1.0 Introduction
When examining the structural condition of aircraft, engineers must consider the
safety of their passengers. In the aerospace industry, a misjudgment of the
condition of a structure could result in fatal consequences. Routine inspections
greatly increase the lifespan of an aircraft. Although such inspections are costly
and time-consuming, they are necessary to ensure the structural integrity of the
aircraft. The considerable cost associated with purchasing new aircraft makes it
impractical for airlines constantly to replace damaged airplanes. Refurbishing
existing aircraft is much more economical. Regular safety inspections and proper
maintenance techniques improve the life of an aircraft and the safety of the
passengers within financial limitations.
The presence and growth of cracks caused by fatigue is one of the most
common causes of aircraft failure. Early cracks may be extremely minute and are
often undetected. Concealment of a crack may cause it to reach its critical crack
length during flight and result in structural failure. The existence of cracks is
common at the area of a fuselage where rivets are inserted to connect lap joints.
The probability of a crack at a rivet hole is higher than at other areas because of
the geometric change that occurs at the countersunk hole and because the
stress concentration at the rivet causes the crack to propagate[1]. For this
reason, aircraft must undergo routine rivet-concentrated inspections to prevent
disasters such as that incurred by Aloha Airlines in 1988.
In 1988, Aloha Airlines Flight 243 experienced a tragedy that restructured aircraft
inspection procedures. While in flight, the aircraft suffered extreme pressure loss
due to the rapid growth of premature cracks located around the rivet locations of
a separated lap joint. Investigations revealed that a longitudinal fuselage crack in
the upper row of rivets along the S-10L lap joint, about halfway between the
cabin door and the edge of the jet bridge hood, caused the catastrophic event [2].
Figure 1 shows a picture of the structural damage of Aloha Flight 243. At the
time, the FAA required inspections of lap joints located at stringers S-4R and S4L but did not require inspections at stringers S-10R and S-10L. The FAA now
mandates inspections for stringers S-4R, S-4L, S-10R, S-10L, S-14R and S-14R
to avoid disasters such as that suffered by Aloha Airlines Flight 243. Figure 2
depicts the inspection areas identified by the stringer locations. Lap joint
inspections currently lack accuracy. One reason these inspections are
inadequate is the inconsistent rivet finishes. Boeing currently recommends an
additional eddy current test to determine the finish of lap joint rivets. Because
eddy currents responses vary with different finishes, inspectors can visually
determine the coating on the rivet. The background information section of this
paper discusses details of this procedure.
Figure 1. Aloha Airlines Flight 243 [2]
Figure 2. Boeing 737 Stringer Layout [3]
In The Design of Everyday Things, Donald A. Norman, speaks of selective
attention, a psychology term that refers to excessive attention on one particular
detail leading to the neglect of other details [4]. One drawback to Boeing’s
inspection method is that the person conducting the test must focus on keeping
the probe aligned with the rivet’s centerline while glancing at the display screen
for the output. Focusing on both tasks throughout the entire experiment most
likely will result in the inspector’s ignoring one of the two tasks at some point
during the experiment. Norman explains “when there is a problem, people are
apt to focus on it to the exclusion of other factors” [4]. Our goal is to develop a
practical and timely design for determining coating finishes that eliminates the
need for simultaneous attention to multiple tasks.