Airworthiness Notice - B041, Edition 4 - 31 March 2005

Piston Engine On-Condition Maintenance Program Requirements

(This Airworthiness Notice (AN) supersedes AN No. B041, Edition 3, dated 18 September 2002)


The purpose of this notice is to provide guidance in the development of on-condition piston engine maintenance programs. Air Operators and Flight Training Unit Operators may incorporate such programs in their aircraft maintenance schedules as an alternative to the manufacturer's recommended "hard time" programs.

It was evident from our CASI experience that the program was not being used as envisioned, for an extension to a useful life, but has been extended, beyond its intended purpose, to an overhaul avoidance program. Due to these misunderstandings on the application of this particular AN, some wording changes were added to reinforce the concept behind this program. In addition, some changes were made to the numbering to make the references more understandable.

Canadian Aviation Regulation Requirements

Canadian Aviation Regulations (CAR) 605.86, and Standards 625, Appendix C.


Many factors affect the wear which takes place in an engine, including the efficiency of the air filter, the techniques used in the engine handling, particularly during and immediately after starting, the type of fuel and oil used in the engine, the conditions of storage when the aircraft is not in use, the length of flights, the atmospheric conditions during flight and on the ground, and the type of operation. Many of these factors are within the control of the operator and the contracted Approved Maintenance Organization (AMO). Continued compliance with the Pilot Operating Handbook, Aircraft Flight Manual, Engine Operating Manual, and the maintenance practices recommended by the manufacturer, may help to extend the time between overhauls (TBO).

The application of this program does not eliminate the need for periodic engine overhaul. This program is a means to extend the useful life of an engine to the point where routine maintenance can no longer ensure the continued safe operation of the engine and overhaul becomes necessary. This consists of repetitive periodic inspections to determine a component's condition so that it can be removed from service prior to failure.

Program Approval

Air Operators and Flight Training Unit Operators must obtain approval to use this on-condition maintenance program in the form of an amendment to their approved maintenance schedules.

A review of all recommendations issued by the aircraft and engine manufacturers, in the form of service publications, and other documents, (i.e. service bulletins, instructions, letters, etc.) shall be completed by the person responsible for maintenance (PRM). The PRM must review the recommendations to determine whether incorporation is appropriate to the new program.

Where necessary, the PRM may obtain technical advice from an AMO familiar with the operator's aircraft and operation.

Program Implementation

Prior to transfer of an engine from "hard time" to "on-condition", an assessment of baseline condition must be made, against which the engine's future condition may be checked. This assessment must be made before the engine reaches the manufacturer's recommended hard time TBO. Once established on the program, accurate records must be maintained.

It is not intended for this program to be used only when an aircraft approaches TBO. An operator is either on-condition or hard time. To have two different programs for the same type of aircraft, the operator would have to demonstrate sufficiently different roles for the aircraft.

At the commencement of the program the operator must arrange for an AMO to perform the following tasks (references are to the "Program Notes" section below):

  1. Run the engine to ensure compliance with the parameters established by the manufacturer (i.e. Type Certificate or Pilot's Operating Handbook) and to establish the reference RPM, or similar datum for power checks as described in "Program Notes" (5) (power checks to be taken after completion of any inspections or repairs to the applicable engine/module);
  2. Perform a complete inspection of the engine in accordance with requirements of the Maintenance Schedule (engine and nacelle);
  3. Check and record each cylinder's compression by the methods described in (4);
  4. Perform a leak check of all cylinders, as described in (9);
  5. Inspect engine oil filters and oil screens for evidence of internal damage or wear, as described in (8);
  6. In the case of an engine that has completed more than 75% of the manufacturer's recommended TBO, remove one cylinder per bay (horizontally opposed engines) for internal visual inspection of crankcase assembly, all camshaft lobes and lifter bodies, etc.;
  7. Inspect the interior of each cylinder, as described in (2);
  8. Implement any oil spectrophotometry or spectroscopic analysis program recommended by the engine manufacturer. (Where the manufacturer has made no such recommendations, this task is at the operator's discretion); and
  9. Commence record keeping; the air operator should keep records of oil consumption, etc. Refer to Appendices A and B for some example forms that can be utilized.

Repeat Inspections

At intervals of 25, 50, 100 and 200 hours air time, the AMO should carry out the following inspections/tests (in addition to other requirements in the Maintenance Schedule). Inspection sheets to cover the following will be developed by the air operator and used by the AMO. (References are to the "Program Notes" section below.)

Aircraft operators who have an approved maintenance schedule that incorporates this on-condition maintenance program, may, through the use of their maintenance schedule approval process, extend the frequency of the 'Repeat Inspections' criteria, or the established threshold of their introduction. This would be based upon demonstrated reliability data through the operator's reliability program.

25-hour Inspection Requirements

  1. Inspect oil screens for evidence of internal damage or wear for aircraft not equipped with filter media oil filters (full-flow or paper type oil filters).

50-hour Inspection Requirements

  1. Inspect oil screens and filters for evidence of internal engine damage or wear;
  2. Check the record of oil consumption as described in (3);
  3. Check and record each cylinder's compression by the methods described in (4); and
  4. Check the reference RPM as described in (5).

100-hour Inspection Requirements

  1. Inspect the interior of each cylinder as described in (2); and
  2. Perform a cylinder leak check in accordance with (9).

