《知識》空中巴士 A320「狗叫聲」是什麼？了解 PTU 動力轉換單元 What Is the Airbus A320 “Barking Dog” Sound? Understanding the PTU (Power Transfer Unit)

 

如果搭過空中巴士 A320 系列（A318/A319/A320/A321），可能在飛機推離登機門或滑行時，聽過一種非常特殊的聲音。那個有點像狗叫的奇怪聲響，其實就是來自飛機上的動力轉換單元（Power Transfer Unit, PTU）。

PTU 的主要任務，是在 A320 的兩套主要液壓系統之間「互相支援」。A320 擁有綠、黃、藍三組液壓系統，而其中綠色與黃色負責最關鍵的飛控與起落架操作。

當綠與黃兩套系統的壓力不平衡時，PTU 就會自動介入，把壓力較高的一方的動力傳給另一方，讓雙方液壓都維持在正常狀態。

PTU 是一套純機械的齒輪機構，兩端各連接綠、黃液壓泵。當壓力差異變大時，它會以「脈衝式」快速啟動，也因此產生了那個聞名的「狗叫聲」。這聲音在地面特別容易聽到，例如：只啟動單邊引擎、飛機後推（pushback）後液壓重新建立壓力、或操作起落架時，聲音可能會連續響幾秒到幾十秒，都是正常現象。

這個代表性的「狗叫聲」，也成為空中巴士 A320 系列的特色之一。其它空中巴士機型雖然也有 PTU，但聲音非常小，旅客幾乎察覺不到。較新的 A350 甚至採用更先進的電動液壓系統（EHA/EPC），完全不需要傳統 PTU，自然不會出現這種聲音。至於波音系列飛機則因液壓設計不同，不需兩邊互帶的 PTU，因此也不會在波音飛機上聽到這種聲響。

下次搭到空中巴士 A320 系列飛機時，不妨仔細聽一下這個奇特又有趣的聲音。

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What Is the Airbus A320 “Barking Dog” Sound? Understanding the PTU (Power Transfer Unit)

If you’ve ever flown on an Airbus A320 family aircraft (A318/A319/A320/A321), you might have heard a very unusual sound while the plane was pushing back from the gate or taxiing. That strange “barking” noise actually comes from a device called the Power Transfer Unit, or PTU.

The PTU’s main job is to let two of the A320’s main hydraulic systems support each other. The aircraft has three hydraulic systems, green, yellow, and blue, with the green and yellow systems handling the most important functions, like flight controls and landing gear.

When the pressure between the green and yellow systems becomes unbalanced, the PTU automatically activates. It transfers power from the system with higher pressure to the one with lower pressure, keeping both systems operating normally.

The PTU is a purely mechanical gear, a driven unit connected to both the green and yellow hydraulic pumps. When the pressure difference gets large, the PTU starts up in short pulses; this is what creates the famous “barking dog” sound. You can hear it most clearly on the ground, such as when only one engine is running, after pushback while the hydraulics are re-pressurizing, or when the landing gear is being operated. The sound may last for a few seconds or even several tens of seconds, and it’s completely normal.

This unique “barking” noise has become one of the signature sounds of the Airbus A320 family. Other Airbus models also have a PTU, but the sound is much quieter, and passengers rarely notice it. Newer aircraft like the A350 use more advanced electric hydraulic systems (EHA/EPC), so they don’t need a traditional PTU and therefore don’t make this sound. Boeing aircraft also use different hydraulic designs, so you won’t hear the PTU noise on them either.

Next time you fly on an Airbus A320 family aircraft, try listening for this strange but interesting sound!