On the Starship Enterprise, when something goes awry, an excitedengineer-type says stuff like, "A warp-core breach is imminent!"This is accompanied by ominous humming and flashing warning lightsto give you the idea that the ship is about to explode.
It never does. That’s because the Captain knows to remind theengineer that in case of warp-core breach, the first thing tocheck is the magneto-resonance frequency in the dilithium crystalchamber. Too bad you can’t similarly advise your avionics techwhen your autopilot suffers the equivalent of a core breach. Whendiscussing autopilots, your technician may utter words thatsound like English but the only things that flash are green dollarsigns.
We can’t promise to tell you enough about your autopilot to fixit yourself, but we can explain the concepts you hear used inreference to the autopilot. And if you can describe its ills interms your technician can understand, you could save some bucksin troubleshooting time.
No Simple Thing
Even the simplest autopilot is a complicated bit of machinerybut all systems can be divided into three parts: A sensing part,a selecting/computing part and a control part.
The sensing part detects the airplane’s status. Are we wings level?On the desired heading? Rolling in turbulence? The system hasto know something about the current attitude and the forces actingon the airplane. These sensors are attitude or rate gyroscopes,pressure sensors, accelerometers, heading bugs, GPS receivers,glideslope pointers. Essentially anything that’s an input to theautopilot computer.
The computing part looks at the information coming from the sensingpart and compares it with the mode selection parts. The computersays to itself, "Okay, this is where we are now. Over thereis where that human in the seat wants to go. I need to roll intoa 22-degree bank to get there. Better haul away on the aileroncable."
So the computer commands a servo to move, yanking the controlsto do its bidding. Those are the controlling bits, things likeservo motors, clutches, capstans, cables and chains. (Look ata Mooney trim system, and you’d swear Orville and Wilbur tooktheir bike shop to Kerrville.)
Flight control is a loop or series of loops. As the autopilotcomputer yanks the chains, it expects to see things happen. Somefeedback is often sent from the servo, in the form of a positionsensor or motor feedback. Plus, if all is going right, the airplanewill go in the direction the computer’s electronic brain wantsit to go. If it doesn’tsay because of turbulencethe computerapplies more muscle until something happens.
It’s the loop-iness of autopilots that gets the technician andpilot into trouble. On the ground, you have no loop. No matterhow hard the autopilot tries, it just can’t roll into a standard-rateturn on the ramp. Therefore, the best way to troubleshoot an autopilotis in its natural habitat, aloft.
A Little Self-Help
Plan on doing some diagnostics yourself. Otherwise, you risk bigbucks by sending the repair person down the wrong path. The autopilotparts aren’t something you can slide out with an Allen wrenchin one minute for a bench check. The servos in an Aerostar withthe added fuselage fuel tank are about 20 man-hours from daylight.That can be a $2000 gamble to change a roll servo, on a hunch.We’d rather go to Vegas!
Here, in alphabetical order, are some terms common to autopilotsystems. Speaking the same language as your avionics technicianis the biggest part of solving any problem.
Adapter boards, modules or sometimes "personality modules"configure the autopilot computer to a specific airplane type andare common in Bendix/King autopilots. Without the right adapters,the computer has a serious personality disorder, with unpleasantflight consequences. It’s not unusual to find that a shop hasstuck a loaner computer in, "Just to see what happens."
Because an A36 flies sort of like a Cessna 210, the real problemmay appear to go away. Yet it’s just buried. The adapter boardin the KFC 200 contains all the aircraft alignment calibrations.Why? Because if you need to exchange the KC 295 computer, youcan install the same adapter in the new unit and not have to re-alignthe system.
If the adapter board in the KFC 200 is replaced for any reason,or the attitude gyro goes bad, you should re-align the entiresystem. If not, you may introduce another problem or mask theexisting one. There can be no mix and match of part numbers inany system. The correct equipment designed for that particularmodel airplane must be installed; no substitutions.
Following service, examine your logbooks and workorders. Verifythat all part numbers, in and out, are recorded and that theymatch specs. If not, get an explanation.
Automatic electric trim or autotrim is controlled by the autopilotand is used to reduce pitch servo effort in the same way a humanuses trim to reduce control wheel forces. Not all autopilots haveautotrim, but all two-axis autopilots need trim. S-Tec and Century2000 autopilots without autotrim incorporate trim "prompting."These are lights that remind Mr. Pilot to crank in some trim sothe pitch servo doesn’t have to work so hard.
