Helicopter Maintenance Blog*
Posted 4 years 233 days ago ago by Scott Skola
How convenient. Just as we start a year of Lama tips, one of the largest operators of SA315B and SA.316B type aircraft decides to retire their entire fleet. The Indian Army cited recent flight incidents and safety concerns as the reasons for grounding their fleet of approximately 44 aircraft.
While not legally the same models, the HAL Cheetah and HAL Chetak are Indian license-built versions of the SA.315B and SA.316B, respectively. The Indian Air Force operates a similar fleet, but it hasn’t been released if they plan to follow suit.
But that’s life. I wonder what will happen with those aircraft and parts….
TIPS and TRICKS:
Airbus (Sud Aviation)
This Lama tip will complement the previous tip on stack bearing management, and will deal with the actual changing out of the bearings.
By pilot report and by “rotating the stacks”, the mechanic should be able to anticipate that sometime soon the aircraft will be unable to conduct normal safe operations due to an increasingly severe notch in the stack bearings.
Eventually the notch renders the tail rotor pitch unable to be controlled because the pilot cannot “push through” the notch to change pitch past a certain point. By the time the pilot suggests the stacks are nearing this point, you should have the parts and tools and a target date for the change.
There are several warnings before proceeding to change the bearings. First, never change just one bearing stack; change all three sets even if you think you know that only one blade has bad stacks. You can’t order just one bearing stack, they come in three sets of three.
The bearings are available from two manufacturers which have different color races. You will receive a matched set from one manufacturer. DO NOT under any circumstances mix the manufacturers as the weight of the bearings are different.
Follow the manufacturer’s instructions and utilize the special tools required. You may not have access to the tools for some reason, but you must at the minimum have a socket that removes the slotted retaining nut (Fig. 1, Item 330) from the spindle (Fig. 1, Item 520).
You must heat up the bearing (Fig. 1, Item 340) and the housing assembly (Fig. 1, Item 410) to knock out the bearings, and separate them from the spindle. An oven is best, a heat gun works good, and other methods such as heating a pot of oil into which you dip the parts, have been applied successfully.
For best results you should change the stack bearing lip seals (Fig. 1, Item 430) when changing the bearings. You WILL need the special tools to install this seal into the housing without damage. Matter of fact you’d better order an extra seal even if you are using the special tools, as it is likely you’ll damage one during installation.
The bearing housing to blade cuff Teflon gaskets/seals (Fig. 1, Item 390) must also be changed to ensure that there are no leaks so that the grease applied to the bearings remains in the housing during flight.
Reassembly requires heating the stack bearing housing, and ideally putting the bearings on ice in order to drop them in, although it can be done with the bearings at room temp. The bearings must be installed in a particular orientation with the big “V” (Fig. 2 & 3) etched on the sides of the bearings facing the blade root, or away from the tail rotor head.
Then you heat the bearings and housing assembly and cool the spindle to drop it in. Install the nut and snug it up to seat the bearings, then torque it after everything cools.
Remove all previous balancing weight from the blade cuffs and balance to achieve less than .2 IPS. Re-service the stacks with grease after each balance run to ensure balance is done with full grease.
The Alouette family of helicopters use T/R p/c link boots (Fig. 1, Item 20). After installing one, cut off the tabs that allow you to grasp it. These tabs stretch and destroy the boot prematurely. If a boot becomes damaged or missing, replace it or remove the other two to preserve balance and aerodynamics.
[Submitted by Lama-Nator]
Here’s a link to recent Robinson Helicopter service and safety documents:
Robinson Helicopter Publications
SUBMITTING MAINTENANCE TIPS/TRICKS/QUESTIONS/INFO:
Have an old tip or trick you’d like to share with your fellow mechanics? Or maybe a question that you can’t seem to find an answer to? Or just some info to pass on? Send an email to: [email protected]
About the author: After 32 years maintaining helicopters in various capacities, Scott concluded a full time career with a major operator in 2014. When not pursuing future writing projects, he can still be seen around the flight line tinkering on aircraft for beer money.
*To keep the hounds at bay, the information contained in this blog is for discussion purposes only.