We all know what TBO means. Time Between Overhauls. But is it?
The word overhaul implies replacing the worn parts but what
also happens is the steel moving parts are Magnafluxed for cracks.
The steel parts are connecting rods, camshaft, valves, valve springs,
etc., and crankshaft. These parts are subjected to reversing stress that
leads to metal fatigue.

It should be called TBI or Time Between Inspections.

This TBO/TBI concept has been developed over the years based
on vast experience with aircraft engines. Sooner or later they all
wear out or one or more steel parts break. The FAA initially
assigns about a 1000 hour TBO/TBI or so to a new aircraft engine design.
This decision is partly based on the measured wear rate after a standard
test of the parts subject to wear and a educated guess on the life of the
steel parts based on the calculated stress. After many engines
have been used in actual service for over 1000 hours the FAA may increase
the TBO/TBI time. The O-320 Lyc is good for 2500 hours. The earlier Cont.
O-470's were 1500 hours.

If the steel parts are not cracked they are put back in the engine. But is that
a good idea? Just because a part did not break or crack in 2000 hours does not
mean it will not crack or break in the next 2000 hours. My O-470 had a 1500 hour
TBO/TBI. When I bought the plane the engine  had been overhauled at least once.
I put another 1000 hours or so on the engine. I then took it apart at 3000 hours
to do an overhaul. My wife Robin is an A&P so it is legal for us to do an
overhaul. We sent the crank and rods to Pacific Continental to be Magnafluxed.
They told us the crank was a mass of cracks and needed to be replaced.

We checked the extraordinary price of a new crank and decided to forget the over
haul. Continental had a good trade in policy at the time so we threw it back
together and took it down to Alabama for a re manufactured engine with a 2000
hour TBO. The difference was a another pendulum damper on the crank bringing the
total up to two. Only $500 dollars additional. Obviously a major factor in the
life of the crank is how it deals with torsional vibrations. Torsional
vibrations can amplify the stress and deflection of a crank shaft. I digress.

The bottom line is one should buy a new aircraft piston engine instead of over
hauling the one you have to be on the really safe side.

In Europe Thielert and Austro Engines AKA Diamond Aircraft  build "aircraft"
engines based on Mercedes A class diesel engines. The A class is the bottom of
the Mercedes line. What the automotive industry calls an "econo box". I think
what happens is the new Mercedes engine is completely disassembled and all parts
checked for cracks and dimensions and then reassembled. This is to insure
quality control as there is no detailed quality control paper trail on the
individual parts as found in the real aircraft engine manufacturing community.

The European FAA equivalent then assigned a 1200 hour absolute life. The entire
engine had to be thrown away after 1200 hours. No overhaul was allowed. I think
this was later modified and the worn parts were allowed to be replaced at 1200
hours until 2400 hours and then the engine had to be thrown away if cracks appeared.

As I recall Rotax engines require the crank and rods to be thrown away
at over haul time.

If I were using a US automotive piston engine for aircraft use other than the
rotary I would have it inspected at 300 hours regardless of the wear if any.
Automotive engines are designed to run for at least 200,000 miles at the typical
automotive duty cycle. Cruise at 80 MPH (50 HP) 90% of the time with an
occasional WOT acceleration and hill climb. During the WOT acceleration run the
engine reaches full power for only a fraction of a second and then the
transmission shifts dropping the RPM to well below peak power. Usually it is
all over in less than 10 seconds.

By contrast the aircraft engine is required to generate full power for 5 minutes
several times and generate 70% max power for the rest of the time in the
standard 150 hour FAA test cycle. Think about what that means. If you ran your
Corvette at 175 MPH (70% power) for 2000 hours you would have covered 350,000
miles.  The typical NASCAR race is 500 miles and they don't use standard parts.
All parts are aircraft quality or better. (Titanium rods for on example.) Don't
ask the cost!

The Wankel rotary is an exception because there are no reversing loads on the
steel parts. Basically that means the parts in theory will last forever and are
not subject to fatigue. We are really lucky to have such a low cost, long
lasting engine for our experimental aircraft.

Paul Lamar



What an excellent summation of the reliability of every type of engine.

Van