I have been observing several aircraft landings and have noticed that when the aircraft's landing gear touches down, it precipitates the rising of a cloud of smoke. Why is this?
(Source: airliners.net)
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10$\begingroup$ Beautiful picture indeed! $\endgroup$– Harshil SharmaMar 18, 2018 at 10:22
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1$\begingroup$ It's from the tires not the landing gear $\endgroup$– Steve KuoMar 19, 2018 at 16:22
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6$\begingroup$ The flight is a non-smoking one. Landing gear is the first one who get to smoke one the ground. $\endgroup$– vasin1987Dec 27, 2018 at 5:31
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5 Answers
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Because the tires are momentarily skidding on the pavement as they rapidly spin up from standstill to the jet’s touchdown speed. It’s just a puff of burnt rubber but the tire tread itself becomes quite warm, reaching 600-700°F for an instant. It’s also the reason a pilot does not ride the brakes upon touchdown as this could cause a blowout as the tire is ground up rapidly.
answered Mar 17, 2018 at 20:26
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16$\begingroup$ to enhance tire life, some airplanes were equipped with pre-rotator motors which were intended to spin the tires up to speed so they would not skid and smoke upon initial touchdown. does anyone know if this is still used today? $\endgroup$ Mar 17, 2018 at 20:44
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15$\begingroup$ @nielsnielsen that question has been explored here: aviation.stackexchange.com/questions/3702/… $\endgroup$– BenMar 17, 2018 at 21:14
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4$\begingroup$ @nielsnielsen: Those systems were long abandoned, and we discussed them here $\endgroup$ Mar 17, 2018 at 21:14
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2$\begingroup$ Basically the same reason that car tires smoke when someone "burns rubber". $\endgroup$– jamesqfMar 18, 2018 at 1:06
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Imagine yourself driving your car at a speed of 60mph (96 kmph) and suddenly you apply the brakes completely and notice that it produces smoke. THIS IS FRICTION.
Now, when the aircraft lands at 180 knots (324 kmph) it creates friction too.
- Exactly before the tyre touches the ground it is static.
- As soon as it touches it takes that tiny bit of time to gain the relative speed with the airplane on the runway, but since it has already touched the rough surface. It drags on it. This is when the smoke appears.
- After that it gains speed and the speed is gradually broken down by the speed brakes and it finally comes to a stop.
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2$\begingroup$ Except that most newer cars have anti-lock brake systems that are supposed to prevent the tires from locking up. $\endgroup$– jamesqfMar 18, 2018 at 18:02
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$\begingroup$ @jamesqf even with such systems fitted under heavy braking - stopping dead on the motorway from standard speeds for example - I've seen puffs of smoke in my rear-view mirror. $\endgroup$– James TMar 19, 2018 at 8:43
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1$\begingroup$ @jamesqf : They work by detecting the lock, though. That means they don't prevent it. When a lock is detected, the "anti-lock" system quickly releases the brake. This allows the wheel to rotate again. It's not 100% efficient, but from a control perspective it's much easier. $\endgroup$– MSaltersMar 19, 2018 at 10:31
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$\begingroup$ @James Trotter: I wouldn't know about that, at least from my own experience, as I try to avoid the need for heavy braking :-) $\endgroup$– jamesqfMar 19, 2018 at 18:14
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$\begingroup$ @jamesqf Unfortunately, as I have learned, it is sometimes unavoidable; When there's a collision a hundred yards in-front of your bonnet you need to come to a stop ASAP. $\endgroup$– James TMar 20, 2018 at 7:42
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Anytime a tire's bottom is moving at a significantly different speed than the roadway, it'll make smoke. An example is a burnout, where the tire is spinning at high speed with a vehicle and road going 0 mph. Roads generally go 0 mph.
Prior to aircraft touchdown, the tires are not turning, so the top and bottom of its tires are going at the same speed - aircraft ground speed, say 120 knots for a 737 in a bit of a headwind. Not all runways have a ground speed of 0 knots, but the ones that land 737s do.
When the 120-knot tire bottom touches the 0-knot runway, that's a big difference, so as said, there'll be smoke. This reveals drag on the tire, which will very quickly spin the tire up so its bottom is going 0 knots relative to the ground. At that point the smoke will stop.
answered Mar 18, 2018 at 5:12
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2$\begingroup$ Even the runways that do go faster than 0 knots typically don't go at the aircraft's landing speed, so there's a difference either way. $\endgroup$– userMar 19, 2018 at 9:43
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Planes land at somewhere in the ballpark of 150mph (230km/h). At the moment when the plane lands, the wheels aren't spinning, and the only thing that can make them spin is the contact with the runway. So, at the moment the plane lands, the wheels are skidding at 150mph, and they'll continue to partially skid until their rotation speed matches the speed of the plane. And it takes a little while for that to happen, because aircraft wheels are pretty heavy: according to Lufthansa, a wheel and tyre on a 737 weighs about 112kg, and about 185kg on a 747.
So, the smoke you see is material being worn off the tyres as they skid up to speed. They're designed to be used this way and are regularly inspected and replaced to ensure that not too much of the rubber is worn away.
answered Mar 18, 2018 at 13:53
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$\begingroup$ Nice link! I would like each answer provide such further readings. $\endgroup$– Manu HMar 19, 2018 at 6:41
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$\begingroup$ For a wheel/tire that large, those are rather light. Contrast with a similar sized wheel/tire on any piece of construction equipment. $\endgroup$ Jul 1, 2019 at 1:25
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$\begingroup$ Sadly, your Lufthansa link no longer seems to indicate anything about tire (or tyre) weight for any sort of aircraft. $\endgroup$– FreeManJul 1, 2019 at 18:08
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$\begingroup$ @FreeMan Thanks -- replaced with a Wayback Machine link to how it was. $\endgroup$ Jul 1, 2019 at 18:56
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It is kind of the reverse of burning tires on a car. There is a limit to static friction and tires spin. Even when they spin there is kinetic friction. When the tires spin they get hot from friction and burn (more like the rubber vaporizes).
The tires have mass and it takes energy to spin them up the speed of the plane. Initially the speed difference is so great they spin and smoke. The kinetic friction quickly brings the tire up to the speed of the plane.
