十公里高空

在长途航班上望向窗外,大地就在你脚下约十公里处,化作田野与云朵拼成的一片图案。然而,航空公司选择这个高度,并不是因为风景壮观,无论那风景有多么惊人。他们选择它,是因为那里的空气稀薄,而稀薄的空气飞起来格外省钱。

Glance out of the window on a long flight and the earth sits some ten kilometres beneath you, reduced to a patchwork of fields and cloud. Airlines do not choose that altitude for the view, however spectacular it happens to be. They choose it because the air up there is thin, and thin air is remarkably cheap to fly through.

在地面附近,空气很稠密,推动飞机穿过它,就像游泳者蹚过水一样费力:每飞一米都要消耗能量。到了十公里高空,空气已经稀薄了许多,飞机穿行时受到的阻力小得多,这意味着发动机要维持同样的速度,燃烧的燃料明显更少。稀薄的空气还带来第二个好处:大多数风暴、湍流和厚重的云层都停留在这个高度以下,这也是为什么那里的飞行往往平稳得多。

Near the ground, air is dense, and pushing a plane through it is rather like a swimmer wading through water: every metre travelled costs energy. Ten kilometres up, the air has thinned considerably, so the aircraft slips through it with far less resistance, which means the engines burn noticeably less fuel to hold the same speed. Thin air offers a second advantage too: most storms, turbulence, and heavy cloud sit well below that height, which is why the ride up there tends to be so much smoother.

当然,这也有极限。飞得太高,空气就会稀薄到机翼无法产生足够的升力,或者发动机吸不到需要的氧气。因此,航空公司总是瞄准一个狭窄的最佳区间,通常在十到十二公里之间:高到足以省油,低到足以让飞机继续飞行。每一次安全带指示灯熄灭的时候,这笔几乎无形的交易都在悄悄进行。

There is, inevitably, a limit. Climb too high, and the air becomes too thin either for the wings to generate enough lift or for the engines to draw the oxygen they need. Airlines therefore aim for a narrow sweet spot, generally between ten and twelve kilometres: high enough to spare the fuel bill, low enough to keep the aircraft flying at all. That quiet trade off is being made, almost invisibly, every single time your seatbelt sign switches off.