TECHNICAL ADI Attitude Directional Indicator

B Y RALPH GRAVES

hile there are many ver- least understood of the flight instru- sions of the attitude direc- ments. This is for good reason on both Wtional indicator (ADI) that counts. It gives the pilot a visual indi- provides the pilot with verity of flight cation of the aircraftʼs attitude in rela- information, one thing that is common tion to the surface of the earth, and has to all of them is their ability to indicate more factors affecting its operation the aircraftʼs attitude in pitch and roll than any instrument in the aircraft. in relation to the surface of the earth To aid us in understanding the oper- The basic functions of this flight in- ation of the ADI we need to start with strument have been with us for a very some basic aircraft instrument theory. long time and have carried a variety of The primary purpose of any instru- names such as, Attitude Gyro, Gyro ment system is the transformation of ADI Horizon, Artificial Horizon and oth- certain information or condition into a ers. I like the name Artificial Horizon useful visual indication on the aircraft lished or known point to the desired because it best describes the primary instrument. In most cases, there are at unknown, and provide us with a vi- function of the instrument. That is, to least two parts to any indicating sys- sual indication of some measurement provide the pilot or pilots with an arti- tem: the “Sensor” that senses or mea- from this known, established reference ficial horizon when the true horizon is sures the information for the indicator; point. If this established, or known ref- not visible to them. and the “indicator” that converts this erence point changes, so does the vi- In the AID indicator (center) is a information into a visual analog or sual indication of the instrument, pro- recent panel-mounted ADI. The two digital display. Like most systems, any viding us with an erroneous pictorial cut-a-way instruments are from an era instrument system is only as good as indication. past, when manufacturers wanted their its weakest point. There is a misconception accepted customers to see what their products I think it could be said that all air- by many of those personnel who use were made of. craft instruments could be classified as and service aircraft instrument systems. It has been my conception that the differential instruments because they In our modern aviation society, and the ADI is the most widely used and the indicate changes from some estab- public in general, there is a general

Gyro Assembly Rotor and Shaft Gimbal Ring and Rotor Housing Horizon Bar Gimbal Ring Caging Mechanism Assembly Gyro Horizon Erection System Attitude Gyro

52 AVIONICS NEWS • SEPTEMBER 2005 consensus that any digital indicator is Type of Gyro Used much more accurate than any analog with ADI Indicators indicator. This theory does have merit; in most cases when the accuracy issue is restricted to the indicators only, the digital indicators are the best in almost every case. In many cases, the sens- ing portion of the instrument system is the same if it is used with a digital or analog indicator and will indicate the same correct or incorrect information on either type of instrument display.

Aircraft Gyros is used in the ADI to establish a ref- tion. When the rotor axis is turned left erence point to measure change in the They may use different names, but or right and parallel to the gimbal axis, attitude of the aircraft around the air- at least in theory there are only three the gimbal will rotate in a direction that craftʼs longitudinal (roll) axis and the basic types of gyros with a rotor that is 90 degrees from the rotor axis. The literal (pitch) axis of the aircraft. Any provide us with a reference point to rigidity of the rotor and the tension of movement of the aircraft around the measure the attitude of the aircraft, in the spring will determine the amount pitch or roll axis of the aircraft will be relation to the earth. and rate of gimbal ring rotation. This transferred mechanically or electroni- One is a universally-mounted three- change and rate of the change is trans- cally to a miniature airplane and an ar- axes gyro with the rotor axes parallel ferred mechanically or electronically tificial horizon on the face of the ADI to the earthʼs surface, used in the di- to the aircraft instruments or autopilot. indicator, mimicking the actual rela- rectional gyro to establish a reference This is most often used in, but not re- tion of the aircraft to the true horizon. point to measure change in direction stricted to, turn and banks or turn co- The third type of aircraft gyro is of the aircraft around its vertical (yaw) ordinators. the rate gyro. This consists of a rotor axis. The directional gyro has no di- In a perfect world, this is all one mounted in gimbal ring 90 degrees to rection seeking properties and must be would need to know about the gyro in- the axis of the gimbal ring. A spring set to or slaved to a magnetic compass strument systems in our aircraft. How- holds the gimbal ring in a neutral posi- Continued on following page heading. Once it has been set or slaved to a heading, it will sense deviations from that reference point. Type of Gyro Used with T&B and T/C Second is a universally-mounted Indicators three-axes gyro with the rotor axes perpendicular to the earthʼs surface. It

