Inspection of Control Systems
(This Airworthiness Notice supersedes AN No. C010 Edition 1 dated 10 October 1997)
Despite widespread educational efforts and the introduction of special inspection requirements, incidents of faulty control rigging continue to occur. In an effort to eliminate such occurrences, this notice is issued to explain the regulations applicable to the maintenance of engine and flight controls and to outline the applicable standards for control system maintenance. The current revision of the notice is in response to incidents that occurred since the original notice was published. The information in this notice may in future be incorporated into Standard 571 of the Canadian Aviation Regulations (CARs), at which time the notice will be withdrawn. For the purposes of this notice"flight and engine controls" includes all controls by which the propulsive force or flight path of an aircraft can be altered, and includes both primary and secondary controls.
Background and Principles
In Canada, the maintenance of engine and flying controls has traditionally been treated differently to other maintenance activities. The specific requirements have undergone several changes in recent years, but all the various developments have had one feature in common-the requirement for a "second set of eyes" to check the work after a control system has been disturbed. This additional inspection is often referred to as "the dual inspection" or "the independent check". The most recent changes, introduced with the Canadian Aviation Regulations (CARs) are intended to retain the benefits of this additional inspection, while at the same time reducing any tendency for complacency that might be introduced by the knowledge that another inspection will take place. To this end, while the current requirements still call for two signatures, the roles of the two signatories are quite distinct.
The primary signature is the maintenance release. This is a certification, issued pursuant to CAR 571.10, attesting that the described maintenance has been performed in accordance with the applicable airworthiness requirements. One of those requirements is outlined in a table of section 571.10 in Standard 571 of the CARs. It calls for the system to be inspected for correct assembly, locking and sense of operation, by at least two persons, both of whom must sign in the technical record. For simplicity, the maintenance release itself may constitute one of these signatures. The maintenance release may not be signed until the dual inspection has been completed and signed for, unless a suitable procedure is in place to ensure that the aircraft cannot be released until this has occurred.
The person signing the maintenance release assumes full responsibility for the satisfactory completion of the work. The responsibility is not "shared" with the other signatory. As a maintenance release, it can only be made by an appropriately rated AME (or, in the case of work performed abroad, by an equivalent person recognized under a bilateral agreement). In addition to ensuring that all the relevant regulatory and technical requirements have been met, the AME must also be satisfied that the person conducting the independent check is qualified to do so.
The signature of the person performing the independent check attests to the satisfactory completion of a quality assurance inspection. In keeping with the CARs principles of focused accountability and independent quality assurance, this activity forms no part of the direct performance or certification of the work. It is not a final approval of the work, as a traditional quality control signature would be. Instead, it indicates that an independent review of the work has been completed, and that no deficiencies have been found. It differs from other quality assurance activities in only one respect-it must be completed before the maintenance release can take effect. Since this signature is not a maintenance release, it is not restricted to licensed AMEs. However, the person undertaking this responsibility must be suitably qualified to do so. Where the work is being done under the control of an Approved Maintenance Organization (AMO) this qualification may be restricted to holders of full Aircraft Certification Authority (ACA). Alternatively, it may be granted to individuals who do not hold a full ACA on type but who have qualified for control system authority after completion of an applicable training program approved under CAR 573.06.
Where no AMO is involved, the AME signing the maintenance release must directly assess the qualifications of the person performing the independent check. In making an assessment of these qualifications, the AME must take into account the individual's training and experience. Completion of a Transport Canada approved course in aircraft maintenance, followed by documented proof (such as entries in an AME logbook) of satisfactory participation in similar control work would be acceptable. In the case of a pilot, an entry in the pilot's logbook by an AME, attesting that the pilot had satisfactorily completed similar control inspections under supervision, would also suffice. The AME may also consider other forms of proof that provide an equivalent level of assurance. These qualifications must, however, be assessed and accepted before the work in question takes place. It is not satisfactory for the AME signing the release to show the person performing the independent check how to perform the inspection at the time the work is completed. Such a procedure would not provide sufficient confidence that the person performing the independent check was capable of independently detecting errors overlooked by the AME.
In the case of AMOs routinely performing heavy maintenance on large aircraft, the procedures for controlling independent checks must be documented and referenced in the MPM. This is especially true when several sub-tasks affecting the same control system may be performed over multiple shifts. To permit the controlled sequential closing of panels, the checks for assembly and locking of a system may be broken into separate sub-tasks, each signed-for individually, with a final check for sense of operation and range of travel of the system as a whole being done on completion of the work. The work record for each sub-task must cross-refer to the task card for the full system check.
