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A flight management system (FMS) is a fundamental component of a modern airliner's
avionics
Avionics (a blend word, blend of ''aviation'' and ''electronics'') are the Electronics, electronic systems used on aircraft. Avionic systems include communications, Air navigation, navigation, the display and management of multiple systems, ...
. An FMS is a specialized computer system that automates a wide variety of in-flight tasks, reducing the workload on the flight crew to the point that modern civilian aircraft no longer carry
flight engineers or
navigators
A navigator is the person on board a ship or aircraft responsible for its navigation.Grierson, MikeAviation History—Demise of the Flight Navigator FrancoFlyers.org website, October 14, 2008. Retrieved August 31, 2014. The navigator's primar ...
. A primary function is in-flight management of the flight plan. Using various sensors (such as
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
and
INS INS or Ins or ''variant'', may refer to:
Places
* Ins, Switzerland, a municipality
* Creech Air Force Base (IATA airport code INS)
* Indonesia, ITF and UNDP code INS
Biology
*'' Ins'', a New World genus of bee flies
* INS, the gene for the insul ...
often backed up by
radio navigation
Radio navigation or radionavigation is the application of radio frequencies to determine a position of an object on the Earth, either the vessel or an obstruction. Like radiolocation, it is a type of radiodetermination.
The basic principles a ...
) to determine the aircraft's position, the FMS can guide the aircraft along the flight plan. From the cockpit, the FMS is normally controlled through a
Control Display Unit (CDU) which incorporates a small screen and keyboard or touchscreen. The FMS sends the flight plan for display to the
Electronic Flight Instrument System (EFIS), Navigation Display (ND), or Multifunction Display (MFD). The FMS can be summarised as being a dual system consisting of the Flight Management Computer (FMC), CDU and a cross talk bus.
The modern FMS was introduced on the
Boeing 767
The Boeing 767 is an American wide-body aircraft developed and manufactured by Boeing Commercial Airplanes.
The aircraft was launched as the 7X7 program on July 14, 1978, the prototype first flew on September 26, 1981, and it was certified on ...
, though earlier navigation computers did exist. Now, systems similar to FMS exist on aircraft as small as the
Cessna 182
The Cessna 182 Skylane is an American four-seat, single-engined light airplane built by Cessna of Wichita, Kansas. It has the option of adding two child seats in the baggage area.
Introduced in 1956, the 182 has been produced in a ...
. In its evolution an FMS has had many different sizes, capabilities and controls. However certain characteristics are common to all FMSs.
Navigation database
All FMSs contain a navigation database. The navigation database contains the elements from which the flight plan is constructed. These are defined via the
ARINC 424 standard. The navigation database (NDB) is normally updated every 28 days, in order to ensure that its contents are current. Each FMS contains only a subset of the ARINC /
AIRAC data, relevant to the capabilities of the FMS.
The NDB contains all of the information required for building a flight plan, consisting of:
* Waypoints/Intersection
*
Airway
The respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa.
Air is breathed in through the nose to th ...
s
* Radio navigation aids including
distance measuring equipment
In aviation, distance measuring equipment (DME) is a radio navigation technology that measures the slant range (distance) between an aircraft and a ground station by timing the propagation delay of radio signals in the frequency band between 9 ...
(DME),
VHF omnidirectional range
Very high frequency omnirange station (VOR) is a type of short-range radio navigation system for aircraft, enabling aircraft with a receiving unit to determine its position and stay on course by receiving radio signals transmitted by a network ...
(VOR),
non-directional beacon
A non-directional beacon (NDB) or non-directional radio beacon is a radio beacon which does not include directional information. Radio beacons are radio transmitters at a known location, used as an aviation or marine navigational aid. NDB are ...
s (NDBs) and
instrument landing system
In aviation, the instrument landing system (ILS) is a precision radio navigation system that provides short-range guidance to aircraft to allow them to approach a runway at night or in bad weather. In its original form, it allows an aircraft to ...
s (ILSs).
