Failure Manager

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You may need to install some extra components, use the latest development (Git) version or even rebuild FlightGear from source, possibly from a custom topic branch using special build settings: .

This feature is scheduled for FlightGear 3.2. 70}% completed

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Failure Manager
Started in 02/2014
Description Failure Management Framework
Maintainer(s) galvedro, Hooray
Contributor(s) User:galvedro
Status First milestone merged to fgdata on 06/2014
Folders

[1]

[2]
Topic branches:
fgdata [3]

Targeted FlightGear versions: 3.20+

Objective

Come up with a framework that allows modeling instrument/equipment failures for systems implemented in scripting space and C++, focus on airborne equipment and prepare hooks for also supporting ground equipment failures such as navaids.

Current Situation

All systems and most instruments implemented in the C++ core support basic failure simulation by means of a serviceable property. Generally, when this property is set to false, the system will stop updating itself. Some of these systems may support additional, more elaborate types of failures.

Other than this convention of using a serviceable property, there is no framework on the C++ side with regards to failure simulation. There is, however, a Nasal submodule that can generate random failures by taking user input from the GUI and controlling the relevant properties.

The approach is good, but the main problem is that the supported types of failures are hardcoded both in the Nasal module and the GUI.

Limitations

  • The GUI presents a fixed set of failures that can be simulated, regardless of what systems are actually implemented in the aricraft.
  • Aircrafts can not add their own implemented failures to the set in a clean way.
  • Failures are considered boolean by the framework, i.e. either serviceable or not serviceable. There is no way to express intermediate states of failure.
  • Only random failures based on time or usage cycles are supported.
  • In general, the framework is not extensible.

Proposed improvements

The proposal is to maintain the current schemma of having a Nasal submodule dedicated to failure simulation, but overhaul it to overcome the limitations stated above. In order to accomplish that, we will raise its status to a full fledged Failure Manager.

Here are some desirable traits for the new module:

  • To start with, the failure manager should definitively _not_ implement any particular system failure by default, but provide the logic for managing random events or on demand failure mode activation.
  • It should also provide a subscription interface so aircraft systems can register their own failure modes. After all, only the aircraft "object" is really aware of what systems are being modeled.
  • It should not make any assumptions on how to trigger failure modes (i.e. do not assume a serviceable property). Instead, the Failure Manager should use an opaque interface for setting a "failure level" and leave the details of activation to user scripts.
  • The Failure Manager should also support a flexible set of trigger conditions, so failure modes can be programmed to be fired in different ways, for example at a certain altitude.
  • GUI dialogs should be generated procedurally, based on the set of supported failure modes that has been declared by the aircraft.

Implementation Details

The current prototype includes three components:

  1. A Nasal submodule that implements the core Failure Manager.
  2. A Nasal library of triggers and actuators for programming the Failure Manager.
  3. A compatibility script that programs the Failure Manager to emulate previous behavior. Currently loaded by default on startup.

The design revolves around the following concepts, all of them implemented as Nasal objects.

FailureMode
A failure mode represents one way things can go wrong, for example, a blown tire. A given system may implement more than one failure mode. They store a current failure level that is represented by a floating point number in the range [0, 1] so non boolean failure states can be supported.
FailureActuator
Actuators are attached to FailureModes and encapsulate a specific way to activate the failure simulation. They can be simple wrappers that change a property value, but they could also implement more complex operations. By encapsulating the way failure modes are activated, the Failure Manager does not depend on conventions like the serviceable property, and can be easily adapted to control systems designed in different ways.
Trigger
A Trigger represents a condition that makes a given FailureMode become active. The current prototype supports the following types: altitude, waytpoint proximity, timeout, MTBF (mean time between failures) and MCBF (mean cycles between failures). More can be easily implemented by extending the FailureMgr.Trigger Nasal interface.
FailureMgr
The Failure Manager itself. Keeps a list of supported failure modes that can be added or removed dynamically using a Nasal API. It also offers a Nasal interface for attaching triggers to failure modes (one trigger per failure mode). While enabled, the FailureMgr monitors trigger conditions, and fires the relevant failure modes through their actuators when their trigger becomes active. The FailureMgr can be enabled and disabled on command, both from Nasal and the property tree.

Roadmap

  1. Replace Nasal/failures.nas with a new module implementing the design proposed above. Wire it to the exising GUI dialogs and ensure backwards compatibility 100}% completed
  2. Help the Canvas team to develop a Nasal GUI API.
  3. Replace the hardcoded dialogs with dynamic ones using whatever comes out from the step above.
  4. Do not load the compatibility layer globally (i.e. by default), but rather load it explicitly from every aircraft (this is gonna be some seriously boring and tedious work).
  5. Aircraft authors can now start customizing the failure features for their crafts in a clean way.
  6. Extend the feature set as needs arise (instructor console, additional triggers, ground equipment failure simulation, etc).

Under consideration

  1. Generalize the trigger system and make it available as a global service. Might be useful for missions/adventures, AI agents, etc.
  2. Introduce the concept of Wear Models.

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