Deset kilometara u visinu

Pogledaj kroz prozor zrakoplova tijekom dugog leta i vidjet ćeš svijet deset kilometara ispod sebe. Zrakoplovi ne lete tako visoko zbog pogleda. Lete tako jer je zrak na toj visini rijedak, a kroz rijedak zrak jeftinije je letjeti.

Look out of a plane window during a long flight and the world is ten kilometres below you. Planes do not fly that high to enjoy the view. They do it because the air up there is thin, and thin air is cheap to fly through.

Blizu tla zrak je gust. Zrakoplov koji se probija kroz gust zrak nalik je plivaču koji se probija kroz vodu: svaki metar košta energije. Deset kilometara u visinu, zrak je nekoliko puta rjeđi. Zrakoplov kroz njega klizi s mnogo manje otpora, pa motori troše mnogo manje goriva da bi održali istu brzinu. Rijedak zrak donosi i drugi dar: većina oblaka, oluja i turbulentnog vremena nalazi se ispod te visine, pa je let i mirniji.

Near the ground, air is thick. A plane pushing through thick air is like a swimmer pushing through water: every metre costs energy. Ten kilometres up, the air is several times thinner. The plane slips through it with much less resistance, so the engines burn far less fuel to keep the same speed. The thin air has a second gift: most clouds, storms, and bumpy weather live below that height, so the ride is smoother too.

Naravno, postoji granica. Previsoko, i zrak postaje prerijedak da bi krila držala zrakoplov u zraku, i prerijedak da bi motori mogli disati. Zato avioprijevoznici ciljaju na idealnu točku, obično između deset i dvanaest kilometara. Dovoljno visoko za uštedu goriva, dovoljno nisko da let ostane moguć. To je tihi proračun koji se odvija svaki put kad se ugasi znak za sigurnosni pojas.

There is a limit, of course. Too high, and the air becomes too thin for the wings to hold the plane up, and too thin for the engines to breathe. So airlines aim for the sweet spot, usually between ten and twelve kilometres. High enough to save fuel, low enough to keep flying. That is the quiet calculation happening every time your seatbelt sign turns off.