Structures, like the door stop components of the Boeing 737-9, may fail in a process known as fatigue, and fatigue failures start with a single failure, followed by many subsequent failures. When jets take off, the fuselage, or the skin of the aircraft, expands, and a new force is slammed against the fuselage during every takeoff. These conditions were in place when the door stop blew off of a Boeing jet. Engineering failures are complicated, and we learn as we move forward.
A History of Jetliner Failures -The Comet
In January 1954, the Comet jet crashed, and that crash killed all 36 passengers ("Why Did The de Havilland Comet Jet Airliner Keep Crashing?", click here). The Comet fleet was temporarily grounded. Placed back into flight in March, another Comet jetliner crashed and killed 21 people in April 1954. These jets failed in a process known as fatigue, where a crack started in the jet fuselage and the top half of the jets unzipped off of the planes, and both jets crashed over the Mediterranean Sea (Figure 1).
Fatigue Failures
Fatigue is a statistical failure process. That is, for a given stress, or loading condition, the number of cycles to cause a crack varies by a factor of more than 100, as shown in Figure 2. The first crack failure can happen long before subsequent cracks at other locations, but those other cracks are on the way.
The Rediscoveries of Fracture Mechanics
As a direct result of these two crashes, technology moved forward in a giant step to understand fatigue failures. Bending a clothes hanger back and forth is an example of a fatigue failure. There two different scientific methods that are used to explain fatigue failures, where repeated, or cyclic, loads cause structures to crack after many loads. Classic Fatigue Theory is purely experimental, and graphs of stress versus the number of cycles to initiate a crack are available for many materials. Fracture Mechanics describes how fast a crack will grow to tear a structure in two - once a crack has formed in a structure, like a jet. For the Comets, cracks were found in multiple jets, and fracture mechanics has been applied to operating jet aircraft to determine how much longer those jets could remain in service.
This new application of fracture mechanics was a bold new application of science. Science was established to eliminate the risk to life during subsequent flights. Fracture Mechanics was discovered by Griffith in 1920, and his discovery stayed in the shadows until the Liberty Ships started to break in two during World War II. Although receiving some use, fracture mechanics then stayed pretty much in the shadows until the Comet crashes of 1954. Today, Fracture Mechanics and Fatigue Theory are two courses that are taught in graduate schools, such as the University of South Carolina where I studied these engineering courses.
The Boeing 737-9 Door Stops
These door stops are installed in door openings, in lieu of secondary doors for the aircraft (Figure 3). As the planes go up during takeoff, the jets expand outward like an aluminum balloon. Stresses increase during these takeoffs, and fatigue stresses are in process. The door blew off as the jet ascended, and fatigue conditions were certainly in process during this door stop failure. The question becomes, can the upcoming investigation be expected to be effective.
NTSB Credibility Concerns
The National Transportation Safety Board (NTSB) investigation team has a credibility problem, based on past performance ("Gas Pipeline Explosions and Deaths can be Stopped! - The San Bruno and Carlsbad Pipeline Explosions - Recent Findings"). In my many years of working as a lead engineer, I have found that engineers will find an answer to a problem, whether the answer is right or not. American industry has a long history of closing investigations without sufficient facts ("Book Publisher Wanted for a New Book", "Industrial Murder for Profit").
The NTSB found an answer to the San Bruno explosions, but they refuse to pursue new information to stop ongoing gas pipeline explosions. Given this lack of performance, malfeasance, or nonfeasance, the ability of the NTSB to properly conduct this 737 investigation is in question. The FAA is stepping into this investigation ("FAA launches probe into Boeing 737 MAX 9 door plugs", click here), and jets have been grounded to ensure safety during an investigation.
Tests to Prevent Deaths
Full-scale tests of the door stops are recommended. Simply finding that cracks are not observed will not ensure that the next door does not fly off of a jet, and maybe kill the passengers on board next time. I cannot state as fact that fatigue was the exact cause of this failure, but based on studies of fatigue failures, fatigue needs detailed investigation to ensure that an airline crash does not occur.
What Dangers Lurk in the Future of Jet Flights?
What is the fallout if fatigue is the failure cause for this door stop blow-out ("2 major airlines find loose bolts, other problems on grounded Boeing jets", click here)? Is there a jet airliner design problem? How many other aircraft contain similar door stops? Are hinges on doors themselves subject to such failures in the future? Until we know the cause of this 737-9 door-stop failure, we do not know what other aircraft are in danger, and consequently we do not know if we are safe during other flights.
Addendum
'An initial assessment provided by NTSB officials indicates none of the door plug's four bolts had been installed ("Boeing faces FAA audit after its 737 Max 9 plane's door plug blew off mid-air", Click Here ). If this statement is true, then fatigue may be a secondary, but important, cause to this failure. If the bolts were not installed, how did the door stop remain in place for earlier flights as the cabin pressure pushed the door stop outward? Alternatively, the bolts could have fallen to the ground at a location different than the door stop.
This failure analysis is still unfolding. There are many questions. Is there a design problem? Was there a manufacturing problem? A material problem? How many flights were there for this aircraft? Are we still in danger when we fly?
(Article changed on Jan 14, 2024 at 12:29 PM EST)
