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Michael R. Hill Mechanical and Aerospace Engineering University of California, Davis *[email protected]
The Role of Design, Maintenance, and Repair in Structural Safety
Some Observations in Firefighting 31 Oct 2016
2 Copyright Michael R. Hill, 2016
Outline
Accidents involving fire fighting aircraft All occurred in older, aging ex-military aircraft
Historical record related to aging aircraft What do engineers know about aging What can we do about it
Describe the response of federal agencies to those accidents US Forest Service programs
Continuing airworthiness Loads monitoring
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Aerial Firefighting
Aircraft provide unique and useful defense against fire
Firefighting aircraft operate in difficult conditions Mostly ex-military aircraft Flying low In gusts Heavily loaded
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Aerial Firefighting Missions
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Firefighting aircraft accidents Large airtanker accidents
C-130 Pearblossom, CA (T-82): August 13, 1994 Aubenas, France (N116TG): September 6, 2000
(investigation pending) Non-USFS mission, operated by INTRATECH, a US
contractor Walker, CA (T-130): June 17, 2002
P4Y Estes Park, CO (T-123): July 18, 2002
Details: All lost in service due to failure of the wings as a result of
fatigue cracks in the wing skin and structure. Actions:
Grounding of C-130s and P4Ys in 2003
Similar problems in other parts of the world e.g., Bombardier 415 loses tail over Corsica 09/05
Walker, CA crash video at this link (next page)
https://www.youtube.com/watch?v=3dgzBjQvqMs
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Firefighting aircraft accidents
Most firefighting aircraft are old Contemporaries of the Comet (slides later) C-130 first flight August 23, 1954 Materials of that era no longer used
High strength aluminum alloys with poor corrosion and fatigue performance
Lessons learned: Government “Public use” operations do not benefit from FAA oversight Ex-military aircraft pose significant problems
May exhibit significant age-related problems Require specific maintenance to mitigate aging mechanisms Have military safety records dependent on Depot System maintenance
(expensive)
Maintenance and inspection critical to safety of crew and aircraft
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Firefighting aircraft accidents Excerpt from NTSB Report LAX02GA201 following the Walker, CA accident (emphasis added):
In 1991, the Department of Interior (DOI) began to doubt the continued airworthiness of the C-130A firefighting tanker fleet and was specifically concerned that the lack of a depot level maintenance program or any requirement for compliance with all military airworthiness technical orders could compromise the safety of the airplane.
The DOI asked the FAA to standardize the type certificate for the C-130A and mandate improvements in the maintenance and inspection requirements. In a written opinion, the USAF agreed and urged the FAA to mandate that operators establish a depot level type continuing airworthiness program for the airplane and mandate compliance with all technical orders.
In a series of meetings held in 1993, FAA management internally agreed that the DOI and USAF positions held merit and began to develop requirements.
In late 1993, in a meeting between the FAA, DOI, USFS, and the airplane operators, the USFS and the operators objected to the idea of depot level maintenance programs and full compliance with all technical orders on the basis of the potential economic impact of these requirements. As of the time of the accident, the FAA had not standardized the existing five type certificates nor had they imposed any additional maintenance or inspection program requirements .
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Historical Context
Incidents that lead to the concept of Aging Aircraft DeHavilland Comets (1954) Aloha Ailrlines Flight 243 (1988)
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Major Airworthiness Incidents The DeHavilland Comet
The first pressurized passenger jetliner. Capable of operating at up to 40,000ft and 500mph. Twice the operating ceiling and speed of any American made airliners, at the
time.
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DeHavilland Comet G-ALYP: Jan 10, 1954
Accident Details Lost contact ~20min after takeoff,
from Rome, while crossing 27,000ft
Crisp, calm winter day. Aircraft only had 3681 flight hrs
(~1200 flights) Witnesses hear a series of loud
explosions and saw fiery wreckage fall from the sky, into the ocean.
The wreckage was difficult to recover because much of it lay 500-600ft under water.
15 bodies were found and examined to help determine the cause of the accident.
The examinations found that the bodies had sustained injuries consistent with explosive decompression of the aircraft s fuselage.
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DeHavilland Comet G-ALYP (cont.)
Immediate Actions The British Overseas Airline Company
(BOAC), the operator, grounded all of its Comet fleet.
A full scale test was set up to try to determine why the fuselage might have failed.
A water tank was used to prevent fragmentation upon failure.
Test involved loading the undercarriage, transferring load to wings, pressurizing and depressurizing the fuselage, and transferring loads back to undercarriage.
Test was setup so 10min in the test rig was equivalent to 3 hrs flying time.
Testing was conducted 24 hrs per day. However, Comets were put back in
service on March 23 because the plane in the tank was showing no signs of failure.
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DeHavilland Comet
G-ALYY: April 8, 1954 Accident Details
Lost contact ~35min after takeoff, from Rome, at ~35,000ft Again, the aircraft disintegrated in midair. Most of the wreckage was unrecoverable (sea floor ~2000ft). All aspects were eerily similar to those of G-ALYP. Again, recovered bodies indicated explosive decompression.
