SAS and ATT mode
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Originally Posted by [email protected]
In the Sea King, you needed differential pedal to disengage the heading hold with the ASE in - since not many pilots sat on the deck with differential pedal pressure any heading change of the ship would cause the yaw channel to try and swing the tail. Hence the ASE disengagement on Sea Kings.
ISTR the 412 has an irritating yaw trim function but I don't think it has a yaw channel heading hold - that is a roll channel function in the upper modes of the AFCS.
Short answer is no, I don't think you need to disengage the AP on a deck unless it is moving a lot but I would go from ATT to SAS mode.
ISTR the 412 has an irritating yaw trim function but I don't think it has a yaw channel heading hold - that is a roll channel function in the upper modes of the AFCS.
Short answer is no, I don't think you need to disengage the AP on a deck unless it is moving a lot but I would go from ATT to SAS mode.
The RAF Shawbury Gazelles had a SAS light but it didn’t really do anything. It was just a confidence light…
Thanks for this. Yes the 412 has no heading hold. My thought process was that the SAS works on providing a opposite force to a roll / pitch rate induced due to wind or (pitch and roll on deck) to try and minimise disc movement caused due to this external stimulus. The ATT mode on the other hand holds the disc attitude in position. But on deck which is heaving, does one want the AFCS in SAS mode to provide these opposing forces to minimise the roll/pitch rate ? Why not just let the force trim do its job of holding the disc. Why allow the AFCS to provide inputs to the disc
Having said that, the gyros don't know the difference between the aircraft movement and the boat movement so you are probably better off taking out the AFCS if the deck is moving a lot and you will be on it for more than a few seconds.
Just don't forget to put it back in before take off!!
My thought process was that the SAS works on providing a opposite force to a roll / pitch rate induced due to wind or (pitch and roll on deck) to try and minimise disc movement caused due to this external stimulus. The ATT mode on the other hand holds the disc attitude in position
On a ship deck, force trim was OFF and the stick just held, because the autopilot might make some strange inputs to counteract pitch/roll.
In a Huey, force trim just held the stick in position, which meant that if you went hands-free, you would eventually roll up in a ball.
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SAS/ATT/APLT on decks or moving platforms
It has always been taught in my experience, that if landing on any deck/platform or surface that moves, turn the "automatics" off after landing and it is normally the last thing to engage before getting airborne again. The rationale being the fundamentals of the two different systems - SAS & ATT (or APLT). But each system has direct control on the tip path plane of the disc with the very real potential of taking heads off.
SAS on most, if not all helicopters is called an "inner" loop system. That is, it acts completely independently of your stick position. It is designed to provide short term dampening and gust alleviation from short term changes in pitch/roll/yaw rate sensors (gyros/accelerometers). This short term change is then processed (in the AFCS computer) and a signal sent to the hydraulic/electric actuators to command an input change to the disc. SAS actuators are the "muscle" to convey the desired disc change that the SAS portion of the AFCS desires. This means the SAS by itself can physical change the tip path plane without any input from the pilot and without any change to the pilot's stick position.....ie an "inner' loop control input. The pilot has no idea how much of an input the SAS is giving to the disc. The SAS normally only has a limited control authority - something like 5-15% only (don't quote me on this ...it has been some time and may vary from system to system). None-the-less, SAS has the power to change your disc's tip path plane without your input and subject to external forces beyond your control....sometimes significantly so depending upon the external disturbance.
ATT/APLT is called an "outer" loop system. It provides long term stability such as Attitude hold and long term navigation. As such, it takes inputs for not only the pitch/roll/yaw rate sensors but the attitude gyro, the navigation inputs, accelerometers, pressure instruments etc. The Autopilot ("long term") part of the AFCS computer calculates the Long Term stability or aircraft positional information eg heading, airspeed, height, ROC, etc and determines where it needs to position the flight controls to achieve that result. The AFCS/APLT does this by sending its processed signal as an output to the Force Trim, which then inputs to the disc via the Force Trim motors....ie the Force Trim is the "muscle" for the APLT to operate....the AFCS is it's brains. Hence you will physically feel your flight controls (cyclic/pedals & collective for 4 axis) move without your input or against your pressure as response to an input commanded by the AFCS/APLT. Similar to the SAS, you will see the tip path plane dance in response to the APLT input (long term stability) BUT you will also feel your flight controls move as well - the major difference to SAS input.
