View source for Phugoid

{{short description|Aircraft motion involving constant angle of attack but varying pitch}}

[[File:Phugoid8.png|400px|thumb|right|A [[diagram]]matic representation of a [[Fixed-wing aircraft|fixed-wing airplane]] in phugoid]]

In [[aviation]], a '''phugoid''' or '''fugoid''' ({{IPAc-en|ˈ|f|juː|ɡ|ɔɪ|d|audio=En-us-phugoid.oga}}) is an aircraft motion in which the vehicle [[Aircraft principal axes|pitches]] up and climbs, and then pitches down and descends, accompanied by speeding up and slowing down as it goes "downhill" and "uphill". This is one of the basic [[aircraft dynamic modes|flight dynamics modes]] of an [[fixed-wing aircraft|aircraft]] (others include [[short period]], [[Aircraft_dynamic_modes#Roll_subsidence_mode|roll subsidence]], [[dutch roll]], and [[spiral divergence]]).

==Detailed description==
The phugoid has a nearly constant [[angle of attack]] but varying [[pitch (flight)|pitch]], caused by a repeated exchange of [[airspeed]] and [[altitude]]. It can be excited by an [[elevator (aircraft)|elevator]] singlet (a short, sharp deflection followed by a return to the centered position) resulting in a [[pitch (flight)|pitch]] increase with no change in [[trim tab|trim]] from the [[cruise (flight)|cruise]] condition. As speed decays, the nose drops below the horizon. Speed increases, and the nose climbs above the horizon. Periods can vary from under 30 seconds for [[light-sport aircraft|light aircraft]] to minutes for [[airliner|larger aircraft]]. [[Ultralight aviation|Microlight aircraft]] typically show a phugoid period of 15&ndash;25 seconds, and it has been suggested{{by whom|date=March 2019}} that birds and model airplanes show convergence between the phugoid and short period modes. A classical model for the phugoid period can be simplified to about (0.85 times the speed in [[knot (unit)|knots]]) seconds, but this only really works for larger aircraft.{{Explain|reason=why larger aircraft? Is there an analogue for smaller ones? What counts as larger?|date=March 2019}} 

Phugoids are often demonstrated to student pilots as an example of the speed stability of the aircraft and the importance of proper trimming. When it occurs, it is considered a nuisance, and in lighter airplanes (typically showing a shorter period) it can be a cause of [[pilot-induced oscillation]].

The phugoid, for moderate amplitude,<ref>Charles Hampson Grant, ''Model Airplane Design and Theory of Flight'', Jay, New York, 1941</ref> occurs at an effectively constant angle of attack, although in practice the angle of attack actually varies by a few tenths of a degree. This means that the stalling angle of attack is never exceeded, and it is possible (in the <1g section of the cycle) to fly at speeds below the known stalling speed. Free flight models with badly unstable phugoid typically stall or loop, depending on thrust.<ref>Keith Laumer, ''How to Design and Build Flying Models'', Harper, New York, 1960</ref>

An unstable or divergent phugoid is caused, mainly, by a large difference between the incidence angles of the wing and tail. A stable, decreasing phugoid can be attained by building a smaller stabilizer on a longer tail, or, at the expense of pitch and yaw "static" stability, by shifting the center of gravity to the rear.{{why|date=July 2022}}{{cn|date=July 2022}}

Aerodynamically efficient aircraft typically have low phugoid damping.<ref name="Stengel">{{cite book |last1=Stengel |first1=Robert F. |title=Flight Dynamics |date=17 October 2004 |publisher=Princeton University Press |isbn=978-0-691-11407-1 |url=https://books.google.com/books?id=dWKYDwAAQBAJ |access-date=6 July 2022 |language=en}}</ref>{{rp|464}}

