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Safety of Composite Structures Being Evaluated After Aircraft Design Already Approved by FAA

Mon, Jan 26, 2009 — David Evans

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Boeing is facing challenges building the composite B787 airliner, which won’t make its first flight until the second quarter of 2009, a good six-month delay. Separate concerns with the composite structure are embedded in recent presentations by Federal Aviation Administration (FAA) officials at the agency’s Technical Center in Atlantic City, NJ.

The briefings by FAA scientists prompt one to wonder why the design was approved and certified by higher officials in the agency

The problems facing Boeing in building the B787 – which will replace the B767 and B757 aluminum designs with a weight-saving all composite structure – comes from arch-rival Airbus. In a candid critique of its chief competitor, Airbus crafted a comprehensive 46-page competitive analysis that touches on virtually every aspect of the troubled program, gleaned from Boeing proprietary data and from supplier and subcontractor documents.

The analysis, titled “Boeing 787 Lessons Learnt,” was compiled by Burkhard Domke, Airbus Head of Engineering Intelligence and was presented internally on 20 October 2008. The presentation examined key issues of weight, engine, production and schedule issues facing Boeing. We will focus here on two safety issues raised by FAA Technical Center experts.

According to Airbus, windows were eliminated on the B787 because of fuselage joints that ran right across the windows. In addition, fabrication of the composite material has been slower than anticipated, preventing the production of 10 airplanes a month and a rate of 7 will be more likely through 2012. The airplane’s quick engine change has also been significantly modified. Originally, an engine swap was envisioned in one hour, but that is now more likely to take 3.75 days, with an ultimate objective of 6 hours.

A couple of slides from the Airbus briefing, which apparently incorporate wholesale Boeing data and illustrative material, give a flavor for the challenges, which one commentator summed up thusly: “Good QA [quality assurance] and fresh, inexperienced and marginally qualified hires don’t mix.” The airplane is built in sections by a network of U.S. and foreign companies, with Boeing performing final assembly.

In addition to cosmetic issues, a Boeing source dated August 2008 indicated that strengthening will be made of the outboard wing, the center wing box, the wing leading edges, the main landing gear wheel well, and the center fuselage as well as enhancing maneuver load alleviation.

In addition to cosmetic issues, a Boeing source dated August 2008 indicated that strengthening will be made of the outboard wing, the center wing box, the wing leading edges, the main landing gear wheel well, and the center fuselage as well as enhancing maneuver load alleviation.

 

Airbus analyzed a public lecture given at the Univ. of Washington in November 2007 by Al Miller, B787 Director of Technology Integration, in which it was asserted that composite material could be laid down at a rate of 80 lbs/hour. Airbus believes that Boeing suppliers were actually only able to lay-down 8-9 lbs/hour at the time production began in 2007 and had gradually increased to 19 lbs/hour. Airbus expects the rate to increase to 30 lb/hr once a dual-head machine arrives, well below the initial goal of 100 lbs/hr with a single-head machine.

Airbus analyzed a public lecture given at the Univ. of Washington in November 2007 by Al Miller, B787 Director of Technology Integration, in which it was asserted that composite material could be laid down at a rate of 80 lbs/hour. Airbus believes that Boeing suppliers were actually only able to lay-down 8-9 lbs/hour at the time production began in 2007 and had gradually increased to 19 lbs/hour. Airbus expects the rate to increase to 30 lb/hr once a dual-head machine arrives, well below the initial goal of 100 lbs/hr with a single-head machine.

The B787 production issues are matched by concerns over the safety of composite materials. As outlined by two briefings at the FAA Technical Center 19-20 November 2008 before the International Aircraft Systems Fire Protection Working Group, the B787’s composite structure presents unique challenges over traditional aluminum structure. One presentation, by the FAA’s Harry Webster, dealt with aluminum versus composite in-flight burn through tests. Test articles were fabricated out of aluminum and out of composite to simulate the top surface of an aircraft with a fire in the cabin/overhead area.

The briefing said this about aluminum:

  • “Aluminum’s high capacity for heat rejection prevents burn through while in-flight due to the cooling effect of the airflow around the fuselage.”
  • “Once on the ground, the ground, the cooling effect of the airflow no longer exists.”
  • “Burn-through can occur within minutes of touchdown.”

As an example, at 200 mph and 300 mph in the test facility, and a 6-inches radius from a 900º F heat source, the temperature on an aluminum test skin was a mere 72º F.

In static (no air flow) conditions, the aluminum panel grew much hotter than under flight conditions, sagging in the center and opening a hole. Time to failure: about 14.8 minutes. Under the same conditions, the composite panel got about four times hotter, reaching a temperature of 550º F.

Under airflow conditions, the composite panel did not burn through, but considerable smoke occurred under the panel and 3:40 minutes into the test, a flash fire occurred under the panel.

The briefing concluded:

  • “In-flight conditions cooled the aluminum panel top surface by 500-600º F.”
  • “In-flight conditions cooled the composite panel top surface by 200-350º F.”
  • “The resin in a composite panel is flammable, however, the exposed fibers act as a fire blocking layer, preventing further damage.”
  • “The resin in a composite panel gives off a flammable gas when exposed to a live fire.”

So there is an interesting, mortal choice here: the aluminum panel can burn through, admitting more oxygen to feed the fire, but the composite panel, while retaining stiffness, gets hotter, smokes and gives off a flammable gas, both of which can kill occupants. The results are preliminary, but they do suggest that more must be done for occupant survivability in a composite fuselage than an aluminum one. An airplane with a composite structure that’s on fire will maintain integrity, but the inside of the cabin, absent smoke hoods and fire suppression, could well become a lethal inferno before the aircraft can land and airport fire and rescue squads could come to the rescue.

For comparison to aluminum, when the heater temperature is at 900º F, the center panel temperature is a modest 120º F. However, in addition to much higher temperatures, the composite panel smokes above 600º F.

For comparison to aluminum, when the heater temperature is at 900º F, the center panel temperature is a modest 120º F. However, in addition to much higher temperatures, the composite panel smokes above 600º F.

A second presentation, by the FAA Technical Center’s Steve Summer and William Cavage, dealt with composite and aluminum wing fuel tank flammability. To summarize some of their key points:

“Next generation aircraft scheduled to enter service in the coming years have composite skin that could change baseline [aluminum] fleet wing tank flammability.

“Testing shows large increases in flammability with composite wing fuel tank skin not seen with aluminum skin when heated from the top during ground conditions.”

Note that the top surface of the composite tank gets about 100º F hotter than the top of the aluminum tank. To mitigate the prospect of a fuel tank explosion, Boeing plans to inert all fuel tanks on the B787.

Note that the top surface of the composite tank gets about 100º F hotter than the top of the aluminum tank. To mitigate the prospect of a fuel tank explosion, Boeing plans to inert all fuel tanks on the B787.

Note that the FAA has just begun studying the effects of composite materials on fuselage and fuel tank safety, but the airplane was certificated by the FAA months ago. We might add that that certification was based on eleven “special conditions” regarding design features the regulations do not address (e.g., lithium batteries, crashworthiness, etc.).

The B787 program, from a regulatory standpoint, seems a classic case of getting the proverbial cart in front of the horse. Serious unknowns are being explored and evaluated in a composite aircraft design the FAA has already approved for production. One wishes these issues had been addressed years ago, not in the months before first flight.

(Airbus briefing; FAA Technical Center briefings; scroll to bottom of page for briefings #12 and #15; other related briefings may be of interest.)


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