I was very interested to learn that the problem with the hollow blades on the 337 was not corrosion as I had previously thought, but sourced to an entirely different problem.
Within a couple of weeks of the January 1950 Stratocruiser blade failures, the Civil Aeronautics Administration (CAA) ordered close inspections of Model 2J17 propeller blades. A tapping test of the blades was intended to detect fissures in the filler material of the shell cavities. The CAA also ordered a magnetic inspection for cracks when the aircraft made its first stop at its main base. Any blades with cracks or large filler fissures had to be removed from service. As noted in the Brazilian accident report of 06/26/53:
“…with less than two years of service experience, longitudinal cracks were found in a number of blades in the outboard blade portions, about four inches from the leading or training edge and parallel to the edge. These cracks were found to be due to the black rubber filler in the blade breaking and allowing the steel shell of the blade to vibrate locally from aerodynamic buffeting.
At this point the blade design was again altered by using nylon-rubber filler instead of black rubber, the nylon filler being tougher and more resistant to shattering. A daily tap test, required as an interim measure, to locate any internal voids that were occurring in the blades with the black rubber filler”.
Another modification was the introduction of a nickelplated hollow steel blade for Boeing 377 aircraft in early 1954. The CAA certified this blade, manufactured by Hamilton Standard, for air carrier operations on September 14, 1953. This blade was a bit heavier than the unplated one
because of the plating. Hamilton Standard hoped that the plated blade would be less subject to damage from foreign objects picked up by the propellers.
The CAA made installation of propeller blade imbalance detectors mandatory as of July 30, 1955 on Boeing 377 aircraft.
The detectors enabled the flight engineer to identify which engine or propeller was malfunctioning and to shut down the engine and feather the propeller.
The C.A.B. administrator stated the final cure for the problematic hollow steel blades in a letter dated June 28, 1955:
“As a result of this investigation, and of the investigations conducted following six other accidents or serious incidents, we have concluded that, in the interest of safety, the Hamilton Standard Model 24260 propellers having 2J17 series hollow steel blades presently used on Boeing B-377 aircraft should be removed from service and replaced with propellers having solid metal blades. This shall be done at the earliest possible date consistent with the ability of the propeller manufacturer to supply satisfactory blades.”.
The final governmental action in the hollow steel-bladed Boeing 377 Stratocruiser was an issuance of an airworthiness directive in the fall of 1958.
Airworthiness Directive
AD 58-19-02 HAMILTON STANDARD: Applies to All Hamilton Standard 2J17 Hollow Steel Propeller Blades installed on Boeing 377 Aircraft.
Compliance required as indicated.
Operators that are using or have used these blades learned through adverse experiences that exceptional maintenance procedures and repair techniques are required to assure the continued airworthiness of these blades. In order to preclude the possibility of additional adverse experiences occurring either under the supervision of the present operators or other operators who will acquire some of the involved aircraft, the following shall apply:
1. Not later than August 1, 1959, remove from service all 2J17 Series propeller blades.
2. Prior to August 1, 1959, no operator who has not had previous experience in the overhaul and maintenance of 2J17 Series propeller blades shall be issued a certificate of airworthiness for aircraft on which these blades are installed.