The APLT control authority is up to 100%. Because you feel the cyclic/pedals/collective move if the APLT makes an input to the disc, you can be made aware of these inputs and try to counter them by pushing against the force trim or pickle off the APLT (remove the brains) or Force Trim (remove its muscle).
Force Trim - is simply a means of holding the flight control position steady to allow the pilot to remove his hands from the flight controls in order to do other things in the cockpit without fear of the cyclic/pedals literally falling over and rapidly having the aircraft go out of control....ie the the UH1 without force trim on. It has up to 100% control authority if you were to continue to beep the coolie hat to change the disc/attitude, it will continue to input a stick change to displace the disc attitude. It is used as the "muscle" by the APLT function of the AFCS.
SAS actuators - Because the SAS requires a limit to it's control authority (eg 5-15% is typical), it has its own set of SAS actuators which have limited control authority, separate from the Force Trim (which has 100% control authority). This system provides NO feedback to the cyclic/pedals/collective if it generates a change to the disc attitude.
Landing on Decks - So when landing on any floaty/moving platform (the smaller the platform the more critical), it is STRONGLY suggested that you turn off the AFCS - both APLT (long term stability) AND the SAS (short term stability - leave Force Trim ON. Next time you are on a small bobbing platform, try leaving the SAS on and observe just how much the tip path plane moves just under SAS inputs without your stick moving. It is enough to take heads off when manoeuvring under the disc with any sort of deck movement. Leave the Force Trim on as it will help hold the disc steady ref the aircraft. By disengaging the SAS/APLT, all you have done is killed the "brains" which are easily fooled by outside disturbances. The Force trim by itself cannot change the disc attitude unless you physically input to it.....which you may have to do if the deck is really moving.
Hope this provides some distinction between SAS & ATT/APLT and why its strongly advisable to get into the habit of disengaging the "brains" of both while conducting deck/platform operations.
cheers
SAS on most, if not all helicopters is called an "inner" loop system. That is, it acts completely independently of your stick position. It is designed to provide short term dampening and gust alleviation from short term changes in pitch/roll/yaw rate sensors (gyros/accelerometers). This short term change is then processed (in the AFCS computer) and a signal sent to the hydraulic/electric actuators to command an input change to the disc. SAS actuators are the "muscle" to convey the desired disc change that the SAS portion of the AFCS desires. This means the SAS by itself can physical change the tip path plane without any input from the pilot and without any change to the pilot's stick position.....ie an "inner' loop control input. The pilot has no idea how much of an input the SAS is giving to the disc. The SAS normally only has a limited control authority - something like 5-15% only (don't quote me on this ...it has been some time and may vary from system to system). None-the-less, SAS has the power to change your disc's tip path plane without your input and subject to external forces beyond your control....sometimes significantly so depending upon the external disturbance.
ATT/APLT is called an "outer" loop system. It provides long term stability such as Attitude hold and long term navigation. As such, it takes inputs for not only the pitch/roll/yaw rate sensors but the attitude gyro, the navigation inputs, accelerometers, pressure instruments etc. The Autopilot ("long term") part of the AFCS computer calculates the Long Term stability or aircraft positional information eg heading, airspeed, height, ROC, etc and determines where it needs to position the flight controls to achieve that result. The AFCS/APLT does this by sending its processed signal as an output to the Force Trim, which then inputs to the disc via the Force Trim motors....ie the Force Trim is the "muscle" for the APLT to operate....the AFCS is it's brains. Hence you will physically feel your flight controls (cyclic/pedals & collective for 4 axis) move without your input or against your pressure as response to an input commanded by the AFCS/APLT. Similar to the SAS, you will see the tip path plane dance in response to the APLT input (long term stability) BUT you will also feel your flight controls move as well - the major difference to SAS input.
The APLT control authority is up to 100%. Because you feel the cyclic/pedals/collective move if the APLT makes an input to the disc, you can be made aware of these inputs and try to counter them by pushing against the force trim or pickle off the APLT (remove the brains) or Force Trim (remove its muscle).
