Labor-saving Devices: Automatic flight control

April 1, 2002
By Jim SparksLabor-saving devices have many applications on modern aircraft. Automatic flight systems are just one.Various high-performance aircraft need automatic flight controls to ensure stability. Development of automatic flight control systems has paralleled the modernization of aircraft. In fact the first autopilots were nothing more than wing-leveling devices. When a pilot was on a cross-country flight and the aircraft was headed in the appropriate direction a device could be engaged that would hold the ailerons in a level situation. Of course today we have auto flight systems that are an integral part of the aircraft and in some cases are engaged prior to aircraft departure and are turned off only after the mission is complete.
Let's take a closer look
What can the autopilot do? In this day and age the possibilities are only limited by the imagination. We can still find the most basic systems which only allow the aircraft to fly straight up ranging to sophisticated systems that control movement around all axis of flight and can even conduct a full stop landing. One thing all these devices have in common is that at some point in time they will require attention from someone like us. Just what does an autopilot look like? Well certainly not like the blowup doll in the pilot suit from the movie "Airplane". In most aircraft the autopilot is an electronic box or circuit card. Obviously this device has no eyes or ears and in most cases no ability to sense what's going on around it. A good way to picture an auto flight system is to look at a pilot in training for an instrument rating. A hood is placed over the pilot's head limiting the field of vision exclusively to flight deck instruments. Pilots learn the "Basic T" concept that teaches them to frequently survey and extract information from the arrangement of flight deck gauges. This includes airspeed, altimeter, attitude, and heading. If this is information the pilot depends on to successfully fly the aircraft, why would an autopilot be any different? Some auto flight systems actually extract data from cockpit displays. There are several terms used today when discussing automatic flight control systems and when you put everything in perspective an autopilot can be compared to the pilot under the hood on a bad day. What would be the outcome if the hood slid down a bit too far? Obviously the pilot would have no information to use to determine how to control the machine. About all that could be done is to continue to hold the flight controls right where they are. This is exactly the way the autopilot may respond. This means in a basic mode with no other data available an autopilot will either keep the aircraft doing what it was when the auto flight system was selected or in some cases it will return the aircraft to straight and level flight. The purpose of the autopilot is best described as a labor-saving device because it saves the pilot from constant manipulation of flight controls. It cannot, however, anticipate the destination or for that matter even the present position of the aircraft. Auto flight systems are often categorized as integrated or non-integrated. This refers to the interface of the flight guidance or flight director system with the autopilot. Just like the pilot, any auto flight system needs to know some information. In an integrated system flight guidance and autopilot orders all come from one device. Non-integrated systems have a means of initiating communication such as an autopilot coupled switch. Flight directors have the ability to take in all of the data made available from the "Basic T" then based on priorities supplied by the pilot, flight guidance will utilize required data and produce a pictorial display for the flight crew. This display can take on several forms including "cross hair pointers" and "V bars" usually displayed on the attitude indicator. Then all the pilot needs to do is hand fly so that the aircraft symbol is kept in sync with the command bars and the aircraft will then follow all the directions provided by the flight director. Priorities the flight crew can usually assign to the flight director include heading, navigation, approach, altitude, and speed. When the autopilot is selected the flight guidance data can be fed into the auto flight computer and the autopilot becomes capable of relieving the pilot from the task of following the flight director's instructions. The flight crew can still observe the flight guidance commands by continually monitoring the command bars and verifying the aircraft is responding accordingly.
Top: Honeywell Primus auto flight control panel.

Middle: HSI, note orange reference at 290-degree position. This is heading cue syncro input to flight director.

