Flights involve all manner of noises from the moment the plane starts to move. Some noises sound perfectly normal but other, very loud ones, can alarm passengers. There is one noise in particular that really sticks out and jars if you hear it. Certain people may well start to panic if they hear it as it sounds highly unusual.
Pilot Patrick Smith has explained what “the staccato WOOF, WOOF, WOOF” plane noise really means when it emanates from the floorboards during taxi or at the gate.
“Almost every frequent flyer has encountered this sounds at one time or another,” Smith wrote in his book Cockpit Confidential.
“Crew rarely make efforts to explain it, leaving passengers befuddled and sometimes worried.
“Because the noise is akin to a motor repeatedly trying – and failing – to start, there’s often the assumption that something is malfunctioning.”
Passengers will most likely hear this on twin-engine Airbus models: the A320 series and the larger A330 – but what actually is it?
“What you hear is a device called the power transfer unit, or PTU, which is designed to ensure adequate hydraulic pressured during single-engine operations.
“To conserve fuel, it’s fairly routine for two-engine planes to taxi with an engine shut down.
“Each engine normally pressurises its own hydraulic system, but with a motor not running, that leaves one system without a power source.
“That’s where the PTU comes in, helping the left system power the right or right system power the left.
“Since is it activated only when the pressure falls below a certain level, it cycles on and off, on and off, on and off.
“Due to pressure fluctuations, the noise will sometimes continue even after both engines and up and running.
“It also does a self-test when the starboard engine is started, so you’ll hear it then as well. Boeing aircraft also employe a PTU, but the operation is slightly different, and it doesn’t bark like a dog.”
An engineer revealed on US knowledge sharing site Quora the noise that should worry a passenger the most – no noise at all. If the plane engines are silent that means they have malfunctioned and an emergency landing may be required.
“[Worry] if you hear all the engines stop making noise,” engineer Dave Lindbergh posted.
“If this happens, it means the plane must land very soon (it can glide, but not forever).
“If the pilots can’t reach an airport in time (or get at least some of the engines restarted), you’re in for at best at forced landing.”
The good news is that if one plane engine fails there is nothing to worry about; however, if both fail the aircraft will need to land.
“If one engine fails mid-flight it doesn’t pose too much of an issue,” one pilot from a UK airline told Express.co.uk. “It’s almost a non-event as aircraft are designed to fly for long periods of time on one engine.”
