PERFORMANCE Advisory and Flight
Management Systems
Systems designed in various forms to carry out performance advisory or
comprehensive flight management functions are now an essential feature of a
number of types of commercial transport aircraft, their development having
stemmed from the need to ensure the most efficient use of fuel, the need to
reduce workload and the need to reduce operating costs overall. Fuel usage and
other economic factors associated with aircraft operations have always been
ones attracting the attention of the manufacturing and operating sectors of the
industry, but in about the early 1970s when certain of the oil-producing states
were creating sharp increases in the costs of crude oil and for political
reasons were imposing oil embargos against some Western nations, the industry
was forced to pay even greater attention to the above factors. As a result,
many research and development programmes were instituted and were centred on
the fuel efficiency of engines, improvements in LID ratios of airframes (e.g. by such means as use of
supercritical wing sections) and on reductions in structural weight by use of
composite materials.
Computer technology, although limited at the time in its application to
aircraft systems, was nevertheless more advanced in overall concepts, and so by
the production of software, which took into account the many operational
variables, computers offered an additional and quicker route to the attainment
of economic flight operations by performing automatic adjustments to relevant
control systems. In conjunction with developments in the areas noted earlier, a
system of computerized flight management has currently become the `elite' of
avionic systems and it is probably not unfair to say that it is the most
developed, besides being the one most readily retro-fitted to aircraft.
FMS development has, of course, resulted in a number of variations on
the original theme of controlling engine power and flight operations consistent
with the most efficient use of fuel at all times and, consequently, a variety
of system designations has been applied by the manufacturers of” systems. These
designations, some of which are interchangeable, while others indicate
distinctly different capabilities, are given in Table 2.1 although Table 2.1 has not been compiled to a rigid scale of
evolution, it does provide some indication of development of system functions
which may be advisory only, or a combination of advisory and control
Table 2.1 System designations
System Function
Performance advisory system (PAS)
- Advises of best altitude and speed to
fly at to save fuel. Flight crew has to transfer values into automatic flight
control system and throttle settings
*Performance data computer system
- Similar to PAS but typically linked
(PDCS) to provide automatic pitch and throttle control
*Performance management system (PMS)
- Similar to PDCS but with additional
lateral navigation capability
*Automatic performance management Similar to PAS
System (APMS)
*Flight management computer system
(FMCS)
- Full performance
and navigation capabilities, flight planning and operation in a
three-dimensional capacity
*Flight management system
- Similar to FMCS (FMS)
In performing an advisory function a system merely advises
the flight crew of the optimum settings of various control parameters, such as
engine pressure ratio (EPR) and climb rate under varying flight conditions, in
order to achieve the most economical use of the available fuel. Such systems
require adjustments of controls on the part of the flight crew if they are to
be utilized to maximum advantage. Examples of advisory systems are the PAS and
PDC systems noted in Table 2.1
A system performing a combined function is one in which
the sensing computer and display units are interfaced with an auto-throttle
control system and pitch channel of an automatic flight control system; thus,
in removing the flight crew from the control loop, an integrated automatic FMS is formed
to provide greater precision of engine power and vertical flight path control.
Early forms of flight management systems, whether purely advisory or combined
function, were limited to supervising control parameters affecting the vertical
flight path. In order to ensure maximum fuel economy it is, however, also
necessary to integrate this optimized flight path management with the lateral
flight path; in other words, a system must also be provided with a navigation
capability. This requires interfacing the computer with such navigation systems
as Doppler, inertial reference system, DME and VOR. The inputs from these
systems permit continuous monitoring of an aircraft's track in relation to a
flight plan, which may be pre-stored in the computer memory and an immediate
identification of deviations. Furthermore, it allows flight plan variants to be
constructed and evaluated. It is thus apparent that by combining these inputs
with those controlling the vertical flight path parameters mentioned earlier,
an FMS can integrate the functions of navigation, performance management,
flight planning and three-dimensional guidance and control along a pre-planned
flight path.
Inputs: In addition to changing
data inputs from such systems as those mentioned above, an FMS system also
requires data bases for storing bulk navigation data, and the characteristics
of an aircraft and its engines, in order that the system will operate in a full
three-dimensional capacity. The navigation data base is capable of storing the
necessary flight environmental data associated with a typical airline's entire
route structure, including pertinent navigation aids and waypoints, airports
and runways, published terminal area procedures, etc. The memory bank also
contains flight profile data for a variety of situation modes, such as
take-off, climb, cruise, descent, holding, go-around and 'engine -out'. The
cruise mode is also sub-divided into sub-mode variants such as economy,
long-range, manual and thrust-limited. The integration of all the foregoing
data, plus other variable inputs such as wind speeds and air traffic control
clearances, permit the automatic generation or modification of flight plans to
meet the needs of any specific flight operation
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