*
Airport
An airport is an aerodrome with extended facilities, mostly for commercial air transport. Airports usually consists of a landing area, which comprises an aerially accessible open space including at least one operationally active surface ...
s
*
Runway
According to the International Civil Aviation Organization (ICAO), a runway is a "defined rectangular area on a land aerodrome prepared for the landing and takeoff of aircraft". Runways may be a man-made surface (often asphalt concrete, as ...
s
*
Standard instrument departure
Standard instrument departure (SID) routes, also known as departure procedures (DP), are published flight procedures followed by aircraft on an IFR flight plan immediately after takeoff from an airport.
Introduction
A SID is an air traffic con ...
(SID)
*
Standard terminal arrival
In aviation, a standard terminal arrival route or standard terminal arrival (STAR) is a published flight procedure followed by aircraft on an instrument flight rules (IFR) flight plan just before reaching a destination airport.
A STAR is an air ...
(STAR)
*
Holding patterns (only as part of IAPs-although can be entered by command of ATC or at pilot's discretion)
*
Instrument approach procedure
In aviation, an instrument approach or instrument approach procedure (IAP) is a series of predetermined maneuvers for the orderly transfer of an aircraft operating under instrument flight rules from the beginning of the initial approach to a landi ...
(IAP)
Waypoints can also be defined by the pilot(s) along the route or by reference to other waypoints with entry of a place in the form of a waypoint (e.g. a VOR, NDB, ILS, airport or waypoint/intersection).
Flight plan
The
flight plan
Flight plans are documents filed by a pilot or flight dispatcher with the local Air Navigation Service Provider (e.g. the FAA in the United States) prior to departure which indicate the plane's planned route or flight path. Flight plan format is ...
is generally determined on the ground, before departure either by the pilot for smaller aircraft or a
professional dispatcher for airliners. It is entered into the FMS either by typing it in, selecting it from a saved library of common routes (Company Routes) or via an
ACARS
In aviation, ACARS (; an acronym for Aircraft Communications Addressing and Reporting System) is a digital datalink system for transmission of short messages between aircraft and ground stations via airband radio or satellite. The protocol was des ...
datalink with the airline dispatch center.
During preflight, other information relevant to managing the flight plan is entered. This can include performance information such as gross weight, fuel weight and center of gravity. It will include altitudes including the initial cruise altitude. For aircraft that do not have a
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
, the initial position is also required.
The pilot uses the FMS to modify the flight plan in flight for a variety of reasons. Significant engineering design minimizes the keystrokes in order to minimize pilot workload in flight and eliminate any confusing information (Hazardously Misleading Information).
The FMS also sends the flight plan information for display on the Navigation Display (ND) of the flight deck instruments Electronic Flight Instrument System (
EFIS). The flight plan generally appears as a magenta line, with other airports, radio aids and waypoints displayed.
Special flight plans, often for tactical requirements including search patterns, rendezvous, in-flight refueling tanker orbits, calculated air release points (CARP) for accurate parachute jumps are just a few of the special flight plans some FMSs can calculate.
Position determination
Once in flight, a principal task of the FMS is to determine the aircraft's position and the accuracy of that position. Simple FMS use a single sensor, generally
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
in order to determine position. But modern FMS use as many sensors as they can, such as VORs, in order to determine and validate their exact position. Some FMS use a
Kalman filter
For statistics and control theory, Kalman filtering, also known as linear quadratic estimation (LQE), is an algorithm that uses a series of measurements observed over time, including statistical noise and other inaccuracies, and produces estimat ...
to integrate the positions from the various sensors into a single position. Common sensors include:
* Airline-quality GPS receivers act as the primary sensor as they have the highest accuracy and integrity.
* Radio aids designed for aircraft navigation act as the second highest quality sensors. These include;
** Scanning DME (
distance measuring equipment
In aviation, distance measuring equipment (DME) is a radio navigation technology that measures the slant range (distance) between an aircraft and a ground station by timing the propagation delay of radio signals in the frequency band between 9 ...
