(CNN) Ukrainian Boeing plane crash in Iran, investigators hunt for clues. Former NTSB manager Peter Goelz remarks on the startling appearance of the fireball.

The estimate is that Iran shot it down by accident.

The main hypothesis that avoids a hostile act is an uncontained engine failure. This occurs when a rapidly rotating fan or turbine explodes from centrifugal force on a defective part.

The instantaneous appearance of a fireball has not been a feature of previous engine explosions.  Of the 13 that have occurred since 1973, three have featured large fires:

• British Airways Flight 2276 at Las Vegas on 9/8/2015.
• American Airlines Flight 383 at O’Hare on 10/28/2016.

Both of the above  occurred on takeoff rolls that were successfully aborted. They featured large wing fires that could have resembled the Ukraine accident if seen at altitude.

• Baikal Airlines Flight 130, which has an atypical pattern, is the only instance of a large fire at altitude.

In the other 10 incidents, fire did not occur, or was  minimal.

A large fire requires a lot of fuel. The 737 has a tank in each wing, and one or two belly tanks along the center. Cellphone videos of the crash may be important, since it is unlikely Iran will release the black boxes before erasing them. Photometric analysis of a video, measuring the brightness of individual pixels, may indicate a belly tank fire. Since the belly area has not in the past examples been impacted by engine explosions, it may indicate:

• An engine explosion with extreme violence, enough to puncture the belly area with shrapnel.
• Explosion of an antiaircraft missile with a proximity fuse.

This is ground for mere suspicion, an atypical accident on an unusual day. But there is a separate factor that allows laying odds.

(NY Times 6/22/2019) U.S. Carried Out Cyberattacks on Iran on 6/20. Quoting,

An additional breach, according to one person briefed on the operations, targeted other computer systems that control Iranian missile launches…Officials have not publicly outlined details of the operation. Air defense and missile systems were not targeted, the senior defense official said, calling media reports citing those targets inaccurate.

But targeting air defense was technically possible.  (Times of Israel) US cyber attack on Iran exploited flaw in heavily guarded network, experts say.

Citing US official sources, American media last week reported that the Army Cyber Command had crippled the Iranian Revolutionary Guard’s air defense units that shot down a sophisticated drone on June 20.

I  take the denials stated to the Times at face value. Someone said, “we could have”, and it morphed by human exaggeration into “we did this.”

For Iran, the nut is not whether air defenses were in fact disabled. It is the fact that the U.S. can jump into air-gapped Iranian networks with ease. (An air-gapped network has no electronic connection to other networks.)

So prior to the missile attacks in the early hours of 1/8, the Iranians did what is prudent in the face of a cyber superpower: they air-gapped their air defense computer network, breaking it into local pieces, reliant on human-to-human communication.

The three notorious shoot-downs of our time, KAL007, MH17, and Iran Air 655, were mistakes. Mistakes happen because radar doesn’t show all the features or paint job of a plane. Now we can work out what I think happened:

• A radar blip is a featureless “blob”. By itself, it’s a mystery.
• Every commercial airplane carries a transponder, a kind of automatic two-way radio.
• When air traffic surveillance radar interrogates a plane with a transponder, the transponder replies. Now a little ID code appears on the screen next to the blip.
• When, in anticipation of retaliation, Iran air-gapped their network, their surveillance radar was unable to send the ID information over to the AA batteries.
• An AA missile battery, disconnected from the larger network, detected the Ukraine flight. In a confused telephone conversation, they were unable to resolve their radar blip with air-traffic control radar.
• Some underling turned a key and hit the big red button.

The cost: 82 Iranian lives. At Soleimani’s funeral, 65 died in a stampede. This is the cost exacted by fate, or your deity if you have one,  for the privilege of killing one American contractor.

At times like these, even the most devout wonder if God is on their side.