Level flight after an aircraft climbs to a set altitude and before it begins to descend
Cruise
is the phase of
aircraft
flight that starts when the aircraft levels off after a
climb
, until it begins to
descend
for landing.
[1]
Cruising usually comprises the majority of a flight, and may include small changes in
heading
(direction of flight),
airspeed
, and
altitude
.
Airliner cruise
[
edit
]
Commercial or passenger aircraft are usually designed for optimum performance around their cruise speed (
V
C
) 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
narrowbodies
like the
Airbus A320
and
Boeing 737NG
cruise at Mach 0.78 (450 kn; 830 km/h),
[2]
[3]
while modern
widebodies
like the
Airbus A350
and
Boeing 787
cruise at Mach 0.85 (490 kn; 900 km/h).
[4]
[5]
The typical cruising altitude for commercial airliners is 31,000 to 38,000
feet
(9,400 to 11,600
m
; 5.9 to 7.2
mi
).
[6]
[7]
[
better source needed
]
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
3?5%
increase in speed.
[8]
It is also a more
stable
speed than maximum range speed, so gives less autothrottle movement.
[9]
However, LRC speed does not take account of winds, or time-related costs other than fuel, so it has little practical value.
[9]
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.
[8]
A higher cost index results in a higher ECON speed. Cost index can be given in "Boeing" or "English" units as
($/hr)/(cents/lb)
, equivalent to
100 lb/hr
.
[10]
[11]
A typical cost index in these units might be anywhere from 5 to 150.
[12]
Alternatively cost index can be given in metric or "Airbus" units of
kg/min
.
[10]
[11]
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.
[12]
In the presence of a tailwind, LRC speed may give a higher fuel burn than ECON.
[9]
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
[
edit
]
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.
[13]
Combustion engines have an optimum efficiency level for fuel consumption and power output.
[14]
[
better source needed
]
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.
[15]
[
better source needed
]
Altitude
[
edit
]
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
[
edit
]
References
[
edit
]
- ^
"Glossary"
.
CAST/ICAO Common Taxonomy Team
. Retrieved
2016-06-19
.
- ^
"A320 Family Technology"
. Airbu. Archived from
the original
on 2016-04-03.
- ^
"Next-Generation 737 Family Backgrounder"
(PDF)
. Boeing. February 2015.
- ^
Fred George (May 22, 2015).
"Flying The A350: Airbus's Most Technologically Advanced Airliner"
.
Aviation Week & Space Technology
. Archived from
the original
on 2015-05-25.
- ^
"787 Airplane Characteristics for Airport Planning"
(PDF)
. Boeing. February 2023.
- ^
Sforza, P. M. (2014). "Chapter 3 - Fuselage Design".
Commercial airplane design principles
. Oxford: Butterworth-Heinemann.
ISBN
978-0-12-419953-8
.
At the normal stratospheric cruising altitudes of 30,000?38,000 ft
- ^
Hacobian, Celine (27 January 2018).
"Here's How High Planes Actually Fly, According to Experts"
.
Time
. Retrieved
23 September
2022
.
- ^
a
b
"AERO ? Fuel Conservation Strategies: Cruise Flight 2"
.
boeing.com
. Boeing
. Retrieved
28 January
2022
.
- ^
a
b
c
Brady, Chris (14 November 2021).
The Boeing 737 Technical Guide
. Blurb, Incorporated.
ISBN
978-1-006-28058-0
. Retrieved
8 October
2022
.
- ^
a
b
"Getting to grips with cost index"
(PDF)
. Airbus
. Retrieved
31 January
2022
.
- ^
a
b
"Top 10 facts or myths about Cost Index"
.
blog.openairlines.com
. 2 May 2019.
- ^
a
b
"AERO ? Fuel Conservation Strategies: Cruise Flight 3"
.
www.boeing.com
. Boeing
. Retrieved
28 January
2022
.
- ^
"Why You Rarely Fly At Best Range Speed In A Prop, But You're Close To It In A Jet"
.
boldmethod.com
. Retrieved
31 January
2022
.
- ^
Cruising speed definition
- ^
Thiel, Richard (2 February 2018).
"How to Find the Best Cruising Speed for Your Boat"
.
Power & Motoryacht
. Retrieved
29 January
2022
.
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