Once the scientists at CSIRO had generated the probability distribution for the plane’s last known location on the 7th arc, the next question they had to answer was: how far did the plane travel from that point before it impacted the water?

As we discussed earlier, their goal was to define a search box within which the plane was likely to be found. The plane’s location along the 7th arc defined the length of the rectangle, and the distance it could have traveled from the 7th arc would define the width of the search box.

So the question of how far the plane could have flown after the last transmission depends on what the investigators thought was going on with the plane at that moment. You’ll recall that the plane took off at 16:42 heading from Kuala Lumpur, Malaysia to Beijing, China.. A flight that normally takes 5 1/2 hours but it was carrying enough fuel to keep it flying until around 00:12 , in case it needed to divert somewhere else and needed extra fuel.

The inmarsat data showed the plane transmitting signals on a regular schedule — either in response to phone calls from the ground, or an hour after the last exchange. The last exchange in his sequence took place at 0:11 universal time. (That’s 8.11am Kuala Lumpur time, but to keep things simple let’s stick to Universal Time from here on out.)

Then at 0:19 , the plane sent a request to Inmarsat to log in again. In many ways this request is similar to the one that was made at 18:25 — the SDU had logged itself off because it had powered down, and then tried to log on after getting turned back on again.

As you’ll recall, no one has a very good answer for why the SDU was turned off and back on again at 18:25 . It’s a very unusual thing to happen in flight. But investigators had a very good idea about why the SDU might have turned on again at 0:19 . They knew that by this time the plane would have been very close to running out of fuel. And once that happened, the engines would stop running, and the plane would lose electrical power. It would start to lose speed and/or altitude. And the satcom would disconnect.

But then an emergency situation a backup system would kick in. A device called a Ram Air Turbine, or RAT, would deploy—it’s a propellor that pops out into the slipstream to generate electricity like a windmill. This would generate enough electricity to start up a powerplant in the back of the airplane called the Auxiliary Power Unit, or APU. Its generatorwould return power to the plane’s electrical system, including the satcom. This is why the plane reconnected to the satellite.

The process from fuel exhaustion to logon would take 3 minutes 40 seconds. So the plane presumable ran out of fuel around 00:16 , or about 5 minutes after the previous handshake at 0:11 .

At 0:19 the plane had been out of fuel for almost four minutes, which meant the engines hadn’t been providing power. At first the ATSB assumed that the most likely scenario was that the plane had flown on a ghost flight, and after losing power had fallen into a descending spiral. Based on previous accidents they estimated that it would have fallen into the ocean within 30 nautical miles of the 7th arc.

Later they looked more closely at the BFO data, and they realized that there was a simple explanation for the seemingly strange values recorded at the time. Since the SDU’s doppler precompensation algorithm doesn’t account for vertical velocity, the BFO data could be explained if the final transmissions were made in a steep and accelerating dive.

This dive would be so steep that it could only be accomplished by a pilot who was actively pushing the plane’s nose down into a deliberate steep suicidal dive.

That being the case, the plane’s wreckage should be found very close to the 7th arc. Perhaps no more that 5 or 10 miles away. But, to give themselves a bit of leeway, the authorities decided to define an initial search area that was 400 miles long and 58 miles wide.

They had every reason to be confident, based on what they knew, that the plane would be within these 23,000 square miles.

Spoiler alert: it wasn’t.

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