As many of you know, I answer many of the avionics questions that you send to the Cessna Pilot’s Association. It’s not unusual to get between six and twelve e-mails a day pertaining to avionics. Many of these questions are about problems that are directly related with the avionics the owner now has or questions about a particular piece of avionics the owner is thinking about purchasing or in some cases, a complete avionics package. Often the questions are related to equipment operation such as; how do I get my newly installed GPS to work with my HSI or 300A autopilot? Believe it or not, I really enjoy helping out; it keeps me sharp and I like the feeling knowing I’ve helped someone; it’s a good feeling. I’d be willing to say that around 50% of the "problem" e-mails that I get are directly related to the pesky Cessna/ARC 400B Navomatic autopilot. What I hope to accomplish in this article is describe just what the capabilities of the Cessna/ARC 400B are, what components are employed in the system along with FAQ’s and fixes for this system. I know what you are thinking; your Cessna doesn’t have a 400B autopilot in it so why read this article? Well, maybe your next one will, thus you will have a better understanding how this system (it is unique) works. 

From what my records show, the first 400B autopilot system was installed in a Cessna twin mid-year 1976. The manual also states that the first single engine aircraft to receive the "coveted" Cessna 400B system was a 1977 T210. The very early model 400B’s in the single engine aircraft used a different servo than later installed in 1978. This autopilot found it’s way into most of single Cessnas excluding the fixed gear 182, 172 and 150 series. The 400B was an option in most Cessna twins even though most 400 series Cessnas came from the factory with the ARC 400IFCS. The IFCS (Integrated Flight Control System) is basically a 400B with a flight director and a mode selector; we’ll dive into the IFCS some other time. By the way, there’s night and day difference between the first 400B computer and the last one built, after all this beast had 52 Cessna Service Letters written against it during the 400B’s history. There're tons of mods that need to be completed to make the computer work properly and I’ll talk more about that at the end of the article.

Just what does the Cessna 400B "Navomatic" supposed to do? I do use the word "Navomatic" very loosely. OK, the Cessna 400B is a two-axis with coupled approach capability that automatically controls the positions of the ailerons and elevators to maintain the commanded aircraft roll and pitch attitudes. The 400B will provide heading tracking, interception and tracking of VOR radials, ILS, glideslope and includes automatic pitch synchronization and trim, manual turn and pitch command, altitude hold and back course switching. Yes, this system will track your GPS/Loran if properly wired and the computer is functioning properly. The yaw damper was an option with the 400B on twin-engine aircraft. Now that you have an idea on just what the ARC 400B is supposed to do in the Cessna, let’s discuss the components and their purpose. Again, we are only discussing the Cessna 400B autopilot, not the IFCS upgrade.

The C-530A Control Unit. This control unit is a panel mounted device and normally located at the bottom of the radio stack. The C-530A includes the primary switches and controls for operating the autopilot. It provides the pilot with the option of selecting the operating mode he/she desires. This panel also provides controls for the manual pitch and roll commands to the autopilot computer. Interlock circuits in the control unit and 400B computer prevent engagement of conflicting modes of operation. A good example of this is you can’t engage altitude without the roll axis being engaged first. Now here’s a problem we see often. A C-530A control unit with MOD 6 incorporated MUST be used with a MOD 7 or higher 400B computer. Control units S/N 3720 and higher include MOD 6. In other words, get the control units mixed up and you’ve got one heck of a mess on your hands. I’ve seen shops do this; they figure if it’s the same part number it will work with all computers but it will not. I learned this lesson the hard and expensive way…the mounting rack for the C-530A also is a dust cover for the unit. This cover is attached to the radio rack via four screws. There are two connectors on the back of the control unit that are full of wires; they connect all the electrons together. Now here’s some good scoop for you. On the back of the C-530A controller, is a switch labeled Gyro "In/Out". Normally the switch should be in the "In" position but it can get knocked easily into the "Out" position during maintenance. When this switch has been placed in the "Out" position, the gyro inputs to the computer will be disabled; thus the autopilot will be all over the sky, much like Uncle Toad’s Wild Ride at Disneyland. If your autopilot is seems very erratic, simply loosen the screw on the C-530A, pull it out of the rack, verify the switch is set in the (In) position. There’s only one switch on the back of this unit so it’s obvious which one to look for. While you are looking, make sure those two connectors are on the controller tight, it will be obvious. Now, simply slide the controller back into the rack and tighten the screw. Checking the switch is easy and it’s very difficult to screw up anything checking it out.

