To most pilots, the airplanes propeller is something routinely taken for granted. Oh sure, preflight may include running a hand over the blades in some pretended attempt to look for something. But many people dont have much of a clue as to what theyre looking for – maybe nicks or leading edge surface roughness from sand or water erosion.
Controllable-pitch props generally have some kind of flight time or calendar TBO, such as requiring an overhaul after 1,200 hours or five years, whichever comes first. Check the logbooks of most older airplanes, and you may find this to be the most commonly ignored manufacturer recommendation for Part 91 airplanes.
Some misguided owners, in an effort to save time and a few bucks while still doing necessary maintenance, even take a file to a propeller ding themselves because they have been told the leading edge must be smooth.
Yet, as the NTSB accident record shows, the condition of your prop and how its maintained is vital to safe flying. Ironically the cause of almost every prop failure mishap is the result of owner/operator ignorance and the resulting neglect.
Troubled Operators
After takeoff a Cessna 182 with five people aboard suffered a complete power loss as it climbed through 800 feet. The aircraft flipped inverted during the ensuing emergency landing but fortunately no one was injured. Investigators found a propeller blade fractured due to a fatigue crack. In addition, internal components of the prop contained substantial amounts of rust and rust residue.
The specified TBO for the prop was 1,200 hours or five years, whichever came first, yet the prop had not been touched in its entire 19 years of service. In addition Airworthiness Directive 91-15-04 had been ignored. The AD required a modification of the prop so that a dyed oil would leak in the event of a fatigue crack.
In another case a Piper J-3 Cub was cruising at 2,500 feet when the engine began vibrating severely. The pilot told investigators I was afraid it was going to shake the engine off the airplane, so I shut the engine down. Investigators found prop delamination on one blades leading edge and tip. In addition traces of rust were found on the wood where the metal leading edge tipping was attached with the manufacturers steel screws.
In a similar case, a Taylorcraft was in cruise flight when it too experienced a severe vibration. This pilot too shut down the engine and made a successful forced landing. Investigation showed the brass abrasion strip and part of the blade were missing. A mechanic found dry rot in a part of the remaining blade.
Striking Out
Neglect – or if it aint broke, dont fix it – is only part of the problem. Like it or not, a propeller strike always requires a non-destructive inspection at a minimum, even if there is no apparent damage. Even with seemingly minor damage it is most likely ruined, but a prop shop should make that determination objectively.
For example, a Rockwell Commander 112TC was cruising at 9,500 when the pilot heard a loud boom and the engine failed. Investigation disclosed that the airplane had experienced two previous blade strikes. Each time the engine checked out with no damage, and the prop had no visible problems. Yet an NTSB metallurgist determined, The (prop) hub failed as a result of fatigue cracking. Damage to the pre-load plate shelf … indicated the hub had previously experienced a blade strike.
In yet another case a Cessna 172 flown by a student on his first solo suffered an engine failure during initial climb out. The pilot told investigators he heard a loud bang and the engine quit. He successfully landed on a golf course, but the airplane sustained substantial damage.
Investigators found the entire prop, flange and forward end of the engine crankshaft missing. These were located later near the mid-point of the departure runway. The FAA investigator found that the engine had two previous prop strikes without any inspection being done.
In yet another case a retired Air Force friend, his eldest son and I were going antelope hunting. For several months we had looked forward to a week of re-living old times. He lived in Washington state and we were flying from his base to our hunting spot in Montana in his Cessna 210.
He had two partners in owning the airplane; both of the other two were successful businessmen. One partner, despite several hundred hours in the airplane, seemed to have an unlucky streak.
On two occasions he had dinged the prop while taxiing too fast from the grass parking area onto the concrete ramp. In each case, the prop was damaged when the nosewheel hit the edge of the ramp. In a third mishap he had landed much too fast and porpoised, again dinging the prop. However this time the engine required inspection and a new three-bladed prop was installed.
As we approached the airplane for our trip to Montana, my host said calmly, Looks like the Doc has done it again. The prop was obviously new, yet none of the partners had been informed of a prop change. In fact in only one of the three other mishaps were the partners informed, and that was when the airplane required several days of down time to accomplish an internal inspection of the engine.
