March 1, 1999
After years of development, the
by Ron Bower
Reporting from Mt. Pleasant,
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.