The Navy has discovered that its new $18.7-million F-18 jet fighter has a serious design flaw that could cause airframe failures after as little as one-third of the aircraft's intended life, The Times has learned.
Top Navy officials were told of the problem in recent weeks after aircraft tests appeared to show that the defect could ultimately cause the aircraft's wings to fall off, Navy officials confirmed Wednesday.
The flaw is located in a large aluminum structure, called a "wing carry-through bulkhead," which holds the wings onto the aircraft, the Navy officials said.
"The cause of it is an inadequate design," said Capt. George Strohsahl, the Navy's F-18 program manager, in a telephone interview late Wednesday. "We are putting a major management emphasis on getting the problem solved."
The F-18 was designed to last 6,000 flight hours, but data from the recent stress tests indicates that the bulkheads may only be capable of withstanding as few as 2,000 hours and at most 5,000 hours of flight stress.
The F-18s in the Navy's fleet have accumulated less than 1,000 hours of flight time, so the Navy believes it has plenty of time to fix the problem before any aircraft or pilots are jeopardized by the defect. The Navy has not placed any restrictions on flying the aircraft, Strohsahl said.
"We don't anticipate seeing any of these cracks in fleet aircraft," Strohsahl said. "I anticipate success in the not too distant future, given that we do have several years before we have any bind in the operation of aircraft."
McDonnell Douglas Corp. and Los Angeles-based Northrop Corp. jointly produce the F-18, which is supposed to be the Navy's primary new attack jet for its aircraft carrier fleet. The aircraft already has been involved in a long series of controversies, including the discovery last year of another structural defect, which has since been corrected.
The Navy plans to buy 1,377 of the aircraft, of which 240 have been delivered with the latest defects. Strohsahl said he is confident that the defective aircraft already in the Navy inventory can be strengthened to provide 6,000 hours of service, but he said it is not yet known what modifications will be necessary or what they will cost.
Strohsahl said all future changes in the design and modifications to existing aircraft will be paid for by the contractors under warranty provisions in Navy contracts.
The problem came to light in recent fatigue tests conducted by McDonnell Douglas. In those tests, an F-18 on the ground was subjected to flight stresses on its airframe that would occur over the projected life of the aircraft.
The simulation was done in a large fixture that pushes and pulls on various parts of the airframe. During the tests, the bulkhead unexpectedly cracked on one side and then fractured on the other side, Strohsahl said.
Firm Disputes Assessment
McDonnell Douglas spokesmen acknowledged that the bulkhead's life is below requirements, but they dispute the Navy's worst-case assessment.
"Our tests show us that it does not have the design life we had hoped for," said company spokesman Jack Cooke. "It is a problem that we have to cope with."
Using the same data, McDonnell Douglas believes the bulkhead can withstand 3,500 to 5,000 hours of flight, a spokesman said.
The variation in all of the estimates results from deciding how much of a safety margin to allow, since the Navy would not want to fly the plane up to the point when the bulkhead would fail. The bulkhead failed after 10,000 simulated hours of flight time in static tests, so the Navy would cut that amount in half when assigning a fatigue life, Strohsahl said.
In addition, the bulkhead was modified in certain late production models and according to one engineering analysis these bulkheads could fail in 2,000 hours, Strohsahl said.
Can Be Strengthened
Strohsahl said bulkheads in future aircraft can be strengthened simply by adding several more pounds of metal. But the repair of existing aircraft is expected to be more complicated.
"We don't know at this time whether it will be a complicated fix or a simple fix," said a Navy engineer at the Northrop plant in Hawthorne where the rear part of the jet's fuselage is produced. The bulkhead was designed by Northrop, Strohsahl said.
The bulkhead, manufactured out of a special aluminum alloy that is supposed to be resistant to fatigue, measures about 6 feet wide, 5 feet high and 6 inches thick. It is machined out of a single piece of aluminum.
It is classified as a "fatigue critical" part, meaning that failure of the part could result in failure of the aircraft and loss of a pilot, Navy officials said.
The bulkhead is produced by Monitor Aerospace, a machine shop located in Long Island, New York. Northrop has reportedly told Monitor to stop production of the bulkhead, but Northrop spokesmen declined comment on the report.
Direct responsibility for the problem rests with McDonnell Douglas, which is the prime contractor of the aircraft, Strohsahl said.
The bulkhead defect is the second significant design error on the fighter to surface in the past year. In 1984, the Navy stopped F-18 deliveries when cracks were discovered to be developing in the tail sections of the jets.
Eventually, the tail sections had to be strengthed in a program that cost about $25 million. McDonnell Douglas paid for that repair and modification program.
In addition to that problem, the F-18 has been severely criticized for not measuring up to Navy requirements on its expected range and payload carrying capabilities.&
The aircraft was also embroiled in a dispute over cost. At one point, the Navy had threatened to cancel the program due to higher than expected costs on the aircraft. More recently, however, the aircraft's costs have been lower than expected.