Feature Rich Autopilot to Compete with GFC 700
Over the years I have had the pleasure of flying and providing instruction in a number of aircraft. During that time I have had the opportunity to fly behind many different autopilots. The list includes Bendix King KAP 140, KFC 150, KFC 200, KFC 225, KFC 250, STEC 20/30, STEC 60-2, STEC 55X, STEC 550, STEC Magic 1500, STEC 2100, Sperry SPZ 500, Collins 106 and 107, Cessna 400, Cessna 800B IFCS, Cessna 1000 IFCS and Garmin’s GFC 700. Over the years as technology has increased greatly, and in the last four or five years at a breakneck speed, the new innovations in avionics have greatly increased safety and when used properly can increase situational awareness. Of course there is no replacement for that moving map every pilot has between the ears, a pilot’s ability to visualize his position is extremely important to becoming a safe pilot especially when operating in the IFR environment.
The PA46 Malibu started out with the KFC150 autopilot in 1984. This autopilot stayed with the airframe until the end of 1998 (this included the transition from Malibu to Mirage in 1989). It was a very good autopilot and remains so to today. It is an attitude based autopilot which means that it relies on the attitude indicator output for operation.
In 1999 Piper began to install the KFC 225 autopilot. The KFC 225 autopilot was only offered in 1999 model aircraft. The KFC 225 is a digital autopilot which is attitude based. The altitude preselect is integral to the KFC 225. The KFC 225 proved to be a capable autopilot. Many PA46 JetProp owners have chosen the 99 model for conversion to the JetProp since it is only year model which used the KFC 225.
In 2000 the Mirage was given the STEC 55 autopilot to replace the KFC225. This was a departure from the previous attitude based system. Attitude based autopilot system are generally more responsive than rate based systems. Does this mean that the STEC 55 could not handle the Mirage, the answer is no but the attitude systems are more responsive and seem less prone to deviations. The downside to the attitude based systems is that they relied on inputs from a mechanical indicator. Enter the glass revolution and now more and more aircraft have AHRS (Attitude Heading Reference System) which provide a much more stable attitude reference to the autopilot.
The DFC90 was made available to the PA46-350P Mirage about 10 months ago (Avidyne or Aspen equipped). It is an attitude based autopilot system which utilized the inputs from the ADAHRS (Air Data and Attitude Heading Reference System). These AHRS systems are embedded in the Avidyne Entegra or Aspen Pro PFD. The AHRS system produces an more accurate and stable output than in previous analog attitude systems which relied on output voltage to drive the Flight Director and Auto Pilot. As a result of these outputs the DFC90 can control the aircraft with accuracy and stability not found before in previous autopilots. The GFC700 also has this capability, but is not available for retro fit to any aircraft separate from the G1000 system.
The DFC90 offers the functionality formerly only available to autopilots found on turboprops and jets. The autopilot has four vertical modes (Pitch, IAS, VS, or Altitude Hold). If the pilot pushes the AP button the autopilot and flight director immediately engage and will hold the aircraft current pitch and bank. I have found this function to be quite useful in the beginning of the missed approach phase. To operate IAS mode the pilot simply uses the knob on the AP to input the desired airspeed and the autopilot will vary the pitch to maintain the selected airspeed. Before takeoff the pilot can set the Flight Director to indicate a pitch to hold an IAS or VS, and also may be set for a HDG, NAV course or GPSS. One thing to remember when using the IAS mode before takeoff is that if you select any airspeed it will show that you should pitch down. This is normal because the FD bars are telling you to lower the nose to accelerate to the selected airspeed. The Vertical Speed mode may be selected by the VS knob on the autopilot or on the PFD. The altitude hold mode simply maintains a captured altitude, but it does have a feature not normally found in GA autopilots. If the pilot adjusts the altimeter setting after capturing the altitude the AP will acquire the new altitude once the altimeter has been changed. One standard operating principle in the DFC 90 is the active and armed modes. The active mode buttons are in green while the armed modes are in blue. This is true for both lateral and vertical modes.
