Yak 18T Flight Operations Manual
Page 27 of 82
Disclaimer: This manual is to be used as a reference only, it is based on translated Yak 18T Flight Operations
Manuals and has not been approved by the Yakovlev Design Bureau or any other authority.
Section 5 – Engine Start Preparation
The following text provides an overview of Hydraulic shock – if you have not read it before,
you are urged to read it entirely before continuing. If you have read it before, it is still worth
reading it as a reminder.
5.1. Hydraulic Lock/Shock Avoidance
This section kindly provided courtesy of Richard Goode Aerobatics (
www.russianaeros.com
)
The importance of preventing hydraulic lock can never be underestimated. The
consequences are at best expensive, at worst –
catastrophic!
One fatal accident was
caused by hydraulic lock, leading to total engine failure and a subsequent stall/spin.
5.2. What is Hydraulic Lock/Shock?
In the normal functioning of the internal combustion engine, the piston moves towards the
top of the combustion chamber. (To avoid confusion we should realise that we are talking
about the lower three cylinders on the M-14P, and therefore this is effectively at the bottom,
at least in relation to the ground!) If, however, a proportion of the combustion chamber is
filled with a liquid – in this case oil, which is incompressible - something must give. If the
engine is turning with any degree of inertia – even being pushed through by the air-starter
(although hand pulling cannot cause problems), the weakest part of the entire assembly is
the connecting rod.
Thus, when we attempt to start an engine with oil in the bottom of the combustion chamber,
and it certainly doesn’t need to be full, this will initially increase the compression and, at its
extreme, simply results in the piston being unable to progress further, hence the connecting
rod bending and shortening.
If you have any doubts about oil descending in these engines, you only have to see the
amount of smoke following a start-up to realise that every engine has a fair amount of oil in
the cylinders – but we of course are talking about more than that. A fundamental cause is
that the pistons are made from an aluminium alloy to a very high coefficient of expansion.
This means that, as it heats up, it expands to a much larger size and, in order to tolerate this,
when cold it is quite significantly smaller than the piston bore, allowing oil in the engine to
drain past the pistons, through the piston rings, and then down into the bottom three
cylinders.
The other issue is that the M-14P has a dry sump, i.e. a separate oil tank, from which oil is
sucked to feed the engine, and to which it is returned by the scavenge pump. Normally a
valve prevents oil from the tank, which is of course higher than the engine, descending into
the engine unless the oil pump is turning. However, this can become stuck, even by a small
piece of dirt, and particularly over a period of time the entire contents of the oil tank can
migrate into the engine and, inevitably, will end up in the lowest place – the lower cylinders.
5.3. The Consequences
Once the connecting rod has bent, it is inevitable that it will ultimately break, although
amazingly engines often continue to run with a bent connecting rod for a number of hours.
In the shorter term however, the bottom piston ring (of course, with inverted cylinders, the
one furthest away from the ground), comes very close to the end of the cylinder at the
bottom of its stroke, i.e. geographical top). With a bent connecting rod it is quite likely that
the final oil-control piston ring will jump out of the cylinder. Then of course, when the piston
is forced back into the cylinder, the piston ring will break, which initially will lead to high oil
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