An aircraft constructed with a push-pull configuration has a mixture of forward-mounted (tractor) propellers and backward-mounted (pusher) propellers.
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An early example of a "push-pull" aircraft was the Caproni Ca.1 which had two wing-mounted tractor propellers and one centre-mounted pusher propeller.
Claudius Dornier was the first aviation designer to heartily embrace the concept for production aircraft, as many of his flying boats used variations of the tandem "push-pull" engine layout :
Push-pull designs have the engines mounted above the wing as Dornier flying boats or more commonly on a shorter fuselage than conventionnal one, as for Rutan Defiant or Voyager canard designs. Twin boomers such as the Cessna Skymaster and Adam A500 have the aircraft's tail suspended via twin booms behind the pusher propeller. In contrast, both the World War II-era Dornier Do 335 and the early 1960s-designed French Moynet M 360 Jupiter experimental private plane had their pusher propeller behind the tail.
While pure pushers decreased in popularity during the First World War, the push-pull configuration has continued to be used. The advantage it provides is the ability to mount two propellers on the aircraft's centreline, thereby avoiding the increased drag that comes with twin wing-mounted engines. It is also easier to fly if one of the two engines fails, as the thrust provided by the remaining engine stays in the centerline. In contrast, a conventional twin-engine aircraft will yaw in the direction of the failed engine and become uncontrollable below a certain airspeed, known as Vmc.
Pilots in the United States who obtain a multi-engine rating in an aircraft with this push-pull, or "centerline thrust," configuration are restricted to flying centerline-thrust aircraft; pilots who obtain a multi-engine rating in conventional twin-engine aircraft do not have a similar limitation with regard to centerline-thrust aircraft.
Despite its distinct advantages push-pull configurations are rare in military aircraft. This is mainly due to the increased risk to the pilot in the case of a crash-landing or the need to parachute from the plane. In a crash the rear engine threatens to kill the pilot by crushing him between itself and the forward engine; in the case of bailing-out the pilot is in danger of hitting the rear propeller. Examples of past military applications include the late WWII Dornier Do 335 heavy fighter, which included a jettisonable tail to facilitate bailing out, and the Cessna O-2, a militarised version of the Skymaster used for observation and forward air control during the Vietnam war.