Type of Gyro Used in Directional Indicators

AVIONICS NEWS • SEPTEMBER 2005 53 ATTITUDE DIRECTIONAL INDICATOR Continued from page 53 ever, we donʼt live in a perfect world so we know we have to accept some bad with the good. Gyro precession is one of those undesirable conditions of a gyro. All gyros precess to some degree and the ADI is no exception. Basically, we have two types of pre- cession—real and apparent. Apparent gyro precession is based on the fact that the earth rotates and the gyro maintains its position in space. In this situation it appears that the gyro is precessing, when, in fact, it is holding Figure #1 Figure #3 its position. It is only apparent that the gyro is precessing as the earth spins by its position. Although it is only appar- ent that the gyro is precessing, it is a factor we have to compensate for. Real gyro precession is that prop- erty of a gyro that causes the rotor axis to be displaced, not in line with the applied force, but 90 degrees away in the direction of rotor rotation. This is caused by friction in the gyro rotor and supporting gimbal ring bearings. This resistance to the rotation of the rotor or the supporting gimbals will result in rotor precession. An out of balance condition of the rotor or supporting gimbal rings will also be the cause of rotor precession. Rigidity is another important prop- erty of a gyro. The primary trait of a rotating gyro rotor is its rigidity in space, or gyroscope inertia. Newtonʼs first law states in part: A body in mo- Figure #2 tion tends to move in a constant speed and direction unless disturbed by some as large and as heavy as is practical. surface as the aircraftʼs center of grav- external force. They are also designed to spin at high ity (CG). The rotor of the gyro instruments speed. All gyros precess to some degree, maintains a constant attitude in space To compensate for real or apparent and the ADI gyros are no exception. as long as no external force changes it. precession in the ADI gyro, we add The precession of the ADI gyro is This stable quality of the rotor depends weight or torque to the gyro gimbals to constantly corrected by the erection on the mass and speed of that rotor. induce a reverse precession. mechanism to the aircraftʼs CG. Vari- Thus, the larger and heavier the rotor In the ADI gyro, we have no direct ous erection mechanisms are used to and the faster we spin it, the greater the connection to the surface of the earth correct this gyro precession, but one stability or rigidity. This is why most to use as a perpendicular position from thing that is common to all of them is aircraft gyrosʼ instrument rotors, both it. In this case we use what should be a their ability to apply a force to oppose electrical and pneumatic driven, are perpendicular position from the earthʼs the force causing the precession. The