The G-519A-1 and G-519B-1, G-549A and G-549B are vacuum horizon indicators that provide an artificial horizon display and a visual indication of roll and pitch references of the aircraft. The G-519A-1 and G-549A (code-0000) or G-549B (code-0010) are used in aircraft with a "zero degree" tint. The G-519B-1 and G-549A (code-0008) and G-549B (code-0018) are used on instrument panels with an eight (8) degree tilt. Pick-off windings mounted in the roll and pitch axes of the gyro sense aircraft deviation from level flight and apply signals to the 400B computer roll and pitch channels through the famous Gyro (In/Out) switch located on the rear of the C-530A control unit. A problem I see often is that someone has substituted a (0) degree tilt horizon indicator with a (8) degree gyro. This causes all types of problems, which often make the avionics technician think there’s a computer problem, but in reality the aircraft has the wrong horizon gyro. Bottom line; make sure your aircraft gets the proper horizon gyro when you install a new one. Check to verify which horizon you should have in your Cessna. As a "general" rule, most twin Cessnas are 8 degree but again, have the shop verify which one you have, it’s in the book. The wrong gyro could have you chasing your tail for days. Yes, I’ve been there too…

The G-502A and G-503A Directional Gyros are vacuum-driven instruments that supply heading-change signals to the 400B computer amplifier and provide a visual heading display. The aircraft heading is indicated by a symbolic aircraft and arrowhead-pointer affixed to the face of the instrument with respect to the gyro-positioned 360-degree compass card. A heading selector knob (it says HDG on it) is used to move the heading bug, associated with the compass card, to select the heading to be held by the autopilot when the heading function is engaged. Heading error voltage, proportional to the deviation of the indicated heading from the selected heading is supplied to the computer. Regardless of this error, the 400B computer should not initiate more than a standard rate turn to null out the difference between the heading bug and the compass card. A gyro adjustment knob labeled (PUSH) is used to align the gyro with the magnetic heading which should be present on the wet compass. Precession may require frequent adjustment of the directional gyro. The directional gyro has two vacuum lines and one electrical connector on the rear. Be sure to get the pins aligned up correctly before pushing the electrical connector on or you will soon learn to hate yourself. Best yet, get the avionics person to install the connector, if he breaks it, he’ll have to fix it. The main difference between the G-502A and the G-503A is the bezel is different. The DG doesn’t care if the panel is level or not, unlike the horizon indicator. Neither of these directional gyros are internally lighted, thus post lamps are required. We’ll discuss optional gyros later in the article if I don’t forget.

The DT-550A Acceleration Sensor, better known as the "G" switch. Not all Cessna aircraft have this accelerometer. The DT550A consists of a switch that is sensitive to acceleration in one direction along one of its axes and an electronic circuit that detects the opening of the normally closed contacts of the switch. The electronic circuit energizes a relay. When the relay contacts are closed, the autopilot interlock circuit is complete, thus the autopilot will engage (in most cases). This acceleration sensor is mounted on the aircraft to sense acceleration along the vertical axis in a downward direction. If the duration of the acceleration keeps the normally closed switch contacts open for more than 625 milliseconds, the electronic circuit de-energizes the relay and causes the autopilot to disengage. This time delay prevents momentary switch contact openings caused by turbulence from falsely disengaging the autopilot. This accelerometer is located under the co-pilots seat in most Single engine Cessnas. The DT-550A sensor has a self-test circuit to permit preflight testing by operating the "TEST BEFORE FLIGHT" momentary switch. I’m sure you test the autopilot before each flight, right? Rest assured we will be discussing this jewel during FAQ’s; it’s a real dog.

