Hydraulic lock is a phenomenon that can occur on piston engines that have downward-pointing cylinders, that is, cylinders orientated such that the piston is moving down during the compression stroke. All radial engines have some cylinders that are pointing downwards and are, therefore, susceptible to hydraulic lock. After a radial engine has been shut down for a period, oil may drain into the combustion chambers of the lower cylinders or accumulate in the lower intake pipes, ready to be drawn into the cylinders when the engine starts. As the piston approaches top dead centre (TDC) of the compression stroke (both valves are closed at this point), the oil, being incompressible, can stop the piston movement . If the crankshaft continues to rotate then damage to the engine will occur – this could result in a cylinder being blown out, a bent or fractured connecting rod, or damage to the gudgeon pin. This phenomenon is known as ‘hydraulic lock’. A partial hydraulic lock can also occur when liquid is inside the combustion chamber, but is not sufficient to fill the space between the cylinder head and the piston when it is at TDC. In this situation, the air gap is still reduced and, therefore, the pressure rise within the cylinder can still be sufficient to stop the piston or to result in damage if the piston is forced through TDC during engine start. Damage resulting from a partial hydraulic lock can be more serious as it could go undetected during the engine start, and then result in failure at some later time in flight.
(See diagram showing the possible effect of hydraulic lock on a piston connecting rod in the AAIB report on the July 2009 accident to Piston Provost G-AWVF via the pdf link below.)
To avoid hydraulic lock during engine start, the propeller should be turned through a few revolutions by hand in the direction of rotation (with the ignition switches off). If any excessive resistance is felt while pulling the propeller through a compression stroke, then liquid is present in one of the cylinders, and the propeller should not be pulled through any further.
As an example the Provost T1 Pilot’s Notes state:
‘Unless the engine has been run during the preceding hour, check for hydraulic locking by having the propeller turned by hand through four revolutions.’
The Pilot’s Notes do not state what to do if hydraulic lock is encountered.
The RAF Ground Handling Notes for the Pembroke, 56 which uses a similar Alvis Leonides engine to the Provost, states:
‘All engines which have NOT been running during the 30 minutes preceeding the intended start, are to have the following ‘hydraulic’ check carried out:’
After ensuring that the magneto switches are off the procedure states:
‘With the right hand cupped about the lower descending propeller blade tip, advance across and forward of the propeller disc, pulling the propeller blade until the right hand releases naturally from the blade. Repeat this exercise until 12 blade tips have passed the lower vertical point. Any resistance to rotation of the propeller is to be reported to the propulsion trade manager. The resistance will indicate excess fluid in the lower cylinders and, in this event, the sparking plugs must be removed from the cylinders and the propeller turned through several revolutions to drain off the fluid.’
With a three-bladed propeller, turning the propeller through ‘12 blades’ ensures that the engine will have been turned through at least four complete revolutions5. The engine manufacturer’s ‘Operation, Maintenance and Overhaul Handbook’, for Leonides 500 and 510 series engines contains the following similar procedure:
5 The engine has a 0.625 to 1 reduction gearbox, so four complete revolutions of the propeller actually equates to 6.4 revolutions of the engine.
‘Anti-hydraulicing procedure. To prevent the possibility of a hydraulic lock occurring when an engine is started ensure that the ignition switches are OFF then turn the propeller through twelve blades. If undue resistance to movement is experienced during the above operation or after installation, after storage or when an installed engine has not been run for seven days or more, proceed with either of the two following procedures as applicable.’
The two procedures which follow the above paragraph both involve removing the spark plugs from the No 4, 5 and 6 cylinders and turning the engine through several revolutions in order to expel the excess fluid.
Ex-military aircraft pilots handling notes do not always provide full information on what to do if hydraulic lock is encountered. Some pilots may not fully understand hydraulic lock nor appreciate its damaging effects.
It is generally known in the Historic aircraft industry that some pilots may employ different procedures to the aforementioned when they encounter a hydraulic lock. These include turning the propeller forward through at least 27 blades and, if any undue resistance is encountered, some pilots would turn the propeller back in order to clear any hydraulic lock. The theory behind this procedure is that, by turning the propeller back, the intake and exhaust ports are opened and the fluid is allowed to drain into these ports. This procedure avoids the more time-consuming and work-intensive procedure of removing the spark plugs to drain the fluid however this is very dangerous for reasons explained below.
Anecdotal evidence suggests that the use of this procedure may be widespread, but it is contrary to the advice from the engine manufacturer, and it has a potential problem. When the propeller is turned backwards, the piston which has encountered the hydraulic lock moves up (assuming it is a ‘downward pointing’ cylinder), and then the first valve to open is the intake valve. As the propeller continues to be rotated backwards the piston moves down and will help to force any liquid out through the intake port. As the propeller continues to rotate, the exhaust valve will open and some liquid might also drain into the exhaust port. Oil in the exhaust port is safe and will either drain out through drain holes in the exhaust, or result in smoke being produced during engine start. However, oil in the intake port is not safe, as it will not drain away and is likely to be sucked back into the cylinder during engine start, potentially causing damage as a consequence of hydraulic lock.
The US Air Force Powerplant Maintenance Manual (AFM 52-12, May 1953), in a section on hydraulic lock involving radial engines, states:
‘Never attempt to clear the hydraulic lock by pulling the propeller through in the direction opposite to normal rotation, since this tends to inject the liquid from the cylinder into the intake pipe with the possibility of a complete or partial lock occurring on the subsequent start.’
Author: WALLY EPTON - ENGINEERING GROUP
Contact: Captain O W (Wally) Epton FRAeS
Council Member – Special Projects
Historic Aircraft Association
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Hampshire GU52 7SS
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