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A diagrammatic representation of a fixed-wing airplane in phugoid. In aviation, a phugoid or fugoid (/ ˈ f juː ɡ ɔɪ d / ⓘ) is an aircraft motion in which the vehicle pitches up and climbs, and then pitches down and descends, accompanied by speeding up and slowing down as it goes "downhill" and "uphill".
The phugoid oscillation is a slow interchange of kinetic energy (velocity) and potential energy (height) about some equilibrium energy level as the aircraft attempts to re-establish the equilibrium level-flight condition from which it had been disturbed.
Since the lift is very much greater than the drag, the phugoid is at best lightly damped. A propeller with fixed speed would help. Heavy damping of the pitch rotation or a large rotational inertia increase the coupling between short period and phugoid modes, so that these will modify the phugoid.
[3]: 319 This greatly excited the phugoid motion, [3]: 291 and the aircraft pitched up, before pitching back down after power was reduced. When power was added again, the aircraft rapidly pitched up to 40°, and the airspeed dropped down to 108 knots (200 km/h; 124 mph) at 6:49:30 p.m., [ 3 ] : 1–6, 291 briefly stalling at 9,000 feet (2,700 m).
An animated illustration of the two motions which combine into a Dutch roll Dutch roll damping technique, scanned from U.S. Air Force flight manual. Dutch roll is an aircraft motion consisting of an out-of-phase combination of "tail-wagging" (yaw) and rocking from side to side (roll).
The aircraft pitched rapidly up and down in a roller-coaster phugoid, oscillating between a nose-up and a nose-down position. As in the case of the 1989 United Airlines Flight 232 disaster in the United States, Captain Genotte could only use thrust to modify pitch, speed, and altitude and vary throttles asymmetrically to control yaw and turn ...
In flight dynamics, longitudinal stability is the stability of an aircraft in the longitudinal, or pitching, plane.This characteristic is important in determining whether an aircraft pilot will be able to control the aircraft in the pitching plane without requiring excessive attention or excessive strength.
The aircraft began to exhibit phugoid oscillations, but the crew countered them and maintained a controlled descent at about 250 to 260 knots (460 to 480 km/h). They were able to bring the plane to 4,000 ft (1,220 m) and begin the approach to Tan Son Nhut's runway 25L.