The pilot has also kept the insurance company and FAA out of it, because technically none of the mishaps were required to be reported. In each case the responsible party simply paid cash for the repairs and kept quiet. Now, apparently, he had done it again.
We flew the trip and fortunately had beautiful weather with no turbulence over the mountains. There was no hint of a problem with the airplane, engine or prop. Yet after the trip my host kept examining the airplane from a distance. Without prompting, he said, Something doesnt look right. The nose looks like it droops.
I couldnt see anything wrong. The airplane had looked good on both our pre-flight inspections. But my friend wasnt satisfied.
The next day the owner and a mechanic took off the cowling and went looking for trouble. In no time they found it. The engine firewall was buckled and the forward frame bent. That dinged prop incident had actually been a major accident. The repair estimate was such that the insurance company declared it not economically repairable and wrote off the airplane as totaled.
Thus our trip to Montana was made in a dangerously unairworthy airplane. Any moderate to severe turbulence over the mountains of Idaho and Montana could have caused structural failure. But the condition of the finely tuned Continental engine was of special concern, since internal damage could have caused engine failure during the critical takeoff and landing phase or over some very rugged terrain.
The engine manufacturers have issued guidelines for when an engine should be torn down for inspection after a prop strike. Continental has published Service Bulletin M84-16, which advises that prop damage requiring only minor dressing of a blade does not require an engine inspection. However if a blade or the entire prop needs to be removed for repair then severe internal damage to the crankshaft, prop components, counterweights or crankcase bearing caps may have occurred. This, Continental says, requires complete engine disassembly.
Lycoming has published Service Letter L163C, which recommends that any engine involved in a sudden stoppage be removed, disassembled and inspected prior to being returned to service. Anything less is a game of you bet your empennage.
Fatigue Cracks
Prop blade failure due to fatigue cracks plays prominently in the accident record. These usually result from a neglected nick or gouge in the leading edge of a blade. For example, a Mooney M20B was approaching the outer marker on an ILS approach. Suddenly the airplane developed a severe engine vibration. The pilot shut down the engine and made a successful emergency landing in a field. Cause of the vibration was separation of a propeller tip due to metal fatigue.
In another instance an Air Tractor AT-401 was damaged in a forced landing when 10 inches of the prop broke off due to a fatigue crack. In fact the blade showed fatigue cracking over 60 percent of the fractured surface. Origin of the crack was a notch created by a previously re-worked leading edge ding.
The record shows that most all models of airplanes have had at least one instance of slinging a piece of prop. Youre not immune, whether you fly a Warrior, Cessna 172, Cessna 421 or Beech 1900 Commuter.
FAA Oversight
The critical importance of ADs and factory-issued service bulletins cannot be over-emphasized. The Cessna 182 mishap described earlier involved the complete ignoring of an AD that could have prevented loss of the aircraft.
The FAA, however, is not the last word on the safe operation of an airplane. Bureaucratic inertia and lobbying have both led to delays in providing crucial information to operators of airplanes that may have some mechanical flaws, and propellers are certainly not excluded.
On August 18, 1992, a Canadian registered MU-2-60 on a Part 135 air cargo flight lost a blade from the left propeller during cruise flight at 6,000 feet. The blade penetrated the fuselage, breaking electrical lines and causing complete electrical failure.
The pilot made a 180-degree turn to get back into visual conditions, then flew for 40 minutes using non-electric instruments until reaching clear weather. Ultimately he made a successful gear-up landing at a small airport.
The cause of the accident was a prop blade separation because of fatigue cracking that originated from the inside diameter surface of one of the four arms of the prop hub. At the time there was no routine or special inspection that would detect a fatigue crack originating inside the prop hub, despite the fact that a similar propeller failure had occurred on September 27, 1991.
Subsequently the NTSB made three safety recommendations to the FAA. First, (A-92-81) was a requirement to develop a non-destructive inspection technique that would detect hub arm cracks stemming from the inside diameter surface of the hub arm at the approximate location of the inserted end of the pilot tubes … and issue an Airworthiness Directive requiring that HC-B4 hubs with 3,000 hours or more be inspected using this technique the next time the propeller assembly is overhauled for any reason, or the next annual inspection (whichever comes first).