) that check the distances from five different DME stations simultaneously in order to determine one position every 10 seconds.
** VORs (
VHF omnidirectional radio range
Very high frequency omnirange station (VOR) is a type of short-range radio navigation system for aircraft, enabling aircraft with a receiving unit to determine its position and stay on course by receiving radio signals transmitted by a network ...
) that supply a bearing. With two VOR stations the aircraft position can be determined, but the accuracy is limited.
*
Inertial reference system
An inertial navigation system (INS) is a navigation device that uses motion sensors (accelerometers), rotation sensors (gyroscopes) and a computer to continuously calculate by dead reckoning the position, the orientation, and the velocity (direc ...
s (IRS) use
ring laser gyro
A ring laser gyroscope (RLG) consists of a ring laser having two independent counter-propagating resonant modes over the same path; the difference in phase is used to detect rotation. It operates on the principle of the Sagnac effect which shif ...
s and accelerometers in order to calculate the aircraft position. They are highly accurate and independent of outside sources. Airliners use the weighted average of three independent IRS to determine the “triple mixed IRS” position.
The FMS constantly crosschecks the various sensors and determines a single aircraft position and accuracy. The accuracy is described as the Actual Navigation Performance (ANP) a circle that the aircraft can be anywhere within measured as the diameter in nautical miles.
Modern airspace has a set
required navigation performance
Required navigation performance (RNP) is a type of performance-based navigation (PBN) that allows an aircraft to fly a specific path between two 3D-defined points in space.
Navigation precision
Area navigation (RNAV) and RNP systems are funda ...
(RNP). The aircraft must have its ANP less than its RNP in order to operate in certain high-level airspace.
Guidance
Given the flight plan and the aircraft's position, the FMS calculates the course to follow. The pilot can follow this course manually (much like following a VOR radial), or the
autopilot
An autopilot is a system used to control the path of an aircraft, marine craft or spacecraft without requiring constant manual control by a human operator. Autopilots do not replace human operators. Instead, the autopilot assists the operator' ...
can be set to follow the course.
The FMS mode is normally called LNAV or Lateral Navigation for the lateral flight plan and VNAV or vertical navigation for the vertical flight plan. VNAV provides speed and pitch or altitude targets and LNAV provides roll steering command to the autopilot.
VNAV
In aviation, vertical navigation (VNAV, usually pronounced ''vee-nav'') is glidepath information provided during an instrument approach, independently of ground-based navigation aids. An onboard navigation system displays a constant rate descent ...
Sophisticated aircraft, generally airliners such as the
Airbus A320
The Airbus A320 family is a series of Narrow-body aircraft, narrow-body airliners developed and produced by Airbus.
The A320 was launched in March 1984, Maiden flight, first flew on 22 February 1987, and was introduced in April 1988 by Air F ...
or
Boeing 737
The Boeing 737 is a narrow-body aircraft produced by Boeing at its Renton Factory in Washington.
Developed to supplement the Boeing 727 on short and thin routes, the twinjet retains the 707 fuselage width and six abreast seating with two un ...
and other turbofan powered aircraft, have full performance Vertical Navigation (VNAV). The purpose of VNAV is to predict and optimize the vertical path. Guidance includes control of the pitch axis and control of the throttle.
In order to have the information necessary to accomplish this, the FMS must have a detailed flight and engine model. With this information, the function can build a predicted vertical path along the lateral flight plan. This detailed flight model is generally only available from the aircraft manufacturer.
During pre-flight, the FMS builds the vertical profile. It uses the initial aircraft empty weight, fuel weight, centre of gravity and initial cruise altitude, plus the lateral flight plan.
A vertical path starts with a climb to cruise altitude. Some SID waypoints have vertical constraints such as "At or ABOVE 8,000". The climb may use a reduced thrust or "FLEX" climb to save stress on the engines. Each must be considered in the predictions of the vertical profile.
Implementation of an accurate VNAV is difficult and expensive, but it pays off in fuel savings primarily in cruise and descent. In cruise, where most of the fuel is burned, there are multiple methods for fuel savings.
