Tucked away in the bowels of many airplanes is a piece of equipment the pilot may count on for survival. But many pilots dont take the time to learn about their autopilots, nor do they check them before flight.
The best place to spot a problem is on the ramp rather than at altitude. Even though many pilots consider the autopilot a luxury, its good to know that it works correctly in cases where the success of a flight depends on turning control of the airplane over to a computer.
Ask any pilot, Whens the last time you preflighted your autopilot? and youre likely to be met with a blank stare. Under pressure, many will admit that they dont even know how to check it or where to look in order to find out. Some, particularly renters, may not even know how the thing works in the first place.
Sometimes its not easy to preflight an autopilot, either. In the POH of a 1982 Cessna 182RG we checked, for example, the supplements describing autopilot operations had no instructions for preflight checks. It merely instructs pilots to turn the unit off for takeoff.
Although a properly functioning autopilot is not a requirement for flight on part 91 aircraft, use of the autopilot during any phase of flight without having first made a functional preflight check is an invitation for the NTSB report to read: inadequate preflight – pilot in command.
In flight is no place to test the device; on approach is even worse. Because an autopilot takes over the flight controls, a malfunction can have a rapid, disastrous impact on the safety of the flight.
The preflight regimen should be in your POH. Even if the autopilot was added after your aircraft was built, the information should be in an addendum. Should and is are two different things, of course. If you dont have one, the manufacturer of the device is the logical place to start to get the necessary supplement.
The PC Generation
The most basic and elderly of general aviation autopilots is the Brattain PC system in older Mooneys. The so-called Positive Control mechanism acts as a wing leveler that is always on, except when the pilot holds down a button on the yoke for maneuvering. Although reliable in their day, many of the aging systems now are quirky. Many old Mooneys now sport rubber caps over the horn of the yoke, thereby keeping the system switched off.
Preflighting the system is easy, because there are really only three things to check. I call them the overpower test, the appropriate output test and the disconnect test.
The system is vacuum driven, so the prop must be turning to conduct a ground check. You have to be able to turn the yoke against the system. Then turn the neutral point turn control left, then right. The yoke must turn in the same direction incrementally and not run away to full stop. As you turn on the taxiway, a real turn (as measured by the turn and bank indicator) in each direction must be accompanied by opposite yoke deflection. Finally, disengaging PC by leaking the vacuum via the yoke button must truly disengage the actuator force.
As you get more functions, the preflight check will be more involved.
Stepping up from the PC system is a unit that combines the basic wing leveling with the ability to follow a nav radios CDI. These step-up units generally have a two in their name, such as Century II, Autoflite II or Cessna 200A. In addition to the wing leveling function, these autopilots have a roll control knob that allows standard-rate turns to be made by the autopilot. Such a feature is useful for VFR pilots who stumble into the clouds and want the airplane to turn itself around.
Take another step up the food chain and the unit will couple with the directional gyro. In that case, the DG will have a heading bug that allows you to set a desired course independent of a nav signal.
Preflight check is similar in that the pilot must determine that the unit engages and disengages properly and can be overpowered by hand. In addition, the pilot must verify that the autopilot captures the signal from the CDI and DG and responds correctly, and also that it responds correctly to input from the roll control knob.
Movin on Up
The next level of complexity is found in models that have altitude hold, such as the Century III and the Altimatic IIIC. Once you get into altitude hold, preflight becomes even more critical because of the interconnection between the autopilot and the electric trim. A runaway trim can lead to structural failure at high speeds.
The preflight of a Century III has a preface and three sections. The preface reminds you that there is an autopilot Vmo (maximum speed for autopilot operation), and flap limitations, and a limitation that you must be belted in the PIC seat for operation. This autopilot does not have appropriate backups to accommodate loss of control with PIC not in the seat.
The roll section check requires placing the radio coupler switch in the heading mode and engaging the roll section rocker switch. Rotation of the roll command knob must be accompanied by the appropriate control response. Directional gyro coupling then must be similarly checked in heading bug mode by moving the bug and verifying appropriate yoke movement. The lateral overpower limits must then be verified. The pitch section requires centering of the pitch command disc, engaging the pitch rocker switch, and verifying that the pitch commands result in the appropriate yoke motion, as well as an overpower check.
In Pipers with stabilators, there is a note that the pilot may have to assist the pitch servo in overcoming the weight of the elevator while the airplane is on the ground. The pilot also must check the disengagement modes by pushing the master disconnect switch in all modes and testing for free control travel.
But wait, youre not done yet. The electric trim system also needs to be tested. The thumb switch needs to be depressed as well as moved. The electric trim motor should not engage if the switch is moved without being depressed, and when actuated must run in the correct direction. The trim system must also pass the overpower test as accomplished by holding the trim wheel or the yoke against the trim servo.
This is also a good time to remind yourself of a number of items.
Know the location of the trim circuit breaker and trim master switch, so that if you have a pitch runaway, you can shut the beast down. Second, make sure you know how to manage the disconnect mode without inducing a sudden large control excursion.
With the Century III system, things go a lot more smoothly if you disengage pitch control with the computer showing near neutral trim position. If you dont plan the disconnect, the instantaneous pitch forces can be quite surprising and it may not be possible to control pitch without creating a distressing excursion.
In addition, you should be familiar with the failure modes for your autopilot. I recently experienced an enroute roll down only failure. Subsequently the local avionics shop diagnosed an open output power transistor in the roll up amplifier, so that even though the computer might be commanding roll up, the message could not be executed.
