Flight management system

ferent sizes, capabilities and controls. However certain

characteristics are common to all FMS.

1 Navigation database
All FMS 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 up-
dated every 28 days, in order to ensure that its contents
are current. Each FMS contains only a subset of the AR-
INC 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

• Airways (highways in the sky)

• Radio navigation aids including distance mea-

suring equipment (DME), VHF omnidirectional
range (VOR), non-directional beacons (NDBs) and
instrument landing systems (ILSs).

• Airports

• Runways
Example of a FMS Control Display Unit
• Standard instrument departure (SID)
A flight management system (FMS) is a fundamental • Standard terminal arrival (STAR)
component of a modern airliner’s avionics. An FMS is a
specialized computer system that automates a wide vari- • Holding patterns (only as part of IAPs-although can
ety of in-flight tasks, reducing the workload on the flight be entered by command of ATC or at pilot’s discre-
crew to the point that modern civilian aircraft no longer tion)
carry flight engineers or navigators. A primary function
is in-flight management of the flight plan. Using vari- • Instrument approach procedure (IAP)
ous sensors (such as GPS and INS often backed up by
radio navigation) to determine the aircraft’s position, the Waypoints can also be defined by the pilot(s) along the
FMS can guide the aircraft along the flight plan. From route or by reference to other waypoints with entry of a
the cockpit, the FMS is normally controlled through a place in the form of a waypoint (e.g. a VOR, NDB, ILS,
Control Display Unit (CDU) which incorporates a small airport or waypoint/intersection)
screen and keyboard or touchscreen. The FMS sends the
flight plan for display to the Electronic Flight Instrument
System (EFIS), Navigation Display (ND), or Multifunc- 2 Flight plan
tion Display (MFD).
The modern FMS was introduced on the Boeing 767, The flight plan is generally determined on the ground, be-
though earlier navigation computers did exist.[1] Now, fore departure either by the pilot for smaller aircraft or
systems similar to FMS exist on aircraft as small as the a professional dispatcher for airliners. It is entered into
Cessna 182. In its evolution an FMS has had many dif- the FMS either by typing it in, selecting it from a saved

1
2 5 VNAV

library of common routes (Company Routes) or via an The FMS constantly crosschecks the various sensors and
ACARS datalink with the airline dispatch center. determines a single aircraft position and accuracy. The
During preflight, other information relevant to managing accuracy is described as the Actual Navigation Perfor-
the flight plan is entered. This can include performance mance (ANP) a circle that the aircraft can be anywhere
information such as gross weight, fuel weight and center within measured as the diameter in nautical miles. Mod-
of gravity. It will include altitudes including the initial ern airspace has a set required navigation performance
cruise altitude. For aircraft that do not have a GPS, the (RNP). The aircraft must have its ANP less than its RNP
initial position is also required. in order to operate in certain high-level airspace.

