A United Airlines 737 MAX 8 diverted after a strange, very specific “sequential beeping” sound onboard triggered fears of a possible bomb.

United 737 MAX 8 Diverts After “Sequential Beeping” Under Floor Triggers Bomb Scare

United Airlines flight UA2092 from Chicago (ORD) to New York (LGA) diverted to Pittsburgh (PIT) after crew reported something unusual: a “sequential beeping” noise coming from beneath the cabin floor: a steady, patterned beep about once per second.

At that point, the crew really had no choice.

“We’re getting a sequential beeping… we’re going to have to start treating this as a potential bomb,” the pilots told air traffic control.

The aircraft, a Boeing 737 MAX 8, diverted, landed, and passengers were evacuated via slides after being warned not to attempt to open any overhead bins (encouragingly, it looked like most complied). Bomb squad swept the aircraft. Nothing was found. The flight ended up continuing to LGA, landing at 5:50 pm.

So what was it?

“Sequential Beeping” Is A Very Specific Clue

“Sequential beeping” suggests something electronic, not structural or mechanical (not that the two are necessarily mutually exclusive). And “under the floor” narrows it further.

On a 737 MAX, that leaves you with a few realistic buckets:

Not A Bomb Timer (Probably)

Let’s start with what it almost certainly was not.

Hollywood has trained everyone to think “beeping = bomb.” Realistically, that’s not how modern devices work. You don’t design something that audibly announces itself every second.

Could it have been some sort of improvised device? Maybe. But a perfectly consistent one-beep-per-second pattern is far more consistent with aircraft systems than anything improvised.

Still, from the cockpit, you don’t get to assume that…you have to assume the worst.

The FBI is also investigating:

More Likely: Something In The Aircraft Itself

“Under the floor” on a 737 MAX is not empty space. You’ve got cargo compartments, wiring runs, environmental control system components, and various control units. And importantly, you’ve got things that can generate alerts.

I spoke to a pilot who provided a few plausible scenarios:

• Cargo smoke detection system – These are electronic systems with alert logic. A fault, intermittent trigger, or degraded sensor could create a repeating tone or signal.
• Equipment bay avionics – The 737 has avionics and electronics below the cabin floor. Faults, relays cycling, or warning modules can produce patterned tones.
• Pressurization or environmental system alerts – Not typically audible in the cabin, but depending on how the sound transmits through structure, passengers or crew might perceive it that way.
• A faulty component – Something as simple as a stuck relay or failing unit cycling on and off could easily produce a steady “beep…beep…beep.”

The key point: airplanes are full of systems that operate in patterns.

Or Something In The Cargo Hold

The other realistic possibility is something loaded onto the aircraft.

Lithium battery packs, trackers, medical devices, even some consumer electronics can emit periodic warning tones when they fault or overheat. And if it’s in a bag under the floor, you’re not going to identify it at 35,000 feet, though I’m be surprised if it was this considering how loud the beeping was.

In any case, once you have a 1.) repeating electronic sound from an 2.) unknown source that is 3.) located under the floor you have to assume the worst, treat it as a potential security threat, and divert.

CONCLUSION

The phrase “sequential beeping” sounds ominous, but it also sounds familiar, like when a some detector goes out or an electronic device overheats.

But from the cockpit, you don’t get to assume that. It seems passengers respected commands to leave carry-on items behind, a Pittsburgh bomb squad met the aircraft and quickly searched it, and the plane got underway again, ultimately reaching LGA several hours late…but safety.

image: Ella Froberg / hat tip: The Bulkhead Seat

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