In this situation, its important that the pilot not merely respond with back pressure on the yoke, as the autopilot will only respond with more pitch down trim. It then becomes impossible to disconnect without a large nose down surge as autopilot disengages. At cruising speeds the large nose down trim could induce enough negative gs to cause a structural failure. In a recent NTSB report, the crew of a Falcon Jet leveled off at 11,000 ft. manually without minding the autopilot. The unbelted stewardess sustained head injuries during the subsequent autopilot disconnect.
The proper response is to immediately disengage the autopilot either at the control yoke trim switch, at the pitch servo, or at the computer/amplifier circuit breaker. In any case theres real potential for substantial altitude deviations, which is why I personally do not ever use the glideslope coupler in real IMC. In cruise, the altitude excursion is disconcerting; on the glideslope, failure of an output transistor may put you into the ground.
But just as all planes are not created equal, so too do autopilots vary. S-TECs line of autopilots is rate-controlled and totally electric. Although preflight checks are similar, failure modes are a bit different – loss of vacuum doesnt affect the system. In addition there are trim lights to verify, as they alert you when the S-TEC pitch servo needs you to re-trim the aircraft main trim to a more centered range.
Got a flux gate compass and an HSI? The NSD HSI requires both vacuum and electric power; a hit in the vacuum and the HSI will go down, but the S-TEC autopilot will continue flying. If you have the new X series, lose the vacuum and the GPS will continue you steadily along your programmed path, right through turns. The HSI will confuse you if you havent thought it out, in advance. Got a Century III? Lose the vacuum and youre partial panel into an ILS or a return to visual conditions.
In addition, some installations combine equipment from different manufacturers. For example, some aircraft with working single-axis autopilots have been upgraded with pitch-control systems from another source. That means a proper preflight involves both units separately. In addition, aftermarket installations are more likely to have missing paperwork, including the supplement needed in the POH.
The most sophisticated light planes naturally have the most sophisticated autopilots. While that makes them more capable, it also makes preflight even more essential.
A Piper Malibu is equipped with the King KAP 150. Although King has copies of the autopilot addendum available for the Malibu and the Malibu Mirage, avionics manufacturers are quick to point out that they dont write the authoritative procedure – the aircraft manufacturer does.
The complex interactions during a disconnect, the authority cutoffs and the rate controllers are always individual to each airframe. The KAP 150 goes one step further than most to ensure you check it out correctly. It has a lockout that prevents the system from engaging unless it has been through preflight.
Just as with the Century III, there is a limitations section wherein the pilot is reminded that there is no out-of-seat redundancy. Stay belted in the left seat. The autopilot and yaw damper must be off during takeoff and landing, and indicated airspeed must remain below 185 ktas. The system will not tolerate more than 60 gallons of fuel imbalance and after and in-flight trim warning the pilot must pull both autopilot and pitch trim breakers and not reinitialize the system.
The preflight itself has 10 steps:
• Gyros up (3-4 minutes).
• Radio master ON.
• Preflight Test button – all annunciators on, Trim annunciator flashing, then all off except for the Autopilot light which is to flash about 12 times and then remain off.
• Electric trim test – This unit has a split switch; both sides must be moved and the thumb bar depressed for anything to occur, and what occurs must be appropriate.
• Engage the Flight Director.
• Engage the Autopilot Yaw Damper
• Hold the control wheel against pitch up on the mode controller. Autotrim runs up after 4 seconds. Then depress and hold the Control Wheel Steering switch and verify autotrim stops. Then release vertical trim and CWS switches and repeat this test in the down direction.
• Verify that the autopilot and yaw damper can be overpowered.
• A/P disconnect Trim interrupt switch – Press and verify that the both the Autopilot and the Yaw damper disconnect, and that the Flight Director goes off line.
• Set takeoff trim.
If the Trim Warning light remains lit after the preflight, the autopilot did not pass the preflight rest. The autopilot circuit breaker must be pulled and manual electric trim cannot be used.
The outline here is an electronic self-verification, a check of manual electric trim, an appropriate direction and overpower tests, as well as a disconnect test.
Because of the built in programming requiring the 10-step check, completing the check on the ground is essential. Otherwise, youll have to do it in the air before engaging the autopilot. Such a detailed test during cruise could be hazardous to your health. Simply the distraction of having to do this enroute would force an already imprudent pilot to now cope with a massive workload.
To make the test a little less onerous, King prints the KAP 150 manual in half-notebook size for kneeboard use in the pilot seat. The systems are complex enough that if you dont perform the preflight in the order required, you may be spending many a minute going through the routine.
The Malibu and its systems are sufficiently sophisticated that the rapid preflight – structure OK, fuel drain/fuel present/no water present, landing gear OK, boots OK, doors and hatches secure, instruments set – will not do. Detailed, by-the-book preflight is the only way to go in more complicated equipment.
A violent disconnect above maneuvering speed can be catastrophic. In aircraft like the Malibu, Va is well below indicated airspeed in most cruise situations. Most pilots dont even calculate Va for todays load, and many are hard pressed to recall whether Va goes up or down with load.
Treating the passengers to a stable g-force flight profile is one of many marks of the proficient pilot, but autopilot operations go far beyond that into the realm of airframe upset. Its bad enough having to apologize as the passengers tidy up a cabin that has just been subjected to pilot-induced turbulence. In another scenario, the airframe gets upset and over-stressed.
Many pilots view preflight as a nuisance that only slows down their trip. Owners are particularly susceptible, especially if theyve flown the same airplane for years and knew everything was working the last time it was up.
Remember that every malfunctioning piece of equipment was working before it broke. If you dont preflight adequately, youre losing the opportunity to exercise proper go/no go decision-making. Its at least negligent. At worst its careless and reckless or even perhaps fatal.
Also With This Article
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-by Bruce Chien
Bruce Chien is a CFI and Airman Medical Examiner.