200-hour Inspection Requirements

  1. Remove rocker covers for inspection of valves, springs, rocker arms, shafts, etc.

Program Notes

  1. External Conditions - The engine should be examined externally for obvious faults such as cracked crankcase, excessive play in the propeller shaft, propeller out of track, overheating and corrosion. Check engine mounts, baffles and exhaust system for any signs of excessive vibrations.
  2. Internal Conditions - Inspect the interior of each cylinder for scoring of the cylinder bores, pitting of piston crown, cracking, erosion or distortion of valve seats, and general condition.
  3. Oil Consumption - Inspect records to ensure that maximum allowable consumption is not exceeded, and to verify any abnormal oil consumption. Either situation is cause for further investigation.
  4. Compression Check - Piston ring and cylinder wear, and poor valve seating could result in a significant loss of power. A cylinder compression check is a method of determining, without major disassembly, the standard of sealing provided by the valves and piston rings. Inspect in accordance with the current recommendations of the manufacturer.
  5. Maximum Static RPM Check (Power Check) - Each manufacturer states the Type Certificate information. A check against maximum static RPM should be completed whenever the engine is run up for a serviceability check. AMOs may use it for trouble shooting purposes, and it is necessary after an inspection, repair or replacement of any component, which may affect the RPM, such as a carburetor or magneto, etc. The check is made by running the engine under conditions, which permit the maximum static RPM to be observed and recorded.
  6. Although normally aspirated engines are often fitted with variable pitch propellers, the engine speed obtained at full throttle is usually less than the governed speed and the propeller remains in fully fine pitch. With supercharged engines, however, the propeller is usually constant speeding at high power settings and small changes in power will not affect engine speed. The power of a supercharged engine is, therefore, checked by establishing a reference speed at prescribed power settings.
  7. Since a supercharged engine is run at a specified manifold pressure regardless of the atmospheric pressure, corrections must be made for both temperature and pressure variations from the standard atmosphere.
    The procedure is to run the engine until normal operating temperatures are obtained, open up to maximum take-off manifold pressure, decrease power until a fall in engine speed occurs (denoting that the propeller blades are on their fine pitch stops), then throttle back to the manifold pressure described by the manufacturer and observe the engine speed.
    The correction factor to be applied to the observed engine speed of a supercharged engine may be obtained from graphs supplied by the engine manufacturer.
    Engine power may usually be considered satisfactory if the corrected speed obtained during a power check is within 3% of the reference speed.
    1. The engine tachometer system shall initially be checked for accuracy using a hand-held stroboscopic tachometer, or other approved method, and thereafter the system shall be checked yearly.
    2. Normally aspirated engines are tested at full throttle. Where a variable pitch propeller is fitted, maximum RPM shall be selected. (The operator should ensure that the propeller is at maximum fine pitch). The magnetos and engine parameters should be checked in accordance with the manufacturer's recommendations.
    3. The engine should not be taken to full throttle for periods longer than are necessary to make these observations. When recorded, the throttle should be eased back to a low RPM position to allow the engine to cool off before it is shut down.
  8. Oil Filter/Screen/Chip Detector Inspection - This inspection is to determine if there is any internal malfunction or wear occurring inside the engine. The quantity, shape, appearance, and condition of the particles found should be inspected for the type of material. A visual inspection and use of a magnet may help to distinguish between ferrous and non-ferrous materials.
  9. Cylinder Leak Check - This check serves as an identifier for conditions, which may not be detectable by visual inspection and also serves as a back-up for conditions, which may be difficult to detect because of visual limitations. This check may be accomplished as follows:
    1. With the compression tester connected, apply 5 psi to the cylinder;
    2. Position the piston as close to bottom dead center on the compression stroke as possible, ensuring that the intake valve remains closed to allow the cylinder to hold pressure;
    3. With propeller secured, increase the pressure slowly to a maximum value of 80 psi. Saturate the entire cylinder assembly with soap and water solution;
    4. Inspect the complete cylinder for leakage. Leakage may be indicated by an accumulation of bubbles;
    5. After complete cylinder inspection, relieve cylinder pressure and remove compression tester; and
    6. Perform this inspection on each cylinder.
    Note: Any cylinder that exhibits external leakage from the head or barrel structure must be replaced prior to further engine operation.

Components Included

For the purpose of this program, all components necessary to run the engine (eg. a magneto or carburetor, but not a vacuum pump or prop governor), may be included in the on-condition program, and overhauled in conjunction with the core engine..

Operators may choose not to include these components in the on-condition program in which case, they will require TBO's for these components to be approved in the Maintenance Schedule.

Failure to Meet Standards

Should the engine require repair, then that component should be overhauled or repaired to the manufacturer's requirements.

Repetitive failure of components is an indication of failure of the engine to meet standards and action must be taken to correct the situation up to and including complete engine overhaul in accordance with the manufacturer's requirements.

Records Technical Entries

A record shall be made at the commencement of the program, including the details of any inspections/checks and any rectification performed. This should be entered and certified in the applicable technical records pursuant to CAR 571.03.

Airworthiness Directives

Where compliance with an airworthiness directive is due "at next overhaul", this time is considered to be the next time the engine or component is overhauled, regardless of whether the overhaul is based upon hard time or an on-condition program.

Appendix A: Sample Form

Appendix B: Sample Form