Aircraft designers have come quite a long way since the dawn of aviation. Calculations and drawings once done by pencil, paper and slide rule are now done by high-speed computer processors and modeling programs. Add in today’s lighter, stronger composite materials and state-of-the-art manufacturing techniques, and aircraft are not just airworthy, they can easily see several decades of service.

Lengthy aircraft service life, however, is a double-edged sword. It’s wonderful for the corporations and individuals who spend millions of dollars buying them, mostly because it keeps them from having to replace them as frequently. The down side is that the longer an aircraft is kept in service, the more problems it is susceptible to; some of which can be chillingly deadly.

Aloha Airline Flight #243

On April 28, 1988 at 13:25 HST, a Boeing 737-200 operating as Aloha Airlines flight 243 departed Hilo International Airport on a regularly scheduled flight to Honolulu. On board were 89 passengers and a crew of five. The departure and climb-out were uneventful, but once they leveled-off at 24,000, things changed… dramatically.

The pilot and first officer reported a loud "clap" or "whooshing" sound, along with some sort of buffeting that jerked their heads backward. One look over their shoulders and they knew they had a serious problem. The cockpit door was gone, and 18 feet of the cabin’s skin just aft of the door had been ripped away. The pilots managed to land at Kahului Airport on Maui, but senior flight attendant Clarabelle Lansing was missing. She had been sucked out of the gaping hole in the cabin while the plane was still at altitude. Many of the remaining people on board suffered injuries, some serious.

The National Transportation Safety Board (NTSB) cited "disbonding and fatigue damage" near the cabin entry door as the cause of the hull breach, which was due in part to the condition of the 19-year-old aircraft and its 89,090 takeoff-landing cycles — 14,000 cycles over the amount it was designed to withstand.

United Airlines Flight #232

On July 19, 1989 at 14:09 CDT, 285 passengers and 11 crew members departed Denver en route to Philadelphia via Chicago aboard United Airlines Flight #232. The tri-engine McDonnell Douglas DC-10-10, built in 1973, had 43,399 total hours on its airframe, and 16,996 cycles at the time of departure.

Approximately one hour after takeoff, and while cruising at an altitude of 37,000 feet, the crew heard a loud bang, and began experiencing severe difficulty controlling the aircraft in all three axes. Their gages read that they had no hydraulics. They declared an emergency and fought to keep the wide-body airliner under control long enough to reach Sioux City Airport.

In spite of the best efforts of the captain, first officer and a check pilot who happened to be aboard, the DC-10 slammed into the runway and tumbled to a stop in a ball of flames. Of the 285 passengers aboard, 111 people lost their lives.

The NTSB determined there was an uncontained failure of the stage-one fan rotor. The debris severed all of the hydraulic lines in the tail. Further investigation revealed a "failure of maintenance personnel to detect a fatigue crack originating from a previously undetected metallurgical defect in a critical area of the stage-one fan disk."

The common denominator in the NTSB’s findings in the Aloha and United Airlines tragedies was fatigue, presumably brought on by old age. But what is fatigue? Of equal importance is the more elusive question: What is an "aging" aircraft?

What Is Fatigue?

In his book Fundamentals of Aircraft Material Factors — Second Edition, the late Charles E. Dole, EdD, noted professor of aeronautical engineering and safety technology at the University of Southern California, defined fatigue as "progressive localized structural damage." He went on to list the three requirements for a fatigue crack to form and spread in metals.

First, there must be local plastic stress. Next, there must be a tension stress. Finally, there must be a cyclic (repeated or fluctuating) stress. Aircraft are notorious for subjecting their structures and components to all three of these conditions, particularly during cabin pressurization cycles. "If we can eliminate any one of these three requirements," wrote Dole, "we can stop the fatigue process." He went on to insist that proper inspections are the key.

What Is An Aging Aircraft?

Technically, everything begins aging the second it comes into being. The more accurate question for aircraft is "What is old?"

Figuring out the chronological age of an aircraft is pretty straight-forward. Subtract the date it was built from the date you asked the question, and that’s how old it is. From a practical standpoint, that formula stops well short of determining the condition of the plane.

