Plastic Pilot

When evaluating the value of an aircraft, one of the key factor is the TBO: Time Between Overhauls. Engine and propeller manufacturers define maximum time-in-service. The closer the engine / propeller is to this maximum age, the lower the plane value. This is rather logical, as the new owner will have to replace them sooner as if the time in service was lower.

This TBO story could be simple, but rules (both FAA and JAA) gives some flexibility to private owners and operators. As this includes all non-commercial operations, flying clubs can rent aircrafts which fly beyond their engine’s TBO. This is the open door to an interesting debate: on time maintenance vs. on condition maintenance.

On one side, the “on time maintenance” clan. Engine reaches its overhaul time ? Replace it by a new one. Dot. There is a reason why manufacturers define a limit, isn’t it ?

On the other side, the “on condition maintenance” clan. Basic rule: why replace an engine that runs well ? Obviously, “runs well” do not simply means starts and operates well. A serious “on condition maintenance” policy it typically based on things like:

• How the engine has been operated
• What kind of maintenance has already been done
• Oil and oil filter inspection
• Compression reports
• Spark plugs inspection

The required analysis are not for free, but they remain low when compared to the cost of a new engine. Because of my technical background, I’d tend to prefer on condition maintenance… if I only could afford to own a plane

Thanks to their black-box / monitoring role, the FADECs do help to gather information about how the engine was operated, and about general engine condition… but not for the Thielert engines. The Thielert maintenance plan calls for an inspection at 1′200 hours, and replacement at 2′400. No other way - not even a full revision.

One of the reasons is that given the high pressures within diesel engines, they are made of a single bloc of metal. It’s not possible to replace just a cylinder, as some do with classical engines. This leads directly to the second argument in favor of “maintenance on condition”: most defective engine parts will fail within the first hundreds of hours. Why introduce the risk of “youth problems” in an engine that ran well for several hundreds of hours and gives no sign of trouble ?

I should may be ask an aviation lawyer, because the exact consequences of an incident with an engine flying legally with a post TBO engine are not clear. If you have any hints on this, or more generally on this topic, feel free to comment !
Read more on this topic:

Aircraft Maintenance Technology online

AvWeb Columnist Mike Busch, fan of overhaul on condition

Thielert Centurion engines (search for TBO)