There have now been at least two instances in which evidence is conclusive that:
An incorrect or out-of-spec spring was present in the nose gear downlock cylinder.
Emergency extension testing was incorrectly performed during Annual Inspection, or not performed at all.There is now an FAQ on the nose gear downlock cylinder that shows some specs for the spring.
With the cylinder rod disconnected from the downlock hook, it should take a minimum of 75 pounds of force to get initial movement of the piston rod into the cylinder. If tested on-airframe, the emergency extension bypass valve must be open. If tested off-airframe, a bathroom scale can be used, and the fluid ports must not be plugged.
If the spring is missing, weak, or incorrect, the nose gear downlock hook will not engage crisply like it is supposed to; or it will fail to engage at all. If this happens, you are at risk for a nose gear collapse, despite having the nose gear green light. Unless, of course, you have the auxiliary downlock switch installed, which will warn you that the hook is not engaged. This is yet another example of why having this additional switch installed is important.
If a spring is bad in one of the main gear uplock cylinders, that main gear will fail to release from the well, during emergency extension. The purpose of the main gear uplocks is to keep the gear from bouncing out of their wells in turbulence, and causing pressure spikes in the system. But the locks have to release properly when the dump valve is opened, so that the gear can free-fall and lock. On the mains, the spring is what releases the uplocks in the absence of hydraulic up-pressure.
I don’t understand what is happening that is failing to identify these kinds of problems during Annual Inspection. Checking the powered retraction, and both the powered and emergency extension, is a required part of every Annual Inspection. If you are a Sierra driver, if I were you I would make it a point to personally observe this testing
I will add that in the case of one of our BAC members, a nose gear emergency extension failure was traced to a repair made eight years prior to the failure event. It was nearly impossible for the nose gear to extend and lock, under the emergency extension springs alone, without some outside forces helping to complete the locking function. During post-event testing, no successful emergency extensions were completed, until the mechanical interference was removed on the incorrectly-installed part. Which in turn means that it is not possible that a good emergency extension test was completed over the course of seven Annual Inspections.
I’m not sure whether the message is to have some Annuals done by someone new to the plane (who makes no assumptions), or whether the message is to make sure the Annual is done by someone who really knows what to check on the plane. But a definite message is to personally view the emergency extension being done. In fact, if possible, do it yourself from inside, then view it being done while you are outside. It is useful knowledge to witness and understand both the powered retraction, plus the powered and emergency extension.