The term "phugoid" was coined by [[Frederick W. Lanchester]], the British aerodynamicist who first characterized the phenomenon. He derived the word from the [[Greek (language)|Greek]] words {{lang|grc|φυγή}} and {{lang|grc|εἶδος}} to mean "flight-like" but recognized the diminished appropriateness of the derivation given that {{lang|grc|φυγή}} meant flight in the sense of "escape" (as in the word "fugitive") rather than vehicle flight.<ref>Frederick William Lanchester, ''Aerodonetics: Constituting the second volume of a complete work on aerial flight'', (London, England: Archibald Constant Co. Ltd., 1908), [https://archive.org/stream/aerodoneticscon02lancgoog#page/n13/mode/2up p. viii] and [https://archive.org/stream/aerodoneticscon02lancgoog#page/n371/mode/2up p. 348.]</ref>

==Aviation accidents==
{{Main|Flight with disabled controls}}

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. At almost 15,000 feet the aircraft suddenly lost 1,200 feet of altitude and its speed dropped.  It developed phugoid oscillations from which the pilots could not recover.  The aircraft struck the ground at a speed of 340 knots, causing the death of the three crew members and all fifteen passengers.<ref>{{Cite web|url=https://aviation-safety.net/database/record.php?id=19720416-1|title=ASN Aircraft accident Fokker F-27 Friendship 200 I-ATIP Ardinello di Amaseno|first=Harro|last=Ranter|website=aviation-safety.net}}</ref>

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. Of the 328 people on board, 153 died, making it the deadliest accident involving a US military aircraft.

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]]. With 520 deaths, it remains the deadliest single-aircraft disaster in history.

In 1989, [[United Airlines Flight 232]] suffered an uncontained [[turbine engine failure|engine failure]] in the #2 (tail) engine, which caused total [[hydraulic system]] failure. The crew flew the aircraft with [[aircraft engine controls|throttle]] only. Suppressing the phugoid tendency was particularly difficult.<ref>{{Cite web|url=https://aviation-safety.net/database/record.php?id=19890719-1|title=ASN Aircraft accident McDonnell Douglas DC-10-10 N1819U Sioux Gateway Airport, IA (SUX)|first=Harro|last=Ranter|website=aviation-safety.net}}</ref> The pilots reached [[Sioux Gateway Airport]] but crashed during the landing attempt. All four cockpit crewmembers (one an assisting DC-10 captain on the flight as a passenger) and a majority of the passengers survived.

Another aircraft that lost all hydraulics and experienced phugoid was a [[DHL Aviation|DHL]] operated [[Airbus A300B4]] that was hit by a [[surface-to-air missile]] fired by Iraqi militants in the [[2003 Baghdad DHL attempted shootdown incident]]. This was the first time that a crew landed an air transport aircraft safely by only adjusting engine thrust.

The 2003 crash of the [[Helios Prototype#Crash|Helios solar-powered aircraft]] was precipitated by reacting to an inappropriately diagnosed phugoid oscillation that ultimately made the aircraft structure exceed design loads.<ref>'Investigation of the Helios Prototype Aircraft Mishap Volume I Mishap Report', Thomas E. Noll, NASA Langley Research Center, 2004, http://www.nasa.gov/pdf/64317main_helios.pdf</ref>

[[Chesley Sullenberger|Chesley "Sully" Sullenberger]], Captain of [[US Airways Flight 1549]] that [[wiktionary:ditch#Verb|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.<ref>Sully Sullenberger: "Making a Difference" Talks at Google, 2012, (40:23) https://www.youtube.com/watch?v=cKuw49KBywA</ref>

On 25 December 2024, an [[Embraer 190]] operated as [[Azerbaijan Airlines Flight 8243]] was likely damaged by a Russian SAM over Grozny. It was flown over an hour to near [[Aktau International Airport]] with failed hydraulics while undergoing phugoid oscillations.

==See also==
*[[Index of aviation articles]]
*[[Maneuvering Characteristics Augmentation System]]
*[[Projectile motion]]

==References==
{{Reflist}}

==External links==
{{Wiktionary}}

*[http://www.math.sunysb.edu/~scott/Book331/Art_Phugoid.html Analysis of phugoid motion]

[[Category:Aerodynamics]]
[[Category:Flight control systems]]