View source for Cruise (aeronautics)

[[File:Qantas Boeing 747-400 VH-OJU over Starbeyevo Kustov.jpg|thumb|right|A [[Qantas]] four-engined [[Boeing 747-400]] at cruise altitude]]

'''Cruise''' is the phase of [[aircraft]] flight that starts when the aircraft levels off after a [[Climb (aeronautics)|climb]], until it begins to [[Descent (aeronautics)|descend]] for landing.<ref>{{Cite web|url=http://www.intlaviationstandards.org/apex/f?p=240:1:146653179369::NO::P1_X:Glossary|title=Glossary|website=CAST/ICAO Common Taxonomy Team|access-date=2016-06-19}}</ref> Cruising usually comprises the majority of a flight, and may include small changes in [[Aircraft heading|heading]] (direction of flight), [[airspeed]], and [[altitude]].

== Airliner cruise == 
[[File:AircraftMissionProfile.png|thumb|The cruise makes the longest part of a Mission Profile.]]

Commercial or passenger aircraft are usually designed for optimum performance around their cruise speed ([[V speeds#VC|V<sub>C</sub>]]) and cruise altitude. Factors affecting optimum cruise speed and altitude include payload, [[center of gravity]], air temperature, and humidity. Cruise altitude is usually where the higher [[ground speed]] is balanced against the decrease in engine thrust and efficiency at higher altitudes. Common [[Narrowbody|narrowbodies]] like the [[Airbus A320]] and [[Boeing 737NG]] cruise at {{cvt|0.78|Mach|altitude_ft=39000|knots km/h|sigfig=2}},<ref>{{cite web |title= A320 Family Technology |publisher= Airbu |url= http://www.airbus.com/aircraftfamilies/passengeraircraft/a320family/technology-and-innovation/|archive-url= https://web.archive.org/web/20160403132507/http://www.airbus.com/aircraftfamilies/passengeraircraft/a320family/technology-and-innovation/ |archive-date= 2016-04-03 }}</ref><ref>{{cite web |url= https://www.boeing.com/resources/boeingdotcom/media/paris2015/pdf/Backgrounders/bkg_NG_737_family.pdf |publisher= Boeing |title= Next-Generation 737 Family Backgrounder |date= February 2015 }}</ref> while modern [[Widebody|widebodies]] like the [[Airbus A350]] and [[Boeing 787]] cruise at {{cvt|0.85|Mach|altitude_ft=39000|knots km/h|sigfig=2}}.<ref>{{cite news |url= http://aviationweek.com/commercial-aviation/flying-a350-airbus-s-most-technologically-advanced-airliner |title= Flying The A350: Airbus's Most Technologically Advanced Airliner |date= May 22, 2015 |author= Fred George |work= Aviation Week & Space Technology |archive-url= https://web.archive.org/web/20150525145400/http://aviationweek.com/commercial-aviation/flying-a350-airbus-s-most-technologically-advanced-airliner |archive-date= 2015-05-25 }}</ref><ref>{{cite web |url= https://www.boeing.com/resources/boeingdotcom/commercial/airports/acaps/787.pdf |title= 787 Airplane Characteristics for Airport Planning |publisher= Boeing |date= February 2023}}</ref> The typical cruising altitude for commercial airliners is {{convert|31,000|to|38,000|ft|m mi|lk=on}}.<ref name="Sforza">{{cite book |last1=Sforza |first1=P. M. |title=Commercial airplane design principles |date=2014 |publisher=Butterworth-Heinemann |location=Oxford |isbn=978-0-12-419953-8 |chapter=Chapter 3 - Fuselage Design |quote=At the normal stratospheric cruising altitudes of 30,000–38,000 ft}}</ref><ref name="timemag">{{cite magazine|url=https://time.com/5309905/how-high-do-planes-fly/|title=Here's How High Planes Actually Fly, According to Experts|magazine=Time|last=Hacobian|first=Celine|date=27 January 2018|access-date=23 September 2022}}</ref>{{Better source needed|date=September 2022}}
The speed which covers the greatest distance for a given amount of fuel is known as the maximum range speed. This is the speed at which drag is minimised.

