Nine people were 34,000 feet over open ocean with the seat belt signs off when their aircraft rolled all the way around, and the wake turbulence that did it came from a jet already miles behind them

Two hundred miles from anywhere over the Arabian Sea, the air is smooth and there is not a cloud in it.

A small jet is at cruise on autopilot. Nine aboard, seat belt signs off, nothing on the radar.

A large airliner crosses overhead in the opposite direction, a thousand feet higher, and disappears behind them.

A minute later the small jet is upside down.

Nothing was there to see. So what hit it?

Why the empty sky behind a big aircraft is not empty

Any wing making lift leaves two counter rotating vortices behind it, spun up by high pressure air escaping around the tip into the low pressure above.

The heavier and slower the aircraft, the stronger they are.

Two properties turn wake turbulence into a hazard. The first is that they sink, at roughly 300 to 500 feet a minute, which drops them directly into the separation layer underneath.

A thousand feet of vertical spacing stops two aircraft occupying the same point. It was never meant to keep one out of what the other trails.

The second is persistence. In still air at cruise altitude there is nothing to break the cores up, so they hold for two to three minutes and can be met as far as 25 miles behind the aircraft that made them.

Then there is size. A vortex core can be wider than the whole span of a small jet, so instead of catching one wing it takes the entire aircraft and rotates it.

Which is why the encounter is not a bump. It is a roll command the pilot did not give.

What the recorder captured in half a minute

The autopilot noticed first, with a slight roll to the right that it tried to correct.

Nine seconds later the bank angle went to 42 degrees in a single second and the autopilot dropped out.

Pitch swung from three degrees nose up to twenty degrees nose down. Vertical acceleration peaked at negative 3.2 g.

The crew reported several complete rotations about the long axis.

Both inertial reference systems lost validity and the flight computer failed, so the instruments that tell you which way is up were gone.

The captain later described flying attitude off the clouds, because sky and ocean were almost the same colour and the horizon meant nothing.

The numbers on the report

The encounter happened on 7 January at 08:38 in the morning, about 500 miles from land.

The jet was at 34,000 feet and the airliner at 35,000, on opposite headings.

The upset ran roughly 36 seconds and cost about 8,700 feet, from 34,000 down to 24,000.

The left engine spooled from 95 percent down to about 40 and its turbine temperature went over 1,000 degrees, so the crew shut it down and restarted it half an hour later using bleed air from the other side.

They declared an emergency at 08:56 and landed at Muscat at 11:05.

Four passengers went to hospital, one with a fractured vertebra, and a flight attendant was hurt. Both pilots were unharmed. The airframe had exceeded its certified load limits and never flew again.

What the rules were and were not built for

Wake turbulence separation standards are mostly built for approach, where aircraft queue in a line at low speed and everyone knows who is behind whom.

At cruise over open ocean there is no queue and no controller watching a final, and the crossing traffic sits a thousand feet away on a track nobody is thinking about.

The recovery holds a second surprise. Investigators warn pilots against large deflections of the rudder, because fighting a roll that way can load the structure harder than the vortex did.

These encounters remain rare, and vertical separation works almost all of the time.

But rare is not the same as understood, and the shear at cruising altitude is already changing in ways that make the ride rougher.

The published account is the source for the sequence above.

What changed afterward

The safety bulletin that followed is short and practical.

Controllers should call out heavy traffic within 15 miles ahead, and should avoid vectoring smaller aircraft through the wake of a larger one.

Crews should leave the seat belt sign on longer, because the injuries in this kind of event happen to whoever is not strapped in.

And on a busy oceanic track a pilot can fly offset a little from the centreline, which costs nothing and moves the aircraft out of the descending path of whatever went by.

Aircraft keep getting heavier and tracks keep getting busier, so the vortices get stronger and the chances of meeting one go up together.

What makes this class of incident hard is not the violence. It is that the cause has already gone over the horizon, much as it had for the crew who slept through a stretch of their own cruise.

Smooth air, clear sky, nothing on any screen, and a rope of spinning air sitting exactly where the separation rules put you, as the reporting laid out.