Phugoid

It can be excited by an elevator singlet (a short, sharp deflection followed by a return to the centered position) resulting in a pitch increase with no change in trim from the cruise condition.

A classical model for the phugoid period can be simplified to about (0.85 times the speed in knots) seconds, but this only really works for larger aircraft.

[further explanation needed] Phugoids are often demonstrated to student pilots as an example of the speed stability of the aircraft and the importance of proper trimming.

[4] In 1972, an Aero Transporti Italiani Fokker F-27 Friendship, en route from Rome Fiumicino to Foggia, climbing through 13,500 feet, entered an area of poor weather with local thunderstorm activity.

[5] In the 1975 Tan Son Nhut C-5 accident, USAF C-5 68-0218 with flight controls damaged by failure of the rear cargo/pressure door, encountered phugoid oscillations while the crew was attempting a return to base and crash-landed in a rice paddy adjacent to the airport.

In 1985, Japan Air Lines Flight 123 lost all hydraulic controls after its vertical stabiliser blew off due to an aft pressure bulkhead failure, and went into phugoid motion.

While the crew were able to maintain near-level flight through the use of engine power, the plane lost height over a mountain range northwest of Tokyo before crashing into Mount Takamagahara.

[7] Chesley "Sully" Sullenberger, Captain of US Airways Flight 1549 that ditched in the Hudson River on January 15, 2009, said in a Google talk that the landing could have been less violent had the anti-phugoid software installed on the Airbus A320-214 not prevented him from manually getting maximum lift during the four seconds before water impact.