Helicopter Maintenance Blog*
Posted 6 years 17 days ago ago by Scott Skola
An interesting development in the investigation of the EC225 that lost its M/R in flight last year.
One would think in today’s world of super computers and complex algorithms certain things could be calculated with some degree of certainty. Then again, maybe that digital reliance is part of a bigger problem?
As quoted in the updated AIBN preliminary report:
"The observed failure mode in this accident, i.e. crack initiation and propagation with limited spalling, seems to differ from what was expected or foreseen during the design and certification of the main rotor gearbox. The fracture propagated in a manner which was unlikely to be detected by the maintenance procedures and the monitoring systems fitted to LN-OJF at the time of the accident."
The entire updated report: https://www.aibn.no/Aviation/Investigations/16-286
TIPS and TRICKS
Airbus (Sud Aviation)
Continuing with the Lama daily inspection and associated tips and tricks. Remember to take a fuel sample before disturbing the aircraft.
After flight, each T/R head stack bearing should take the same number of pumps to purge. If one stack takes more pumps, it has a leak from the seal or Teflon blade cuff gasket under the centrifugal conditions of flight. When one stack leaks, it weighs less than the other two until they are all greased again, thus causing an imbalance that contributes to stack bearing and other wear.
If the leather T/R boot is soaked and dripping gear oil at statically, the pitch change shaft O-ring in the T/R head cap guide is defective, requiring removal of the tail rotor head for replacement. If the T/R G/B is consuming oil without leaving a trace, it is usually the output seal, unless the filler cap is missing!
Check the T/R G/B magnetic plug which normally accumulates black mung, but no metal. Make sure to seat the plug and verify it’s pulled back down into the détente. A mag plug that escapes can--and will--end up damaging or imbedding itself in a tail rotor blade.
Moving around the back of the helicopter, we’re grabbing the T/R guard as we go by, checking for security. Feel under the bottom of the guard for recent use by the pilot as the initial touchdown point. The French put the guard in Chapter 32, Landing Gear, understanding its occasional use. If it’s scuffed, look at the guard closely as it may have hidden damage like cracks in the tube or fittings.
Grab the T/R pitch change cable drum and move it axially to check for looseness on the pitch change shaft. Work the control cable back and forth on the drum, wiping the exposed cable off and inspect it for cracks in the clear cable sheath. Keep the drum worm screw grooves clean of grit.
If the cable sheath has cracks, that’s okay as long as the elements have not penetrated it and discolored a 10mm (3/8 in) stretch of cable. If a chunk of cable sheath is gone replace the cable as soon as practical. If a cable strand is broken reject the cable. Inspect the T/R drum for “knife-edging” of the raised ridges of the drum screw threads. This knife-edging has been known to pick up the cable, ride it up onto a ridge, and cut it, causing loss of T/R control.
Moving down the tailboom, wipe the T/R cable off as you go with a paper towel dampened with T/R hanger bearing oil to keep the cable clean and prevent the sheath from drying out and cracking. Check each cable pulley for free rotation and flat spots. Settle the pulley in a new spot to spread the pulley wear out and prevent flat spots. Stiff pulley bearings can be greased with a special bearing greaser set.
Grab the tail rotor driveshaft with both hands and move it back and forth, checking the axial play. Should be 5 mm (almost ¼ inch) MINIMUM free float. Notice it does not mention a maximum. If the driveshaft makes a metallic clunk at the end of its travel, it is not reason for re-packing the T/R driveshaft end couplings. You shouldn’t have to pack the driveshaft unless a coupling slings grease.
Check the Intermediate Bearing that joins the T/R driveshaft to the Incline Shaft under the engine. If it’s not slinging grease or loose, it’s fine.
[Submitted by Lama-Nator]
A little known safety information resource for mechanics from the FAA:
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.