After investigating a few aluminum propeller failures, the NTSB has issued a Safety Alert and it’s particularly addressing backcountry pilots. “Aluminum propeller blades can be susceptible to fatigue cracking and fracture if a small nick, pit, or corrosion on the surface or edge is not found and repaired during preflight inspection or maintenance,” the NTSB said in the alert. “Such damage can concentrate stress from normal airplane operation loads, resulting in fatigue crack initiation and growth followed by propeller blade fracture.”
The board wants to remind pilots to carefully inspect the prop during each walkaround and get the prop fixed if nicks or cracks are found. It’s also trying to prompt mechanics to make thorough prop inspections part of every annual. Backcountry and aerial spray aircraft are more susceptible because of the increased risk of damage on unimproved runways. But not all blade failures are on Cubs and Cessna taildraggers. The board recently dealt with a Beech 58 Baron that shed a blade on a recent flight. It had last been overhauled eight years previously when it should have been serviced after five years. “If the recommended overhaul had been performed, the corrosion pits that led to fatigue crack initiation would likely have been detected and removed, preventing the blade separation,” the alert said.
My opinion: Aluminum makes a terrible propeller material. A prop is subject to resonant vibration, which can cause cracks and sudden loss of material resulting in an out of balance rotating mass. In simple terms the out of balance can shake the engine right off its mounts. Wood is better, composites are best.
Don’t composite blades still have a metal leading edge?
It’s usually for wear purposes. It’s much easier/cheaper to replace the leading edge than the whole blade (which, with a few exceptions, can’t be filed down like a metal blade).
Aluminum is a great material for props, as seen with the outstanding safety record in normal GA flying.
This report is mainly for non-normal GA flying where off-airport landings and unimproved runways and “safety” is not what it is in the lower 48.
Personally I think composites are even more brittle and prone to small nicks that promote internal cracks that cause catastrophic failures. Probably that is why they us aluminum in Alaska!
A composite is a wood core with a composite overlay. It’s lighter but is still subject to the rotational forces of an aluminum prop. A propeller blade flexes fore and aft a lot in every rotation and it helps to see a video of the movement in flight.
I’m not happy with composite. I worked on a single that had a gear collapse on a taxiway that had been fitted with an MT prop and when a blade was damaged the entire prop had to be shipped to Germany for repair. The cost was enormous.
The other issue is the hub. If the prop is serviced regularly it will last a very long time but corrosion is the enemy, and lack of lubrication. Few people ever see the inside workings of a prop but the dynamic and flexing is amazing. I have less confidence in composite and the delamination concern. Horsepower has a lot to do with this — low horsepower engines put less impact on a prop.
Aluminum is for me.
I suspect that the risk of low temperature brittle fracture of composites is why aluminum is preferred in Alaska, combined with the infrastructure (autoclaves) needed to repair composites. Field repair of an aluminum prop can be done with a file (and proper training). Aluminum has good low temperature properties and is the metal of choice for cryogenic tanks and heat exchangers even though its high thermal conductivity isn’t helpful in some aspects of cryogenic service.
The metal leading edge on composites is not a structural component but rather a sacrificial shield to absorb damage and protect the composite.
I removed the 4,000+ hour and 60 year old prop on my Cessna and sent it for overhaul. They found a crack in the center bore, adjacent to a bolt hole, and red tagged the prop.
I got an overhauled prop and the shop suggested that I apply ‘prop guard’ tape to the leading edge, a pma part.
So, after 3 years now, the prop looks new with only a few ‘dings’ in the tape, which can be replaced in 2 inch segments, if it is damaged.
So, no more ‘sand blasting ‘ of the leading edge.
BTW, a structural engineer that designed props, indicated that my prop was 10x that number of stress cycles for aluminum to likely failure.
Seaplane props are aluminum and they take a beating. Flying through rain will roughen the leading edge a bit, just imagine the effects taking off in choppy salt water.
The discussion on what prop material is best notwithstanding, I hope the FAA doesn’t get carried away with the NTSB findings and start issuing AD’s. A simple SAIB is all that’s required. As usual, the NTSB has to have a “finding” — years later — but there’s special circumstances in this situation that normally (and protected) airplanes don’t ‘see.’ Given the number of airplanes that have been flying with aluminum props, I don’t see any reason to start worrying beyond the normal inspection process. It’s kinda like the Piper rudder post proposed AD … a few roughly treated airplanes have a problem and suddenly thousands of rudders need replaced … gimme a break. I fought a FAA engineer in Atlanta over the escutcheon plate proposed AD and won; the AD wasn’t issued. Hopefully, nothing more will come of this issue than “keep an eye on your prop, folks.”
Canada has a mandatory 10 year overhaul for variable pitch props. I get that props on seaplanes flying in salt water or guys going into gravel bars should probably get their props looked at more frequently but my prop on a land based always stored in a heated hangar airplane hit the calendar limit with 468 hours on it since new.
Overhauling it at an arbitrary calendar limit was an expensive waste of time and money.
Sandy (desert) environments eat aluminum props and wallets!
Aside from a prop strike or other obvious damaging incidents a close visual examination of the propeller and all fastening hardware including prop spinner bulkheads and fasteners should be adequate for fixed pitch units. Erratic operation and prop seal leakage would certainly require inspection and repair for C/S units. Mandatory tear down and overhaul of non commercial units are probably not indicated based on calendar time.
My prop with 4,000 hr on it had a hidden crack, in the center bore.
It was found during an overhaul and an eddy current test.
McCauley recommends a 7 year time between overhaul, or 2,000 hrs.
Mine is due in 2028, and the $1,200 is worth the cost vs a RUD …!
Yeah, I had one of those as well and is why I switched to another brand of manufacture. If there is a history of problems with a specific propeller/airframe combination then it needs to be followed up but not to tar the entire activity with goofy inspections when not warranted.
I have seen 19,000 hr fixed pitch props. I would say aluminum is a pretty good material and well proven. If you treat your prop like a lawn mower expect a less then desirable outcome.