Propellor Blade Failure
One of the most frightening failures that can happen in flight and unfortunately one of the most common, is propeller blade failure.
Many years ago I had this happen to me. I was flying an IFR twin engine pressurised Aero Commander 680FLP when suddenly there was a loud explosive decompression, passengers screaming, one passenger lying in the aisle, loss of all electrical power and, worst of all, blood spattered around the cabin. It was just coming on dark, I was just entering controlled airspace and above 8/8ths cloud – a tricky situation to say the least! Well to cut a long story short I managed to conduct an emergency descent through cloud to LSALT where I broke out into the clear. Fortunately I spotted Ballarat about 10 miles away on my left where the runway lights were already on (I had no radios to activate the PAL lighting). I diverted there and landed safely without further incident. It was only after landing that I realised the full extent of the damage. A large hole in the upper cabin where a 150mm length of propeller blade had penetrated. The broken tip of the propeller blade was found under a passenger’s seat after landing. The blood, by the way, came from passengers who had been injured by flying debris and the propeller blade tip: fortunately injuries were not serious and the injured passengers were released from hospital next day.
Even during our initial flying training we all have it drummed into us that, during our pre-flight inspection, we must always check the propeller blades for damage, particularly the leading edge for nicks. Well I have to say, leading edge damage was not the cause of my propeller blade failure! Unusually, my incident resulted in both parts of the broken propeller blade being available for the failure analysis. This analysis showed that the failure was a fatigue failure that had been caused by a small nick on the rear face of the blade approximately 50mm aft of the leading edge. Microscopic examination showed that the fatigue crack had started from this small nick and progressively propagated in both fore and aft directions from the nick. As the crack increased in size, the stress on the remaining uncracked portion would have increased with the crack propagation accelerating. Finally, the stress in the remaining blade cross-section would have increased until an overload failure occurred and the blade tip separated.
So what can be learned from my experience? Most importantly, propeller blade failures do not always originate from damage to the leading edge! The Aero Commander is a high wing aircraft with an engine mounted on each wing. In this type, the propellers have a much greater ground clearance than is the case with most single engine aircraft and, therefore, low wing twins, are much less prone to propeller damage from stones and the like. During my pre-flight inspection I had examined the propeller for damage: it was virtually unmarked. What I was not aware of, and could not see, was that at some time in the propeller’s history a nick had occurred on the rear face of the blade. This had been dressed out during subsequent maintenance and the area re-painted with black paint – a standard repair procedure. Unfortunately the engineer who carried out the repair did not dress all of the depth of the nick: he had left a residual small sharp depression. When he painted over it, the paint filled the remaining nick and all evidence of the underlying damage was hidden.
Interestingly enough the propeller blade that failed had only about 600 hours total time in service. As a result of having both parts of the failed blade available for microscopic examination the investigation was very thorough and complete. It was subsequently determined that the propeller blades used on that particular aircraft (it had been modified to an STC with bigger engines and different propellers) were marginal for the application and this contributed to the ultimate early failure. These blades were later replaced with thicker cross-section blades. The aircraft is still operating today, nearly 30 years after my failure and has had no more propeller trouble.
The moral of the story is – don’t just look at the leading edge of the blade when doing your pre-flight inspection: examine the whole blade! Some minor damage to the leading edge is inevitable and common, and some must be tolerated, otherwise we would be grounded most of the time. The trick is to know when the damage is bad enough to warrant grounding the aircraft and writing the damage up on the maintenance release. Propeller manufacturers provide inspection and repair guidance in the manuals supplied with each propeller they sell and this should be consulted whenever possible. As well as the manufacturers, the American FAA publish some excellent guidance material on this subject. The aviation repair ‘Bible’ of course is AC 43.13-1B, commonly available through aviation bookshops and suppliers. Chapter 8, sections 4, 5 and 6, covers propellers. An even more thorough (and free) FAA Advisory Circular is AC 20-37E ‘Aircraft Propeller Maintenance’. This AC is not so readily available and must be downloaded from the FAA web site.
Fatigue failures, as I have described here, only occur with metal blades. Wooden and composite propellers suffer different failure modes and I will discuss those in a later article.