Scope of Inspection
Like all other maintenance, control system inspections must conform to the performance rules outlined in CAR 571.02. In particular, this applies to the requirement to follow the type certificate holder's Instructions for Continuing Airworthiness (ICA) paying attention to variations in wiring diagrams and component part number between aircraft serial numbers of a particular model. However, the type certificate holder's instructions are usually limited to the particular features of the aircraft type, and assume a level of basic knowledge and standard practice on the part of the AME. This section will attempt to summarize the main points to consider when inspecting aircraft control systems that have been disturbed by maintenance. This is not intended to be a comprehensive list. The technical standards and procedures outlined are general in nature, and are intended to supplement the ICA. Where these general procedures are in conflict with the ICA, the type certificate holder's recommendations shall prevail. When checking control systems that have undergone maintenance, the person signing the maintenance release and the person performing the independent check should consider the following points independently 1:
- All those parts of the system that have actually been disconnected or disturbed should be inspected for correct assembly and locking.
- The system as a whole should be inspected for full and free movement. This check should take into account the effects of airframe flexing in flight, the effects of occupants, cargo and baggage, and the full range of positions of other movable items (e.g. ensure that full rudder deflection does not interfere with elevator up travel, and vice-versa).
- With cables tensioned as specified in the ICA, and the primary control stops in contact, there should be adequate clearance at the secondary stops.
- Due to variations in design, rotary wing aircraft are extremely vulnerable to control system maintenance errors. In particular, vertical and lateral vibrations may have a critical effect on flight control clearances in flight. What may appear to be acceptable linear movement in a static position, may very well become unacceptable under certain high power, high torque conditions.
- The range of movement of the controls should be as specified in the ICA or on the aircraft type certificate, as applicable.
- The operation of the control system as a whole should be observed to make absolutely certain that the controls (including, in the case of flying controls, each individual control surface) are operating in the correct sense (i.e. that the devices concerned respond in a direction consistent with the desired intent of the control input). This check is the single most critical part of the entire procedure, and there can be no excuse for not completing it correctly.
- Some turbo-propeller-powered aircraft have a single control for the propeller and the fuel system, with the relationship between the two determined by electrically controlled trim devices. These controls should be checked through the full range of available trim.
- Some controls rely on the end stops for specific position settings. Others have the settings determined by adjustment of the control rod or cable length-make sure you know what kind of adjustment applies to the control you are dealing with.
- Trim tabs, balance tabs, anti-balance tabs, spring tabs and servo tabs all have distinctly different characteristics. Make sure what kind of tab system you are dealing with and check that it operates as specified in the ICA. Remember to check that geared tabs operate in the correct relationship to the main control surfaces.
- Some control systems are duplicated to provide redundancy (i.e. the systems are in parallel, so that either system can operate alone). Check each system separately, as well as checking both systems together. Check the operation of the control disconnect devices, and ensure after the check that the devices are properly re-engaged. With some engine controls that employ an electrical system to provide redundancy for a mechanical system, checking the electrically operated system may involve disconnection of the mechanical system. Following reconnection, the check on correct assembly and locking must be repeated.
- Some control systems have units that are duplicated to guard against inadvertent operation (i.e. the units are in series, so that both must be serviceable for the system to work). Dual element elevator trim switches to protect against trim runaway are an example of this type of system. These systems must be checked to ensure that closure of a single switch will not cause the system to operate.
- Some duplicated systems are intended to allow input at one pilot station to cancel out input at the other in certain circumstances (e.g. runaway trim protection and some fly-by-wire systems). The operation of these features should be checked as specified in the ICA. In the case of fly-by-wire systems, the operation of the systems must be verified from each pertinent flight crew station by observing that the respective cockpit indication and actual system's output are the applicable responses. It is equally imperative to confirm that interaction between the flight crew stations is in accordance with the applicable standards for the type and model. If special test equipment is specified to verify the operation of the systems, precautions must be taken to ensure the equipment used is applicable to the aircraft configuration.
- Different control systems may be interconnected so that they affect each other. In helicopters, these include collective/cyclic control interaction and linkages between the collective and power controls. In aeroplanes, they include rudder/aileron and nose-wheel-steering/rudder interconnections. Flap position may be designed to alter the operation of spring tabs or spoilers, or to modify the range of aileron movement. All these interactions must be checked through the full range of the applicable controls.
- When working on systems that are equipped with position indicators, determine if the work has affected their operation and, if so, observe the operation of the indicators to ensure they correspond with the actual control positions.
- On completion of the independent check, all tools and measuring devices, including protractors, inclinometers, tension meters, rigging pins, and part-power stops, etc. should be removed and accounted for, and the area cleared of all foreign objects. All access panels should then be replaced, after which a final check for full and free movement should be carried out.
- Some control adjustments can only be checked completely by flying the aircraft. In such cases, the maintenance release may be made subject to the satisfactory completion of a test flight, pursuant to CAR 571.10(4). This type of release is only permitted where all checks that can be carried out on the ground have first been satisfactorily completed, and only those items that cannot be verified without flight are outstanding. When a test flight is required under these provisions, a total of three signatures will be involved. These must be completed in the following order:
- Independent check.