Force Trim - is simply a means of holding the flight control position steady to allow the pilot to remove his hands from the flight controls in order to do other things in the cockpit without fear of the cyclic/pedals literally falling over and rapidly having the aircraft go out of control....ie the the UH1 without force trim on. It has up to 100% control authority if you were to continue to beep the coolie hat to change the disc/attitude, it will continue to input a stick change to displace the disc attitude. It is used as the "muscle" by the APLT function of the AFCS.
SAS actuators - Because the SAS requires a limit to it's control authority (eg 5-15% is typical), it has its own set of SAS actuators which have limited control authority, separate from the Force Trim (which has 100% control authority). This system provides NO feedback to the cyclic/pedals/collective if it generates a change to the disc attitude.
Landing on Decks - So when landing on any floaty/moving platform (the smaller the platform the more critical), it is STRONGLY suggested that you turn off the AFCS - both APLT (long term stability) AND the SAS (short term stability - leave Force Trim ON. Next time you are on a small bobbing platform, try leaving the SAS on and observe just how much the tip path plane moves just under SAS inputs without your stick moving. It is enough to take heads off when manoeuvring under the disc with any sort of deck movement. Leave the Force Trim on as it will help hold the disc steady ref the aircraft. By disengaging the SAS/APLT, all you have done is killed the "brains" which are easily fooled by outside disturbances. The Force trim by itself cannot change the disc attitude unless you physically input to it.....which you may have to do if the deck is really moving.
Hope this provides some distinction between SAS & ATT/APLT and why its strongly advisable to get into the habit of disengaging the "brains" of both while conducting deck/platform operations.
cheers
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For a few years I flew a twin engined Squirrel which had the entire SAS system removed to get the weight down. Interesting to fly at night or in poor visibility, especially at very low speed or in an OGE hover for extended periods.
By disengaging the SAS/APLT, all you have done is killed the "brains" which are easily fooled by outside disturbances.
Ring Gear provides good information in his post.
It has always been taught in my experience, that if landing on any deck/platform or surface that moves, turn the "automatics" off after landing and it is normally the last thing to engage before getting airborne again. The rationale being the fundamentals of the two different systems - SAS & ATT (or APLT). But each system has direct control on the tip path plane of the disc with the very real potential of taking heads off.
SAS on most, if not all helicopters is called an "inner" loop system. That is, it acts completely independently of your stick position. It is designed to provide short term dampening and gust alleviation from short term changes in pitch/roll/yaw rate sensors (gyros/accelerometers). This short term change is then processed (in the AFCS computer) and a signal sent to the hydraulic/electric actuators to command an input change to the disc. SAS actuators are the "muscle" to convey the desired disc change that the SAS portion of the AFCS desires. This means the SAS by itself can physical change the tip path plane without any input from the pilot and without any change to the pilot's stick position.....ie an "inner' loop control input. The pilot has no idea how much of an input the SAS is giving to the disc. The SAS normally only has a limited control authority - something like 5-15% only (don't quote me on this ...it has been some time and may vary from system to system). None-the-less, SAS has the power to change your disc's tip path plane without your input and subject to external forces beyond your control....sometimes significantly so depending upon the external disturbance.
ATT/APLT is called an "outer" loop system. It provides long term stability such as Attitude hold and long term navigation. As such, it takes inputs for not only the pitch/roll/yaw rate sensors but the attitude gyro, the navigation inputs, accelerometers, pressure instruments etc. The Autopilot ("long term") part of the AFCS computer calculates the Long Term stability or aircraft positional information eg heading, airspeed, height, ROC, etc and determines where it needs to position the flight controls to achieve that result. The AFCS/APLT does this by sending its processed signal as an output to the Force Trim, which then inputs to the disc via the Force Trim motors....ie the Force Trim is the "muscle" for the APLT to operate....the AFCS is it's brains. Hence you will physically feel your flight controls (cyclic/pedals & collective for 4 axis) move without your input or against your pressure as response to an input commanded by the AFCS/APLT. Similar to the SAS, you will see the tip path plane dance in response to the APLT input (long term stability) BUT you will also feel your flight controls move as well - the major difference to SAS input.
The APLT control authority is up to 100%. Because you feel the cyclic/pedals/collective move if the APLT makes an input to the disc, you can be made aware of these inputs and try to counter them by pushing against the force trim or pickle off the APLT (remove the brains) or Force Trim (remove its muscle).
