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Flight Management Systems for Singles and Light Twins
Stephen B. Miller

Re-printed with permission from the Aug. '97 issue of Avionics News magazine.

Flight Management Systems (FMS), while normally found only on larger aircraft (i.e., bizjets and up), have a potentially unique application for singles/light twins, largely due to the availability of small "embedded processor" systems. Historically, the FMS is used in a "multi-engine/multi-crew" environment, whereas single-engine aircraft are virtually always operated by a single pilot and light twins are frequently operated in the same way. This brings up some interesting "what-if's". For example, what if an aircraft of this class had an FMS coupled to the autopilot (including all the usual nav sources) and could communicate via the MODE S transponder? What if this system also possessed a relatively high-level of software such that all the necessary "human-like" decisions could then be made? The answer to these questions is that you'd have a full-time "virtual" co-pilot, capable of unassisted flight from A to B, including approaches.


In a "single" operating enroute in IMC, getting the aircraft down "dead stick" is no mean feat, to say the least. The multi-tasking capability of a system such as this would enable the aircraft to quickly set up the best available approach at the best airport within gliding range and proceed to execute it on a "how-goes-it" basis, with constant updates for both altitude and direction. Communications would be handled simultaneously through the MODE S data link. All decisions and current status could be visually displayed to the pilot in real-time. The FMS would position the aircraft so that a manual landing could be accomplished at the appropriate time.


In this case, particularly in IMC, passenger survival is highly unlikely without the sort of back-up the FMS could provide. This emergency has a distinctly different "wrinkle" from the previous one, in that both power and fuel are available, allowing the aircraft to select the best available precision approach at the airport having the best weather as well. In this case, however, "auto-land" capability is also required (more about that later).


In order to provide these capabilities, the FMS needs to sense/interpret all of the inputs available to the pilot and to respond with "pilot-like" skill and intelligence. This would obviously include such things as frequency/autopilot mode selection and the ability to control the engine and all other "ancillary" devices (gear, flaps etc). Providing the necessary "pilot-like" decision making capability is by far the most difficult requirement, but is feasible through extensive software development. The level of "knowledge-based" software required dictates that it be developed by people who are also experienced pilots. These folks exist, but they don't exactly grow on trees. Can you envision anyone else attempting to write the code to manage a dead-stick instrument approach, for example? The real difference between an existing FMS and this version is the combination of autopilot coupling, total control of the aircraft and the high level of integration software, the goal being to greatly enhance both the safety and utility of singles and light twins.


Many system configurations are possible, though in a mature design there is no good reason to separate the autopilot/controller functions. It makes more sense for the controller to perform all autopilot tasks, issuing servo commands based on instrument/pilot inputs over a common bus. For a "conceptual" prototype design, however, the simplest initial approach is to envision the system in terms of modifications of existing hardware wherever possible. Viewed in this light, we have the following components:

  1. SYSTEM CONTROLLER: This is the "embedded processor" mentioned previously; think of it as the "brains" behind the operation. This unit also has sufficient I/O capability to communicate with all other system components using a standard interface, such as RS232, RS449, one of the ARINC busses etc. It is beyond the scope of this article to deal with the details of particular system architectures. Communications can be implemented through individual RS232 ports from the controller to each system component or through a common parallel bus, for example. The controller gathers and analyzes the data from the other system components and issues the necessary commands. The point is, there are a number of these OEM processor systems available, readily adaptable to this task, either in present form or with packaging modifications, for example. They generally are rack-mounted or simply "stacked" PC boards, ranging in size from extremely compact to "a little larger than I'd like". Many of these will mount remotely behind the baggage compartment or in the nose of a typical light twin, for example.

  2. AUTOPILOT "SYSTEM": This refers to the autopilot and all associated instrumentation/servos, with the following modifications:

    1. The autopilot needs a communications interface as explained in the previous paragraph in order to "talk" to the controller, which must be able to control the autopilot in exactly the same fashion as the pilot can.

    2. The FMS must also be able to control the engine, gear, flaps etc as mentioned previously in order to handle the job in the case of pilot incapacitation. For ease of visualization, consider a typical "Piper-style" control quadrant incorporating gear and flaps as well. This can be viewed as a single "servoized" package, representing an additional autopilot component, although a complex one. Actually, if autopilot redesign is to be avoided, this "component" would be controlled directly by the system controller, operating in conjunction with the autopilot, until such time as the autopilot and controller functions are eventually merged, as probably would be the case in future designs.

  3. NAV SOURCES: In addition to the usual "steering" inputs to the autopilot, the controller would also communicate with these components for status/control (mode/freq. selection etc). Again, the controller's job is to "emulate" the pilot. Whereas GPS receivers would typically provide, say, an RS232 port, older VHF NAV sources would not, leaving them out of the picture. Considering the rapidly accelerating dominance of GPS, this is not likely to present a problem in the long run.

  4. TRANSPONDER: Obviously, this would be the controller's only means of air-ground communication using the MODE S data link. Similar coupling to a TCAS unit would likewise be a definite "plus".


This capability has never existed on singles or light twins. The development costs/technical problems have always exceeded any perceived need. In the case of the FMS described herein, however, the need is real enough if the aircraft is to be "smart" enough to extricate the passengers from certain disaster should the pilot become incapacitated. I don't think anyone would argue that controlling a small aircraft near the ground in a typical turbulent crosswind is a far cry from doing the same thing in a '747. The problems of precise, instantaneous lateral (DGPS?) and vertical (RADIO ALTIMETER?) guidance/control are substantial, to say the least. No doubt, other fast-reaction sensors such as accelerometers would also be needed. Controller response times would also have to be fast enough to take care of sudden, unexpected changes in flight path (i.e., the vertical gust that leaves the aircraft "high and dry", for example). All of of this would have to be combined into a system whose reactions could not be discerned from those of a human pilot, a daunting challenge if ever there was one.


After all this effort to develop a system which is "always ready" to provide whatever assistance is required, power for its operation must be assured, even on a single with a failed engine/alternator. This means a redundant power bus must exist, invoked automatically when needed. The backup power source could be a high-energy battery, air-driven alternator or other means, but must be free from "contamination" by other system failures. This aspect of the design is not trivial and must be thoroughly thought out if it is to be commensurate with the system goals.


It depends. If a viable manufacturer feels the market is ready and that this system could be made "affordable" (can anyone really define this?), the attempt will probably be made. It could also fit in well with NASA's efforts to create safer aircraft. Incidentally, my guess is that only an existing autopilot manufacturer could even begin to think of a development such as this, so the potential field of players is limited right from the beginning. If this system were available today at the right price, how big do you think the retrofit market would be? How about the new market, now that the industry appears to be "on the mend"? Obviously, there are more questions than answers here, but I think you get the picture.

This overview has been presented in an effort to point out ways in which existing technology might be channelled to provide greater utility/safety for small, high-performance aircraft. Hopefully, a sufficient market exists which will make these things possible in the not-too-distant future.



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