- Conditional maintenance release.
- Pilot's acceptance signature (following successful test).
If the test is not satisfactory and further adjustments are required, then the whole procedure (including both inspections of the items affected) must be repeated.
Extent of Inspection
Both the AME signing the maintenance release and the person performing the independent check must separately decide on the extent of the inspection required, depending on the type of control and the nature of the disturbance. The independent check need not cover the same range of detail as the check leading to the maintenance release. However, as a minimum, it must include an inspection for correct assembly and locking of any parts of the system disturbed by the maintenance performed, an operational check for proper sense and range of operation.
Checking Sense and Range of Operation (both inspections)
It is usually not possible for the person checking the range and sense of operation to observe both extremes of the system at the same time. It may be necessary to involve two persons working as a team, one in the cockpit operating the controls and one at the control surface or other device, observing the effects of the control input. Some AMEs have assumed that where the other person involved is the one responsible for the independent check, this team effort meets the requirement for both inspections. This assumption is risky, as it may not be possible for the person at the control surface to be certain what control input has been applied, neither is it possible for the cockpit occupant to be sure that the movements observed by the outside observer are the ones intended. There is a serious potential for errors in communication. To reduce the likelihood of such errors, after completion of the check the individuals should reverse their roles and carry out the check a second time, each now observing the other end of the system.
Exemptions to the Requirement for Second Inspection
The standards call for two separate signatures whenever an engine or flight control system is disturbed. In this context, "disturbed," means actual disconnection, adjustment or disruption of the system itself. It does not include adjustment of travel stops, and simple adjustments of this kind do not require a dual inspection. The removal and installation of co-pilot control wheels, and rudder pedals that have been designed for rapid replacement without the use of hand tools, is also exempt from the need for two inspections, as is the replacement of sidesticks, electronic flight control computers and full authority digital engine control computers, provided the electrical connectors include provisions to protect against incorrect assembly.
When checking for sense of operation, many technicians rely on mnemonic devices or rules of thumb to guard against errors. These aids should be used with caution, as they may not apply under all circumstances. For example, one common rule of thumb is to check that "the control surface rises to meet the stick". Unfortunately, with the rudder, the control surface does just the opposite-it moves with the pedals, not in opposition to them. Furthermore, with high-wing and high-tail aircraft, even the aileron and elevator surfaces move away from the control. An additional complication arises in aircraft that have servo controls, where the operation of the servo tab (often the only surface that moves) is in the reverse sense to the main control.
A better approach is to visualize the aerodynamic and other forces acting on the controls and mentally follow the resultant chain of events. In the case of an elevator control, this method would involve the following thought sequence "Let's see, stick back, control surface moves up, that will tend to push the tail down, which brings the nose up, so the aircraft climbs-OK". While this may seem simplistic, it has the very real advantage of reducing the task to its absolute basics, and may just detect an error that would be overlooked in a more sophisticated procedure.
Another important point to remember is to check all the surfaces affected. Cases have occurred where only one aileron was observed, and was found to be operating correctly, while the aileron on the other wing was rigged wrongly, and was moving in symmetry with its partner, instead of in opposition.
Signing a maintenance release for an incorrectly assembled control system constitutes a direct violation of CAR 571.10, and may lead to suspension of the AME's license pursuant to 6.9(1) of the Aeronautics Act. The AME's supervisor, if aware that the proper procedures were not being followed, could also be liable under the same regulation. An AMO certificate holder who knowingly allowed employees to use unacceptable practices in the conduct of control inspections could also be proceeded against pursuant to 8.4(4) of the Aeronautics Act. The absence of an adequate training program or quality assurance program that addresses control checks could lead to suspension of an AMO certificate pursuant to 7.1(1) of the Aeronautics Act. Signing for an independent check of wrongly assembled controls is not an offence in itself, but may constitute evidence of incompetence, leading to suspension of a license pursuant to 7.1(1)(b) of the Aeronautics Act. Finally, where faulty control rigging results in an accident, all those involved could be open to charges of negligence.
While aircraft control systems themselves are often extremely complicated, the kinds of errors in the assembly of these controls that lead to accidents are often extremely simple, so much so that, with hindsight, it can be difficult to see just how the oversight could have occurred. These are simple human errors of the most basic kind, involving poor communication, inattention, distraction, faulty assumptions, and overlooking the obvious. Of all the problems encountered in aviation maintenance, these are among the most avoidable. If all of us involved in the maintenance of control systems were to simply resolve to treat the task with the attention it deserves, regardless of how simple it may appear, control-rigging accidents could be completely eliminated.
1 Not all these points will necessarily apply in a given case - the AME should consider which ones are applicable, together with any other aspects not addressed here.