全美航空哈德逊河迫降过程中襟翼为什么不完全打开？ 第1页

这个问题很有意思，刚好之前几个朋友也曾一起讨论过，后来查阅了一些资料，分享给题主供参考。

水上迫降（ditching）有一些基本准则，比较重要的例如：

▪️风向、波浪的选择（into the wind， along the swell）

▪️保持光洁的机身。意味着landing gear处于收起状态，有的POH还会强调把gear circuit breaker拔开。

▪️Wings level 防止侧翻

▪️尽可能降低touch down的速度，这个很好理解，减少冲击的能量，但是前提是不能小到失速。

再来看flaps。

flaps可以降低失速速度，使飞机尽可能低速降落。然而很多文章，包括

的Emergency Procedures里描述"Use no more than intermediate flaps on low-wing airplanes"，都提到了这个问题，low wing的飞机在水上迫降时使用中间档位的flaps。

AOPA有篇文章

对此的解释是，low wing飞机在水上降落时，full flaps很容易被水冲击损坏，造成机翼结构受损，不对称性的机翼容易使得机身侧翻。

High wing飞机没有这个顾虑，POH通常推荐使用full flaps以获得最小速度。

此外还有一些不同的解释，例如认为full flaps姿态不好，下降率过大，以及power off时full flaps更难flare。仁者见仁，智者见智。

我对空气动力学和飞机系统都知之甚少，查阅到的这些解释仅供参考，也希望看到更专业的回答。

这个问题有趣。特地查了一下真实事件中NTSB的调查报告：

其中第2.3.4节是关于襟翼选择的问题：

In the accident scenario, the NTSB notes that the selection of flaps 3 would have allowed the airplane to fly at a lower airspeed.
...
The first officer then stated that they were at flaps 2 and asked the captain if he “want[ed] more?” The captain replied, “no, let’s stay at 2.” About 1 minute later, the airplane was ditched on the Hudson River.
During postaccident interviews, the captain stated that he used flaps 2 because there were “operational advantages to using flaps 2.” He stated that using flaps 3 would not have lowered the stall speed significantly and would have increased the drag. He stated that he was concerned about having enough energy to successfully flare the airplane and reduce the descent rate sufficiently. He stated that, from his experience, using flaps 2 provides a slightly higher nose attitude and that he felt that, in the accident situation, flaps 2 was the optimum setting.

机长当时的考虑是3档襟翼并不会显著降低失速的速度，还会增加很多阻力。对于一架失去动力的飞机来说，最后一刻的拉平会很难。使用2档会略微提高仰角（个人猜测是指拉平前姿态已经更仰，不需要作太多的修正）。总之，就是多年作为飞行员的经验。

NTSB对此的评价也是肯定的：

The NTSB concludes that the captain’s decision to use flaps 2 for the ditching, based on his experience and perception of the situation, was reasonable and consistent with the limited civilian industry and military guidance that was available regarding forced landings of large aircraft without power.

无动力前提下下滑道角度与升阻比成反比，升阻比过低就需要更低姿态角近进，到最后改平不仅需要更大的姿态变化，也会因为更高阻力导致速度衰减过快。

总的来说就是接地（水）风险更大

另外大襟翼角度会改变零升迎角，会造成更低的接地姿态，外加襟翼触水的额外阻力，显然增大风险

近进升阻比过大和过小飞起来都非常麻烦