by: Paul D. Faltyn – Commercial Pilot and A&P Mechanic
A common misleading scenario concerning engine instrumentation among piston engine helicopter operators is that you do not repair or calibrate them unless they fail or there accuracy is suspect. All mechanically driven instruments have a ( TBO ) ” time between overhaul “, most manufacturers recommend that there instruments be overhauled between 1500 to 2500 hours in service.
Question: Why address this issue:
I recently had the opportunity to look at a Bell 47D-1 that was beautifully restored, but the owner was having a
problem with his instruments. After reviewing the logbooks we could find no indication that the tach generators or indicators were ever removed from the helicopter. After removing the rotor/tach indicator, the instruments still had
a Kollsman seal on it dated 1951.
When I removed the indicator from the panel I grabbed the manifold pressure line and it crumbled in my hands and the oil pressure hose and pitot/static hoses were not in much better condition. I also found several frayed wires behind the panel as well.
After having the tach generator, rotor-tach indicator, and manifold pressure overhauled and replacing all the hoses, the helicopter performed perfectly to specification.
Most of the Bell 47’s are flying today are at least 30 plus years old and the instrumentation will not last forever. This is a simple preventative maintenance program that can prevent unwanted head aches or unnecessary maintenance.
A helicopter operator I was associated with that operated 5 or more Bell 47’s at any given time had a strict maintenance policy of verifying the engine instrument calibration at every engine overhaul or 1200 inspection. We would generally find that the instruments required overhaul at 2500 hours and 50 percent of the time minor calibration was required at 1200 hour or engine overhaul.
Suggestion: If your helicopter is going through engine overhaul or restoration consider some preventative maintenance on your instruments. It is cheap insurance and can reduce or prevent maintenance issues. At your next inspection, have your mechanic take a look behind the instrument panel, you may be surprised what you find.
Remember, we are operating vintage helicopters and sometimes you have to take the maintenance to new levels not published in the maintenance manuals to ensure safety and reliability.