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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).
This yaw-roll coupling is one of the basic flight dynamic modes (others include phugoid, short period, and spiral divergence). This motion is normally well damped in most light aircraft, though some aircraft with well-damped Dutch roll modes can experience a degradation in damping as airspeed decreases and altitude increases.
Longitudinal dihedral can influence the nature of controllability about the pitch axis and the nature of an aircraft's phugoid-mode oscillation. When the term "dihedral" (of an aircraft) is used by itself it is usually intended to mean "dihedral angle". However, context may otherwise indicate that "dihedral effect" is the intended meaning.
Pilot-induced oscillation rating scale, with start position at bottom left. Pilot-induced oscillations (PIOs), as defined by MIL-HDBK-1797A, [1] are sustained or uncontrollable oscillations resulting from efforts of the pilot to control the aircraft.
December 4, 1977 – Malaysian Airline System Flight 653, a 737-200 registered as 9M-MBD, crashed following a phugoid oscillation that saw the aircraft diving into a swamp after both its pilots were shot following a hijacking attempt. The crash happened in the Southern Malaysian state of Johor.