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FFS in Motion, or not


A few weeks ago we were requested to assist a Customer who was having problems getting their Full Flight Simulator (FFS) motion system to engage. While we were troubleshooting this we realised that, during our combined careers supporting customers, motion problems were some of the most challenging issues for training centres. So this month we thought we would have a look at motion systems and what goes wrong with them.



What makes up a motion system?


Before looking at the problems though let’s look at a typical 6-axis, hexapod, system containing;


  • Motion actuators - six actuators that physically move the motion platform/FFS.

  • Motion cabinet - for electrical/hybrid systems. These will include a motion computer, electrical servo drive units and the associated power transformers and rectifiers.

  • Hydraulic power unit (HPU) - for hydraulic systems, located in a separate hydraulic room (they don’t make good neighbours). This will typically consist of a large tank fitted with supply pumps and a return pump with associated filters and pressure relief valves. In most cases connected to a building chilled water system.


The first thing to note is that, while most FFS are now delivered with all-electric systems, there are also some hybrid systems being delivered and there is a large number of hydraulic systems still in service (and likely to remain so for a long time).


There are two debates we will avoid in this blog, the first is the relative merits of the different types of motion systems (although all electric seem to have won out on this). The other is whether or not motion systems are required at all; for those of us charged with maintaining a system faced with trying to get it back up and running at 03:00 in the morning it is really a moot point.


Motion won’t engage - Common factors


Whatever the type of system there are some basics that apply to all systems;


Interlocks - the device manufacturers and motion manufacturers include in their systems designs a large number of interlocks that must be made before the motion will engage; these protect not only the personnel using the device and in the training centre but the device itself. If copyright allowed we would share a picture of an FFS where motion was accidentally engaged with the drawbridge down, the drawbridge was very quickly further down, on the floor along with a large portion of the rear of the FFS. Fortunately a large number of “won’t engage” problems are solved by ensuring doors are shut and correctly latched and pressure pads are not stuck on.


Position sensors - to avoid the actuators overrunning and hitting hard stops all actuators include a position feedback system. These used to be all linear transducers (LVDTs) running the length of the actuator that measure the absolute extension of the actuator; the mention of “Tempsonic” transducers can induce trepidation in many a simtech. Newer electric rotary actuators tend to utilise rotary encoders. These work by measuring the speed of rotation and integrating to derive position (and differentiating for acceleration) within the motion computer. Whatever the technology, without valid signals from all actuators the motion will not engage.


Electric system issues


Mechanical brakes - a lot of electric actuators have integrated mechanical brakes, on these systems it is common that when motion is first engaged, as a test for brake wear, the brakes are applied and a force applied by all actuators. Any slippage of a brake will prevent the system fully engaging. We have seen systems where a badly over tuned motion system has been causing regular overspeeds resulting in brake actuation that has, overtime, led to brake wear leading to the situation whereby slippage occurs (around midnight on a Friday or public holiday of course).


Return to Rest (RTR) batteries - some electrical motion systems have a battery pack in the power cabinet that will under power loss conditions power the actuators enough to return the motion to its rest position. If these do not have enough charge in them the motion will not engage.


Hydraulic Systems (pure hydraulic)


Servo valves (and associated Abort Valve)- each actuator will have an electrically actuated hydraulic servo valve that ports pressure to the correct side of the hydraulic ram according to the demanded movement. Unfortunately these are prone to sticking, any fault will prevent engagement (hopefully - think drawbridge on the floor). However often the servo valve failure is sometimes a symptom of dirty, neglected, hydraulic oil. Between us we’ve all seen systems with hydraulic oil the colour and viscosity of molasses; it is imperative with these systems that filters are changed on time, patch tests done and oil changed as recommended. And no, this can’t be done from a keyboard, someone will have to get their hands dirty. That said we have seen some devices fitted with off line filtration systems which were advertised by the manufacturer at FSEMC conferences some years ago. These systems have substantially improved the overall reliability and prevented oil degradation.


Precharge pressures/accumulators - the pressure in these should be checked regularly as part of the preventative maintenance programme, however if they aren't properly precharged the motion won’t engage.


Chilled water - most of the HPUs in service rely on a chilled water supply to cool the hydraulic oil. Between us we have seen more than once motion systems where the oil has overheated causing the system to trip and the fault has been simply debris in an inline filter to the heat exchanger.


Fortunately the motion system manufacturers will provide fault finding systems that will normally assist in identifying the problems but they are not always 100% effective, as in the case we were called in to assist on. Of course the really difficult issues to trace are where motion trips in use for seemingly random reasons, the fault not being present when diagnostics are checked (a lot of these go back to the interlocks). A lot of systems have a fault log that is sometimes only readable by the system manufacturer, sometimes called “dying seconds logs”; make sure that all the simtechs on your team know how to download these and are briefed to do so, it will save a lot of time. That said it didn’t help on a fault we saw a few years ago where rats were chewing the ethernet cables under the floor!


Support Factors


If there is one area where training is essential it is in motion systems; the systems can be dangerous if not treated properly and people don’t understand what they are doing (things like adjusting hydraulic servo valves on a live system -again think drawbridge). Not that long ago there were photographs being shared of a full flight simulator on the ground, we don’t know the full circumstances of this but it looked suspiciously like a maintenance error during an actuator change.


A final word on motion; a common question all the partners at Sim Ops have been asked many times over the years is “do I need to buy a spare actuator”; of course the answer is “it depends''. Spare actuators are very expensive, eye-wateringly expensive, so to have one sat in a box for five years unused is effectively dead money. What we recommend is talking to the motion system supplier direct to see if they have, or are willing to put, one in your region on a guaranteed availability.


How can Sim Ops help?


As in the case discussed we have resources who can assist remotely or, if needed, on-site. We can arrange training and safety courses for personnel engaged in motion system maintenance and we can also provide solutions for Off Line filtration systems.





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