Helicopter Maintenance Blog*
Posted 4 years 320 days ago ago by Scott Skola
Thanks again for the questions and comments.
Appears my last article on helicopter annual inspections spurred a few inquiries on pre-buys. I condensed them below with a quick answer. Might make for a good article next year. Stay tuned.
Also, looks like there’s light at the end of the offshore helicopter market tunnel. It’s a dim light, but at least there’s light. With all the massive cost-cutting taking place on both sides, I hope it works out for all concerned.
And now more from the Lama channel…
TIPS and TRICKS
Airbus (Sud Aviation)
Last time we were ready to adjust a MRH eccentric.
The eccentrics are an offset center adjuster that allows the taking up of play at the MRH damper fixed-lever, or damper piston anchor point. They are set at overhaul of the MRH, however it has been demonstrated repeatedly that the factory adjustments change after the MRH has been “run-in.”
Most eccentrics will probably need re-adjusting at some time because one-thousandth of an inch play at the eccentric equals about one-tenth of an inch of dead, un-dampened space measured at the blade tip. Enough play to affect blade phase and excite vibration and ground resonance--especially if the play is more than that.
The eccentric’s adjustment is crucial to the smooth, balanced operation of the main rotor.
To adjust, remove the sealant from the eccentric so you can view the serrations. You will need a heavy duty, battery terminal-sized two arm puller that will grab the lips on either side. Lift the eccentric up about half-way.
Often times they are seized so you may have to soak it with Aerokroil, heat it up, tap on it, strike it a dead blow, etc. At half way up, it’s not all the way off the post, but it’s off the splines. Tighten it counter-clockwise to the next spline and knock it back down on the serrations.
Moving back to the MRH dampers, the zerks at the piston fork-end and at the attachment of the upper and lower arms are prone to not accept grease. This can be caused by migration and misalignment of the lubrication hole in the bushing to the zerk hole. You must remedy this condition right now and any other place you find a zerk that won’t take grease.
Let’s suppose one of our drag dampers has a low fluid level. If a damper leaks out the inboard seal it will be wet on the outside of the damper. If the outboard seal leaks the fluid will exit out the hole in the end of the rubber piston boot, often without a trace.
If it doesn’t appear to be a seal, take the plastic reservoir off (be careful not to move the damper with the reservoir removed, it’ll draw air). Check it by sucking on it with your finger over the filler port and the bleed port--see if it holds vacuum. Put it back on, fill it and check it again.
The damper reservoir will commonly leak at the rubber cap. The caps get brittle with age and crack. Or they are too soft when new and stretch slightly and leak.
Note the engine oil tank, T/R G/B, and hydraulic reservoir use the same cap. Exchange your cracked, too soft, or too hard rubber cap for a better one. A side tip: this same cap also fits on the bottom of the rear seat legs and makes a good cushion and spare caps stash.
Topping it off means to fill it to the little weep hole on the side of the reservoir. On occasion the top rim of the reservoir can get a defect that might be dressed out with sandpaper.
If you have a damper that the level goes up after shutdown, you’ve got air in that damper. You can check a damper on the helicopter by disconnecting the blade spacing cables from that sleeve and sweep the blade back and forth stop-to-stop. Feel for a dead spot and watch the level to see if it changes. If the level changes, you’ll have to remove the damper to bleed it.
Remove the damper by disconnecting the forked end of the piston from the fixed lever and swinging it out. Remove the big cap nut from the stack-up on the drag hinge. Remove the upside down bolt that attaches the damper to the lower arm. Pop the upper arm loose from the drag hinge and lift the damper away with the reservoir and upper lever. This keeps the damper from drawing air if it is bumped.
[Submitted by Lama-Nator]
Received several emails about complying with a pre-buy inspection on turbine helicopters. The answer can be simple or complex depending on how you plan to operate the aircraft. Plus, each aircraft will present its own unique circumstances. But the top three tips are:
1) Use the person or entity you plan to have maintain your “new” aircraft for the pre-buy. While you can save a buck or two using someone closer to the location of the aircraft, they are not the ones who will send you that eye-shocker bill after its first scheduled inspection.
2) Use some sort of written guide to review the aircraft AND its records. There are a number of examples online from various sources like operational audit forms or OEM inspection forms. Even though pre-buys are an important due diligence in buying an aircraft, there is no official or regulatory pre-buy inspection criteria.
3) Complete a written Service Agreement with your pre-buy mechanic, listing what you expect from the inspection. Believe me, you’ll be glad you did.
A fourth item that is not maintenance related is your agreement with the current owner on how the inspection will be performed and how any discrepancies will be noted.
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.