NEW YORK—Your flight comes in for a landing so smooth you can barely feel the wheels touch down. Nice job by the pilot, the passengers think.

Your next flight lands with a resounding thump. The passengers assume the pilot blew that one.

In both cases, the passengers could be wrong.

A jarring landing (pilots prefer to say ''firm'') can be the safest way to bring down an airplane, depending on wind, rain and runway condition, experts say.

And some smooth landings, though sure passenger-pleasers, entail risks of their own.

Landing a modern jet is not a simple matter. In a typical case, it means bringing 60 tons of plane and people, flying 10 kilometers above the earth at 650 km an hour, gently down to a runway ''touchdown zone'' less than 900 meters long.

In the process, dozens of factors affect the pilot's last-minute landing calculations and the gentleness of the touchdown.

If a big jet makes a smooth-as-silk landing on a runway covered with water, there's a risk of the aircraft hydroplaning along the runway instead of stopping, pilots say.

''You don't want a smooth landing. You want to drive through the water so the plane gets a firm grip so it stops faster,'' says John Cox, a Boeing 737 pilot for US Airways.

Quick, fast contact with the runway can also be the best course when the surface is covered by ice or snow.

On some aircraft, anti-skid brakes and the spoiler panels that pop up atop the wings only start working when the wheels get traction on the runway surface.

Pilots also like to touch down quickly, even at the risk of jarring the passengers, when the plane is being buffeted by wind from one side.

Trying for a slow, gentle landing in a strong crosswind could lead to the plane being blown sideways across the runway.

And at some airports, runways are rougher than at others, meaning no landing can be completely smooth.

When runway and wind conditions are good, the pilot has a better chance of a smooth landing--a ''grease job,'' some call it. But only if the plane's speed is right: A too-fast descent will always have a bumpy conclusion.

Some pilots will ''float'' above the runway trying to reduce their descent speed enough for a smooth touchdown. (''Greasing down'' an Airbus 320, for instance, requires getting the descent rate below 30 meters per minute.) While floating is fine, the pilot can't skim above the runway for so long that not enough concrete remains for a safe stop after touchdown.

If he lands ''too long,'' he has no choice but to slam on all the braking devices he has, jarring the passengers at least as much as in a firm landing, to keep the plane from going off the end of the runway into grass, mud or obstructions.

''We'd like all our landings to be supersoft smoothies because that's what the people in back want,'' says Craig Holmes, a chief Airbus pilot for Air Canada. ''But there comes a time when it's not appropriate to trade off the remaining runway surface for a nice, smooth landing.''

It's easy for the pilot's ego to get involved in touchdown smoothness, especially at airlines where, by tradition, the pilot who makes the landing is the one who stands in the cockpit doorway facing the passengers as they depart.

Pilots could make a loudspeaker announcement after a firm landing to explain why it was the safest thing. But few ever do.