The DFC90 has five lateral modes. They are Roll, HDG, NAV, GPSS, and APPR modes. The roll mode is engaged when the AP is engaged with no other lateral modes selected. As with Pitch mode it engages at the current roll attitude when the autopilot was engaged. So if you are in a 10° bank when engaged it will hold that bank angle. The only way to change Pitch/Roll is via the Control Wheel Steering button. Simply hold the CWS button while setting pitch and roll to desired setting when released the autopilot will maintain that pitch and bank. The Pitch/Roll mode is very helpful on a missed approach as the pilot can pitch up and initiate a bank/wings level until the GPS is sequenced to the MA procedure. HDG mode is standard in that it maintains the HDG much as other AP’s do.
The DFC 90 is very smart when it comes to transitioning from GPSS mode to APPR mode. One of the shortfalls in the 55X is that when you were flying either a LPV or LNAV+V you need to make sure you were not in GPSS mode or the autopilot would not capture the GP. The DFC 90 will automatically cycle from GPSS to NAV/APPR mode when the GP comes alive (sometime before the FAF). When the GPS begins producing the GP the DFC90 automatically switches out of GPSS mode and switches to NAV/APPR mode and arms the GS/GP.
The DFC90 also has an additional mode not found on any other autopilot. The Straight and Level button can aid a pilot in recovering from an unusual attitude. The Straight and Level button does have its limitations though. The maximum demonstrated limits for engagement of the straight and level mode are 60° bank and 30° bank. Above this limit the mode MAY work but no guarantees. While testing this mode I put the aircraft in a 50° bank and 20° pitch up attitudes, the straight and level button returns the airplane crisply to a wings level and 2° pitch up attitude. It should be noted that in a steep pitch down attitude the autopilot may exceed Vne during the recovery process.
The DFC90 has built in speed based envelope protection. This protection makes it very unlikely that the pilot could get into an auto pilot induced stall. The system is designed to keep the airplane at or above 1.2 VS1. The autopilot will reduce bank and pitch attitude to keep the aircraft within the protected envelope. In addition to low speed protection the autopilot also has the capability to adjust for high airspeed protection. At higher airspeeds near Vne the autopilot will reduce pitch to keep the aircraft near maximum speed. In addition to this protection the autopilot is also able to monitor and alert the pilot with envelope protection even when the autopilot is not engaged. With the autopilot engaged theoretically the aircraft should not stall, but there are exceptions. Since the system monitors only airspeed there is a possibility that with ice accumulation the stall speed will increase, and the airplane will stall at a higher airspeed. All in all the system is a good reminder and should increase safe operation both with the autopilot engaged and not engaged.
Avionics have advanced very quickly, and it seems that the majority of the developments have come in the way of panel mounted GPS, MFD’s, weather detection (XM, Stormscope), and traffic detection (Skywatch, TAS610, etc) systems. Autopilots have lagged somewhat behind in new developments (don’t believe me look at the majority of twin Cessna fleet still flying with original autopilots). Avidyne has developed a relatively low cost feature rich autopilot which could end up with multiple applications. Currently the autopilot is available or in process of approval for Cirrus, Piper Malibu Mirage, Piper Matrix, Cessna 182, Beechcraft Bonanza, and Beechcraft Baron. In the foreseeable future I would not surprised to see this working its way into such aircraft as the Cessna 340, Cessna 414, Cessna 421, Cessna 210 (T210&P210), and then possibly into the light turbine market (King Air, Cheyenne, Conquest).
Here is a top ten list of my favorite GFC 90 features.
- IAS hold mode is an excellent way to manage climbs.
- GPSS to APPR automatic switching. Works very well.
- AHRS compatibility. The digital AHRS signal helps the autopilot be smooth and accurate.
- Rock solid performance. Hold courses, headings, and altitudes perfectly.
- Attitude based autopilot. This allows it to be used in a wide range of aircraft from fast to slow.
- FD/AP annunciation. When FD only is on the V-Bars are green engage the AP they turn magenta.
- When the altimeter is reset the autopilot will automatically recapture the altitude preset.
- The autopilot is integrated with altitude preselect and vertical speed on both the Avidyne Entegra and Aspen systems.
- Envelope protection is a nice safety feature.
- In a pinch the straight and level button may come in handy.
The author Daniel Moore provides insurance approved initial and recurrent training in PA46 Malibu, Mirage, Matrix, Meridian, JetProp. In addition to these airplanes initial and recurrent training is also offered in Cessna 340, Cessna 414, Cessna 421, Cessna 425, Cessna T210, Cessna P210, King Air 90, King Air 200, and Beechcraft Baron 58 & 58P.