The K-550 Electronic Switch is only used on the Cessna P210R, 210R and the T210R (1985 and up) as a flap modulator. The purpose of the K-550A is to interrupt flap motion anytime the autopilot is on and is commanding electric trim. Interrupting flap motion is accomplished by energizing a contactor near the flap motor, which interrupts flap motor current and stops the aircraft flaps from extending or retracting. The contactor is normally closed and in series with the flap motor. The K-550A receives 28Vdc when the autopilot is engaged and 28Vdc when the flaps are in motion. If, under these conditions, the autopilot commands a pitch trim, the trim actuator drive signals will trigger the electron switch, which provides a ground to one side of the flap modulator contactor. The K-550A also contains a variable resistor that is placed in series with the TA-495A trim actuator when the autopilot is on. This resistor controls actuator speed during autopilot operation and is switched out of the circuit during when the autopilot is not on. Why only the "R" models have all this monkey motion? Easy, the 210R’s have that massive horizontal stabilizer and weird pitch oscillations can be induced without the K-550 electronic switch. Again, only the 210R models have this switch. I’ve discovered from flying the 210R’s that it’s a different machine than any other single engine Cessna. I personally like the way it feels. The 210R’s are responsive on the controls but light. It doesn’t fly like a truck. Up high the T210R is very stable. Now how did I get on this subject, back to the 400B.

The PA-495-( ) Roll and Pitch Actuators. The roll servo in the 400B system is the PA-495A-1. The roll actuator provides a means of moving the ailerons from signals generated via the 400B computer. The roll actuator is placed in the roll actuator mount, thus the bride cables do not have to be removed should the servo be removed for servicing. Not all roll actuators are created equal. Even though you have a PA-495 actuator these actuators are coded according to the actuator gear ratio, the type of connector installed and the primary power required. In other words, you can’t use any roll actuator just because it fits, the actuator must be approved for your aircraft. Inside of the servo is the hated thermostatic switch monitor. What is supposed to happen is if the actuator overheats due to excessive use such as in turbulence, the thermostatic switch will open; the autopilot will disengage until the actuator switch cools off. Normally after ten minutes or so the actuator switch will cool down and the 400B system can be reengaged and the system should function normally. This so called thermostatic monitoring switch is often the cause of what we call "Random Disconnects", we’ll dive into that problem during our troubleshooting tips later in the article. The average roll actuator weighs 3.9 pounds; that’s hefty by modern autopilots. In 1979 ARC made a major design change in the roll and pitch actuators. The connectors are different along with other changes.

Now let’s talk about the PA-495-2 pitch actuator. This pitch actuator has a built-in torque limiting and over current interlock circuit in conjunction with the thermostatic switch monitor as mentioned above. When the signal out of the 400B computer exceeds a determined level, the torque limiting circuit limits the current drawn by the motor and thus limits the maximum torque that the actuator can develop. If the actuator motor exceeds the torque limiter current by a preset amount, an over current interlock circuit is triggered; a relay is opened and the autopilot disengages. This over current protection is wired in series with the thermostatic switch, which is nothing but double trouble. Normally if the autopilot has disengaged itself and will immediately reengage, the problem often can be found in the pitch servo. The average pitch actuator weighs 3.9 pounds; yes it’s a porker for sure.

