There has been a lot of talk around high timed engines and the requirements for operating aircraft engines once they exceed their recommended overhaul interval time or “Time Between Overhaul” (TBO). I decided to take a look through the CAR’s and Airworthiness Notices and try to interpret the true requirements.

Engine overhaul intervals can vary greatly for any given model due to a number of factors such as oil change interval, oil used, use of oil filter or screen, calendar time since the last overhaul, environmental conditions (dusty, humidity, air quality, average temperature, etc), frequency of use, how it’s operated, etc. Some engines can exceed the manufacturers TBO by 100% and still be OK while others only make it to 50% of the TBO before needing to be rebuilt.

The manufacturer’s recommended TBO is based on the number of hours it takes an average engine to wear components beyond their service limits. For most engines, this typically means how long it takes for the bearings or cam lobes to wear out. With proper care and regular use, wear can be minimized in the engines bottom end. Combustion gasses will tend to wear out the top end by TBO no matter how well you care for it. However, a top overhaul is typically far less expensive and time consuming than a full overhaul. In fact, a four-cylinder engine can have a complete top end replacement in about a day’s worth of labour. If you check the logs of most engines operating beyond their TBO’s you will likely find evidence of a top overhaul in its past.

The first document I ran across is the following:

Airworthiness Notice – B041, Edition 4 – 31 March 2005 Piston Engine On-Condition Maintenance Program Requirements

Which states:

“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.”

“Flight Training Units” are fairly obvious, however, “Air Operator” is not. So, I went to CAR 101.01 for the following definitions:

“air operator” – means the holder of an air operator certificate.

“air operator certificate” – means a certificate issued under Part VII that authorizes the holder of the certificate to operate a commercial air service

OK, so the Airworthiness Notice that defines the on condition program is very specific to certain classes of commercially operated aircraft. However, it does not specifically exclude privately operated aircraft from having to adhere to a “hard-time program”.

COPA’s “Aviation Guide” section was my next stop. The guide on Certified Aircraft made reference to CAR 625 Appendix C. Looking up that reference, the following is stated:

“6. Engines

All piston and turbine engines installed in aeroplanes and helicopters operated pursuant to CAR 406, in large aircraft operated pursuant to CAR 604, and in aircraft operated pursuant to Part VII, shall be overhauled at the intervals recommended by the engine manufacturer, or in accordance with an alternative hard time interval or an engine on-condition maintenance program approved in accordance with Appendix D.

Information Note:

No hard time, including calendar time, between overhauls need be observed in the case of small aircraft reciprocating engines in non-commercial private operation.”

So, my interpretation to this is that a piston engine installed on a non-commercial aircraft can be operated beyond the manufacturer’s TBO with no change in maintenance or reporting procedures. From a policy point of view, the TBO is ignored.

These are the current regulations, but I believe that there was a time when privately operated aircraft were required to go on an on-condition program once the TBO was exceeded. Unfortunately, I could not find any evidence of this within the current CARS (some of the CAR’s include the previous versions of the regulation, but not in this case)

Although you can legally ignore the TBO once it’s reached, it may not be a good idea to do so. If the plane is based in a dry area (like Alberta), has been well maintained, has an oil filter, is flown over 100 hour per year) and the last overhaul was less than 10 years ago, then it’s probably a good candidate for continued operation. If the last overhaul was in the 70’s and it’s been flown 5 hour per year and based in Florida, it’s probably a very good idea to start budgeting for an overhaul in the near future.

The original engine that came with my Cessna 150 had 2200 hours when it was removed. However, it had been overhauled in 1991 and operated at a flight school for the first 10 years and 1800 hours. It had been topped around 1850 hours and, except for a worn thrust bearing, it showed little signs of wear. However, I was still much more comfortable when the new engine was hung on the nose! The engine gave an additional 400 trouble-free hours beyond TBO, but it was certainly the right time to so the swap.

I hope this helps clear up the “on-condition” issue with certified aircraft.