Bottom Line: Revise The Maintenance Manuals
From the NTSB
Report:
On November 17, 2002, at 1800 eastern standard time, a Canadair
CL-600-2B19 (CRJ-2), N868CA, operated by Comair as Delta Connection
flight 5109, a scheduled passenger flight from Atlanta (GA) to
Washington(DC), encountered severe turbulence while in a descent
near Rockville (MD). There were no injuries to the crew or
passengers. The airplane was returned to revenue service on
November 18, 2002, after it was visually inspected for damage in
accordance with the procedures for severe turbulence or extreme
maneuvers outlined in the Canadair Regional Jet (CRJ) Aircraft
Maintenance Manual (AMM).
The National Transportation Safety Board’s examination of
information from the flight data recorder (FDR) indicated that
large vertical accelerations occurred during the turbulence event.
Further analysis by Canadair indicated that during the event, the
wing, pylon, and horizontal stabilizer to vertical stabilizer
attachment structure experienced loads outside their certificated
design envelopes.
Specifically, this analysis revealed that the airplane
experienced vertical accelerations ranging from 4.3 G1 positive to
1.9 G negative, resulting in internal loads well in excess of the
certificated limit load for these structural components. The
inspection procedures in the CRJ AMM define a minimum positive G
threshold, depending on the airplane’s gross weight, above
which a visual inspection for damage is required in order to return
the airplane to service. For an airplane at the approximate gross
weight of the incident airplane, the AMM requires that inspections
be performed if the vertical acceleration exceeds 2.5 G positive.3
No minimum threshold or inspection criteria are provided for
negative G excursions, nor is any minimum threshold provided for
lateral G excursions. 1 A G is a unit of measurement of force on a
body undergoing acceleration as a multiple of its weight. The
normal load factor for an airplane in straight and level flight is
about 1 G. 2 Structures are required to be designed to withstand
the highest loads expected in service, referred to as limit load.
See 14 Code of Federal Regulations (CFR) 25.301(a).
The structure is
required to be designed to sustain these loads without detrimental
permanent deformation and any deformation may not interfere with
safe operation. See 14 CFR 25.305(a). 3 2.5 G positive is the
threshold for inspection at an approximate operating weight of
46,000 lbs. The operating weight of the aircraft at the time of the
incident was estimated to be 45,519 lbs.
On December 20, 2002, as a result of its engineering and loads
assessment(CA)nadair determined that inspections in addition to
those already specified in the CRJ AMM were required to ensure the
airplane’s structural integrity. As a result, the operator
performed a more detailed series of supplemental inspections, but
no damage was identified.
On May 12, 1997, an Airbus Industrie A300 B4-600R series
airplane, N90070, operated by American Airlines as flight 903, was
involved in an upset event. During the upset, the stall warning
system activated, the airplane rolled to extreme bank angles left
and right, and rapidly descended more than 3,000 feet. One
passenger sustained serious injuries. After the event, the airplane
was visually inspected for damage in accordance with the procedures
for excessive turbulence or large vertical G excursions in the
Airbus A300-600 AMM, which specified threshold criteria for
positive and negative vertical, but not lateral G excursions.
During this inspection, damage to the engine pylons and engines was
noted and repaired and the airplane was returned to service.
In March 2002, approximately five years after the American
flight 903 incident, as a result of the Safety Board’s
ongoing investigation of the accident involving American Airlines
flight 587,4 further analysis of FDR data from American flight 903
indicated that the airplane experienced external and internal loads
outside its certificated design envelope. The airplane’s
vertical stabilizer was removed and the composite attachment lugs
were subjected to ultrasonic nondestructive inspection (NDI).
The results of the NDI of the right-hand aft attachment lug
indicated a delamination in the composite material. This type of
damage can be caused by extreme loading conditions such as those
associated with the upset. Based on these findings, the vertical
stabilizer has been permanently removed from service. Inspection
Procedures Following High Load Events The investigations of
American flights 903 and 587 revealed that those airplanes
experienced lateral accelerations in excess of 0.4 G,5 resulting in
internal loads well in excess of the certificated ultimate loads6
for the vertical stabilizer attachment structure. However, until
recently the A300 AMM contained no inspection procedures for high
lateral load events. As a result of the American flight 587
investigation, the Federal Aviation Administration (FAA) issued
Airworthiness Directive 2002-06-09, for Airbus model A300,
A300-600, and A310 series airplanes, which contains inspection
procedures applicable to lateral accelerations exceeding 0.3 G but
not exceeding 0.35 G. If lateral accelerations exceed 0.35 G, the
aircraft is not 4
On November 12, 2001, an Airbus Industrie A300-605R, N14053,
operated as American Airlines flight 587, crashed into a
neighborhood in Belle Harbor(NY), several minutes after taking off
from Kennedy International Airport. During the accident event, the
vertical stabilizer and rudder departed the airplane in flight. Two
pilots, seven flight attendants, 251 passengers, and five persons
on the ground were fatally injured. The cause of the accident is
still under investigation.
