People are mourning for the plane crash victims at Dvortsovaya (Palace) Square in St. Petersburg, Russia, on Monday, Nov. 2, 2015. In a massive outpouring of grief, thousands of people flocked to St. Petersburg's airport, laying flowers, soft toys and paper planes next to the pictures of the victims of the crash of a passenger jet in Egypt that killed all 224 on board in Russia's deadliest air crash to date. (Ivan Sekretarev/AP)

Prevailing wisdom holds that once an airplane reaches cruising altitude it’s clear sailing, but at least half a dozen times a commercial jetliner has simply fallen to pieces without help from a bomb or missile, as some have suggested in the case of the Russian plane that crashed Saturday, killing all 224 on board.

In one famous case the thin aluminum skin ripped open, in another a cargo hatch blew open, and the worst accident in aviation history, the 1985 crash of Japan Airlines Flight 123 that killed 520 people, a faulty repair caused the plane’s tail to fall off near cruising altitude.

Despite the statements from the Russian airline Metrojet on Monday that the plane that crashed Saturday was in perfect working order, and their speculation that renegade militants may have launched a missile at it, the inquiry into what caused the Airbus A321-200 to crash in a rugged area of the Sinai Peninsula is in its nascent stage. Western investigators would take weeks or months before making a final determination of a cause.

Russians investigators, who have had plenty of opportunity to examine commercial crashes on their own soil, cautioned the airline against a rush to judgment. Once they have downloaded data from the flight data recorder and cockpit voice recorder, both of which were recovered intact, they should have a reasonably good idea of the cause.

Here's what we know about the deadly Russian plane crash that killed all 224 people on board Saturday, Oct. 31. (Monica Akhtar/The Washington Post)

At a news conference in Moscow on Monday, Metrojet’s deputy general director flatly denied there was any problem with the plane that took off from Sharm el-Sheikh airport Saturday, bound for St. Petersburg.

“We rule out a technical fault of the plane or a pilot error,” said Alexander Smirnov. “The only possible explanation could be an external impact on the airplane.”

If it were downed by an external impact, attention immediately would turn to a missile. But the ISIS militants who control some territory in the Sinai, and claimed responsibility for the crash are believed to have shoulder fired anti-aircraft missiles that have a maximum range of about 11,000 feet, and not the more potent truck-mounted Buk surface-to- air missiles believed to have taken down a Malaysia Airlines plane flying at 32,000 feet over Ukraine last year.

Smirnov said the Metrojet plane had reached 31,000 feet before its speed dropped by 186 miles per hour and it lost 5,000 feet in altitude before disintegrating into pieces that spread out over more than six square miles on the Sinai.

A missile strike was suspected in the third deadliest airplane crash in U.S. history — the 1996 crash of TWA flight 800 off Long Island — and there are still those who argue that a missile brought down the plane and killed 230 people on board. But the most exhaustive investigation in National Transportation Safety Board history concluded a fuel tank explosion as the plane climbed to altitude was responsible.

Though Smirnov discounted the possibility that the Metrojet plane could have fatally malfunctioned, history suggests otherwise, and it supports several possibilities.

Planes climb to a cruising altitude six to seven miles above the earth’s surface because there the air is far thinner and against that lessened resistance they can fly faster and use less fuel.

When they reach that altitude, however, they must maximize the air pressure in the cockpit and cabin. In 1988, a hole opened in the fuselage of an Aloha Airlines flight in Hawaii, sucking out a flight attendant. Metal fatigue caused it to give way when the cabin was pressurized.

“Typically, if there was that type of defect, you would expect it to manifest just as it reached the peak [altitude],” said Steve Wallace, a former crash investigator for the Federal Aviation Administration.

Poor maintenance 22 years earlier leading to metal fatigue was the culprit again in 2002 when China Airlines Flight 611 disintegrated after takeoff from Taiwan airport, killing all 225 people on board.

“That airplane had an improper repair after a tail strike,” Wallace said, “and I think this Russian airplane that was in the accident had had a tail strike.”

The Aviation Safety Network reported the Metrojet plane suffered a ‘tail strike’ in 2001, a type of incident where the tail of an airplane hits the runway. The damage took three months to repair, but the plane was certified as airworthy this year by regulators in Ireland, where it was registered.

Aviation Safety said the plane, which previously had been owned by the Turkish airline Onur Air and the Lebanese airline Middle East Air, was 18 years old and had flown about 21,000 times.

In 1974, A Turkish Airlines DC-10 went down in a forest near Paris, killing 346 people. Investigators found that locking pins on a cargo door had been filed down to make the door easier to close. When the cabin was pressurized, the door blew open, control cables became tangled and the plane crashed.

The death count was 110 in 1981 when a Boeing 737 flown by Far Eastern Air Transport disintegrated after takeoff from Taipei. Investigators found that corrosion had eaten through pits, holes and cracks in the plane’s hull and the stress of 33,313 landings had so weakened the hull that it broke under pressure.

In the Japan Airlines disaster, a bulkhead in the tail section of the plane that had been improperly repaired years earlier finally collapsed when the cabin pressure was increased at altitude.

“In that case the tail section was not designed to hold pressure so it broke apart and the plane flew around for about 18 minutes, because it wasn’t controllable,” Wallace said. “And then it crashed. That was the worst single airplane accident ever.”

The Russian news agency Ria Novosti quoted someone at Sharm el Sheikh airport who said the plane had experienced “engine start failures several times over the past week.”

The wife of the co-pilot, Sergei Trukachev, told Russian television that he had complained for the plane’s condition.

“He complained before the flight that the technical condition of the aircraft left much to be desired,” Natalya Trukachev said on the state-controlled NTV network.

If the plane, despite claims to the contrary by Metrojet, was in poor working order the fact that it reached altitude and disintergrated suggests that some sort of structural breakdown was more likely than engine failure.

Engine failure is more common as planes takeoff and climb, and when a plane suffers from it, it generally hits the ground or ocean largely intact.

Wallace warned against reaching a conclusion before the facts — particularly those from the black boxes — are in, but he said, “there are plenty of examples of airplanes that came apart, became uncontrollable at altitude, without it being a terrorist act.”