The PA-495A-3 Pitch Actuator is a Modified PA-495-2 Actuator. This unit produces less torque in one direction than the other. This asymmetrical torque output is produced by connecting a five-ohm resistor in series with the motor on one side of the actuator drive bridge circuit only. It has been my experience that the current limiting in one direction was counter-clockwise (nose down trim). In some of the Cessna T210’s you may see this big resistor mounted near the pitch actuator. Normal torque on the pitch actuator is 12.0 +/-1.0 inch pounds; when the resistor is added torque is reduced to 8.5 +/- .65 inch pounds. As with the roll actuator, the actual actuator can be removed from the mount without disturbing the chain and bridle cables. Believe me, you don’t want to remove the chain and cables, they are one huge hassle to properly reinstall and set. A clutch is installed in all of the actuator mounts. Should the motor freeze up, the pilot could still override the actuator; this is a good thing. The pitch actuator part numbers are coded according to the actuator gear ratio, type of connector used and the torque output. Do not get the pitch actuator confused with the Trim Actuator. They are different and have different functions. In a nutshell, the pitch actuator moves the horizontal stabilizer, not the trim tab.

The TA-495A Trim Actuator physically moves the trim tab (or tabs), yes the same trim tab that moves when you manually turn the trim wheel. In most cases this trim actuator is mounted in the horizontal stabilizer and comes with a mount much like any other ARC servo. This actuator receives a signal from either the electric pitch trim switch located on the pilot’s yoke or from the ARC 400B autopilot computer. The unit includes a gear train, a module subassembly that contains a switching relay, a solenoid-operated clutch and a D.C. motor. When the 400B autopilot is not active and the pilot’s electric trim switch is not being used, the clutch solenoid is de-energized. The de-energized solenoid-operated clutch provides a positive mechanical disconnect between the trim actuator gear train and the output sprocket, thus allowing the pilot to control the pitch trim manually. Once the pilot presses on the trim switch (autopilot is off) the solenoid is energized, the actuator motor is now physically connected to the clutch and the gear train. Should the actuator motor freeze and the motor stay engaged the pilot could over-ride the trim because the clutch would slip. The part numbers of different trim actuator models are coded according to the type of connector used, value of trim speed resistor used and the "no load" speed of the motor. When the autopilot is engaged, the 400B computer will determine when the TA-495A trim actuator will run. Most Cessna’s have a "Trim On/Off Switch" and a trim circuit breaker that you can pull should the trim decide to run on it’s on. I’ve had run-away trim and until you figure out what’s going on it’s a scary situation. Become familiar with the location of these safety devices. The TA-495A actuator weighs only 2.0 pounds. Now here’s something NOT to forget. The FAA in my region has determined the TA-495A is part of the autopilot system. If you remove the 400B system and update to another autopilot brand, you have to remove the electric trim also. This isn’t an issue with a system like the S-Tec 55 with auto-trim but upgrading to a System 30 would mean you have to remove the electric trim portion along with the normal autopilot components. Check with your local shop before installing another autopilot system about their FAA interpretation on this issue.

The Airspeed Switch Assembly. Most Cessna’s require a normally open airspeed switch that senses the differential pitot-static pressure. As the aircraft reaches a predetermined speed, the airspeed switch closes which in turns increases the gain of the 400B autopilot computer in the roll and pitch axis error circuits. The airspeed switch part number contains a "dash" code that identifies the airspeed the switch is set to operate at in knots. Not all Cessna’s have an airspeed switch; Cessna 404, R/TR-182’s and many of the Cessna 210 series do not have airspeed switches. These switches often leak, thus this leaks show up during the pitot, static test. At that point the airspeed switch must be replaced; it is not repairable. If this switch is replaced make sure the shop checks to verify exactly which switch is required for your Cessna. Often we find the wrong airspeed switch has been installed, they all look the same but have different part numbers.

