Fix Failures, Don’t Prevent Them

The Federal Aviation Administration (FAA) chases after flaws, one by one, instead of eliminating at one fell swoop an entire class of potentially fatal deficiencies.

It is the ultimate reactive approach to safety, which is why the FAA is often criticized as taking a tombstone approach — ordering selective fixes after the bodies are buried.

The FAA’s selective attitude to structural durability illustrates the agency’s activism after, not before, failures have hazarded passengers and crew in flight.

Recall the Southwest Airline captain’s radio message to ground controllers after his B737 experienced almost a 5-foot fracture of a fuselage lap joint in the top of the plane, “Request an emergency descent. We’ve lost the cabin and we’re starting down.”

By “lost the cabin” he meant the pressurization that kept the airplane safe for passengers to breathe without emergency oxygen.

Passenger Shawna Redden, seated in row 8, heard an ear-splitting BANG. Oxygen masks deployed from the overhead consoles, and the airplane pitched into a dive. As the cold blast of air at 36,000 feet rushed into the cabin, she took the hand of the man next to her. “If I’m going down,” she later recalled, “at least I want to feel connected to somebody.”

Southwest Flight 812 was able to safely land at Yuma, AZ, on that April Fool’s Day in 2011. Investigators from the National Transportation Safety Board (NTSB) arrived the next morning, cut out the failed lap joint and sent it back to their Washington DC laboratory for analysis.

lapjoint

The failed lap joint in the NTSB laboratory

Cracks in the same area were found on five other B737s. The B737 that made the emergency landing had last been subject to a maintenance “heavy check” in March 2010, when cracking should have been — but wasn’t — detected and repaired.

The FAA was sufficiently energized to issue an emergency airworthiness directive (AD) on 5 April 2011, just four days after the lap joint on Southwest Flight 812 with 118 passengers aboard split open with a startling crack. For the FAA, this was light speed for issuing a Band-Aid.

Emergency AD 2011-08-51 was effective on receipt and required detailed instrumented inspections — not a visual look-see — of all B737-300, -400 and -500 series models, with different schedules:

* For airplanes that had accumulated less than 35,000 flights, conduct inspections and any necessary repairs within 20 days. Since the FAA usually allows months or even years to conduct inspections, again this was requiring a light-speed response.

* For B737s that had flown more than 35,000 flights, inspect the lap joints within 5 days and, if necessary, effect repairs before further flight. The accident airplane had accumulated 39,781 flights when the lap joint tore apart and allowed hurricane speed winds into the cabin.

* For all effected airplanes, repeat the inspections every 500 flights thereafter.

The FAA has no idea about the state of all B737s covered by the AD, as it did not require reporting of findings. Whether the cracking was limited to the six airplanes mentioned by the NTSB, or 60 of the airplanes, the FAA to this day remains sublimely ignorant. It’s as if the FAA issued the emergency AD and considered the problem solved. The agency reflected a shocking lack of curiosity; just why was the cracking missed during a heavy maintenance check just months previously, when everything on and in the airplane is supposed to undergo detailed scrutiny?

The AD is just one of dozens issued over the years on Boeing lap joints. As various models age in service, the frequency of structural ADs increases.

A 2006 paper, titled “Can We Hope to Make Today’s Concern About Aging Aircraft a Thing of the Past?”, provides a positive answer.

The paper recounts the history of the DC-8, a four-engine jetliner designed in 1955. The airplane had a long history in passenger service.

dc-8

The original DC-8, shown here in Pan American Airways livery

The paper contains an astounding revelation:

“The [fuselage] splice was so effective that throughout the entire service history of the DC-8 not even one AD ever was issued against this splice. Many of the DC-8s are in use as freighter [aircraft] even today. Their passenger service was discontinued because their systems became obsolete long before the airframes showed any sign of age.”

Well, there may have been some age-related corrosion problems, but no AD was issued because of the flawed (or over optimistic) design of the rivet installations on skin joints.

One of numerous concepts developed at Douglas Aircraft  for improved durability of fuselage joints. 'Minimized eccentricity' was  essential. Note that the skins do not overlap.

One of numerous concepts developed at Douglas Aircraft
for improved durability of fuselage joints. ‘Minimized eccentricity’ was
essential. Note that the skins do not overlap.

To what practices are owed the trouble-free joints? First, the skins were butted together smoothly, not overlapped. The result of the Douglas practice was the absence of eccentricity in the load path between the two skins. The presence of rivet holes worn to a loose-fitting oval, as found on many Boeing lap joints, was a sure sign of uneven load paths between the two, lapped skins.

The Douglas butt-splice design also featured rivets that were driven from the inside of the joint, not from the outside in. The flush, countersunk head of the rivet was formed during the process of riveting. This “cold working” of the rivet installation process filled the countersunk recess in the skin so well that corrosion was practically eliminated. Excess material protruding on the outside of the countersunk hole was shaved off for a nice, smooth appearance.

The design imposes far less risk of failure. As the paper describes:

“Unlike a butt joint, one skin is concealed under the other in the lap joint. It is, therefore, necessary to design the lap joint in such a way that the first fatigue cracks must appear at the visible location, i.e., the outer skin must crack in the first row of rivets. At the same time, care should be taken to ensure that the first row of rivet cracking does not lead to the problem of MSD.” [Multi-site damage, which prompted the latest AD on B737s]

Douglas Aircraft was eventually merged into Boeing, but the highly successful Douglas joint design was apparently lost in the shuffle — or the extra cost of the final step to shave off excess rivet material was deemed too costly. Remember, no aircraft manufacturer has ever warranted its aircraft with the equivalent of Hyundai Motors’ 100,000 mile guarantee for its autos.

Reworking thousands of Boeing jetliners to feature the superior design on the DC-8 would doubtless prove more costly than the “worth” of remaining service life. But here’s betting the less-durable overlap of the Boeing design was blessed by the FAA, despite the agency’s role in certifying the DC-8 for service with its superior butt joints.

Certainly there is nothing in FAA regulations today that requires skin joints of the trouble-free type installed on the DC-8. Obvious success stories are not replicated throughout the fleet, irrespective of manufacturer, because the FAA only watches for failures, not successes.