The Board also recommended considering the possibility of periodic inspections of prop hubs with more than 3,000 hours (A-92-82), and a check to see if Hartzell model HC-B3 and B-5 propeller hubs, based on similarity of design and fabrication process with the HC-B4 propeller hub, should be inspected for cracking in the hub arms (A-92-83).
In a Jan 4, 1993, letter to the FAA Administrator, then-NTSB Chairman Carl Vogt wrote that The Safety Board notes that the FAA is reviewing the service history of the Hartzell propeller hubs to determine the magnitude of the problem. Regardless of … other examples of cracking or fractures … the Safety Board believes that a once-through-the fleet inspection of the subject hubs is necessary. The Safety Board urges the FAA to reconsider the actions planned in response to Safety Recommendations A-92-81 through 83.
Then in a March 4, 1993, letter to the FAA, the Safety Board re-stated once again its concern that no action had been taken to find a better method of inspecting the hub arms. In addition, the Board was concerned that the FAA saw no need to review the design and fabrication process of other Hartzell propeller hub models to determine if similarities in design might indicate the need for inspection of these other hub models.
The Board then classified FAAs rebuttal of Safety Recommendations A-92-81 through -83 as Open – Unacceptable Response.
A few weeks later, on April 19, another Mitsubishi MU-2B-60 carrying the governor of South Dakota crashed near Zwingle, Iowa, killing all eight aboard. The crash stemmed from a fatigue failure in an identical propeller hub that rendered the airplane incapable of maintaining altitude. It descended and crashed into a silo in IMC.
More than a month after the governors fatal crash, the FAA acquiesced in a letter to the Safety Board dated May 21, 1993. The agency had decided to take action and consider a wide range of actions designed to be responsive to the subject recommendations, from the original MU-2-60 accident on September 27, 1991. Meanwhile the FAA issued AD 93-09-04 on April 28, 1993, and AD 93-12-04 on June 10, 1993, which required prop hub inspections for cracks. Repeat inspections were required every 600 hours.
People Strikes
No discussion of propeller mishaps would be complete without addressing human injuries from prop strikes. The first rule is to assume when you walk up to an airplane that the mags are on. Otherwise, a conscientious move to pull the prop blade from vertical to horizontal to put on a tow bar could cause the engine to kick over – with potentially disastrous results.
The pilot of a Navion was accomplishing a walk around and pulled the prop from vertical to horizontal. The engine fired and the prop literally cut his legs out from under him. Quick surgery managed to prevent a permanently crippling condition.
To preclude this kind of accident it is wise to perform a mag grounding check before shutdown to determine if the magneto p-leads have broken. These leads are connected to the cockpit ignition switch and ground the mags to keep the engine from running.
At idle power, check each mag by slowly selecting left, right, then off. When you hear the engine quit, quickly return the mag switch to both. To prevent spark plug fouling, increase power to 1,000 RPM for at least 30 seconds and lean the engine to idle/cutoff. Make the test at idle power and you will not get a backfire if the idle and procedure are performed correctly.
Another accident hazard involves loading or off-loading passengers with the engine(s) running. In a word: Dont. I once watched the eight-year-old daughter of my next-door neighbor run through the prop of her fathers newly purchased Ercoupe. He was taking the five kids to ride one at a time and didnt want to shut down the engine.
The little girl lost a big chunk of brain tissue and survived as a mental and physical cripple until about age 23. It can happen to anyone in a hurry. Float planes are particularly vulnerable, especially when loading passengers, casting off or docking. Hand propping an airplane when the batterys dead or the starter fails provides another good place to lose body parts.
As the general aviation fleet ages, propeller neglect is likely to become an ever-greater problem. Neglect and ignoring the overhaul guidelines is becoming more common. Yet, as the accident record shows, your life depends on an airworthy prop.
Also With This Article
Click here to view “Prop Safety Quiz.”
-by John Lowery
John Lowery is a former Air Force pilot, accident investigator and corporate pilot.