A Question I Get Often is: "My aircraft doesn’t have a 400B autopilot installed, can one be installed? I found one cheap." It depends on the model and serial number of the aircraft. At one time Cessna offered kits to install the 400B in our Cessnas but those days are long gone. You would have to remove the 30 some different components from another aircraft, have these parts inspected and identified along with "Serviceable" tags. At that point, you could install these pieces as a kit with just a logbook entry but there’s more. For the amount you would have to pay in order for someone install a 400B, you could have a good autopilot such as the S-Tec System 55 installed and still have money left over. If you are thinking seriously of installing an ARC 400B in your Cessna, have someone smack you around until you come to your senses.

The AS-895A Altitude Sensor senses altitude change in the form of pressure differential. This sensor produces an electrical error signal proportional to the deviation from the altitude at which the autopilot altitude hold switch is pressed. The error signal is fed to the computer amplifier (located in the 400B computer) altitude hold error circuit, causing the pitch channel signal to drive the pitch actuator until the aircraft altitude is corrected. A modified altitude sensor P/N (44400-0091) is used in the 210 "R" series, 1985 and later to vent the altitude gain scheduling switch to the aircraft static system. This assures proper operation in a pressurized aircraft cabin environment or at least is supposed to.

Some Cessna models have an altitude switch located in the altitude sensor. This altitude switch is not part of the actual altitude sensor circuit but is wired to pins on its connector and is located in it for convenience. The altitude switch contacts are in series with the airspeed switch, located somewhere in the machine. The altitude sensor is responsible for maintaining a rate of climb/decent along with enabling altitude hold once selected. Having said that, here’s the way it works. Let’s say you are climbing out at 600Ft/min and you turn on the 400B autopilot; this module will aid in maintaining that rate of climb. If you turn the trim wheel on the controller you will increase the rate of climb/decent. Once you press the altitude button on the controller, this sensor aids in maintaining the altitude you selected. The AS-895A altitude sensor gets it static source from line connected to the aircraft static system.

Next time we will cover the ARC 400B computer, autopilot options and FAQ’s. Got a 210 that oscillates in pitch or the autopilot disengages whenever it wants to? Stay tuned; we will dive deep into the mysterious 400B.

The ARC CA-550A/FD is the Computer Amplifier of the 400B system; it’s this autopilot system. The CA-550A/FD computer contains the autopilot computation circuitry and most of the system preset adjustment controls. It receives and computes pitch and roll information for the horizon indicator and processes that signal. Heading information is sent to the computer from the directional gyro or HSI (Horizontal Situation Indicator). Let’s not forget navigation information is fed to the computer via the GPS, VOR’s or whatever other navigation system you may have. Altitude error signals are fed from the AS-895A altitude sensor. In the computer amplifier, relays, controlled by the switches on the C-530A controller usually located at the bottom of the radio stack. What you would like the autopilot to track (heading or navigation information) along with other inputs is selected on this controller. The computation circuits produce drive signals, which are routed to the autopilot actuators. The actuators in turn apply torque to the aircraft control surface to fly the machine on the commanded flight path.

The computer amplifiers are tailored to fit the individual requirements of the various models of Cessna aircraft. Each computer amplifier is identified by a code included in the part number of the unit. For example, CA-550A/FD part number 42680-007 is what we call a code 007 and is used only in the Cessna 210’s prior to 1985 along with the associated turbo and pressurized models. Each configuration of the computer amplifier includes a specific group of assemblies or what we call "Cards" which numbers determine the value of certain components on the cards along with omission of some of the components on the cards. These card variations match the performance of the computer amplifier to the specific aircraft model. We will dive into the cards later but a huge problem I found with the 400B computer in the past was inexperienced technicians mixing up the cards while trying to troubleshot a problem. How is one supposed to know which cards should be installed in the computer? Easy, just look in the book, that’s the only way to tell if the computer is properly configured to the proper aircraft. My bet is 25-40% of the 400B computers in use today have at least one wrong card installed. The 400B computer is the only piece of avionics that I’m aware of that had a service letter written against it before Cessna ever installed the first system. There’s over 57 service letters on the 400B system and 21 modifications on the computer alone. The 400B computer has always been a problem child and continues to be so today.

