Case Study Analysis: Accident Research: USAir Airlines Flight 427, Boeing Model 737-3B7

Introduction/Executive Summary

Since air travel was first commercialized in the mid-20 th century, the world has
experienced a jaw-dropping number of fatal accidents, often claiming the lives of all crew
members and passengers on board. The aftermath of these crashes has always been
comprehensive inquiries, usually commissioned and headed by licensing and regulatory boards
like the Federal Aviation Administration (FAA) and the National Transportation Safety Board
(NTSB), to establish the causal factors and recommend remedies for preventing similar cases
from reoccurring. Although accidents still occur, the recommendations by these inquiries to
aircraft manufacturers and airlines have significantly shaped the aviation industry; these changes
have given rise to safer and more airworthy planes and new policies for airlines and crews,
making smooth sailing through the skies a possibility. The USAir Flight 427 crash roughly six
miles northwest of the Pittsburgh International Airport on September 8, 1994, is one of the
deadliest events that shook the aviation industry. NTSB was specifically tasked to investigate the
accident (National Transportation Safety Board, 1994).
This paper examines the primary causal, contributing, structural/mechanical, and
human/organizational factors that led to Flight 427’s crash, the outcomes of the NTSB inquiry,
and the potential risk mitigation strategies that could have averted the event or can likely prevent
similar crashes from occurring in the future.

Primary Causal Factor(s) of the Accident

Boeing Model 737-3B7 crashed on September 9, 1994, while approaching Pittsburgh
International Airport due to loss of control and aircraft stall triggered by an encounter with the
wake turbulence of a preceding Boeing Model 727 aircraft. In a report published by the NTSB,

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the loss of control resulted from a “full rudder deflection” to an opposite direction commanded
by the flight crew in response to the wake turbulence upset during the encounter, the flight crew
commanded the elevator to its “full-up position,” causing the aircraft to stall and further intensify
the loss of control. From then on, the pilots never regained control of the plane. It crashed
roughly six miles (9.65 km) northwest of the airport, claiming the lives of all 132 crewmembers
and passengers. After the accident, Boeing redesigned Model 737’s rudder systems to exclude in-
flight reversals. Additionally, the company introduced a training guideline, the Upset Recovery
Training Aid (URTA), which was developed by regulators to assist in training crew members to
recover from in-flight upsets.

Contributing Factors to the Accident

An inquiry into the crash by the NTSB revealed that the rudder surface’s shift to the
blowdown limit precipitated or contributed to the events (mechanical damage and loss of
control) that led crash. The report showed that the rudder most likely changed directions (moved
in the opposite direction) to that commanded by the flight crew after the main rudder power
control unit jammed, forcing the aircraft to roll and dive into the ground. In a detailed report
issued by the NTSB on March 24, 1999, a rudder system design flaw was found as one of the
main contributing factors. Tests done by the inquiry found that the rudder system was vulnerable
to an uncommented and sudden movement, which likely caused the plane to roll and dive.
The NTSB also pointed out a lack of “Rudder system Redundancy” to keep the rudder in
check as one of the main factors that precipitated the accident. The absence of the “Rudder
System Redundancy” meant only one thing: the 737-3B7 would automatically crash if the rudder
system failed. Aircraft maintenance issues by the airline, flight data recorder limitations, and lack

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of crew training on responding to rudder system failure were also outlined by the NTSB as
potential causes of the USAir Flight 427 crash.

Structural and Mechanical Factors Related to the Accident
As highlighted above, structural/design and mechanical failures of the rudder system
during the flight caused USAir Flight 427 to crash while maneuvering to land at Pittsburg
Airport. In its report, the NTSB revealed that a sudden and unpreventable movement of the
rudder surface to the blowdown limit was primarily responsible for the aircraft’s loss of control,
forcing the airplane to descend and crash.
The report showed that the rudder surface most likely shifted in an opposite direction to
that instructed by the flight crew after the main “rudder power control unit servo valve” jammed.
The NTST concluded that the rudder system of the Boeing 737 had design and structural issues
because investigations into other accidents involving the 737, including the Eastwind Airlines
Flight 517 in 1996 and the United Airlines Flight 585 in 1991, revealed similar ruder system
structural failures. Investigators later found that the rudder system for Boeing 737s was
oversensitive to bench tests compared to other aircraft models. The failure’s exact mechanism
encompassed the servo valve, which the investigators found to abnormally remain cold and
dormant for the most part of the voyage at high altitudes. Injecting the servo valve with hot
hydraulic fluid was found to seize or jam the motion of the rudder power control unit.
During the investigation, investigators also found that the flight data recorder (FDR) and
cockpit voice recorder (CVR) provided insufficient information about the events that precipitated
or occurred before the crash. Because the FDR recorded limited parameters, the investigators
concluded that one potential cause of the accident was the structural inability of the FDR to
accurately monitor, record, and transmit critical information about the rudder, elevator, ailerons,

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and many others. These crucial recordings could likely have alerted the crew about the damaged
rudder system.

