121pilot, a commercial airline pilot in the USA, has graciously written a three-part series on the history of aircraft groundings. Today, we explore the propeller era.
In evaluating the FAA’s decision to ground the 737MAX, it’s helpful to review the history of aircraft types grounded by regulators and operators and the underlying reasons for those groundings. Before we begin, a couple of caveats. Where possible I have reviewed the accident reports of the incidents in question. But that has not been possible in every case forcing me to rely on other sources to understand the root cause and actions taken. Also, this is going to be lengthier than most of what you read here because breaking down an aircraft accident (especially one that leads to a type grounding) isn’t something that happens in 25 words or less.
The first airliner grounded was the Fokker F-10 after one operated by TWA crashed on March 31, 1931 with Notre Dame coach Knute Rockne onboard. The crash occurred when one of the wings separated in flight. The F-10 used wooden wing spars and it was found that over time as the wing and the spar got wet the glue bonding the wing to the fuselage weakened eventually leading to the wing separating. The accident essentially ended the commercial career of the F-10 due to not only a loss of confidence but primarily because the inspection regime mandated after the accident by the CAA (The Civil Aeronautics Authority, the predecessor agency to the Federal Aviation Administration [FAA]) made the aircraft uneconomical to fly.
Lockheed L-049 Constellation
The second was the Lockheed model 049 Constellation. A TWA Constellation on a routine training flight on July 11, 1946 caught fire in flight and eventually crashed in a field leading to the CAA grounding the aircraft the next day. To quote the report:
The Board determines that the probable cause of this accident was failure of at least one of the generator lead through-stud installations in the fuselage of the forward baggage compartment which resulted in intense local heating due to the electrical arcing, ignition of the fuselage insulation, and creation of smoke of such density that sustained control of the aircraft became impossible. A contributing factor was the deficiency in the inspection systems which permitted defects in the aircraft to persist over a long period of time and to reach such proportions as to create a hazardous condition.
In layman’s terms, wires rubbing against the structure caused a fire which quickly got out of control. The Constellation returned to flight on August 23, 1946 after the installation of smoke detectors and inspections of the electrical system which had spawned the fire.
The third aircraft grounded was the Douglas DC-6. On October 24, 1947, United 608 had a fire break out in the cargo compartment and ultimately crashed attempting an emergency landing, killing all aboard. Three weeks later on November 11, 1947, an American flight caught fire but in this case was able to make a successful emergency landing. With an intact aircraft to examine, investigators were able to determine that the fires were being caused by the location of a cabin heater air intake and fuel tank vent. If flight crews allowed the #3 tank to become overfilled during a routine fuel transfer it could lead to fuel being sucked into the cabin heater causing a fire. In this case, after the second fire, airlines operating the DC-6 voluntarily grounded the type for four months until corrective measures could be implemented. The crash was especially noteworthy for being the first one in which the wreckage was reassembled as part of the investigation.
De Havilland Comet
The fourth aircraft grounded was the De Havilland Comet. The case of the comet is particularly interesting. There were two early accidents (October 26, 1952 and March 03, 1953) where the aircraft failed to become airborne as a result of pilots over-rotating upon takeoff, leading to a loss of lift and thrust. Modifications were made and the type continued to fly. Then on May 2, 1953 a third Comet crashed in a thunderstorm six minutes after takeoff. The investigation concluded that the pilot may have inadvertently overstressed the aircraft pulling out a dive through pulling too hard on the flight controls. Because the Comet’s controls were hydraulically powered, they did not give the same level of feedback as previous mechanical systems. As a result, an artificial feel system was introduced along with weather radar and the type continued to fly.
Then on January 10, 1954, BOAC 781 broke up in-flight crashing into the Mediterranean. BOAC voluntarily grounded its comet fleet and De Havilland, essentially throwing darts at a board, recommended 60 changes. But with no apparent fault having been found BOAC returned their Comets to flight on March 23, 1954. Then on April 8, 1954 SA 201 also went down in the Mediterranean, and the type was immediately grounded. An extensive investigation determined the cause was stress concentrations around the rivet holes. Extensive modifications were made to early models that returned to flight, and De Havilland made major design changes to the follow-on Comet 3.
Tomorrow: the jet era.