In their book Aircraft Acquisition and Planning, authors Al Conklin and Bill de Decker warn their readers to look at several areas, in addition to chronological age, to get a feel for the actual condition of an aircraft. They cite total time, time since overhaul, cycles, operating environments (i.e., salt water, extreme temperatures, altitudes, etc.), load, and block speeds (the distance from origin to destination, divided by the time from "door open" to "door closed").

Two other factors that help to determine how "operationally old" an aircraft is are also not apparent on the Hobbs meter or in the logbooks. They are abuse and neglect.

As is the case with any piece of machinery, aircraft are more prone to failure when they are handled roughly, operated outside of the manufacturer’s instructions or poorly maintained. The older an aircraft gets, the more opportunities it has to experience life-shortening limit transgressions, structural abuse, and maintenance that falls short of the manufacturer’s recommendations (even if the records magically fail to indicate it).

The FAA and Congress React

In response to the 1988 Aloha Airlines crash, Congress enacted the Aging Aircraft Safety Act of 1991. The Act, which was sponsored by Rep. James L. Oberstar, D-Minn., was written to "… preserve the structural integrity of the aging airplane fleet." It requires the FAA to perform an Inspection and Records Review of all airliners, 14 years or older, used by air carriers to provide air transportation. The primary focus of the inspections was to locate structural fatigue, and to ensure that "…maintenance of the aircraft’s structure, skin, and other age-sensitive parts has been adequate and timely."

Several years later, with the crash of TWA Flight 800 off the coast of Long Island (1996), and the loss of Swiss Air Flight 111 over the Atlantic near Halifax (1998) — both of which were fatal accidents believed to involve corroded wiring — the FAA convinced legislators to expand the program to cover non-structural systems. So, in 2005, the Aging Airplane Safety Rule was issued to require certain operators to conduct some of the inspections on their own fleets.

On September 7, 2006, Todd J. Zinser, the acting inspector general of the U.S. Department of Transportation, provided a written follow-up report to Rep. Oberstar, saying:

"[The] FAA’s Aging Airplane Safety Rule requires that FAA inspectors perform reviews of aircraft maintenance records and visual spot inspections of certain aircraft; therefore sub-surface cracks or hidden corrosion would be impossible to detect."

To support that assertion, Zinser cited the December 2005 crash of a Chalks Ocean Airways Grumman G-73T Turbo Mallard. The seaplane lost a wing in flight near Miami killing all 20 people aboard. Zinser wrote, "This incident shows that for those aircraft only covered under FAA’s Aircraft Inspection and Records Review process, the structural integrity of the aircraft cannot be assured," referring to the program’s incapacity to detect the fatigue cracks that the NTSB found as the cause of the structural failure.

Other rule-making in this area is still being examined the FAA.

The Search For Problems

With greater emphasis being placed on the careful inspection of aging aircraft, maintenance providers are extra vigilant when it comes to spotting the potential problems that time and wear can generate. OEM’s are especially concerned when they are called upon to conduct scheduled maintenance and even non-required upgrades.

Occupying several large hangars at New Castle County Airport outside of Wilmington, Del., Dassault Falcon sees its fair share of jet aircraft coming in for service. On a recent visit, Aviation Maintenance found them working on a Falcon 20-5 that was beginning to show its age.

"This one came in back in December," advised John Rahilly, VP of sales and marketing, pointing to a sickly looking twin-engine jet in the far corner of a brightly lit hangar. "It’s 24 years old, and came in for an inspection and equipment upgrade." The senior Falcon jet on the property at the time, it had already been stripped of its paint, had its interior gutted, and was being tended to by a small army of technicians installing wires and sensors for the new avionics suite.

"Let me check the book," said Ken Poplos, a 28-year A&P with the Dassault Falcon and the project manager on the model 20-5 I was walking around. "This one has 9261.3 hours total time, and 6314 landings," he reported. It was in for a C-check, hot section, and an upgrade from the original instruments to an IDS-3000 glass cockpit.

Climbing up an inspection platform up to the nose, I could see three technicians tracing wires and inspecting cables. I could also see that a section of frame was missing. "That’s one of the problems we found," said Poplos. "An eddy current test detected a crack in that windshield frame. We have the piece down here, if you want to see it."

On a nearby table, Poplos showed me the section of aircraft that once held the left-side cockpit glass. Like the plane itself, it had also been stripped of paint, and, in some places, even the primer. "It says here that cracks were detected at holes #7 and #11," he declared as he perused the red tag attached to it. "We would have never found them just by looking."

