Tag: TechStories

We have an international airport nearby, and planes are almost always in the sky (plus, even closer by, a small airfield where small private planes are constantly taking off and landing). And I’ve noticed something interesting several times—a plane virtually hanging in the air. Just driving along, you look up—at some point in the sky, there’s a plane. Look up a few seconds later, and it’s exactly in the same spot!

Yes, it’s almost always flying almost directly towards me, so the angular velocity is not noticeable. But again, there are many planes flying, and I have something to compare it with. Encountered it again today, but I went the extra mile and checked on flightradar to see what this four-engine aircraft was.

I investigated. It was an Airbus A380—the largest airliner in the world. It’s just big, the wings are huge, and with a strong headwind, and specifically the one I saw was decelerating to 155 km/h according to flightradar24 while approaching for landing. But the following B777 also landed at a similar speed, give or take. The difference is, the B777 is significantly smaller than the A380 (30% shorter in fuselage height and nearly 20% in wingspan), and the same visual angular size for A380 appears at a higher altitude, where angular velocity is lower. Meaning, I saw the A380 when it was further away than I am used to seeing Boeings, and therefore its speed seemed very slow.

Perhaps, in addition to this, there might be a headwind. It can really drastically reduce the plane’s speed relative to the ground. Here I’m “out of my depth” and don’t know what pilots do when landing against the wind.

While I was figuring this out, I read that apparently, the A380 engages its thrust reversers (which direct the engine’s thrust forward) quite late, when the speed drops to 80 km/h. Although a fully loaded Airbus A380 weighs 573 tons, its braking system does not rely heavily on reversers. They are not a mandatory element for airliners, and on the A380, they are installed only on the two inner engines.

Although reversers help slow down the A380, their contribution to braking is not as significant as on most airliners, including the Boeing 747, where thrust reversers do significantly reduce the braking distance compared to brakes and spoilers.

It turns out, A380’s main braking effort is performed by huge Honeywell composite brakes on 16 of the 20 main landing gear wheels. The brakes on the A380 are equipped with a car-like anti-lock braking system. The aerodynamic braking, thanks to 16 large spoilers on the wings, which create drag and reduce lift, also plays an important role. Reducing lift improves mechanical braking, increasing pressure on the wheels.

The A380 slows down from cruising speed of 500 knots (926 km/h) to a complete stop within a few minutes thanks to the overall design of the plane. The descent begins at cruising altitude at about 0.85 Mach (1235 km/h). Below 10,000 feet, the speed decreases to 250 knots (463 km/h), and on the approach glide, the plane moves at 180 knots (333 km/h). The A380 crosses the landing threshold at 140 knots (259 km/h) and touches down at 130 knots (240 km/h).

By the way, here’s how beautifully the landing gear of the A380 extends: