The Ethiopian Airlines 787 Dreamliner that caught fire at Heathrow on Friday has now been towed out of public view into a hangar, while investigators from the U.K., Europe, and United States pore over it to discover what happened.

One reason investigators will be giving acute attention to the Heathrow incident is that it gives them a unique opportunity to see how an airplane built with composites rather than metal is affected by fire.

And there is, by pure chance, a very pertinent and current comparison to be made between two Boeing models, the 787 and the 777. The 777 model involved in the San Francisco crash of Asiana Flight 214, designed in the 1990s, was the last virtually all-metal airplane designed by Boeing.

Working with Alcoa, the aluminum manufacturer, Boeing developed new alloys that were stronger, and as a result of lessons learned in crashes, the integrity of the 777’s fuselage represents the apogee of safety technology using metal. That is why so few people died in what was an extremely violent impact, although after passengers were evacuated fire did quickly gut the center of the cabin.

Experts have long debated how an all-composite fuselage would respond to a crash—designers believe that it would prove every bit as robust as a metal structure. But there has been no experience, until now, of what fire does to the composite fuselage of an airliner.

Close-up photographs of the Ethiopian 787 while it still remained in public view on a stand at Heathrow reveal two large burn-through holes in the left side of the rear upper cabin just ahead of the vertical stabilizer. The fire was intense enough to bare the skeletal structure, called stringers, beneath the fuselage’s outer skin.

The source of the fire, which was detected after the airplane had been parked for eight hours between flights, has yet to be established—because of its location, it could have started in the rear galley or could also be related to the air-conditioning system.

Whatever the cause, this was severe damage. The first and very scant statement from the British regulator, the Air Accidents Investigation Branch, twice used the term “serious.” In that condition the airplane could not fly; the cabin could not be pressurized, and the damage is in an area of the airplane dense with wiring and critical controls.

Before the 787 (given the name Queen of Sheba by Ethiopian Airlines) can leave Heathrow, it will need very extensive and expensive repairs. In fact, Gene Doub, a former air-crash investigator for the National Transportation Safety Board, told me he thought the airplane might be beyond repair.

The AAIB described “smoke throughout the fuselage … extensive heat damage.” It will be particularly important for investigators to establish the exact nature of that smoke. Composites—in effect, laminations of many layers of carbon fiber, other plastics, as well as adhesives—are a chemical cocktail. Whether the smoke has uniquely toxic properties will be vital to establish, as will how the fire gained hold and how fast it was able to spread.

“It’s going to be very interesting to see how they assess the damage and future airworthiness of the airplane,” said Gene Doub. “It will be a textbook lesson for everybody. The AAIB are going to keep that airplane until they are absolutely through with it. This is not a battery problem. It might go back to the basic electrical system.”

(The Daily Beast put a series of questions to Boeing about the Heathrow event. Spokesman Marc Birtel responded: “We have Boeing personnel on the ground and are working to fully understand and address this event. This is the extent of the information we can provide at this time.”)

On the first news of this new alarm over the 787, Boeing stock fell by nearly 5 percent, but the price began to recover when it became clear that the lithium-ion batteries that were the cause of an unprecedented three-month grounding were not apparently involved.

However, the Heathrow incident is troubling both in itself and in the context of the airplane’s record so far. It’s not simply that the 787 has had a lot of technical glitches—that’s normal in a new airplane. The problem is that the 787’s most serious failures involve fire. You don’t expect to see this in an airplane fully certified as safe. Fire aboard an airplane is anathema to airplane designers and airlines. It’s the last thing you need to see. And a fire that needed airport firefighters to put out, meaning that it was beyond the airplane’s own fire-suppression system to prevent, should be a red flag.

Both Boeing and the AAIB were very quick to assert that the Heathrow fire was not related to the batteries.

The AAIB went so far as to say that “this heat damage is remote from the area where the aircraft main and APU (auxiliary power unit) batteries are located.” That’s a strange definition of “remote”—the rear electronics bay, where one of the two sets of batteries are placed, is under the cabin floor quite close to where this fire originated, probably no more than 25 feet apart.

To be sure, the batteries do not seem to be implicated, but there is absolutely no need to portray them as “remote” when they are not.

Then there is the issue of ‘when is a fire not a fire?’ There are no reports of anyone actually seeing flames at Heathrow, but a smoke-filled cabin is reminiscent of what happened when the batteries on a Japan Airlines 787 caused an emergency at a gate in Boston in January.

Boeing’s chief project engineer for the 787, Mike Sinnett, has not been beyond employing some engineering sophistry in describing the Boston fire. At a major briefing in Tokyo in March he said that the nature of the Boston incident had been exaggerated and that there was “no fire at all.”

This was startling: The NTSB’s detailed account of that incident reported a flame about three inches long around the battery case and a firefighter reported that the “fire would knock down and flare back up.”

There is another little-reported problem in the rear of the 787 that has been identified but not yet fixed and it, also, involves excessive heat.

This involves the auxiliary power unit, used to keep the airplane’s systems, including the air conditioning, working on the ground when the main engines are shut down. Airlines have found that when the APU itself is shut down and, as it is supposed to do, the inlet through which it draws air is shut down, heat continues to build up in the tail.

So serious is this heat that it can distort a major APUS engine component, the rotor shaft, and cause significant damage.

Pratt & Whitney, which makes the APU, is working on a fix. In the meantime airlines are changing the way they operate the APU to minimize the buildup of heat. But this has meant that they cannot turn 787s around as fast between flights as they would like.

As the Dreamliner’s problems continue, the absence of public statements from Boeing’s chairman and CEO, Jim McNerney, is surprising. McNerney’s priority seems to be stockholders, not airline passengers—his only regular comments on the 787 have been in conference calls to Wall Street, airlines, and major investors, who continue to believe in the company’s stock.

That public reticence has left the job of damage control to chief project engineer Sinnett and his boss, Ray Conner, the head of the commercial airplane division, who have briefed reporters and the public at carefully orchestrated events, replete with graphics, as they did in Tokyo, to assert that the battery problem has been fixed—even though they admit its causes may never be known.

McNerney has run Boeing since 2005, during which time the Dreamliner has been his principal headache, with prolonged delays in its production that led to it being delivered to the airlines three years late. During that time he has mostly chosen to deploy his authority out of sight, reassuring regulators and Congress that the 787 will live up to its promise as the future of commercial airliners.

He will have to step down when he reaches Boeing’s mandatory retirement age of 65 in 2014. According to calculations made by the Portland Business Journal, McNerney, whose compensation for 2012 was more than $27 million, will collect a pension worth more than $47 million.