Helicopter Maintenance Blog*
Posted 2 years 254 days ago ago by Scott Skola
Unfortunately, the last month hasn’t been kind to the industry. Several tragic events: AW139 hit a zipline in flight, a pilot was struck by 230 M/R blades during ground ops, a midair between an Astar and fixed wing, and an EMS 407 hit trees during cruise. Luckily, in the fifth incident, where a Skycrane contacted water during fire ops, everyone made it out okay.
On a more positive note, the US Army’s Joint Multi-Role Technology Demonstrator Program competition is heating up with Bell’s entry, the V-280 Valor, hitting 280 knots during a test flight. The Sikorsky entry, the SB>1 Defiant, is currently undergoing ground testing.
Being a big fan of the old Lockheed AH-56 Cheyenne, and having seen one up close and personal, I’m leaning toward the Defiant to make its mark with the pusher prop. But then again, after watching the Bell XV-15 put on a show at the local ramp years ago plus a tour of a V-22… Decisions, decisions.
In the history corner:
TIPS and TRICKS:
I think we’ve pretty much beaten the T/R D/S topics to death. So let’s move onto the engine compartment area.
One item to watch is with the engine cowling itself. The Dzus mounting studs will break over time along with the Dzus receptacles. In general, a Dzus stud should be replaced with a stud of the same material. Some metals “react” with certain composite materials and cause the stud hole in the cowling to enlarge. Not a good thing. Alternative type studs can be used temporarily to get the aircraft back home or to get the proper studs on order.
On the Dzus receptacles, the most common failure is the lock bar/spring assembly dislodges from the riveted cup. If you’re lucky, and still have the two springs and retainer housing (Fig 1, blue), one quick fix is to slide a cotter pin or .040” safety wire through the cup holes and retainer (Fig 1, green). This will give the stud something grab and lock onto.
But if you’re missing the springs and/or retainer, one option that does not require riveting, is to rob the lock pin, retainer, and springs from a spare receptacle and fit them to the riveted receptacle cup. However, as with the studs, ensure you replace any receptacle with the proper type and material. Or you’ll just create more problems in the future.
Another item keep an eye on is the exhaust stack. For some reason they seem to crack more on a 407 than those on a 206. The stack mount flange weld is usually the place the cracks start. While some cracks run parallel to the weld (Fig 2, red), there are other cracks that take off perpendicular (Fig 2, blue) to the weld. Sometimes these cracks can grow quickly and be rather extensive.
If you suspect a crack, one trick to verify is take a small brush or Q-tip and apply MEK or Acetone to the crack area. If the solvent bleeds through to the other side it’s cracked. This check can be done with the stack installed and also used to verify cracks on the exhaust collector assembly provided you can see both sides of the area.
And while we’re here, a couple tips on changing exhaust stacks. Forget trying to reuse the stack mount hardware unless you are in a remote location or in dire straits. Order new mount hardware when ordering the replacement stack. When possible spray all stack hardware with penetrating oil prior to removal. Aero Kroil being the preferred oil.
However, the most important thing to do first is cram several red rags below the mount flange and into the spaces between the exhaust collector and the power turbine and compressor housings, respectively. Those rags will be your life saver if and when a bolt snaps in half or drops from your grip. Nothing ruins a day more than pulling an engine to chase some hardware that has gone astray.
And don’t forget to remove the rags when you’re done!
A couple FAA SAFOs: cabin light fixtures and external load HEC devices.
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]
AND A LAUGH:
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.