Yak 18T Flight Operations Manual
Page 28 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.
consumption, and almost inevitably to pieces of the ring being found in the front oil-filter,
with an indication also from the chips-in-oil. If nothing is done at this stage, the connecting
rod will break.
The consequences of this can be truly dramatic, in one case the main crankcase bolts shot
forward not only through the gills (quite easily), but also right through the passing propeller
blade!
It is also quite likely that mechanical mayhem on this scale could cause the engine to seize
totally, and if this happens suddenly it will very likely tear itself out of the frame, with a
consequent problem of a quite serious shift of C of G to the rear.
5.4. What can be done?
The obvious answer is to not start the engine with oil in the bottom cylinders. What one
should realise, however, is that there is oil not only in the cylinders, but also in the three
bottom intake-tubes. An examination of the engine will show you that a tail-wheeled aircraft
(e.g. Yak-50 or Sukhoi) will have a much higher tendency to collect oil in these tubes than a
nose-wheeled aircraft.
For this reason those owners who think that merely by turning the propeller through 20 or
whatever blades before starting, they are totally avoiding hydraulic lock are deluding
themselves. Certainly this will dislodge oil already in the cylinders, but it will do nothing to
remove the oil in the intake tubes, which immediately after starting, is sucked into the
engine, then possibly causing hydraulic lock.
For this reason the designers put drain plugs in the three bottom cylinders, as well as
thoughtfully providing a special spanner with which to remove these plugs. The problem with
this is that it does take some time to remove the lower engine cowl and drain these intake
tubes. However, you ignore this at your peril.
Thus, for the original operators of these aircraft, as soon as an aircraft had not been used for
a day or so, it was absolutely routine to undo the cowls and drain oil out of these intake
tubes. Few of us however have our trained mechanic to do this before flying, and all too
often it is neglected.
A final way of minimising the problem is to ensure that every engine is run for at least 30
seconds at 60% before shutting down (remembering of course that the engine should not be
shut down at above 1500 cylinder head temperature), which will scavenge excess oil from the
engine into the oil tank, as well as get rid of oil in the supercharger, which is after all, the oil
that ultimately drains into the intake tubes.
5.5. Other Solutions
Given the reluctance of the average pilot in the West to spend more than a couple of minutes
with an aircraft before flying, it is inevitable that people have been thinking of ways of
banishing hydraulic lock.
Sergio Dallan
method
Sergio Dallan, an Italian Sukhoi owner and an exceptional engineer, pondered the problem
at length, and his solution comes in two parts:
i) A master oil-tap at the bottom of the oil tank, which is (thankfully) connected to a warning
light and an external visual signal, whereby after flight the oil supply is switched off and
therefore cannot drain out of the oil tank. Hopefully of course the oil is switched on again
before starting.
ii) A clear window on the gearbox inspection plate enabling the propeller to be turned after
shutting down to the position whereby the crankshaft throw is in its highest position. In this
way oil cannot drain through the crankshaft and then slowly leak out of the big-end bearings
and then into the lower cylinders.
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