Why certain aircraft make a distinctive ‘barking’ or ‘grinding sound’ before takeoff

It is the sound of a hydraulic pump operating automatically to maintain pressure. The Power Transfer Unit (PTU) causes this during single-engine taxi operations.

According to Rajgopalan, to save fuel during taxing and before takeoff, an aircraft switches on one engine and continues taxing when it is near a holding point before takeoff. “When one engine is operating, then only one hydraulic system is working. The other one (yet to be switched on) becomes unbalanced, creating pressure. That is the reason why this particular sound occurs,” he said.

In simple terms, when the PTU activates, it rapidly switches between the two hydraulic systems, creating a reciprocating flow of hydraulic fluid. This reciprocating action generates the barking or grinding noise.

Can you hear the sound? (Photo: Freepik)

The sound is not a sign of any engine problems or danger, and it’s a regular part of the A320’s hydraulic system operation. “It is nothing to be worried about. It is part of the aircraft’s mechanism,” asserted Rajgopalan.

The PTU onboard the A320 is a hydraulically powered unit that exists to maintain a minimum pressure across two of the aircraft’s hydraulic systems, unlike traditional motors or pumps, which are most likely electrically operated, said Rajgopalan.

What more should you know? 

Rajgoapalan also mentioned that because of the PTU’s location on the aircraft, you’re more likely to hear the sound if you sit close to the aircraft’s wings. According to him, the PTU is located near the main landing gear, which is positioned close to the wings, allowing passengers to hear the sound distinctly.  

From the homepage

Bar Council of India suspends lawyer for ‘throwing shoe’ at CJI Gavai: ‘Conduct inconsistent with court’s dignity’

On Ladakh after Wangchuk arrest, LG says normal, protesting groups disagree

Dalit man lynched over ‘drone chor’ rumours in UP’s Raebareli; Cong slams ‘jungle raj’ under Yogi govt