Graham Warwick/WASHINGTON DC

Flight management systems (FMS) are no longer luxury items found only on large airliners, but essential equipment on commercial aircraft of all sizes and ages. The reason is the navigation accuracy now possible and the cost benefits available to airlines in the form of fuel and time savings.

More operators are specifying FMS in new aircraft and retrofitting them into older ones, at a time when the specifications for many elements of the future communications, navigation, surveillance/air traffic management (CNS/ATM) are not fully defined. FMS manufacturers, therefore, are having to meet near-term demand while providing customers with a long-term growth path.

"The FMS is the most complex instrument in the aviation industry," believes Mike Otis, FMS marketing manager for Honeywell's air transport systems division. Evolving from a relatively simple navigation system, the FMS has become the heart of the modern commercial and corporate aircraft cockpit.

The fitting of an FMS was once difficult for operators to justify, given the cost. The navigation accuracy possible with modern multisensor systems has tipped the cost-benefit equation in favour of the FMS. The shift has been accelerated by airspace access changes that provide incentives to operators that equip their aircraft to provide the required navigation performance (RNP) only achievable with an FMS.

RNP limits define the performance required of a system if it is to be used as the primary means of navigation in a specific phase of flight. In oceanic airspace, for example, where there are no ground-based navigation aids within line of sight, RNP is 22km (12nm). In the approach phase, RNP limits narrow to 0.5km - an accuracy that can be exceeded by the latest FMS, using the global positioning system (GPS) aided by an inertial reference system (IRS).


Alaska Airlines is leading the charge in exploiting the capabilities of the latest GPS-based FMS. The Seattle-based carrier is the first airline to receive approval for enhanced approach and departure procedures based on the RNP capability of a new-generation FMS.

The airline has received US Federal Aviation Administration approval for reduced-visibility approaches to Juno, Alaska, using Boeing 737-400s equipped with Smiths Industries' latest FMS. "Alaska has been able to develop approaches which significantly enhance departure and arrival reliability, as well as achieve fuel savings," says Mike Grady, vice-president of FMS business operations at Smiths Industries Aerospace in Grand Rapids, Michigan.

"The ability to navigate very precisely in four dimensions, including time, opens up new departures and arrivals that are safer and more efficient," he says. "Our new-technology FMS in the Next Generation 737 can make airlines a lot of money."

Smiths offers to conduct cost-benefit studies for airlines to illustrate the potential operational advantages. "We have demonstrated to a number of airlines that precision navigation capability can make money quickly," Grady says. "Cost-benefit studies take a year or more, but a retrofit can pay for itself in less than a year."

A major benefit of the increased navigation accuracy possible with a GPS-based FMS is the ability to operate at lower-visibility minima. "The study we did with Alaska looked at the impact of irregular operations on airline performance," says Grady, noting that the knock-on effect of delayed or diverted flights can last for days. "It can take two to three days for the ripple to settle down, and that keeps costing the airline money," he says.

In the case of Juno's runway 26, the newly certificated FMS-based approach has allowed the decision height to be reduced from 1,100ft (335m) to 300ft. "The old approach is 10min longer and uses 600lb [270kg) more fuel," Grady says. The curved approach follows the Gastineau Channel, with mountains on either side, and approval required certification of Alaska's FMS-equipped 737s for an RNP of just 0.15nm. Grady says the FAA required the demonstration of twice this accuracy - 0.07nm - to approve the approach.

Navigating for an extended period with an accuracy of "a couple of hundred feet" required a combined GPS/IRS-based system, he says. The GPS provides the accuracy while the IRS provides the integrity. "The drift rate is such that they can lose the IRS on the approach and still continue," Grady says. When insufficient GPS satellites are available, the RNP increases to 0.3nm. "Alaska has a computer model to predict how many satellites will be in view during the approach," he says.

Alaska plans to retrofit its older 737s with the same FMS and hopes to achieve the same operating benefits, Grady says. Southwest Airlines, meanwhile, is close to completing the retrofit of Smiths' FMS on to all 208 of its 737-300s and -500s, providing commonality with the 737-700s now being delivered. Work is under way to develop FMS-based approach and departure procedures at key airports served by Southwest. "It doesn't take the entire route network [to justify installing an FMS], just one or two major networks," Grady says.

