Ron Rapp, the blogger behind the House of Rapp blog, has a lengthy post about a topic that will probably bore most pilots or aviation enthusiasts: Proper or improper maintenance on a constant-speed propeller – a critical safety issue. Ron began his post by telling how Rob Cable, who was the grandson of the founder of Southern California’s Cable Airport, billed as “the world’s largest family-owned public-use airport,” was killed during a post-annual test flight in his twin-engine Beech 95 Travel Air.
The cause? A 2.5 foot-long portion of the right engine’s propeller blade had failed – meaning the hub ended up attached to a three-foot blade on one side and a broken 6″ stub on the other. This imbalance caused a vibration which severely overstressed the engine mount and tore the right engine off the airframe – rendering the aircraft uncontrollable.
Although what happened to Cable was a massive failure, Ron commented that he has seen cases of props shedding just an inch or so off a blade tip – enough to cause a vibration severe enough to shatter instruments in the cockpit, tear cowlings away and cause other serious damage.
However, the NTSB delved the maintenance records of Cable’s aircraft and found that the props had just been overhauled at an FAA-approved Repair Station. The problem was the investigation revealed a whole series of discrepancies between the overhaul procedures specified in maintenance manuals and the physical evidence found in the propellers – meaning the mechanics were criminally negligent in the performance of (or lack of performance in) their work.
Ironically, Ron himself had his propellers serviced at the same shop and was forced to spend another small fortune to have them re-serviced at another shop – meaning the “FAA Certified Repair Station” designation actually means nothing.
Ron also mentioned that props have a recommended Time Between Overhaul (TBO) and for most constant-speed props, it’s 2400 hours or six years. However, there are instances where a TBO is not mandatory – meaning it’s not uncommon to have aircraft where its been 10, 20 or even 30 years since the prop and/or governor were overhauled. And yet, all of those tiny parts or seals inside the propeller are exposed to both the elements (one way or the other) and to age or use related wear and tear (Ron himself would actually rather fly behind a 30 year old engine than a 30 year old prop).
If you fly an aircraft with a constant-speed propeller, Ron’s entire post will be a must read and will encourage you to check up on the maintenance of your constant-speed propeller.