Sure enough, I could not see any deformation. In fact, holes #7 and #11 looked just as good, if not better, than the other two dozen machined holes. Poplos said the cracks were not there when the aircraft was in for its last inspection six years ago. "It might have been a bird strike, or maybe age that caused it," he surmised.

When asked what the most common problems were in older aircraft, Poplos didn’t have to think very long. "Corrosion is the most common problem," he said. "You look for that anywhere that something can leak and sit. Lavatories and galleys are the most common places, but corrosion can appear around windows, too," offered Rahilly. "The second most common problem with cracking and chaffing comes from a lot of cycles and parts wearing, especially flight controls," continued Poplos. "Third are lubrication problems."

Although the 24-year-old Falcon I was looking at had been properly maintained by Dassault throughout its life, Rahilly and Poplos agreed that they sometimes inherit problems when an aircraft — particularly an older one — has been taken to a less-than-reputable shop before coming to them. "We’ve seen some workmanship from mom and pop shops that we had to repair," said Poplos. "We were fixing history."

Rahilly said, "The problem for us is that once we open [the aircraft], we own it," referring to their responsibility to ensure that every abnormality they discover is properly repaired according to OEM and FAA standards, even if was caused by poor workmanship at a non-affiliated shop.

Dassault has noticed an increase in the maintenance and upgrade business in recent years. They attribute much of it to the increasing popularity of bizjets in the post-9/11 world, and the longer service life aircraft are receiving by way of better engineering and life-extending component upgrades.

But with the increase in aging aircraft, maintenance providers are searching for better ways to identify problems caused by structural fatigue and corrosion. Aging electrical wiring is also of great concern.

PASD Inspection Technology

To help technicians search miles of wiring inside of aircraft, where accessing every inch of the run can be time consuming and very difficult, New Mexico-based Sandia National Laboratories, a Lockheed Martin Corporation, has developed a helpful diagnostic device.

According to the company, their newly patented Pulse Arrested Spark Discharge (PASD) can be plugged into an aircraft’s wiring harness 40 conductors at a time. It then shoots a high-voltage pulse down the line for one billionth of a second. If that pulse runs into a break, it will arc to the aircraft’s frame or another wire with broken insulation. The unit will then measure the time it takes for the pulse to return, thus pinpointing the location of the break, reportedly within inches.

"Rather than ripping apart the fuselage for access to a faulty harness that may run the length of the plane, airline mechanics will be able to use this new tool to efficiently locate and repair the fault," said Larry Schneider, Sandia’s project lead. "Wiring insulation [that has] grown defective over time can cause malfunctions or even fires, but is devilishly hard to spot and even harder [once located] to [exactly] locate," he said in an official company statement.

Astronics Advanced Electronic Systems of Redmond, Washington, who was tasked by Sandia to downsize the prototype to the dimensions of a small suitcase, is excited about PASD’s potential.

"We really value PASD technology," said Astronics team leader, Mike Ballas for Sandia’s June 23, 2006 newsletter Sandia Lab News. "We licensed it into a practical portable test unit targeted for the aviation industry to find intermittent faults, and we believe it’s the best way now to do the job. We’re advertising the system now, and we’d love to take orders."

The FAA, however, is not so optimistic about PASD. In the same Sandia Lab News release, Mike Walz, who was, a the time, the FAA’s project overseer, said, "There’ll be problems just the same in getting the method accepted. What PASD looks like is an electrostatic discharge — something aircraft manufacturers work hard to keep out of their wiring system."

Back at Dassault Falcon in Wilmington, Rahilly and Poplos agreed that tracking down wiring problems in aircraft, especially older ones, is very time-consuming for the 220 technicians assigned to their facility.

"We stay busy. We’re always looking for A&Ps, people in avionics and paint," said Rahilly. "[With the current shortage of skilled people] we’re always looking for about ten of everything!"

Room for Comfort?

As it stands, the aviation community and the flying public can expect aircraft to enjoy a longer life expectancy. They may also find additional comfort in knowing that the FAA and private industry are searching for new ways to ensure the safety of aging aircraft. Whether or not the ravages of time on an airplane can be discovered before they can lead to tragedy remains to be seen, but there appears increased interest in making that happen.