The pilot uses the FMS to modify the flight plan in flight
for a variety of reasons. Significant engineering de-
sign minimizes the keystrokes in order to minimize pi-
4 Guidance
lot workload in flight and eliminate any confusing infor-
mation (Hazardously Misleading Information). The FMS Given the flight plan and the aircraft’s position, the FMS
also sends the flight plan information for display on the calculates the course to follow. The pilot can follow this
Navigation Display (ND) of the flight deck instruments course manually (much like following a VOR radial), or
Electronic Flight Instrument System (EFIS). The flight the autopilot can be set to follow the course.
plan generally appears as a magenta line, with other air- The FMS mode is normally called LNAV or Lateral Nav-
ports, radio aids and waypoints displayed. igation for the lateral flight plan and VNAV or vertical
Special flight plans, often for tactical requirements in- navigation for the vertical flight plan. VNAV provides
cluding search patterns, rendezvous, in-flight refueling speed and pitch or altitude targets and LNAV provides
tanker orbits, calculated air release points (CARP) for ac- roll steering command to the autopilot.
curate parachute jumps are just a few of the special flight
plans some FMS can calculate.
5 VNAV
3 Position determination Sophisticated aircraft, generally airliners such as the
Airbus A320 or Boeing 737 and larger, have full per-
Once in flight, a principal task of the FMS is to deter- formance Vertical Navigation (VNAV). The purpose of
mine the aircraft’s position and the accuracy of that po- VNAV is to predict and optimize the vertical path. Guid-
sition. Simple FMS use a single sensor, generally GPS ance includes control of the pitch axis and control of the
in order to determine position. But modern FMS use as throttle.
many sensors as they can, such as VORs, in order to de-
termine and validate their exact position. Some FMS use In order to have the information necessary to accom-
a Kalman filter to integrate the positions from the various plish this, the FMS must have a detailed flight and engine
sensors into a single position. Common sensors include: model. With this information, the function can build a
predicted vertical path along the lateral flight plan. This
• Airline quality GPS receivers act as the primary sen- detailed flight model is generally only available from the
sor as they have the highest accuracy and integrity. aircraft manufacturer.
During pre-flight, the FMS builds the vertical profile. It
• Radio aids designed for aircraft navigation act as the
uses the initial aircraft empty weight, fuel weight, centre
second highest quality sensors. These include;
of gravity and initial cruise altitude, plus the lateral flight
• Scanning DME (distance measuring equip- plan. A vertical path starts with a climb to cruise alti-
ment) that check the distances from five dif- tude. Some SID waypoints have vertical constraints such
ferent DME stations simultaneously in order as “At or ABOVE 8,000”. The climb may use a reduced
to determine one position every 10 seconds or thrust(derated) or “FLEX” climb to save stress on the en-
so.[2] gines. Each must be considered in the predictions of the
• VORs (VHF omnidirectional radio range) that vertical profile.
supply a bearing. With two VOR stations the Implementation of an accurate VNAV is difficult and ex-
aircraft position can be determined, but the ac- pensive, but it pays off in fuel savings primarily in cruise
curacy is limited. and descent. In cruise, where most of the fuel is burned,
• Inertial reference systems (IRS) use ring laser gyros there are multiple methods for fuel savings.
and accelerometers in order to calculate the aircraft As an aircraft burns fuel it gets lighter and can cruise
position. They are highly accurate and independent higher where it is generally more efficient. Step climbs or
of outside sources. Airliners use the weighted av- cruise climbs facilitate this. VNAV can determine where
erage of three independent IRS to determine the the step or cruise climbs (where the aircraft drifts up)
“triple mixed IRS” position. should occur to minimize fuel consumption.
 3

Performance optimization allows the FMS to determine 7 References

the best or most economical speed to fly in level flight.
This is often called the ECON speed. This is based on the [1] Sam Miller, et als (2009). “Contribution of Flight Sys-
cost index, which is entered to give a weighting between tems to Performance-Based Navigation”. AERO Maga-
speed and fuel efficiency. Generally a cost index of 999 zine (Boeing) (34; Qtr. 2). Retrieved 31 December 2012.
gives ECON speeds as fast as possible without consider-
[2] Spitzer, Carl (2007). “20.2.1”. Avionics, Element, Soft-
ation of fuel and a cost index of Zero gives maximum ef-
ware and Functions. Boca Raton, FL: CRC Press. pp.
ficiency. ECON mode is the VNAV speed used by most 20–6. ISBN 0-8493-8438-9.
airliners in cruise.
RTA or required time of arrival allows the VNAV system
to target arrival at a particular waypoint at a defined time. 8 Further reading
This is often useful for airport arrival slot scheduling. In
this case, VNAV regulates the cruise speed or cost index • ARINC 702A, Advanced Flight Management Com-
to ensure the RTA is met. puter System
The first thing the VNAV calculates for the descent is the
top of descent point (TOD). This is the point where an • Avionics, Element, Software and Functions Ch 20,
efficient and comfortable descent begins. Normally this Cary R. Spitzer, ISBN 0-8493-8438-9
will involve an idle descent, but for some aircraft an idle • FMC User’s Guide B737, Ch 1, Bill Bulfer, Leading
descent is too steep and uncomfortable. The FMS cal- Edge Libraries
culates the TOD by “flying” the descent backwards from
touchdown through the approach and up to cruise. It does
• Casner, S.M. The Pilot’s Guide to the Modern Air-
this using the flight plan, the aircraft flight model and de-
line Cockpit. Newcastle WA, Aviation Supplies and
scent winds. For airline FMS, this is a very sophisticated
Academics, 2007. ISBN 1-56027-683-5.
and accurate prediction, for simple FMS (on smaller air-
craft) it can be determined by a “rule of thumb” such as • Chappell, A.R. et al. “The VNAV Tutor: Address-
a 3 degree descent path. ing a Mode Awareness Difficulty for Pilots of Glass
From the TOD, the VNAV determines a four- Cockpit Aircraft.” IEEE Transactions on Systems,
dimensional predicted path. As the VNAV commands Man and Cybernetics Part A, Systems and Humans,
the throttles to idle, the aircraft begins its descent along vol. 27, no.3, May 1997, pp. 372–385.
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 9 External links
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 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 opti-
mum descent path to the airport, at this time. Thus the
use of idle descents is minimized by Air Traffic Control.

6 See also

• Acronyms and abbreviations in avionics

• Strategic Lateral Offset Procedure

4 10 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES

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