54 AVIONICS NEWS • SEPTEMBER 2005 attitude of the aircraft and the G forces The slower speed when the erection ing at various speeds and attitudes, on the aircraft will vary this CG. To process is taking place allows the the G forces change the friction of the add insult to injury, as the CG changes erection mechanism to erect the rotor gyroʼs bearings and the balance of the and G forces are added to the formula, quickly because of less rotor rigidity. gimbals. The corrections or calibra- we have changes in bearing friction, On the other hand, electrical rotors tions that were made on the ground balance of the gyro and its gimbals to gain speed much faster, so the rotor ri- are no longer valid under these condi- contend with. These factors can cause gidity is at its peak very quickly, mak- tions. the erection mechanism to precess the ing the erection process much slower During take off and climb out, rotor to position to the invalid CG, and than pneumatic rotors. the aircraft experiences the greatest this provides the ADI indicator with To aid this slow erection of the elec- amount of thrust and G force. This a less than true indication of the air- trical rotors many manufactures have force changes the load of the bear- craftʼs actual attitude. added a “Quick Erect” or “Caging” ings in the rotor and its gimbals, which As we increase mass and speed of mechanism. These systems will erect changes the friction of the bearings. the rotor, the rigidity of the rotor will the rotor and its gimbals to a zero de- These forces also change the balance increase. Greater rigidity will slow the gree pitch and roll position of the rotor of these gimbals and rotor. precession of the rotor. This is a con- axis in the ADI gyro. Under these con- Now, the calibration that was made dition that is good for maintaining our ditions the ADI will be indicating level when the ADI was tested on the reference point in space. While it is flight regardless of the attitude of the ground is no longer valid. These forces a good factor for maintaining the ri- aircraft. The caging or quick erect also change the aircraftʼs CG, and the gidity of the rotor, it is a negative for should never be used unless the air- erection mechanism of the ADI will our erection mechanism when using craft is in an attitude of “0” roll and be correcting for precession from an reverse precession to compensate for “0” pitch. The erection mechanism invalid CG; resulting in erroneous at- precession errors. will be applying a force to precess the titude indications often undetected by As the name indicates, the ADI erec- rotor axis to be in line with the CG. In the flight crew. tion mechanism on start up also erects time (some erection systems will pre- This description is not about a poor- the rotor to the perpendicular position cess at a slow rate of less than three ly manufactured or serviced ADI. I am from the surface of the earth—in line degrees per minute), the erection as- referring to one that has been manu- with the CG position it has been cali- sembly will correct any error that had factured or serviced to meet all the brated to operate in. been induced by actuating the caging specifications that are required by the The exhausting air of the pneu- process. FAA to certify it airworthy. This ADI matic erection vanes (Figure #3) or During manufacture or servicing of will never operate more accurately the controlling mercury switches for these ADI gyros, balance of the gim- than it did when the manufacturer or the torque motors (Figure #2) on the bals is used to compensate for some service shop last tested it. The quality gimbal axis is the choice for most ADI unavoidable friction of the axis bear- of this ADI indicator can only decline rotor erection systems. Regardless ings. While high precision bearings with age. of what system is used, the erection and special lubricants are used in all As a technician, did you ever have a system is continually correcting the three gyro axes, there is still some fric- pilot report an attitude directional in- rotor axis to be in line with CG. ADI tion to deal with. The rotor and gim- dicator (ADI) error that he or she had with a pneumatic driven rotor uses bals are precision balanced using qual- encountered in flight that you could the pneumatic vanes, while we find ity balancing equipment and skilled not reproduce with the aircraft on the both the pneumatic vanes and torque operators. ground? Or as a pilot, have you ever motors used with ADI having electric To compensate for this friction, the been unable to duplicate on the ground driven rotors. The electrical ADI lacks bearing axis weight or the static bal- what you thought to be a faulty ADI the airflow used to power the rotor in ance of the gimbals supporting the indication while in flight? Maybe the pneumatic type. Instead, it uses an gyro is moved to an unbalanced posi- some of the above information can impeller developing air pressure that tion. This works well when the gyro is shed some light on why we have some flows over the rotor, cooling it, and tested in the factory or repair station, of these conditions. exhausts through pneumatic erection where the center of gravity is perpen- Several years ago, I was giving a vanes (Figure #1). dicular to the surface of the earth, and lecture to a group of aircraft pilots. In Pneumatic driven rotors gain their we have a G force of 1. With this gyro reference to the ADI, I made the state- operating speeds at a very slow rate. mounted in the aircraft, and while fly- Continued on following page

AVIONICS NEWS • SEPTEMBER 2005 55 ATTITUDE DIRECTIONAL INDICATOR Continued from page 55 ment, “The ADI has the privilege of being the aircraft instrument with the highest failure rate. Any instrument rated pilot that is not capable of fly- ing the aircraft without using the ADI should not have an instrument rating.” This brought many comments, and not too many in my favor. Several pilots said they had been flying for years and never had an ADI fail, and would be lost without it. I wonder if some of these brave pilots are still with us. Af- ter several years in the servicing end of many gyro instruments I will still stand by the statement I made to these pilots. While the ADI is less than perfect, it is the best we have (to date) for do- ing what it does. After many years of service and many fine tuned modifica- tions, the ADI is a very reliable flight instrument. With the proper knowledge of the ADI limitations a pilot should be able to determine if it is operating within its designed specifications. It is a hard sell trying to list all the short- comings of the ADI to the pilots, with- out destroying their overall confidence in their flight instruments. With proper care, all our calibrations of the ADI made by the manufacturer or repair station will remain intact after it is properly installed in the aircraft. As soon as the aircraft starts to taxi, the calibrations that were made during manufacture or overhaul are no longer valid. The “G” force and the center of gravity on the aircraft and instrument have changed. ❑

56 AVIONICS NEWS • SEPTEMBER 2005