Why do planes vibrate on takeoff?

A takeoff roll takes an airliner from standstill to roughly 270 km/h (145 kn) in about forty seconds, and for almost all of that time the aircraft is a 200-tonne vehicle on rubber tyres with a very stiff suspension. Runways are not smooth. Concrete slabs meet at expansion joints every few metres, many surfaces are transversely grooved to shed water, and centreline lights sit proud of the pavement. The nose wheel hits each of these first, followed a fraction of a second later by the main gear, and the shocks travel straight into the fuselage. At low speed the effect is a rhythmic thump; as the speed rises the frequency climbs into the range where the cabin fittings, overhead bins and seat backs resonate, and the ride becomes a continuous shudder. Passengers who have felt a smooth takeoff from a freshly paved runway and a rough one from an older concrete strip have felt the difference the pavement makes.

Underneath that, the engines add their own signature. At takeoff thrust the fans are turning at between about 2,500 and 5,000 rpm depending on the engine, and the whole nacelle transmits a low hum through the pylon and wing. The wing itself is in its least clean configuration, with slats and flaps extended to generate lift at low speed; air spilling off flap edges and through the slots creates turbulence that buffets the flaps and produces a soft rumble felt in seats behind the wing. Tyres spinning up to more than 1,000 rpm with any imbalance, and a nose-wheel that can develop a shimmy on some types, add higher-frequency components. All of this peaks in the last seconds before rotation, when speed is highest and the wheels are still carrying most of the weight.

The moment the main gear leaves the ground the runway input disappears and the aircraft becomes a body suspended in a fluid, which damps vibration rather than transmitting it. The gear retracts, the flaps and slats are gradually pulled in as speed increases, and the engines are throttled back to climb thrust within a minute or two. What remains is a much lower level of vibration from the engines and the boundary layer, most of it filtered out by the wing structure and the cabin's mounting. None of the takeoff shaking is a sign of a problem: certification requires the airframe to withstand far larger loads than a rough runway can produce, and the gear is designed to absorb a landing at 3 m/s (10 ft/s) descent rate on every flight. A vibration that persists after the gear is up, or one that changes in character abruptly, is what crews are trained to watch for; the ordinary rattle of the roll is simply the aircraft telling you what the pavement is like.