Information to date indicates that the vertical stabilizer was
subjected to large aerodynamic structural loading during the
accident event. Data gathered as part of the Federal Aviation
Administration operational loads monitoring program indicates that
0.2 G lateral accelerations are rarely encountered. See American
Airlines flight 587 Public Hearing Exhibit 7F. 6 Structures are
required to be designed to withstand limit load multiplied by a
safety factor (referred to as ultimate load) without failure for at
least 3 seconds. (See 14 CFR 25.301.) The minimum safety factor is
1.5. (See 14 CFR 25.303.) Operators of the affected airplanes have
incorporated these inspection procedures in their AMMs. Following
the American flight 903 and the Delta Connection flight 5109
events, the operators performed the required inspections as
specified in the applicable AMM. However, neither the CRJ nor the
A300 AMM included inspections for damage caused by high loads due
to extreme lateral accelerations, such as those encountered by
American flight 903, and the Canadair AMM did not include
inspections for damage caused by extreme negative vertical
accelerations, such as those encountered by Delta Connection flight
5109.
A review of several AMMs from other manufacturers revealed
similar deficiencies. Extreme lateral accelerations and both
positive and negative vertical accelerations can occur as a
consequence of severe turbulence, system failures, or pilot-induced
yaw, pitch, or roll maneuvers. Because encounters with such
conditions could result in reduced structural integrity of the
airplane, the Safety Board is concerned about these inspection
procedure deficiencies. Therefore, the Board believes that the FAA
should require all manufacturers of transport-category airplanes to
review and, if necessary, revise their maintenance manual
inspection criteria for severe turbulence and extreme in-flight
maneuvers to ensure that loads resulting from positive and negative
vertical accelerations, as well as lateral accelerations, are
adequately addressed.
The CRJ and A300 AMMs established only a single, minimum
threshold for high load events that would require an inspection for
damage before the airplane could be returned to service. Implicit
in this high load and inspection formulation was the presumption
that the specified inspections would be adequate to identify and
address damage caused by any such high load event, regardless of
the degree to which the threshold was exceeded. However(CA)nadair
and Airbus determined, after engineering reviews of the internal
loads, that their published inspection criteria were inadequate to
ensure safety after the high load events encountered by Delta
Connection flight 5109 and American flight 903, and that additional
broader and more detailed inspections were required to ensure
safety.
In light of these
determinations, the Safety Board is concerned that the successful
accomplishment of currently prescribed inspections following high
load events that greatly exceed the manufacturer’s threshold
may be inadequate and that, as a result, airplanes that have
encountered such high loads may be returned to service in an
unairworthy condition.
Further, in light of the circumstances surrounding the high load
events discussed in this letter, the Safety Board is concerned that
airplanes may be exceeding design and certification standards more
frequently than was previously known or expected and, therefore,
recommends that all such events be tracked and evaluated.
Therefore, the Safety Board believes that the FAA should require
all manufacturers of transport-category airplanes to establish and
validate maximum threshold values for positive and negative
vertical and lateral G accelerations beyond which direct
manufacturer oversight and intervention is required as a condition
for returning the airplane to service. The Safety Board also
believes that the FAA should require all operators of airplanes
that have experienced accelerations exceeding these threshold
values (or that the operator has reason to believe might have
exceeded those thresholds), as determined from FDR and other
available data, to notify the FAA immediately of such high loading
events and provide all related loads assessment and inspection
results.
The Safety Board also believes that the FAA should require
manufacturers of transport-category airplanes to immediately notify
the appropriate certification authority of any 4 event involving
accelerations exceeding the threshold values (or that the
manufacturer has reason to believe might have exceeded those
thresholds) necessitating the intervention of the manufacturer, and
provide all related loads assessment and inspection results.
Therefore, the National Transportation Safety Board recommends
that the Federal Aviation Administration:
A-03-41
Require all manufacturers of transport-category airplanes
to review and, if necessary, revise their maintenance manual
inspection criteria for severe turbulence and extreme in-flight
maneuvers to ensure that loads resulting from positive and negative
vertical accelerations, as well as lateral accelerations, are
adequately addressed.
A-03-42
Require all manufacturers of transport-category airplanes
to establish and validate maximum threshold values for positive and
negative vertical and lateral G accelerations beyond which direct
manufacturer oversight and intervention is required as a condition
for returning the airplane to service.
A-03-43
Require all operators of airplanes that have experienced
accelerations exceeding the threshold values established as a
result of Safety Recommendation A-03-42 (or that the operator has
reason to believe might have exceeded those thresholds), as
determined from FDR and other available data, to notify the FAA
immediately of such high loading events and provide all related
loads assessment and inspection results.
A-03-44
Require manufacturers of transport-category airplanes to
immediately notify the appropriate certification authority of any
event involving accelerations exceeding the threshold values (or
that the manufacturer has reason to believe might have exceeded
those thresholds) necessitating the intervention of the
manufacturer, and provide all related loads assessment and
inspection results.