《知識》MD-11 從客機到貨運之王，三引擎時代的終章 McDonnell Douglas MD-11: From Passenger Jet to Cargo King — The Final Chapter of the Trijet Era

在航空史上，曾經有一個特別的時代，只要走進機場停機坪，就很容易被一種造型獨特的飛機吸引，這類被稱為三引擎客機的代表作之一，就是 McDonnell Douglas MD-11。它誕生於航空業從三引擎逐步走向雙引擎的轉折年代，也成為這種設計的巔峰之作。

與早期三引擎飛機不同，MD-11 的中央引擎設計相當獨特。像 Boeing 727 或 Lockheed L-1011 TriStar，機尾引擎通常採用彎曲的 S 型進氣道，氣流必須繞彎才能進入發動機。但 MD-11 卻採用了「直通式」設計，把引擎直接安裝在垂直尾翼基座上，讓進氣口、管道與風扇幾乎呈一直線。這樣做不但簡化了結構製造，同時還能幫助整架飛機取得更好的氣動平衡。不過它也帶來特別的維修挑戰，因為引擎位置很高，維修人員有時需要進入尾部結構內部才能接近設備。

MD-11 的誕生，其實是為了優化前代機型 McDonnell Douglas DC-10。它導入延長機身、翼尖小翼，以及先進的玻璃駕駛艙，最重要的是只需要兩名飛行員操作。

在當時，航空業正從三人機組轉向兩人制，而 MD-11 採用的高旁通比渦輪扇發動機具備高度自動化能力，讓飛航工程師的角色逐漸消失。憑藉這些技術，MD-11 能飛行超過一萬三千公里，巡航速度接近每小時一千公里，最多可搭載約四百名乘客。延伸閱讀：《知識》飛機引擎運作原理及何謂「旁通比」？

雖然它在客運市場很快受到新世代雙引擎廣體客機的衝擊，例如 Boeing 777，但 MD-11 在貨運領域卻找到第二生命。對貨運公司而言，燃油效率劣勢變得不那麼重要，反而看重酬載能力與成本彈性。像 FedEx 與 UPS Airlines 都長期把 MD-11 當作主力貨機，因為它能承載超過九十公噸貨物，且在重載下仍能維持長航程。

為了承受這種驚人的重量，MD-11 的起落架設計也相當特殊。整架飛機共有十二顆輪胎，能有效分散地面壓力，確保在滿載狀態下仍能安全起降，這也是它能長期執行跨洋貨運任務的重要原因。

然而，2025 年 UPS Airlines 一場事故促使美國聯邦航空局（FAA）發布緊急適航指令，導致 MD-11 機隊一度停飛。之後 UPS 決定加速除役，而 FedEx 則選擇完成檢查後繼續使用，使這款傳奇機型走向不同的結局。

回顧整個航空發展史，MD-11 可以說是三引擎時代的最後餘暉。雖然如今天空幾乎已被雙引擎客機主導，但那具直通機尾、宛如第三隻眼的設計，仍然是航空史上最令人難忘的經典之一。

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McDonnell Douglas MD-11: From Passenger Jet to Cargo King — The Final Chapter of the Trijet Era

In aviation history, there was once a special era when a walk across an airport ramp would almost always reveal a very distinctive type of aircraft. These were the three-engine jets, and one of the most famous examples was the MD-11. It was born during a turning point, when the industry was gradually shifting from three-engine designs to twin-engine aircraft, and it became one of the most advanced trijets ever built.

Unlike earlier trijets, the MD-11 had a very unique center engine design. On aircraft such as the Boeing 727 or Lockheed L-1011 TriStar, the tail engine usually used an S-shaped air intake duct, meaning the airflow had to bend before reaching the engine. The MD-11 instead used a straight-through layout, mounting the engine directly at the base of the vertical tail so the intake, duct, and fan sat almost in a straight line. This simplified manufacturing and also improved aerodynamic balance. However, it created a unique maintenance challenge, because the high engine position sometimes required technicians to enter the tail structure to reach it.

The MD-11 was essentially designed to modernize its predecessor, the McDonnell Douglas DC-10. It featured a longer fuselage, winglets, and an advanced glass cockpit. Most importantly, it could be operated by just two pilots.

At the time, the airline industry was transitioning from three-person crews to two-pilot operations. The MD-11’s high-bypass turbofan engines included advanced automation, which gradually eliminated the need for a flight engineer. Thanks to these technologies, the aircraft could fly more than 13,000 kilometers, cruise at nearly 1,000 km/h, and carry up to about 400 passengers.

Although it soon faced strong competition in passenger service from new twin-engine wide-body aircraft like the Boeing 777, the MD-11 found a second life as a cargo aircraft. For cargo operators, fuel efficiency was less critical than payload capacity and operating flexibility. Companies such as FedEx and UPS Airlines relied heavily on the MD-11 because it could carry over 90 tons of freight while still maintaining long range.

To support such heavy loads, the MD-11 also had a special landing gear design. The aircraft used a total of twelve wheels to better distribute ground pressure, ensuring safe operations even at maximum takeoff weight. This was one of the key reasons it performed so well in long-haul cargo missions.

However, in 2025, an accident involving UPS led the Federal Aviation Administration to issue an emergency airworthiness directive, temporarily grounding MD-11 fleets. Afterward, UPS chose to accelerate the retirement of the aircraft, while FedEx decided to continue operating it after inspections, giving the legendary jet two very different endings.

Looking back at aviation history, the MD-11 can be seen as the final glow of the trijet era. Today, the skies are dominated by highly efficient twin-engine aircraft, but the MD-11 remains one of the most unforgettable designs ever created.