Bottom: Aileron autopilot servo from Falcon 50.
Troubleshooting Autopilot maintenance and troubleshooting will vary between aircraft models. Even similar aircraft may have different auto flight capabilities. In the recent past, problems with automatic flight systems were usually referred to an avionics technician. Current production as well as future aircraft will provide unique challenges to the aviation maintenance community. With the current state of integration it is already difficult to tell where airframe stops and avionics begin. In fact, many perceived autopilot problems actually turn out to be caused by airframe issues. The key to troubleshooting is knowledge of the aircraft and auto flight capabilities. Situations periodically arise where flight crews will note a discrepancy where a system in one aircraft does not react like one in another aircraft. This does not necessarily mean there is a problem. In fact, identical systems in consecutive serial number aircraft may operate in different fashions. These differences can sometimes be attributed to requested configuration changes during installation. Even for computerized devices, different levels of software may have an effect on how the system operates. In many cases problem verification is only a telephone call away. When in doubt contact the aircraft manufacturer as well as the auto flight system maker and describe the reported faulty condition. Usually manufacturer's technical support representatives are a very good source of information as real-world problems often occur more than once. In fact, the discrepancy that you are doing battle with may have already had its weak point exposed in a prior event. If in doubt, turn it off One of the first questions that I have learned to ask is, "how does the aircraft perform with the auto flight system disconnected?" If the problem had been related to the roll pitch or yaw axis, turning off the autopilot and then seeing the problem re-occur will provide definitive proof that the problem may be related to an airframe issue rather than the autopilot. One example would be that the yaw damping system might cause the auto flight system to disconnect or indicate some type of failure. In the event of an improperly rigged flap where one side extends a bit more than the other in the retracted position, asymmetric lift would require a yaw condition to make the aircraft fly straight. It would be this continuous demand on the autopilot trying to correct an abnormal aircraft condition that would lead to the auto flight system disconnect or indicated failure. In this situation if the auto flight system was disconnected, the aircraft would probably experience a bit of a roll if the flight controls were all trimmed to neutral and the aircraft was flown hands off.
Repairing auto flight systems
Autopilots have many common features whether on airliner, business jet or small single-engine pleasure aircraft. In the event of a reported problem it is often worthwhile to review specific capabilities of the system in question. There are several steps that can be taken to hasten the repair of auto flight systems. The first is a thorough debrief of the pilot. "Autopilot Inop" is not really a good report. If the system can be engaged what does it do? Is the problem constant or intermittent? Find out if it occurs only in one phase of flight; ask about any abnormal cockpit indications. Secondly, being able to provide separation between flight guidance and autopilot systems as well as determining the normal operating axis will give the attending technician the correct direction to follow for an accurate diagnosis. So if the pilot reports the automatic landing system is inop and the aircraft is not equipped with auto land capability there is no point in troubleshooting the problem. If the aircraft does include multiple flight directors find out if the problem occurs with one or the other coupled. Once a problem is confirmed ask the technical service representative, "how does the system know?" If the autopilot cannot do its job what is lacking and what tripped it off? It's just like if the altimeter fails, the pilot will have a tough time finding and maintaining specific altitudes. Some systems will use data taken from the flight deck instruments. Most electro-mechanical cockpit displays have electrical outputs that will feed the flight director. What this means is a problem in the instrument might surface as an autopilot problem. One example could be the horizontal situation indicator. The heading reference cue not only serves as a visual reference for the pilot, an internal position sensor will feed selected heading to the flight guidance computer which also observes actual heading from the same instrument. Any difference between actual heading and selected heading, and the flight director will command a turn. This command can be viewed by looking at the command bars as well as observing the autopilot negotiating the turn. In the event the autopilot is unable to perform the action it is important to find out if the command bars responded to the selection. If the command bars work then the flight guidance system is capable of processing data. The problem could then be related to the autopilot or possibly the communication link between the autopilot and flight director. Many systems have the ability to function in both the vertical and horizontal worlds. Frequently there are several flight guidance modes in each world, in the horizontal realm there may be a heading (HDG) as well as navigation (NAV) selection. The vertical realm has altitude hold (ALT) and approach (APPR). If one mode won't work, what about another? In the long run what does work can often provide a direct route to concluding what has failed. Many current production aircraft include a Digital Automatic Flight Control System (DAFCS). In these systems the autopilot circuits are in the same box with the guidance system. The result can be a digital signal going directly to the flight control servo. Once it interprets the signal it has the ability to react and make the necessary adjustments. Once the operation is complete the servo can report to the computer that everything is working as advertised. Digital circuits typically include a means of electronic fault finding. In the event certain malfunctions occur, up to and including failures of digital buses, the system can record the fault in a memory and report the findings when interrogated in either a coded format or in plain English. Even though digital systems are more complex than their analog counterparts they are nevertheless simpler to troubleshoot based on the internal fault diagnostics (sometimes). So an autopilot is a "labor-saving device" for the pilot but they sure can mess up a weekend for those of us that have to keep them functioning. Might be nice if someone would devise the "Auto Technician" to solve such problems.