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1. 07:11 AM - What is zm Overhaul? (George Coy)
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| Subject: | What is zm Overhaul? |
M14P overhauls
There is a lot of information and mis-information as to what is an overhaul
on the M14P series engines.
I hope the following is useful in determining when an overhaul is required
and what an overhaul consists of.
First, it is necessary to note that when operating the engines here
in the U.S. the engines are operated just like any other aircraft that is in
non commercial use. That is they are operated "On Condition". This means
they are subject to an annual condition inspection and the mechanic makes a
statement that the engine is airworthy at the time. The manufacturers
recommended time between overhauls is a guide to the mechanic and is not
mandatory.
If you look at the Lycoming and Continental manuals and service
bulletins, they list their overhaul times as "recommended time between
overhaul" They also have a recommended calendar period between overhauls.
The Russian design bureau originally gave the M14P 360 hp engine
both a calendar life of 5 years and an operational life of 500 hrs time in
service between overhauls. This means an overhaul was required every 5
years. This was true even if the engine sat in a crate for those 5 years and
was never operated.
This has been later modified to allow longer times in storage. They
additionally allowed 500 hours of operation between the overhauls and a
maximum life of the engine of 2250 hours. The fourth overhaul allowed only
250 hours as the crankshaft and the crankcase have a maximum life of 2250
hours and the overhaul facilities were not authorized to replace certain
parts. These periods have been officially increased in recent years. The new
engines are allowed 750 hours and if on a Yak-18T allowed 1000 hours before
the first overhaul.
Please keep in mind that these periods were developed based on using
the Russian oils available at the time. They were also based on a pattern of
heavy use in aerobatics. The use of western oils, regular oil changes and
lower use in heavy aerobatics allows us to expect a longer life between
overhauls. At this time we are recommending 1200 hours as a guide to
consider overhauling the engines. This is based on the experience of many
operators here in the U.S.
So what did a factory overhaul consist of?
The factory overhaul has a listing of mandatory parts to be
replaced. They also had a listing of parts to be NDT tested, a listing of
parts that needed to be dimensionality checked.
The engines were disassembled, the NDT work was performed, the measurements
were made and rework of some parts was carried out if needed. The parts
which were not complying with the technical specifications from the overhaul
manual were replaced. The engines were reassembled and "run in" on a special
stand that used an electric motor to turn the engine over while the metallic
content of the oil was examined as it was turned over. It was then put on a
test stand for the preliminary test. It was then removed and disassembled
again to check the wear patterns from the run in. It was then reassembled
tested, and the data for the overhaul page collected. Then the engine was
preserved with special oils and hermetically sealed in a plastic bag with
desiccant bags.
There are 132 parts on the mandatory replacement parts list. They
include simple items like safety tabs, gaskets, seals as well as some more
complex items like bearings, ignition wire and the piston rings.
The NDT testing includes 67 parts for Magnetic particle inspection,
12 parts for florescent dye penetrant inspection, 9 parts for X-Ray
inspection and 4 parts for pressure testing.
There are 68 parts that require dimensional inspection and possible
rework to standard or oversize dimensions.
During the reassembly there are many measurements of fits and
clearances and even "bluing" on some gears to check the fit.
The engine is then run in, tested twice and preserved.
To the best of our knowledge there are very few facilities making the
overhauls per the original requirements.
I hope this clears up many misconceptions regarding the M14P engines.
George Coy
MotorstarNA.com
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