The computer amplifier housing is secured to a mounting base via a clamp and knurled nut. Normally the computer is located under the rear seat of the aircraft, in the baggage are on under the right part of the instrument panel in the case of the 337. Two connectors located at the front panel provide an interconnection between the computer amplifier and the aircraft wiring. A third connecter is covered by a removable plate is located at the front of the panel and provides access to our test and alignment equipment. The average computer weighs six pounds. The average ARC 400B system weighs a massive 42 pounds including the wiring and current draw is a whopping 15.6 amps at 28Vdc. The 400B is a porker no doubt. Today the average avionics stack only draws 15 amps including the autopilot and the entire panel mounted avionics. My, times have changed for the better. The weights and current draw in this paragraph do not include options such as an IFCS option or HSI. The 400B computer contains around 12 separate PC boards, depending on the model.

Let’s talk about some optional equipment that was available for your 400B system. Any of four optional slaved compass system or a non-slaved HSI could have been in stalled in your Cessna as a substitute for the standard G-502A directional gyro. The CS-504A slaved compass system contains a slaved directional gyro which the CS-832A slaved compass system contains a slaved horizontal situation indicator. The remaining components of the systems are the same. There are two versions each of the slaved system; one version of each includes a heading bootstrap. The bootstrap is used to provide heading information for equipments such as RMI, Stormscopes, Sandel EHSI’s and many other pieces of avionics equipment.

The non-slaved IG-832C HSI may also be used as a substitute for the G-502A directional gyro. This non-slaved HSI is a good economical way to get an HSI in the cockpit without busting the budget.

The DV-106A Dynaverter is a method of making 400Hz AC power that is required for some avionics equipment. The DV-106A was a dinosaur the day it was produced. Today you can get the same amount of power from inverters that weigh just ounces not to mention they are much cheaper to purchase. The dynaverter was an option on some 1970-1981 models. I’ve only seen them in a couple of Cessna 210 series and a handful of twin Cessnas. Equipment that uses 400Hz is: some heading syncros, Sandel EHSI’s and some air data computers.

The G-504A slaved directional gyro is a vacuum-driven instrument with a magnetically stabilized gyro, which provides a directional display and supplies heading change signals to the computer amplifier. Aircraft heading is indicated by a symbolic aircraft and a heading index (lubber line) affixed to the face of the instrument with reference to a gyro-positioned, 360 compass card. A slaving signal, supplied by the SA-832A or SA-832B slaving accessory orients the compass card to maintain magnetic heading and eliminates the need for constantly setting the compass card due to precession. A small slaving indicator on the front of the instrument monitors the heading displacement error signal between the CT-504A flux detector and the internals of the G-504A slaved directional gyro. Something you should be aware of on the Cessna HSI. This HSI needs vacuum AND electricity to operate. Loose either and the heading system is dead. Keep this in mind while flying partial panel.

The 400IFCS was an option on most Cessna models. The 400B IFCS system consisted of a flight director/horizon indicator along with a mode controller; the CA-550A/FD computer is the same one used with or without the flight director. The flight director is a nice option; it enables the pilot to visually see what commands the autopilot is sending to the servos. Yes, it would be possible to update a 400B autopilot to an IFCS system but it wouldn’t be cost effective in my opinion. Besides the flight director, the pilot is able to select nav tracking from the #2 VOR system and a couple of other small features. All IFCS models came with a slaved HSI system or at least that’s what my records show. The 400IFCS mode controller has been a piece of avionics that is full of problems. Repairs on the 400IFCS normally are double what the standard 400B system is. They are bazaar and difficult to work on.

Enough on the boring components of the 400B system; I thought it would be a good idea to describe what parts are associated with this complex autopilot. This will allow you to have a better idea of what I’m talking about when we get into the FAQ’s and discuss some of the 400B’s problems. Our goal in the FAQ section is to answer the questions we get the most often along with some ideas on how the components function as a system.