Autotrim is a frequent cause of problems. You can have too muchor too little. In flight, you should pay attention to the trimwheel. Does it run every few seconds or rarely? The frequency,even in a perfectly functional system, depends on many factors,not the least of which are the handling characteristics of theairplane. However, if the airplane has a pitch oscillation (seeporpoise), you may have overactive trim, and mis-set trim sensors.
Trim sensors are not in the trim servo. They’re associated withthe pitch axis. They sense elevator loads, either from insidethe pitch servo, or on the elevator cable. Exactly where dependson the brand and type of autopilot and airplane. The lesson is:If the trim is too active, don’t replace the trim servo! If theautopilot technician recommends it, get a second opinion.
Analog autopilots contain lots of transistors and resistors andplenty of integrated circuits, but their stimulus and responseare not controlled by a microprocessor. Century, S-Tec and someBendix/King systems such as the KFC 200 are analog. The Bendix/KingKFC 150 and later autopilots, as well as current Honeywell andCollins systems are digital.
Analog systems are easier to troubleshoot. Sometimes. But theyhave more components and therefore are more failure-prone. Airplanesare analog machines and don’t lend themselves to digitalizationeasily. But so is music.
With enough computational power, anything is possible.
Attitude-based autopilots such as the Bendix/King and Centurysystems (except for the Century I) use a sensor that detects theaircraft position from the attitude gyro. They manipulate thecontrols to put the airplane in an attitude that will satisfythe pilot-requested input for pitch (usually altitude) and roll,(heading). Attitude-based autopilots also use the rate of changeof the attitude to control the airplane smoothly.
A single-axis autopilot has aileron control only: typically wingslevel, heading hold or heading select and some sort of couplingto a nav. Single-axis autopilots from S-Tec and the Century 2000can be upgraded as your needs and desires change. Bendix/Kingautopilots are installed from the git-go as single axis. By thetime you change all the components necessary to upgrade the system,you might as well have bought a new system.
Two axis autopilots have aileron and elevator control, possiblyincluding pitch trim. You set altitude hold and pitch commands,such as an attitude or rate hold. Most will include glideslopecoupling. It’s important to understand how the glideslope modeworks, what the prerequisites are and if the beam can be capturedfrom above and below. You can find this stuff in your AFMS orapproved flight manual supplement. To be perfectly ramp-checklegal, you must have a current supplement aboard.
A three-axis autopilot includes a rudder servo. Most are yaw-dampingsystems that keep the tail where it belongs. Some airplanesBonanzas,for exampleneed yaw damping more than others.
A slip clutch allows the human to overpower the autopilot. Clutchesrepresent a compromise between performance and safety. Clutchesare strength limiters and therefore degrade the ability of theautopilot to efficiently fly the airplane. But in order to protectthe occupants in the event of an autopilot malfunction, manufacturersinstall slip clutches to give you some control.
It won’t be a picnic to fight a clutch, but you ought to be ableto arrest the undesired control movement while disconnecting thesystem. The slip clutches are sized during certification to balancethe malfunction controllability with performance. For this reason,the tolerance of the clutch is important. If the clutch torqueis too low, the autopilot will be sloppy. Too high, and you won’tbe able to wrestle the airplane away from a deranged autopilot.
Will you be able to detect a clutch set too tight? Probably not.We recommend that autopilot clutches be checked annually. It willcost money, so you might want to wait until something goes wrongto see if you can control the airplane. Again, we confine ourgambling to Las Vegas (or maybe Atlantic City, which is wherethe FAA does its gambling).
Some Century (a.k.a. Mitchell, Edo-Aire or Piper) systems usea shear pin, which has no sense of humor. Avionics shops love’em. One slip and the autopilot is in for service.
Digital autopilots employ a microprocessor to fly the airplane.They have fewer parts, because the microprocessors take over formany discrete circuits. The problem is most avionics techniciansare analog devices. They have an inherent mistrust of computers(some even mistrust transistors). The troubleshooting processfor microprocessor-based autopilots is different than doe an analogunit. You have to think like an autopilot.
