The Lake is a "hydraulic" aircraft in that the landing gear, flaps, trim and brakes are hydraulically actuated. Normally, hydraulic systems are the most trouble free of all power transmission methods, and far less susceptible to problems than electrical systems. Hydraulics were chosen for the Lake because hydraulics are generally impervious to the effects of water immersion, and as opposed to mechanical systems, are simple to seal between water tight compartments.

Unfortunately, for various reasons, the hydraulic system on the Lake is also unnecessarily troublesome and problematic, mostly because of deficient design criteria. We review below each of the major components and the most common problems.

Landing gear cylinders

Because all the gear cylinders have pressure on them continuously, any very small leak in the shaft seal will be immediately evident. There is no wiper seal, so that sand and dirt can easily get imbedded in the shaft seal. A small amount of leakage in the gear actuator shaft seals (two or three drops per day) is normal and tolerable. More than that requires repair. The seals used are standard O-rings with the prefix MS28775-xxx, meaning they are compatible with 5605H hydraulic oil. Depending on the type of cylinder, the replacement of seals is straightforward with some cautions. The internal diameter of the cap should be inspected for wear and any cap with wear of more than about 0.010" (less on the nose gear) should be replaced.

On the cylinders which have threaded ends secured by small set screws, make sure to totally remove the set screws before attempting to unscrew the cap. If the threads are corroded or are resistant to motion, lubricate well before attempting removal. These threads are very susceptible to galling. When reinstalling the set screws and after tightening the cap, drill through the set screw hole only to the thread depth, such that the set screw does not distort the thread.

Most landing gear piston oring grooves are too wide for a standard oring. The design of the grove requires a back up ring. Use a MS28774-xxx back up ring if the width of the groove is more than 1.3 times the nominal cross section of the oring. (I.E. more than 0.1625" for an 1/8" oring). Most back up rings are about 0.032" thick. Use of the back up ring will prevent rolling of the oring which causes premature failure of the oring.

On some cylinders there is a transverse hole at the base of the cylinder (Especially those manufactured by Aerofab). Unless you deburr the internal edge of this hole (the factory did not do so), it is inevitable that you will cut the piston oring during assembly. This hole MUST be deburred!

Whenever you have a cylinder apart you should inspect the shaft for straightness, especially the threaded end. Nose gear shafts have a propensity to break at the base of the thread and this area should be inspected carefully for bending or fracture. The shaft should also be polished to remove any scratches or pits.

Nose gear cylinders should have a modulating restrictor installed at the lower ( Gear up pressure) fitting to prevent the nose gear from "crashing" down. A restrictor size of approximately 0.032" is required to maintain a gear actuation time of approximately 5 seconds.

Accumulators

There are numerous types of accumulators used in the Lake. The very old spherical types used in the very early aircraft are not repairable and should be discarded and replaced. Lake produced a welded steel version. which also cannot be easily repaired except by cutting and subsequent re welding. It is not recommended that these be repaired in the field. There are two types of field repairable aluminum accumulators. One has a large axial bolt through the center that holds the ends on. Although these are repairable, our experience is that the bolt and the ends are not consistently concentric, and they are very subject to leaking. However, they can be modified to accept (radially) bolted ends, which is the design of the latest version.

When repairing the type with the radially bolted end caps, it is critical that the holes in the tube be de burred on the inside, or the orings will always be chipped when inserting the piston. The factory neglected to do this and almost all factory accumulators will be subject to early failure. Almost ALL types of accumulators will require MS28774-xxx back up rings on the moveable piston, since all the o-ring grooves were manufactured too wide for a single o-ring.