Force Trim - is simply a means of holding the flight control position steady to allow the pilot to remove his hands from the flight controls in order to do other things in the cockpit without fear of the cyclic/pedals literally falling over and rapidly having the aircraft go out of control....ie the the UH1 without force trim on. It has up to 100% control authority if you were to continue to beep the coolie hat to change the disc/attitude, it will continue to input a stick change to displace the disc attitude. It is used as the "muscle" by the APLT function of the AFCS.
SAS actuators - Because the SAS requires a limit to it's control authority (eg 5-15% is typical), it has its own set of SAS actuators which have limited control authority, separate from the Force Trim (which has 100% control authority). This system provides NO feedback to the cyclic/pedals/collective if it generates a change to the disc attitude.
SAS on most, if not all helicopters is called an "inner" loop system. That is, it acts completely independently of your stick position. It is designed to provide short term dampening and gust alleviation from short term changes in pitch/roll/yaw rate sensors (gyros/accelerometers). This short term change is then processed (in the AFCS computer) and a signal sent to the hydraulic/electric actuators to command an input change to the disc. SAS actuators are the "muscle" to convey the desired disc change that the SAS portion of the AFCS desires. This means the SAS by itself can physical change the tip path plane without any input from the pilot and without any change to the pilot's stick position.....ie an "inner' loop control input. The pilot has no idea how much of an input the SAS is giving to the disc. The SAS normally only has a limited control authority - something like 5-15% only (don't quote me on this ...it has been some time and may vary from system to system). None-the-less, SAS has the power to change your disc's tip path plane without your input and subject to external forces beyond your control....sometimes significantly so depending upon the external disturbance.
ATT/APLT is called an "outer" loop system. It provides long term stability such as Attitude hold and long term navigation. As such, it takes inputs for not only the pitch/roll/yaw rate sensors but the attitude gyro, the navigation inputs, accelerometers, pressure instruments etc. The Autopilot ("long term") part of the AFCS computer calculates the Long Term stability or aircraft positional information eg heading, airspeed, height, ROC, etc and determines where it needs to position the flight controls to achieve that result. The AFCS/APLT does this by sending its processed signal as an output to the Force Trim, which then inputs to the disc via the Force Trim motors....ie the Force Trim is the "muscle" for the APLT to operate....the AFCS is it's brains. Hence you will physically feel your flight controls (cyclic/pedals & collective for 4 axis) move without your input or against your pressure as response to an input commanded by the AFCS/APLT. Similar to the SAS, you will see the tip path plane dance in response to the APLT input (long term stability) BUT you will also feel your flight controls move as well - the major difference to SAS input.
The APLT control authority is up to 100%. Because you feel the cyclic/pedals/collective move if the APLT makes an input to the disc, you can be made aware of these inputs and try to counter them by pushing against the force trim or pickle off the APLT (remove the brains) or Force Trim (remove its muscle).
Force Trim - is simply a means of holding the flight control position steady to allow the pilot to remove his hands from the flight controls in order to do other things in the cockpit without fear of the cyclic/pedals literally falling over and rapidly having the aircraft go out of control....ie the the UH1 without force trim on. It has up to 100% control authority if you were to continue to beep the coolie hat to change the disc/attitude, it will continue to input a stick change to displace the disc attitude. It is used as the "muscle" by the APLT function of the AFCS.
SAS actuators - Because the SAS requires a limit to it's control authority (eg 5-15% is typical), it has its own set of SAS actuators which have limited control authority, separate from the Force Trim (which has 100% control authority). This system provides NO feedback to the cyclic/pedals/collective if it generates a change to the disc attitude.
So take a raw aircraft and let's give it a role - a private helicopter to take your gran up for a flight. What do you want from it? You want to be able to land in her garden, take off, flight around smoothly so she doesn't get ill then come back safely to the field. She natters a bit which is distracting so you want an aircraft that stays pretty much the same way up and pointing the same way as when you left it. You want nice predictable responses too.
So we basically want a really stable helicopter. When it's disturbed by an external force, you want it to resist and not be blown about. You want stability. Fine, get one of those contra-rotating rotor toys which pretty much stay still.
But this is no good as we want to move around. Therefore we need control. We want to be able to tilt the rotor to tilt the lift vector, we want to control heading via the tail rotor and control up and down/power with the collective.
