Deset kilometara u visinu

Baci pogled kroz prozor tijekom dugog leta i zemlja se nalazi otprilike deset kilometara ispod tebe, svedena na mozaik polja i oblaka. Zrakoplovne kompanije ne biraju tu visinu zbog pogleda, koliko god on bio spektakularan. Biraju je zato što je zrak ondje gore rijedak, a kroz rijedak zrak izuzetno je jeftino letjeti.

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.

Blizu tla zrak je gust, pa je guranje zrakoplova kroz njega slično plivaču koji gazi kroz vodu: svaki prijeđeni metar troši energiju. Na visini od deset kilometara zrak se znatno prorijedio, pa zrakoplov kroz njega klizi uz mnogo manji otpor, što znači da motori primjetno manje troše gorivo za održavanje iste brzine. Rijedak zrak nudi i drugu prednost: većina oluja, turbulencija i gustih oblaka nalazi se znatno ispod te visine, zbog čega je let ondje gore obično mnogo mirniji.

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.

Neizbježno, postoji granica. Penješ li se previsoko, zrak postaje prerijedak da bi krila stvarala dovoljno uzgona ili da bi motori mogli usisati potreban kisik. Zrakoplovne kompanije stoga ciljaju na uski idealni raspon, obično između deset i dvanaest kilometara: dovoljno visoko da se uštedi na gorivu, dovoljno nisko da zrakoplov uopće može letjeti. Taj se tihi kompromis donosi, gotovo neprimjetno, svaki put kada se ugasi znak za sigurnosni pojas.

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.