In the late 1970s,
the Model 841 was redesignated the 841H when modified
for helicopters. The 841H was a two-axis unit (pitch and bank), and was found
mostly in Bell 206 JetRangers and LongRangers.
Some were even certified for single-pilot IFR flight.
At around the same time, SFENA, AlliedSignal, SFIM, and Sperry all
manufactured helicopter autopilots. Today, nearly all IFR-certified helicopters
(
Autopilots provide ample challenges for avionics and electronics
manufacturers. Because of high R&D and certification costs, prices will be
high if manufacturers forecast sales of only a few units. A high market demand
allows the manufacturer to amortize those costs over a longer production run,
thereby bringing down prices. If the unit price is set too high, then demand,
regardless of the need, is likely to evaporate.
There is a considerable certification effort and expense difference between
FAA approval for VFR autopilots and IFR autopilots. Increased redundancy-not
just in the autopilot, but in aircraft systems-leads to a complex product that
is expensive to manufacture and to certify for IFR. Both the regulatory
authority and the manufacturer carefully examine the flight-test criteria for
pilot workload during an autopilot failure in IFR. Pilot workload becomes an
even more critical issue if the manufacturer is attempting to obtain
single-pilot IFR certification.
My own helicopter autopilot experience (other than force trim on UH-1B Hueys in the mid-1960s) was in certifying the last
single-pilot IFR Bell 206B JetRanger in early 1983
under SFAR 29-4, which was an experimental program to evaluate helicopter IFR
capability.
Flying this autopilot-equipped 1980 JetRanger
convinced me of the functional value and safety of helicopter autopilots for
both VFR and IFR flight. I never would have attempted the grueling 1994 solo
around-the-world speed record flight, which comprised 229 hours of flight time
over 24 days, without an autopilot.