Immediate Actions April 12,1954: British Air Registration Board withdraws Comet s Certificate of
Airworthiness
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DeHavilland Comet End of June 1954: Comet in test rig fails suddenly, after only 9000 hrs. of equivalent flight time.
Why did the fuselage fail so early? DeHavilland designed Comet to 2x cabin pressure differential of 8.25psi Development tests found fatigue life in excess of 18,000 flights (~55,000 hrs)
But only tested cyclic pressurization!
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DeHavilland Comet
Results: Fatigue cracks starting at rivet holes near the escape hatch and at the corners of the square ADF antenna cutout caused the fuselage to undergo explosive decompression, breaking apart midair.
No more passenger flights of the Comet, only a few modified Comets remained in service with the RAF.
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DeHavilland Comet Lessons Learned:
Cutouts in pressure type cabins need to be small, with round corners or circular in shape, to minimize stress concentrations.
Pressurized fuselages need thick skin reinforced with cross webs, or fail safe straps, to stop the growth of fatigue cracks in the skin.
FATIGUE DESIGN IMPERATIVE
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Aloha 243 Aloha Airlines Flight 243: April 28, 1988
Accident Details Boeing 737-200, owned and operated by Aloha Airline Inc. A 6m section of fuselage peeled off the passenger cabin ~25 min in flight at 24,000ft. Pilots were able to maintain control of the aircraft and make an emergency landing on
Maui. One stewardess was swept from the plane and was the only fatality.
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Aloha 243 More Pictures .
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Aloha 243 Failure description
Pressurized fuselage failed due to fatigue cracking. Multiple site fatigue cracking at countersunk rivet holes along a lap joint Cracks were initiated due to skin disbonding
Production problems, and corrosion NTSB Report AAR-89-03: The National Transportation Safety Board determines that the probable cause of this accident was the failure of the Aloha Airlines maintenance program to detect the presence of significant disbonding and fatigue damage which ultimately led to failure of the lap joint a S-10L and the separation of the fuselage upper lobe. Contributing to the accident were the failure of Aloha Airlines management to supervise properly its maintenance force; the failure of the FAA to require Airworthiness Directive 87-21-08 inspection of all the lap joints proposed by Boeing Alert Service Bulletin SB 737-53A1039; and the lack of a complete terminating action (neither generated by Boeing nor required by the FAA) after the discovery of early production difficulties in the B-737 cold bond lap joint which resulted in low bond durability, corrosion, and premature fatigue cracking .
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Aloha 243: Lap Joint Detail
Multiple site fatigue cracking at countersunk rivet holes along lap joint
Cracks initiated by skin disbonding at the lap joint due to
Poor manufacturing quality Corrosion
This was a known problem for these aircraft at the time of the accident
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Further reading on Aloha 243
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Aloha 243 Outcomes
NTSB Recommendations: Provide better, more up to date, training for aircraft mechanics, technicians and
inspectors. Require operators to periodically test staff on proper inspection skills. Require that all turbojet transport category airplanes certified in the future receive
full-scale structural fatigue testing to a minimum of two times the projected economic service life. Also, require that all currently certificated turbojet transport category airplanes that have not been fatigue tested to two lifetimes, be subjected to such testing. As a result of this testing and subsequent inspection and analysis, require manufacturers to identify structure susceptible to multiple site damage and adopt inspection programs appropriate for the detection of such damage . (Class II, Priority Action (A-89-67)
This event marks the beginning of the aging aircraft movement Congress instructs FAA to roll out National Aging Aircraft Research Program
FAA engages OEM, NASA, DoD, and other entities
Lessons Learned: Proper maintenance and inspection critical to aircraft safety Aircraft designed to specific lifetime If used beyond the initial design lifetime
Reevaluate and possibly redesign aircraft, maintenance, and/or inspection
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July 16, 2009 The NTSB has released photos of the section of a Southwest Boeing 737-300 fuselage that let loose in flight earlier this week. The photos show a remarkably clean fracture where the outside skin of the aircraft blew out at 34,000 feet. The section of aluminum is being examined by metallurgists while Southwest fixes the airplane in Charleston, W. Va. where the crew made the emergency landing. The plane was on its way from Nashville to Baltimore when the hole appeared. None of the 131 passengers and crew were injured.
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Southwest 812 “On April 1, 2011, Southwest Airlines flight 812, a Boeing 737-300 experienced a rapid depressurization The flight was at 34,000 feet when the depressurization occurred. The flight crew diverted the flight to Yuma, AZ. At the time of the accident, the aircraft had accumulated 48,740 hours of service and 39,781 cycles (a cycle is a takeoff and landing). The aircraft was delivered on June 13, 1996.”
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Continuous Improvements in Design pay-off in Safety
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What is being done to avoid similar accidents
Develop understanding of the fire fighting mission Measure the loading on aircraft during service
Develop maintenance plans that are mission specific
Educate the working staffs in aircraft maintenance and safety
