Why you want to avoid blade failure
The next time we fly together, I am going to watch you very closely during the preflight inspection--especially when you go the near the propeller. Not just to make sure that you're careful of your extremities in the vicinity of the air-slicing device. I want to see if you're really looking closely at the prop for any sign that it's an unsafe object for us to be flying behind.
Every pilot who has ever preflighted a training airplane knows what the leading edge of a prop blade feels like beneath a moving finger. All but the newest blades deliver an occasional gentle nip to your fingertip--but how many times have you heard of the pilot calling a timeout from the day's scheduled flying to take a closer look at the rough area? Or calling off the flight and squawking a propeller discrepancy? Many pilots--perhaps they participate in blindfolded wine tastings on their day off--do a variation of the prop check that could be called a blind smoothness test. Watch them as they let their fingers do the sensing while their eyes wander off to more sightly surroundings of the airport environment.
Maybe feeling is believing, but a dinged spot on the flat face of the blade, or even a crack there, will escape notice if all we do is touch the leading edge. Nor will you catch problems with the spinner or prop hub unless you really look at the entire prop assembly.
Inspecting the prop blade doesn't get a pilot very far if he doesn't know what he's looking for. Pilots learn from textbooks to look for nicks, but they also know from their very first preflight that perfect smoothness isn't to be expected. Pilots' operating handbooks (POHs) give some guidance. "Small nicks on the propeller, particularly near the tips and on the leading edges, should be dressed out as soon as possible since these nicks produce stress concentrations and if ignored, may result in cracks," warns the single paragraph on propeller care to be found in Section 8 (Handling, Service, and Maintenance) of the POH for a 1985 Cessna Cutlass RG.
More guidance comes from knowing an aircraft's track record. Where does it go, and who takes it there? Is your trainer flown from an unimproved strip, or a paved airport with areas of broken-up asphalt scattered about the runway or taxiways? Is it employed for real-world training in short- and soft-field work? That's a heads-up. "Stone damage to the propeller can seriously reduce the fatigue life of the blades," says the Cutlass POH. A great place to gather expertise on how to preflight a propeller, and what seriously no-go damage can look like, is the AOPA Air Safety Foundation's Propeller Safety Safety Advisor.
What if a blade does let go? You're going to know in a big hurry that something's wrong, but you may not realize exactly what it is. What's the checklist for a prop blade failure? There isn't one in most POHs. But your first probable diagnosis of your problem--some form of engine failure--might as well be correct in terms of how you are going to have to respond to it. You will need to get your aircraft on the ground immediately, and with the severe vibrations caused by having more blade on one side than the other (or others, if you fly behind a three-blade prop) and possibly damage to the airframe or engine mount, controllability may not be on your side.
If there's any good news about propeller failure, it would have to be that it doesn't happen very often. Using that term as a basis for a search of the AOPA ASF Accident Database produced 173 cases, most of which were malfunctions of mechanical components of constant-speed prop assemblies, rather than blades breaking off in flight. A prop hub can also fail, or a damaged, unbalanced prop can commence a disconcerting shuddering--for instance, after striking the ground on a bounced landing. Once in a great while, an entire prop departs the aircraft. When you think about how you'd react to that, ponder the stability implications of abruptly losing 30 pounds or more of propeller and associated hardware from the forward-most weight-and-balance point on the aircraft.
Catastrophe? Sometimes, but not always. On June 6, 2006, a Cessna 172S with a flight instructor and a commercial pilot aboard was executing a missed approach from a practice instrument approach under visual conditions at Lawrence (Massachusetts) Municipal Airport. The flight had arrived on an instrument flight plan from Mansfield, Massachusetts. "According to the flight instructor, the flight was a proficiency check for the commercial pilot. The commercial pilot had just flown the VOR Runway 23 approach, and had initiated a missed approach one-half mile from the runway due to traffic landing in the opposite direction. Passing through 1,700 feet, a 'loud pop' was heard and the airplane began to shake violently. The flight instructor took control of the airplane and shut down the engine to prevent further damage. He then made a Mayday call with his intentions, and glided the airplane to a landing on Runway 32," said a National Transportation Safety Board accident summary.
Postflight examination showed that the McCauley fixed-pitch prop blade separated "about 12 inches from the center of the hub." The CFI said that "both he and the commercial pilot visually inspected the propeller blade, and ran their hands along the leading and trailing edges with no anomalies noted." It was also recorded in the report that a recent 100-hour inspection of the propeller blade occurred less than a month before the failure, "about 53 operating hours before the incident. No anomalies were noted."
Bringing us back to the beginning of the discussion are the NTSB's metallurgical analysis of the blade fragment; its detailed description of prefailure prop damage, which emanated "from an indentation at the blade's leading edge;" and the probable cause of the event. This was described as "foreign object damage to the leading edge of the propeller blade, which resulted in fatigue cracking and subsequent blade failure." Factors that probably helped the pilots to make the successful emergency landing were the spaciousness of two-runway Lawrence Municipal and the fact that they had terminated their approach early because of the opposing traffic.
It was noted earlier that not all prop damage begins with a leading-edge nick. A prop mounted on a Piper PA-28-140 incurred its damage on the blade's face, eight-tenths of an inch away from the leading edge. This was repaired on March 8, 2002. Failure occurred on August 19, 2003, 30 minutes into cruise flight in the vicinity of Winchester, Tennessee.
"According to the pilot, while in cruise flight at 4,500 feet mean sea level, the airplane began shaking violently. The pilot saw smoke coming from under the dashboard. The pilot shut down the engine and saw part of one propeller blade missing.... The pilot executed a forced landing into a field approximately 12 miles east of Winchester, Tennessee," said the NTSB accident summary. The pilot and passenger suffered minor injuries.
"A review of the engine logbook showed that the leading edge of the propeller had a nick, and was filled during an annual inspection on March 8, 2002. Examination of the airplane revealed that the propeller was fractured 17.5 inches from the hub center. The National Transportation Safety Board Materials Laboratory examination of the propeller reported fracture features typical of fatigue. The fatigue features originated from a nick on the flat face approximately 0.8 inch away from the leading edge of the propeller. Examination of the propeller blade showed paint from the flat face extended into the nick."
The report also noted a potential for ambiguity in repair methods. "According to a representative from the Sensenich Propeller Manufacturing Company, the repair manual does not specify a minimum size for damage requiring repair. The representative suggested that any visible damage to the propeller should be blended. The nick was of sufficient size that it would have been visible to the naked eye. The shape of the nick boundary indicates that it was not blended."
The NTSB concluded that the probable cause was "a maintenance personnel failure to follow propeller inspection procedures that resulted in a fatigue failure of a propeller blade and the subsequent forced landing."
Remember that a prop blade flexes in flight. Stress concentrations work to further weaken any damaged area, and the recipe for a failure takes shape. A nick deeper or wider than a mere 1/32 of an inch needs immediate attention, as would "a preponderance of nicks," says the Propeller Safety Safety Advisor.
Armed with that level of detailed information plus the foregoing examples of what it is you hope to avoid, my guess is that you will never look at your propeller the same way again--whether I'm there looking over your shoulder as you preflight, or not.
Dan Namowitz is an aviation writer and flight instructor. A pilot since 1985 and an instructor since 1990, he resides in Maine.
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