Cincinnati Aviation Heritage Society & Museum

Preserving aviation's past for future generations.

REACHING   BACK - “Soft Steel”

By Dave Nichols

     On February 21, 1967 an Allegheny Airlines Convair 580 made a routine landing at Newark but the right propeller would not go into reverse pitch.  Upon examination of the internal prop hub workings, steel powder and small bits were found in the propeller hydraulic fluid.  The malfunctioning prop was removed and sent by truck to an overhaul facility in Kansas City.  This prop shop was about to come across a “tip of the iceberg” scenario where other CV580 propellers were ticking time bombs.

      The overhaul company had received and dismantled the offending four blade windmill on March 2.  They quickly found the cause.  Each blade’s angle is controlled by a torque piston.  This steel piston is located in the prop hub and is machined with a spiral groove so that as the piston slowly moves in it’s cylinder case, it rotates on the groove to change propeller pitch.  Each blade has its own torque piston.  The steel must be very hard and the piston must withstand high hydraulic pressure behind it.  Incredibly, two of the four pistons were softer than normal and the grooves had worn almost away.  The pistons must have careful nitriding and heat treatment to bring the machined piece of steel to extreme hardness so that it may function for 2000 hours without grinding down.  The Allegheny propeller had only 450 hours of use since new.  The shop knew that if one torque piston failed in flight, the affected propeller blade would almost instantly slam to low (fine) pitch and set up a horrific imbalance with the other blades.  They quickly called the FAA and propeller manufacturer.

      March 3rd found all operators of Convair 580s poring over a telegram.  The instructions said to check propeller hydraulic fluid for the presence of steel powder or filings and do it at the earliest possibility.  These operators were Frontier, Allegheny, North Central, Lake Central and a number of corporations.  Sure enough, another Allegheny and one corporate 580 had metal contaminated prop fluid.  Lake Central had two of their four 580s in the Indianapolis hangar on March 3 and 4.  N73125 (msn 52) and N73130 (msn 23) were both checked out and found to have clean propeller hydraulic fluid.  An Allison engine and prop specialist was on-site and verified the clean green fluid.

      The evening of March 5 found Lake Central N73130 flying flight 527.  A consistently strong revenue trip, it departed Chicago-O’Hare at 5:00 p.m. destined for Lafayette, Indiana (home of Purdue University); Cincinnati, Columbus, Toledo, Ohio and on to Detroit-Metro.  Flight 527 did especially well in boardings at Cincinnati and Columbus.  At the controls this chilly overcast evening was captain John Horn, age 45, and a long time pilot with Lake Central.  He was one of the few pilots who had flown  LCA’s cantankerous former C46 charter airplane.  John was very well liked by a variety of employees and was a solid mentor to first officers.  Co-pilot Roger Skillman, 33, had flown with Captain Horn many times and they functioned together like a fine time piece.  Young Barbara Littman was the stewardess, hired at the minimum age she was excited to be a two year employee.

      All was well and on-time through Columbus.  The flight had just started a shallow descent out of 10,000 feet on the quick leg to Toledo.  The air traffic radar controller located at Cleveland Center saw the blip for Lake Central 527 simply disappear from the screen.  Not a single transmission was made by the crew.  Some residents of Marseilles, Ohio heard a sound similar to an air raid siren, then silence, followed by a cacophony of heavy pieces thudding onto the ground.  These pieces were Lake Central’s first fatal crash and contained 38 people.

      Portable floodlights illuminated the battlefield-like scene of an aircraft sliced into two or more pieces while at altitude.  The newly commissioned National Transportation Safety Board (NTSB) found the right propeller had gone into complete fine pitch at 250 knots.   The blades were ripped from the hub by excruciating centrifugal force.  Three of the blades flung away from the fuselage but the fourth blade entered and exited the cabin with incredible force, slicing through all the electrical cables and severing much of the fuselage support members.  The investigators determined that the blade entered the cabin sharp end first, then rotated 180 degrees and exited butt first.  The resulting rapid yaw to the right broke the weakened fuselage into three components.  Total time from normal to breakup was estimated at seven seconds.

      The NTSB metal lab soon found that one of the propeller torque pistons was soft, that it had not been heat treated for hardness.  The total time on the failed piston was 1055 hours.  There was a curious number “12” stamped in heat resistant ink way in the corner of this piston.  No other prop pistons they had ever seen bore those numbers.  The propeller manufacturer researched and found that their in-house lab uses the numbers “12” to denote parts that were removed from the assembly line, received special testing, and then placed back on the line.

      The two failed pistons from the first malfunctioning Allegheny propeller were checked with a magnifying glass and they both were stamped “12”.  Then on to the other two affected 580 props.  Lo and behold, those torque pistons bore the tiny numbers “12”, also.  The propeller manufacturer traced all of them back to a single batch of ten torque pistons that their lab had performed some research analysis on and dutifully stamped them with a “12”.  There were still several unhardened pistons that were in the field.  The FAA issued an immediate Airworthiness Directive and hurried phone calls were made by all parties to operators who were unknowingly flying with the soft parts.  The defective pistons were found and removed before any more accidents occurred.

     One of the bad Allegheny props was run on a test stand until it failed.  The rpm of the propeller went ballistic in seconds.  This increase in rpm was so fast and forceful that the pitch lock mechanism was not strong enough to stop the disintegration of the blades.  All four blades tore loose from the hub.  Just before blade separation, a scream like an air raid siren was heard.

 It is still not fully known why most of the affected Convair 580 propellers did not visually show steel filings in the fluid.  Sadly, the well intentioned prop fluid test only partially did its intended job.

      The prop manufacturer found that their research lab removed the ten pistons from the assembly line for a routine size test.  Somehow, they surmised, when those ten were placed back on the assembly line, it was at a point after the heat treating phase.  Those ten soft parts somehow got past final inspectors who are supposed to check for hardness.  The unhardened torque pistons were innocently installed in new propeller hubs.  The FAA ordered the manufacturer to tighten assembly line vigilance and the final inspection process.  A design change was also ordered that beefed up the runaway prop preventing pitch lock device.                                                

Dave Nichols                                  March 18, 2001    

Webmaster's Comments

The NTSB Report of this accident where I (Charlie Pyles) lost three friends may be found here:

This crash was my first experience losing friends in accidents. I was the operations agent when 527 passed through Cincinnati that evening. John Horn always called in range at Cincinnati by saying "Marion, 2-7 In Range". My first name (which hardly anybody used) was the same as Marion, Indiana (MZZ) and the first time he did that I thought he was calling in range up there. Roger Skillman's son Mark is still a good friend of mine to this day (October 10, 2009).

N73130 Taxiing Photo