Helicopter Maintenance Blog*
Posted 4 years 90 days ago ago by Scott Skola
A follow up to last month’s blog tidbit on the Airbus 225 accident in Norway. Ran across an interesting article detailing the inner workings of the Super Puma main transmission and how it relates to the failure of the Norway aircraft.
Makes you wonder how any aircraft stays together.
A programming note: I’ll be off on my annual off-grid Alaskan beer-fishing-airplane-fest till August. So the Lama-Nator will keep the tips flowing. Still send your emails and I’ll get caught up on them when I return.
And away we go…
TIPS and TRICKS
Airbus (Sud Aviation)
Continuing with the Lama daily inspection. Remember to take a fuel sample before disturbing the aircraft.
Moving to the engine L/H side, look at the engine oil in the tank. If it shows over-filled, when it was not on the previous day, you may have fuel in the engine oil. This is the one drawback of having an engine internal centrifugal fuel delivery system. There is a seal (flexi-box) where the fuel supply turns 90° and enters the fuel tube. This tube is inside the engine’s rotating shaft and goes to the injection or “slinger” wheel.
When this carbon seal goes bad, you get fuel in the oil. You’re allowed 10% maximum dilution in 25 hours, so that’s a ¾ quart rise in the 7 quart engine system. You can smell the kerosene in the oil tank. You can always change the oil each time the limit is reached, to buy some time and prepare for the repair. It’s rather expensive, but it lessens the damage to bearings and gears caused by severely diluted oil.
This tip will only work for a slow dilution situation. Usually when the flexi-box goes, it makes a quart of fuel in a half day of flight or less, resulting in an AOG situation. Back in the day, someone could come replace the flexi-box seal during one day of down time. However, those kind of services are getting harder to find for the Lama.
Looking for other leaks on the engine, be advised that several leaks can appear as drips off the bottom of the reduction gearbox at the split line. The source of this leak is never the split line itself. An engine output seal leak oozes out into the clutch flange and either puddles there or leaks on down to the bottom of the engine gearbox.
If no leak appears traceable from the clutch flange, stick a pipe cleaner in the drain hole and see if it comes back wet. If so, the engine output seal is leaking into the clutch housing and staying there. Normally the friction ring that contacts the carbon seal needs lapping.
The starter can leak externally at the gasket, or at its seal, which permits engine oil into the inside of the starter and an oil mist into its cooling hose to the cooler. Gasket leaks are usually due to the mounting surface on the starter having a defect, often a raised area at the edge where the starter was bumped in handling.
Dress the starter flange flat with a fine file. Coat the gasket with Fuel Lube (WWII radial engine rocker cover grease) to stiffen and bolster it. A starter gasket leak can also end up dripping off the bottom of the engine reduction gearbox, like the engine output seal.
Moving all the way around the cabin to the other side (RH) where there are no obstructions (on the left there are the engine and hydraulic reservoirs), let’s have a look at the upper flying control bellcrank as it is called--the three-lever flight control relay at the front of the transmission deck.
Check it for cracks in the tube. It has a steel tube doubler cemented inside to prevent disintegration if a crack does develop. This can be caused by extreme ground resonance, a hard landing, or severe turbulence that stresses the flight controls.
Next, try to move the Freewheel Shaft back and forth. It should move at least one millimeter. If it is axially stiff, remove it and determine why. Give the clutch a spin and listen and feel for rough bearings.
[Submitted by Lama-Nator]
A couple FAA SAFOs on transponder/ADS-B testing and UH-1 hydraulic switches:
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.