Smiths is the sole FMS supplier on new 737s and the company has extended its reach in recent years. Rockwell-Collins has licensed Smiths' FMS software for use in its GPS navigation and landing unit (GNLU), offered for retrofit on older non-FMS aircraft such as the Boeing 727 and McDonnell Douglas DC-9. The software is hosted on a circuit card which Smiths builds for the GNLU.

An agreement was signed with France's Sextant Avionique in 1998 for joint development of an advanced FMS for Airbus Industrie aircraft. Sextant is integrating the Smiths FMS into its flight management and guidance computers for the A320 family and the A330/A340. Grady says that the Sextant/Smiths team has won "more than half " of the competitions on the A320 family.

The competing supplier is Honeywell, which is adapting its Pegasus advanced FMS for the A320 family and the A330/A340, having won certification on the Boeing 757, 767 and MD-90. Approval is pending on Boeing's MD-11

Honeywell's thrust has been to provide a common FMS platform, with growth to handle the move to CNS/ATM. "We've developed FMS for 20 years and we have 25 different permutations," says Otis. "Airlines are looking for commonality in technology and maintenance, and to position themselves for free flight, although it's some time off."


The navigation database is common across all Pegasus applications. "The database is cheaper and larger, and it is just one part number to update," Otis says. The Pegasus is certificated already with Future Air Navigation System (FANS-1) capability, the initial increment of CNS/ATM. Follow-on certifications are adding software features including required time of arrival, which allows 4-D navigation. A new control display unit was introduced late last year which paves the way for datalink communications.

Full release of the Pegasus on the A330/A340 is scheduled for January 2000, followed that August by release on the A320 family. The initial stage will provide FANS-A capability, equivalent to Boeing's FANS-1. The Pegasus is also a "viable and inexpensive option" to upgrade earlier Honeywell FMS. The company offers a card replacement programme that allows airlines to drop Pegasus circuit boards into their existing FMS. "We have 35 customers wanting the retrofit," says Otis. "The upgrade costs about 10% of a new FMS."

The Pegasus is part of Honeywell's overall WorldNav line of CNS/ATM products. Others include the Honeywell/Trimble HT-series of GPS-based navigation and navigation management systems for the retrofit market. Although there is not much call for FANS capability in the market served by the HT series, these retrofit FMS are being developed to work with both wide-area and local-area GPS augmentation systems, Otis says.

If airlines want a glimpse of where FMS are going, they should take a look at the corporate aircraft market, where customers are more willing to embrace - and pay for - new technology. The latest feature introduced on Honeywell's corporate FMS is graphical weather, and work is under way on new user interfaces such as cursor control devices, says Vince Sadd, FMS marketing manager for the company's business and commuter aviation systems division.

The problem, as with the airline sector, is to offer a system that can grow incrementally as the CNS/ATM environment is defined. "We are trying to educate our customers that an FMS is like a PC. The technology is not stagnant, and is going to require significant changes in hardware and software," Sadd says. "Fortunately, corporate customers are willing to spend more for a faster processor and more memory."

Another trend in the corporate and regional sectors is towards integration of the FMS into the aircraft's avionics suite. In Honeywell's latest Primus Epic system, the FMS and GPS are hosted on modules in an integrated avionics computer. "The FMS is not a dedicated card in the Primus Epic," says Sadd. "We see that happening with the Pegasus." Both the corporate and commercial sectors are heading in the direction represented by Primus Epic, towards more integration, he and Otis believe.

Although the corporate community is moving faster in developing a new user interface for the FMS, "Windows-style" operation is still a few years away, Sadd says, because of issues with pilot acceptance. But both arms of Honeywell are working on developing new control languages, although Otis admits that the air transport sector is moving "at a slow pace".

The pace at which the aviation community at large is turning to the GPS-based FMS is not so slow, however. There may still be lingering concerns about the GPS itself, but the cost benefits of a multisensor FMS offering precise navigation are apparently increasingly well understood and readily accepted by operators.

Source: Flight International