Relevant Human/Organizational Factors Related to the Accident
Human errors by the pilots and mistakes by the airline (organization) could also have
contributed to the crash. For example, an analysis of the control-yoke data retrieved from the
flight data recorder (FDR) showed that the pilots made a critical mistake by pulling the yoke
back throughout the plunge – the “stick shaker” was audible on the cockpit voice recorder
immediately after the aircraft began the nosedive. Pulling back the yoke was an error because it
lifted the plane’s attack angle, displaced aileron authority, blocked the aircraft’s capacity to
regain from the roll triggered by the rudder, and resulted in an aerodynamic stall.
According to Bill (2002), since the plane was slipping, pulling the yoke back
compounded the “bank angle.” Boeing ran tests on a simulator to replicate the accident using
similar parameters recorded by the USAir Flight 427’s FDR. The experiment found that
recovering from a fully deflected rudder at 190-knot speed was only achieved by turning the
wheel in the opposite direction of the roll, and not pulling the yoke back to recoup aileron
authority (Bill, 2002). The FAA also wrote a report later remarking on the failure of the pilots to
employ standard crew resource management during the dive, continuing to use a “full up
elevator” even after being warned.
The pilot’s limited experience and training in “unusual attitude” and “aerobatic” were
also highlighted in the report released by the National Transportation Safety Board in 1999
(FAA, n.d.). The report noted that the captain received aerobatic training while serving in the Air
Force. The first officer had no military flight experience, although he had obtained general
aviation spin recovery training without “unusual flight attitude” and “aerobatic” experience.

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Outcomes of the Accident

In response to a perceived lack of aerobatic and unusual attitude training, a cabal
comprising industry players from the airline, regulatory bodies, and aircraft manufacturers
released an AURTA (Airline Upset Recovery Training Aid). Officially announced in 1998, the
training aid provided a basis for airlines and individual organizations to design their “own”
training regimens and programs to equip their pilots and flight crews with high-angle-of-attack
and unusual attitude training. The aviation industry recognized that upset recovery skills,
knowledge, and experience were insufficient. Admittedly, since then, most airlines are
developing their training programs based on the material contained in the Airplane Upset
Recovery Training Aid.
Overall, NTSB made the following recommendations/suggestions relative to USAir
Flight 427 accident: (1) a directive that all 373 aircraft models be installed reliably redundant
rudder systems, (2) a requirement that the Federal Aviation Association assembles an
independent commission, Engineering Test & Evaluation Board, to look into the certification and
design of the Boeing’s 737 rudder system, and (3) that the Federal Aviation Association
mandates continued safe landing and flight after a flight control jams at any deflection by
revising paragraph [(c) (3) (14 CFR 25.671(c)(3))]. The NTSB also recommended that training
in flight control jams be enhanced, the operating speeds for Boeing’s model 373 are amended to
lower the impacts of rudder jams, and the improvement of flight data recorders to provide crew
members with flight control commands (inputs) and potential control deflections.
The FAA also issued multiple other airworthiness directives. For example, an amendment
to AD 80-07-02 required that Boeing performs a one-off, manual input test of the flight control
systems (ailerons, elevators, and rudder) before handing over the plan to the customer (airlines).

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In 2002, the FAA issued directive AD2002-20-07R1 requesting Boeing to install a redesigned
rudder system within six years from November 12, 2002. The directive recommended that
Boeing installs a rudder system based on the method approved by the FAA, including separate
hydraulic inputs, two PCUs, and a standby actuator.

Risk Mitigation/Reduction Strategies

In retrospect, it can be argued that addressing the issues that led to the USAir Flight 427
accident and preventing similar events in the future can be achieved through training of crew
members on standard flight practices, implementation of organizational training programs (such
as those based on Airline Upset Recovery Training Aid), reengineering and redesigning the
aircraft’s rudder system and the FDR, and commanding airlines and aircraft manufacturers like
Boeing to guarantee the airworthiness of their fleet. For example, policy changes for Boeing
should include revising the FAA-approved AFM (Airplane Flight Manual) protocols to clarify
instructions for recovering or correcting a restricted or jammed flight control condition (FDA,
n.d.).

Conclusion/Summary

USAir Flight 427 presents a typical case whereby multiple failures – structural and
mechanical, human, and organizational – can spell catastrophe to an airline, crewmembers, and
passengers. A “full rudder deflection” to the opposite direction of the command by the flight
crew in response to the wake turbulence upset was found by the NTSB to be the primary cause of
the crash. The report also highlights a lack of experience by the captain and the first officer in
“unusual attitude” and “aerobatic training” to have contributed to the accident. It is recommended
that mitigating accidents similar to USAir Flight 427 require a multipronged approach that

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addresses all causative and contributing factors, from design flaws to limitations in in-flight data
recording technology to maintenance issues to inadequate crew training.

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References

Adair, Bill (2002). The mystery of Flight 427: Inside a crash investigation.
https://archive.org/details/mysteryofflight40000adai
Federal Aviation Administration. (n.d.). Boeing Model 373-3B7 USAir Flight 427, N513AU.
https://lessonslearned.faa.gov/ll_main.cfm?TabID=1&LLID=1
National Transportation Safety Board. (1994). Aircraft accident report: Uncontrolled descent
and collision, USAir Flight 427 Boeing 737-3000, N513AU near Aliquippa,
Pennsylvania. Washington, D.C.: NTSB.
https://lessonslearned.faa.gov/USAir427/usair427_ntsb_report.pdf