No, take a good look at the airspeed in the CPA magazine prior to this one. Notice that this switch has two round line connections; one of these attaches to the pitot system and the other to the static system. These little switches are prone to leakage and cost a small fortune. The switch changes the gain of the autopilot at a predetermined airspeed. Your airspeed indicator also has pitot and static connection. We will dive into the "Why" question some other time.

This problem is the largest one we see with the 400B. Here’s what I’ve found in the past. First thing to check is what we call the "bridle cable" tension. Your Cessna service manual will call out the tension required. The bridle cables go from the pitch servo mount and are attached to the main pitch cables. We often find these cables so loose that the cable tension meter will not read, check for the proper tension before any other repairs, it must be within the manufactures specifications.

Now let’s check for feedback. While in level flight, with the altitude hold pushed on, slowly pull back on the yoke to raise the nose of the aircraft. Take note that the manual trim wheel is running the trim nose down. Turn off the autopilot and manually trim the machine; now reengage the autopilot and push in on the altitude hold button. Slowly press down on the yoke; the manual trim wheel should trim up. If the automatic trim does not run chances are the problem is in the computer; this is assuming the trim will run in both directions using the electric trim on the yoke. I’d recommend being at least 5,000Ft AGL before completing the above test.

Another problem that will cause pitch porpoise is a bad output. The autopilot computer gets its pitch data from the horizon indicator. If the autopilot pick off is dirty inside the horizon, at some pitch angles there is no output and the aircraft will start to pitch porpoise. If you install a new horizon indicator you must adjust the autopilot computer to the null of the new horizon indicator. If you don’t do this then chances are the aircraft will develop a pitch porpoise. I couldn’t begin to count the number of times I’ve seen maintenance personal "just" install a new horizon and not align it to the computer; often this is where pitch problems originate.

The power supply in the 400B is also an area that will cause pitch porpoise. If the +/- 10Vdc regulator isn’t set properly, you stand a big chance of pitch porpoise in your Cessna. The +/-10Vdc adjustment must be exact; any deviation is not permitted. A dirty power supply will cause the above problem; you may have to have mods 12 and 17 completed in order to cure the power supply problems. Be prepared to take out a loan to pay for these mod updates. Some shops will not touch a 400B computer unless all the mods are up to date; from what I know, I can’t blame them.

Erratic outputs from the AS-895A altitude sensor can cause a pitch porpoise. What we normally see with this sensor, as far as problems is a rapid change in movement of the control wheel. The problem turns out to be the diaphragm inside the altitude sensor is stick and causes massive changes with very little input. You’ll know when this sensor fails; the aircraft will sky rocket straight up and down; much like an "E" ticket ride at Disneyland. We’ve also seen a few cases where static ports induced pitch porpoise at some attitudes. This is the most difficult problem to troubleshoot as related to the 400B.

This is the second largest problem we see with the Cessna 400B system. If your aircraft is a Cessna 210 series or a model that contains the DT-550A accelerometer, chances are that’s the problem. The best way to troubleshoot the problem is install a known good accelerometer, fly the aircraft and see if the problem goes away. Another cause of random disconnects is the roll or pitch servo. These servos have a thermo link located inside that will disconnect the autopilot should the servo overheat. The problem is, the servos seldom overheat but the thermo link inside the servo opens up and the system disengages. If you cannot reengage your autopilot for a minute or so after it disengages, then the problem could very well be either the pitch or roll servo. The best way again to troubleshoot the problem is install a known good servo and fly the machine to see if the problem is cured. Another cause of random disconnects is the power supply located inside the 400B computer. Any kind of noise generated by the power supply or low D.C. voltage outputs will cause disconnects. A good example of this is the +/- 10Vdc regulator inside the computer. If that voltage drops to 9.5Vdc, then the system will disconnect whenever it decides to. If you are having power supply problems with your 400B computer the real and only fix is installing modifications 12 and 17. In fact, few shops will work on a computer unless these modifications are installed. Modifications 12 and 17 help in stabilizing the power supply which is a must in my opinion. Unless you have the proper servos, it would be difficult to troubleshoot the 400B system for random disconnects. I’d try and find a shop that knows the system but as you may already know, that is very difficult anymore. If your system seems to disengage when you pull or press the yoke, chances are the problem is the cable running down the yoke. This cable often chafes, shorts out and disconnects the autopilot.