Accumulators are most easily pre charged in the aircraft. To do so, first make sure that ALL hydraulic pressure is depleted. After connecting the Nitrogen hose to the accumulator, bring the pressure on the nitrogen tank up to 350 PSI. Use the hand pump of the aircraft (two or three strokes maximum) only until a positive reading is attained on the aircraft hydraulic pressure gauge. Without removing or changing the nitrogen supply, once again deplete the pressure in the system such that the aircraft pressure gauge reads zero. The pressure reading should drop from a positive reading to zero almost instantly, and again indicate a positive reading with no more than two or three strokes of the hand pump. This has the effect of pushing the accumulator piston just barely off its seat so that you can read the total pressure in the accumulator on the aircraft pressure gauge without significant volume compression. Finally increase the supply pressure on the nitrogen line so that the aircraft gauge reads 375 PSI +50,

Hydraulic bleed down

Since the system is pressurized full time, any leakage in any of the pressurized cylinders will cause the hydraulic pressure to bleed down when the aircraft is inactive. The problem is to determine which of the several potential leakage points are at fault. To help determine this, follow the following procedure.

1. Fully pressurize the system and record the time it takes for the pressure to drop 100 PSI. If this time is over one hour, it is best to do the following isolation process over night. Check for obvious external leaks, and if none detected, proceed as follows.

2. Place the gear lever in the neutral position. For safety it might be wise to clamp each of the gear locks with vise grips in case you inadvertently get the gear handle in the UP position. If the bleed down stops, then the leakage is in one of the gear actuators. The nose actuator is the most likely culprit. It is usually easier to rebuild the actuator than to try to isolate the individual actuator. Depending on your success, it may be necessary to rebuild all three gear actuators.

3. Assuming no success by isolating the gear system, return the gear handle to "DOWN" and similarly isolate the flap system. Again, if the bleed down ceases by this action, then the internal leak is in the flap cylinder.

4. Since the trim cylinder is designed to be in neutral when at rest, it is unlikely that the bleed down is a result of internal trim actuator leak. However, since the neutral position of the trim handle is determined by two springs, make sure that the trim valve is indeed in the neutral position when at rest, and that it does not drag on the trim cover allowing the handle to sit in the bypass position.

5. If the above isolation procedure has had no effect, then the most likely source of the bleed down is the check valve on the pressure side of the hydraulic pump. Although all hydraulic pumps have an internal check valve, they are prone to leakage and failure. Most aircraft have an external check valve located at or very near to the pressure outlet of the pump. In most cases this check valve is field repairable with installation of a new O-ring. If not, then install, or replace it with, any suitable check valve compatible with 5605H hydraulic oil and a operating pressure of at least 1500 PSI.

6. Finally, it is possible but improbable that the leak could be through the emergency hydraulic hand pump. This would require simultaneous failure of both inlet and outlet check valves. A symptom here would be continuous pressure on the hand pump subsequent to operating the hand pump in the down position. There should be no propensity for the hand pump to return to the up position by itself. The solution, of course, is to rebuild or replace the check valves.

Prestolite Hydraulic pumps

The Prestolite pump used in most Lakes as well as many other General Aviation aircraft was originally designed in the late 1940's as a hydraulic power source for automotive convertible tops. Many aircraft manufactures used these pumps because they were surplus and very cheap. They also found service as Marine pumps for outboard motor tilt systems, etc.

However, they were NEVER designed for aircraft use and have generic problems, particularly in the Lake. The pump gears are very light duty and are designed for intermittent duty. They DO wear out, although it is normal to give five to ten years of service.

The biggest problem with these pumps as installed in the Lake is the pump drive shaft seal. The Lake requires a much larger reservoir than built into the pump, and in every case, the reservoir tank is located higher than the pump seal. The O-ring type pump seal was never intended to seal in this manner, and thus often leaks allowing oil to enter the motor. This is the major cause of failure of these pumps as installed in the Lake. Some pumps have been modified to accept a lip seal and this works better.

Another problem with these pumps as installed is that they often leak externally. This is especially true in some models where the additional external tank was not welded to the pump base. We have seen some very ugly attempts to seal this junction. If this is the case, we leave it up to the individual ingenuity of the repair shop to correct this leak.

Although virtually all Prestolite pumps have already been modified, in those that have not, it will be necessary to install helicoils in the mounting flange of the pump body to reinstall the pump in most aircraft. They were originally installed with bolts and nuts, before the aircraft deck cover was installed, and it is virtually impossible in LA-4 and early 250 installations to re install the pump using nuts.

This page last updated on April 21, 2003