How are the main gear up-lock hooks supposed to be adjusted? How about the nose gear down-lock hook? How do you bypass the airspeed safety switch, during retraction tests?
Technical Editor:
The pressure switch activation is a necessary part of each swing test, including for maintenance. That switch is what keeps the gear from folding, if the gear switch is moved Up, and the plane isn’t moving at least 62 MPH.IAS. I like having the opportunity to ‘test it’ any time the plane is on jacks.
The ‘tool’ I use for it is a length of Armaflex pipe insulation (solid, not split), with a standard plumbing angle-stop valve clamped on one end. I slip the Armaflex hose over the Pitot far enough to cover the water drain hole. I open the valve, and gently blow into the hose until the Airspeed Indicator is showing near red-line. Then I close the valve while maintaining that pressure. The Pitot system will generally hold the pressure above 62 MPH long enough for the testing. This also gives a good test of the Pitot system leakage. You can also just hold mouth pressure to keep the pump running, while leaving the gear switch Up. That makes it very easy to stop the pump at any point you wish, during testing. Any similar tubing system will work.
The main gear are not supposed to rest on the up-locks during flight. The locks are there only to keep the gear from bouncing below the gear wells in turbulence, with the resulting yaw and drag (and structural stress). The gear are supposed to be adjusted up against the rubber-nosed stop bolts. The up-lock hook is supposed to have at least .060” clear air gap from the contact point on the fork; and close to one-half inch clearance at the top corner above the fork. You may see rub marks on the fork contact point, where the hook has been dragging across it. While some marking is normal, that often means that someone has mistakenly adjusted the gear down onto the hooks.
If the gear are set up so that they rest on the hooks while up, the hooks may fail to fully engage on retraction. The weight load on the hooks may also make it impossible for the emergency retraction springs inside the uplock cylinders to retract the hooks when the emergency extension bypass valve is opened. The slight clearance allows those springs to get the hooks moving out of the way before the gear weight fully hits them. I’m not talking about the external spring cartridge, which allows the hooks to be moved out of the way by the fork contact surface as it passes by the hook while going up. I’m talking about a double spring inside the lock cylinder. That double spring is what move the piston to the unlock position, pushing the fluid back through the bypass valve, so that the gear can clear the lock on the way down.
There is a similar single spring inside the nose gear down-lock cylinder. Its purpose is to engage the nose gear downlock during gear extension. Many people have sluggish locking, and never know it until the nose gear folds some day. By now, every Sierra should have been equipped with the auxiliary lock microswitch, on the downlock hook itself. Otherwise the green nose gear light just means that the nose drag-link limit switch has turned off the pump. It provides no assurance that the nose gear down-lock is actually engaged. There is an illustrated Download available for this on the BAC website.
There is a design limitation in the nose gear down-lock cylinder. That cylinder is simply a Gerdes brake cylinder, with an external cartridge assembly on the end of the rod. There is a spring under the piston, inside the cylinder. There is no travel limit for the piston, in the Up position. As a result, the internal stop becomes the spring itself, when the coils stack up solid from the Up pressure. Any compression spring that is operated in a stacked-coil condition WILL take a ‘set’ over time; that’s “Spring Design 101”. There should be a spacer inside the spring that acts as an internal stop. The result of the ‘spring set’ is that there is no spring pressure at all, when the rod is fully extended. That means that the spring is not assisting the locking function at the end of the travel limit; a particularly risky situation during emergency extension. Delayed locking or no locking is the result; and without the aux microswitch, the pilot has no clue. When you read of cases of back-to-back nose gear failures in a Sierra, the lack of the aux lock switch and incorrect lock cylinder function are virtually always the primary culprits.
I have been working on a replacement stainless-steel spring, but no PMA yet. As a temporary solution, you can put a couple of standard washers under the spring. There should be no ‘free play’ in the rod, with the cylinder disconnected from the hydraulic lines. In many cases you will find that the rod can be ‘wiggled’ in and out by .200” to .250”. Use washers to eliminate the play. Make sure that the hook still clears the passing block during gear retraction. If you install too many washers to remove spring free play, the lock cannot retract far enough.