How a digital autopilot flies is like this: The sensors receivethe analog position (attitude, rate, CDI deviation, etc.) andinput that to the autopilot computer. An analog to digital conversiontakes place inside the box. The microprocessor compares the inputwith the desired result and outputs a data stream to command thecontrols to move. A digital-to-analog converter on the outputchanges the data stream to analog voltages to spin the motorsand yank on the controls. Got it?
One word of advice, though. If you think the microprocessor isbad, or has a software problem, think again. Barring lightningstrikes, the processors are the most rugged things in the box.
Manual Electric Trim
Manual electric trim is controlled by the pilot, an electricallydriven trim wheel. Not much to say except that the switches wearout and more frequently, the wires inside the yoke get broken.In any case, manual trim problems tend to be simple and annoying,if not expensive to fix.
The best wiring is a coiled cord from the wheel, but for somereason (probably aesthetics), airplane manufacturers keep shovingthis mass of wires through a tiny tube. It’s important to describetrim problems accurately. If you grab the wheel or crank and trimthe airplane, this is manual trim. If you run the trim with alittle button on the control wheel, it’s manual electric trim.If the autopilot trims, while you get 40 winks, that’s autotrim.
A wise autopilot technician once said, "Fish porpoise, airplanesoscillate." Okay, there are purists everywhere, but aftera couple of, ahem, oscillations, you will say, "Whoa, whata porpoise!"
What you are experiencing is an inability of the autopilot tomaintain a stable flight path. There are two root causes. Toomuch control and too little control. It’s your ability to classifythese gyrations in flight, between onset and the time you disconnectthe autopilot (which you must do for a safe conclusion of theflight), that will determine the relative success of the technician’stroubleshooting.
The best cue is the deviation from the desired path. Under-controlling"fish" deviate from altitude a long way and over a periodof some minutes. You can watch the controls as the autopilot driftsoff farther and farther, then catches itself and wanders backon track. Typically, it will overshoot and go off in the otherdirection. The trim may run, but tend to be too little too late.Trim may aggravate the situation. What’s more important, trimwill be behind the autopilot. This kind of problem is typicallya slipping clutch, miss-aligned or defective attitude sensor oreven a dead pitch servo.
At the other extreme, an overactive autopilot will stick closeto the desired flight path, but will be like riding a canteringhorse. Avionics technicians look forward to these, because there’susually loose change in the seat cushions. Rapid oscillationsare often the result of mis-alignment (too much gain), or a failurein the smoothing portion of the autopilot loop. Overactive trimwill induce a porpoise as the pitch servo acts to counteract amomentarily out-of-trim condition. Pitch porpoises that inhabitthe roll axis are often termed "wing rock" or "Dutchroll." Again, amplitude and period of the phenomenon arecritical bits of information. Get your watch out and time it.Describe the amplitude in terms of degrees of roll.
Rate-based autopilots such as the S-TEC, Century I and Brittainsystems depend on acceleration, movement along the longitudinalaxis and altitude changes sensed by a transducer to determineattitude changes. They essentially ignore aircraft position, controllingthe roll and pitch to put the airplane in a steady state whilesatisfying the command for heading or altitude.
The advantages are an independence from the attitude gyros, whichhave rarely set records for reliability, particularly when thereliability of the vacuum system as a whole is factored in. Theygive a true standard rate turn, instead of the approximation thata position-based system can provide. In addition, an attitude-basedsystem can, in severe turbulence, attempt to bend the airplanetrying to maintain an attitude or altitude.
In the S-Tec system, based on rate, motion extremes are sensedand overcontrolling is thus avoided. The biggest advantage issimplicity. The biggest disadvantage is precision in putting theairplane in the correct attitude. In 90 percent of general aviationairplanes, in 90 percent of flying situations, you’ll never noticethe difference.
If you know your autopilot system well enough to describe it,you’ll be surprised how quickly a technician might be able tozero in on your problems. Communicate the facts carefully, makenote of your observations and pass them along. Worst case, you’llat least understand what he’s talking about when he says you’vegot an autotrim problem. If possible, take your shop guy for aride before pulling panels and boxes.
But until the autopilot is working 100 percent, do the right thing:Keep it turned off and breaker pulled until you know what’s wrong.