As an aircraft burns fuel it gets lighter and can cruise higher where there is less drag.
Step climb
A step climb in aviation is a series of altitude gains that improve fuel economy by moving into thinner air as an aircraft becomes lighter and becomes capable of flying in the thinner air at a higher altitude.
Description
Since the early days of ...
s or cruise climbs facilitate this. VNAV can determine where the step or cruise climbs (in which the aircraft climbs continuously) should occur to minimize fuel consumption.
Performance optimization allows the FMS to determine the best or most economical speed to fly in level flight. This is often called the
ECON speed. This is based on the cost index, which is entered to give a weighting between speed and
fuel efficiency
Fuel efficiency is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical potential energy contained in a carrier (fuel) into kinetic energy or work. Overall fuel efficiency may vary per device, wh ...
. The cost index is calculated by dividing the per-hour cost of operating the plane by the cost of fuel.
Generally a cost index of 999 gives ECON speeds as fast as possible without consideration of fuel and a cost index of zero gives maximum fuel economy while disregarding other hourly costs such as maintenance and crew expenses. ECON mode is the VNAV speed used by most airliners in cruise.
RTA or required time of arrival allows the VNAV system to target arrival at a particular waypoint at a defined time. This is often useful for airport arrival slot scheduling. In this case, VNAV regulates the cruise speed or cost index to ensure the RTA is met.
The first thing the VNAV calculates for the descent is the top of descent point (TOD). This is the point where an efficient and comfortable descent begins. Normally this will involve an idle descent, but for some aircraft an idle descent is too steep and uncomfortable. The FMS calculates the TOD by “flying” the descent backwards from touchdown through the approach and up to cruise. It does this using the flight plan, the aircraft flight model and descent winds. For airline FMS, this is a very sophisticated and accurate prediction, for simple FMS (on smaller aircraft) it can be determined by a “rule of thumb” such as a 3 degree descent path.
From the TOD, the VNAV determines a four-dimensional predicted path. As the VNAV commands the throttles to idle, the aircraft begins its descent along the VNAV path. If either the predicted path is incorrect or the downpath winds different from the predictions, then the aircraft will not perfectly follow the path. The aircraft varies the pitch in order to maintain the path. Since the throttles are at idle this will modulate the speed. Normally the FMS allows the speed to vary within a small band. After this, either the throttles advance (if the aircraft is below path) or the FMS requests speed brakes with a message such as “ADD DRAG” (if the aircraft is above path).
An ideal idle descent, also known as a “green descent” uses the minimum fuel, minimizes pollution (both at high altitude and local to the airport) and minimizes local noise. While most modern FMS of large airliners are capable of idle descents, most air traffic control systems cannot handle multiple aircraft each using its own optimum descent path to the airport, at this time. Thus the use of idle descents is minimized by Air Traffic Control.
See also
*
Acronyms and abbreviations in avionics
Below are abbreviations used in aviation, avionics, aerospace and aeronautics.
A
B
C
D
E
F
G
H
I
J
K
L
M
N
N numbers (turbines)
O
P
Q
R
S
T
U
V
V speeds
W
X
Y
Z
See also
* List of avia ...
*
Strategic Lateral Offset Procedure
References
Further reading
* ARINC 702A, Advanced Flight Management Computer System
* Avionics, Element, Software and Functions Ch 20, Cary R. Spitzer,
* FMC User's Guide B737, Ch 1, Bill Bulfer, Leading Edge Libraries
* Casner, S.M. ''The Pilot's Guide to the Modern Airline Cockpit''. Newcastle WA, Aviation Supplies and Academics, 2007. .
* Chappell, A.R. et al. "The VNAV Tutor: Addressing a Mode Awareness Difficulty for Pilots of Glass Cockpit Aircraft." ''IEEE Transactions on Systems, Man and Cybernetics Part A, Systems and Humans'', vol. 27, no.3, May 1997, pp. 372–385.
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Avionics
Aircraft instruments
Flight management