For jet aircraft, "long-range cruise" speed (LRC) is defined as the speed which gives 99% of the maximum range, for a given weight. This results in a {{nowrap|3–5%}} increase in speed.<ref name="Boeing pg2"/> It is also a more [[Drag curve|stable]] speed than maximum range speed, so gives less autothrottle movement.<ref name="Brady">{{cite book |last1=Brady |first1=Chris |title=The Boeing 737 Technical Guide |date=14 November 2021 |publisher=Blurb, Incorporated |isbn=978-1-006-28058-0 |url=https://books.google.com/books?id=u7G-zgEACAAJ |access-date=8 October 2022 |language=en}}</ref> However, LRC speed does not take account of winds, or time-related costs other than fuel, so it has little practical value.<ref name="Brady"/> Instead, the speed for most economical operation (ECON) is adjusted for wind and the [[cost index]] (CI), which is the ratio of time cost to fuel cost.<ref name="Boeing pg2">{{cite web |title=AERO – Fuel Conservation Strategies: Cruise Flight 2|url=https://www.boeing.com/commercial/aeromagazine/articles/qtr_4_07/article_05_2.html |website=boeing.com |publisher=Boeing |access-date=28 January 2022}}</ref> A higher cost index results in a higher ECON speed. Cost index can be given in "Boeing" or "English" units as {{math|($/hr)/(cents/lb)}}, equivalent to {{math|100 lb/hr}}.<ref name="Airbus">{{cite web |title=Getting to grips with cost index |url=https://ansperformance.eu/library/airbus-cost-index.pdf |publisher=Airbus |access-date=31 January 2022}}</ref><ref name="Openairlines">{{cite web |title=Top 10 facts or myths about Cost Index |url=https://blog.openairlines.com/top-10-facts-or-myths-about-cost-index |website=blog.openairlines.com |date=2 May 2019 |language=en-us}}</ref> A typical cost index in these units might be anywhere from 5 to 150.<ref name="Boeing pg3">{{cite web |title=AERO – Fuel Conservation Strategies: Cruise Flight 3 |url=https://www.boeing.com/commercial/aeromagazine/articles/qtr_4_07/article_05_3.html |website=www.boeing.com |publisher=Boeing |access-date=28 January 2022}}</ref> Alternatively cost index can be given in metric or "Airbus" units of {{Math|kg/min}}.<ref name="Airbus"/><ref name="Openairlines"/>

In the presence of a tailwind, ECON airspeed can be reduced to take advantage of the tailwind, whereas in a headwind, ECON speed will be increased to avoid the penalty of the headwind.<ref name="Boeing pg3"/> In the presence of a tailwind, LRC speed may give a higher fuel burn than ECON.<ref name="Brady"/> As the aircraft consumes fuel, its weight decreases and the ECON speed decreases. This is because a heavier aircraft should fly faster to generate the required lift at the most efficient [[lift coefficient]]. ECON speed will also be higher at higher altitudes because the density of the air is lower.

==Propeller aircraft==

For propeller aircraft, drag is minimised when the [[lift-to-drag ratio]] is maximised. However, the speed for this is typically regarded as too slow, so propeller aircraft typically cruise at a significantly faster speed.<ref name="BoldMethod">{{cite web |title=Why You Rarely Fly At Best Range Speed In A Prop, But You're Close To It In A Jet |url=https://www.boldmethod.com/learn-to-fly/aerodynamics/best-range-speed-prop-vs-jet/ |website=boldmethod.com |access-date=31 January 2022}}</ref> Combustion engines have an optimum efficiency level for fuel consumption and power output.<ref>[http://www.collinsdictionary.com/dictionary/english/cruising-speed Cruising speed definition]</ref>{{Better source needed|date=April 2022}} Generally, gasoline piston engines are most efficient between idle speed and 30% short of full throttle. Diesels are most efficient at around 90% of full throttle.<ref name="Motoryacht">{{cite web |last1=Thiel |first1=Richard |title=How to Find the Best Cruising Speed for Your Boat |url=https://www.powerandmotoryacht.com/boats/how-find-best-cruising-speed-your-boat |website=Power & Motoryacht |date=2 February 2018 |access-date=29 January 2022 |language=en-us}}</ref>{{Better source needed|date=January 2022}}

==Altitude==

As the aircraft consumes fuel, its weight decreases and the optimum altitude for fuel economy increases. For traffic control reasons it is usually necessary for an aircraft to stay at a cleared [[flight level]]. On long-haul flights, the pilot may ask [[air traffic control]] to climb from one flight level to a higher one, in a manoeuvre known as a [[step climb]].

== See also ==
* [[Fuel economy in aircraft]]
* [[Range (aeronautics)]]
* [[Lift-to-drag ratio]]
* [[Loiter (aeronautics)]]
* [[Supercruise]]
* [[Takeoff]]
* [[Visual flight rules]]

[[Category:Cruise (aeronautics)]]
[[Category:Aircraft Cruise Performance]]
[[Category:Cruise Speed]]
[[Category:Cruising Altitude]]
[[Category:Cruise Flight Planning]]
[[Category:Long-Range Cruise]]
[[Category:Cruise Fuel Efficiency]]
[[Category:Cruise Phase of Flight]]
[[Category:Optimal Cruise Altitude]]
[[Category:Flight Management Systems]]
[[Category:Autopilot Cruise Control]]
[[Category:Cruise Flight Dynamics]]
[[Category:Jet Engine Efficiency at Cruise]]
[[Category:Cruise Speed Optimization]]
[[Category:Flight Envelope]]
[[Category:Aircraft Operating Conditions]]
[[Category:Aircraft Fuel Consumption]]
[[Category:Cruise Thrust Settings]]
[[Category:Aircraft Drag at Cruise]]
[[Category:Specific Range]]
[[Category:Aircraft Endurance]]
[[Category:True Airspeed]]
[[Category:Mach Number]]
[[Category:Aircraft Performance Monitoring]]
[[Category:Air Traffic Separation at Cruise]]
[[Category:Cruise Altitude Selection]]
[[Category:Step Climb Procedures]]
[[Category:High Altitude Operations]]
[[Category:TAS vs IAS]]
[[Category:Range vs Payload Tradeoff]]
[[Category:Cruise Weather Considerations]]
[[Category:Aviation Safety X]]
[[Category:ASXWiki]]