We see this problem often. During maintenance under the instrument panel, (changing vacuum filters and sorts) the electrical connector on the horizon will get jarred loose. With the horizon disconnected electrically, the autopilot does not have a reference for pitch or roll, thus the aircraft will roll all over the sky. In fact, it’s possible to roll the aircraft upside down if this connector comes loose on some model Cessna. The next area to check is the C-530A autopilot controller. Pull the C-530A autopilot controller out of its case by loosening the screw at the bottom of the controller. Slide it out of the rack and notice a slide switch on the back of the unit. The slide switch can be placed in either "IN" or "OUT" position. If this switch is placed in the OUT position, then the autopilot computer is disconnected from the horizon indicator. This switch MUST be in the "IN" position for flying the aircraft. This switch does from time to time get moved and it causes one heck of a ride.

What is happening is the autopilot for whatever reason is not running the trim tab; the autopilot is trimming the aircraft using the pitch (horizontal stabilizer) only; thus when you disengage the system the pitch servo lets go and the aircraft goes where it was trimmed to go and who knows where that may be. See, the autopilot moves the horizontal stabilizer to trim the aircraft. Once a determined amount of force is placed on the stabilizer, the autopilot commands the trim system to run thus relieving the pressure on the control surface. Henceforth, when you disengage the system, the aircraft should be trimmed. Here’s a way to check things out. While high above the ground, engage the autopilot and altitude hold. Now pull back on the yoke, the manual trim wheel should run nose down, it’s trying to relieve the pressure on the stabilizer. Disengage the system and turn the autopilot back on and again reengage the altitude hold. Now press down on the yoke and notice if your trim wheel runs nose up, it should. If not, you need to get the system repaired. Be sure your electric trim breaker is pressed in and the on/off switch on the yoke is in fact on. I couldn’t began to count the number of trim problems that have been flown hundreds of miles to our shop only to find the trim switch was turned off.

Normally, the computer not being properly aligned to the horizon indicator causes this problem. This problem normally shows up after a horizon indicator has been replaced for whatever reason. The 400B computer should be aligned to the new horizon anytime it is replaced.

For over 10 years I repaired the Cessna 400B system and became quite good at it. The problem is economics; for every 400B I repaired it ended up costing me $100.00. Yes, they are money losers and for this reason alone I quite repairing them. Few 400B owners are willing to leave the aircraft for a week or two and spend several thousand dollars to get it working properly. Most shops do not want to pay thousands for the proper test equipment to repair the 400B only to lose money on each repair; they would rather invest their money in test equipment that will give them a decent return. Like it our not, the 400B is only going to get more expensive to repair and fewer shops will even let one in the door. If you want your system repaired find a shop that has the proper test equipment to repair the system. Expect to leave your aircraft and pay dearly for the repair when you pick up the machine. Become good friends with the avionics manager and put him/her on your Christmas card list, you’ll become great friends or bitter enemy's.

In most cases the Cessna 400B system is an unreliable, difficult and costly to maintain. Some of the power supply components and cards are no longer available, thus repair in the future will be very limited. I’m sure someone will write me and state how they have had years of trouble-free use from their system. All I can say is "Outstanding"; you’re a very fortunate person. We’ve covered a lot of areas of the Cessna 400B autopilot; I hope you now have a better understanding of its components and theory of operation. I know I do!