A British Airways A320 Lost 800 Feet Over London on July 6. But Did It Actually Stall?
A stall warning, an 800-foot drop and screams in the cabin: what passengers experienced aboard BA919.
Pilot Nick - 6 min read
Around 180 people were sitting on a British Airways A320 at 3,000 feet over London on 6 July, on their second attempt at landing at Heathrow, when the stall warning went off for the second time that evening.
The aircraft lost roughly 800 feet, swung left of the approach path, then came back onto it, lined up with the runway and landed. Passengers described a violent manoeuvre and screaming in the cabin. Nobody was hurt. Nothing was damaged.
I’ve watched the coverage build all week, and reporters have run a long way ahead of the facts. The official account of that flight is about forty words long. Almost everything dramatic you’ve read came from somewhere else.
So this is the flight in the order it happened, from someone who flies that approach for a living.
What the investigators have actually said
British Airways flight BA919 was inbound from Düsseldorf. On the first approach, one of the aircraft’s air data units failed. That triggered a stall warning, the flight control system dropped into a simpler mode called alternate law, and the crew declared a PAN.
They broke off the approach and went into a holding pattern to work the problem. Coming back in, a second stall warning sounded at 3,000 feet, followed by what the AAIB calls “a potentially abrupt recovery manoeuvre.” The crew upgraded the PAN to a MAYDAY, which cleared them straight in.
They landed about thirty minutes after abandoning the first approach.
That’s the whole official account. The AAIB has opened an investigation and published no cause. It has not said the aircraft stalled. It has not attributed anything to any crew member.
That last sentence has gone missing from most of the coverage.
What actually broke
An air data unit is the aircraft’s source of truth about the air it’s flying through. Speed, altitude, pressure. Those numbers feed your instruments, and they feed the computers that decide what the aeroplane is allowed to do.
Normally those computers will not let you fly the A320 outside its safe limits. You can pull the stick fully back and the aircraft simply refuses to stall. All of that protection is built on trusting the speed reading.
When an air data unit fails, the computers lose confidence in the speed they’re being fed, so they hand more control back to the pilots. The aeroplane still flies exactly as it did a second earlier. Some of the automatic safety net thins out, and the crew flies it more by hand, following a procedure every airline pilot practises in the simulator.
There’s an awkward consequence. Once that safety net is gone, the aircraft can no longer prevent a stall, so it warns you about one instead. And that warning is calculated from the same suspect speed reading.
So a stall warning in that situation might be real, or it might be a ghost produced by bad numbers. The crew gets a few seconds to work out which. Responding to a real one costs altitude, and guessing wrong in either direction is expensive.
That ambiguity is the hard part of this flight. I suspect it’s most of what the investigation is about.
The four things that crew got right
Read the sequence again with a pilot’s eyes and you can see a procedure running underneath it.
They flew the aeroplane first. When the speed reading can’t be trusted, the opening move is to set a known attitude and a known power setting, from memory, no discussion. You put the aircraft in a shape you know produces flight, and you sort out the instruments afterwards.
They stopped trying to land. A go-around with a degraded aircraft over west London is the cheapest decision on the menu. Every problem gets easier with height and time, and both of those you can only buy by climbing away.
They bought time in the hold. This is the part passengers never see. That holding pattern is where the checklists get run, the fault gets identified, the landing numbers get recalculated, and two people say out loud what they’re about to do next. Circling for twenty minutes is what a crew does when nothing is on fire and they’ve decided to spend time rather than luck.
They said the words that clear the airport. PAN is the international call for an urgent problem that isn’t yet life-threatening. MAYDAY means distress, and it hands you the runway, the radio and every fire truck at Heathrow. Upgrading from one to the other mid-approach is a crew deciding they’d rather have too much help than too little.
None of that would make a decent film. That’s rather the point of the job.
Below the line: what alternate law actually removes from an A320, for those of you who want the systems detail. Then the four claims the newspapers ran that investigators have not corroborated, and why the manoeuvre that terrified everyone on board was the correct one.
This is the part I’d want a nervous flyer to read before they board.