But we don't want too much control. Consider the R22. If you take your eye of the ball for 2 secs off it wanders. But when you put an input in, boy does it move. But it takes a definite amount of cognitive effort to process the visual cues, (attitude change), think what needs to be done, command your limbs to move, make the movement and then process the cues again (the pilots control loop). Due to human limitations, this long loop can get out of synch with the aircraft giving a PIO.
SAS (stability augmentation system) is designed to help you. Without a SAS, the response of many aircraft to intentional or unintentional changes is an accelerating response (the rate of change of attitude gets bigger over time) towards a relatively large final attitude state. This is hard for your brain to process (although with a lot of attention (R22 hovering) you can). What SAS does through some type of rate sensing gyro (which could be a solid state AhRS in the modern world) is to sense changes in attitude rate and damp them down. Thus with a gust of wind, the rate is damping such that actually the aircraft doesn't react much. Of course the vertical fin does the same thing aerodynamically (in some case a SAS is sed to compensate for an inadequately sized fin - eg the T3/P3 variant of the EC135). But of course what if you make an intentional input and the SAS damps that out: too much stability and not enough control!
So SAS has to recognise when you make an input too and damp this appropriately. What you feel is that the response is lower in magnitude but it gets to this steady state faster. You would describe this as the controls feeling crisp.
SAS actuators do operate on an inner loop to achieve this. They have their own internal sensors which sense the parameter (usually rate of change of attitude), formulate a response, drive the actuator and then sense the parameter. There's your inner (fast) loop. You want the actuator to response quickly (so it does not get into a PIO like you) but what if this ran away by accident? That is why SAS actuators are restricted to 5-15% of travel. However this means they can get saturated (reach full travel). Generally SAS will have some indication for the pilot so he knows this has happened and he can move the appropriate control...this helps put the SAS actuators back in the centre of their travel and let them work again. Some pilots won't like SAS as it has a damped feeling and they cannot achieve the rates their used to but they have to work harder.
I will follow on with autopilot in next post.
SAS = fast acting linear actuators with limited authority for rate damping.
ATT = slower acting parallel actuators with 100% authority than can re-centre themselves in an 'open-loop' condition needed for upper modes such as HDG, NAV, IAS.ALTA, BAR and RAD hold etc
ATT = slower acting parallel actuators with 100% authority than can re-centre themselves in an 'open-loop' condition needed for upper modes such as HDG, NAV, IAS.ALTA, BAR and RAD hold etc
So that's SAS (yaw is usually the first thing that's SAS'd following by pitch and roll but rarely collective). The actuator for SAS is in line with the controls so you cannot feel it working but it definitely moves the same control surfaces (in 99% of cases). They can be either electromechanical or hydraulic -mechanical in nature . They might be called SEMA or series actuators or something like that. Designers of aircraft need to demonstrate that a runaway of these fast acting actuators cannot cause such a significant upset that the pilot cannot safely recover from it.
Generally SAS takes a challenging or spritely aircraft and transforms it into a more predictable but still VFR machine. The sensors in the system usually sense or read the attitude rate of the aircraft so actually have no ideal what attitude you're in. However since it stops unwanted rates building up you get what's called pseudo attitude hold. It feels like it is holding your attitude but that's not strictly true.
Now with this system you can still have some sort of associated trim system. In anything other than small helicopters where you have hydraulic main rotor control, there's no feedback on the controls. So generally a trim system is included to: one, hold the stick where you leave it, two, provide artificial feel (so the pilot has feedback about how far he is moving away controls away from trim). This may then give the pilot the option of temporarily removing the forces (force trim release) and/or beeping the trim position (beeper trim) and/or moving the control to a new position through against friction (typically in yaw or collective). Some aircraft have all three optkons, some less, some different for different axes.
All these various trim options do is change the neutral point. It so happens that the trim system is generally also a core part of the autopilot which is where we go next.
Generally SAS takes a challenging or spritely aircraft and transforms it into a more predictable but still VFR machine. The sensors in the system usually sense or read the attitude rate of the aircraft so actually have no ideal what attitude you're in. However since it stops unwanted rates building up you get what's called pseudo attitude hold. It feels like it is holding your attitude but that's not strictly true.
Now with this system you can still have some sort of associated trim system. In anything other than small helicopters where you have hydraulic main rotor control, there's no feedback on the controls. So generally a trim system is included to: one, hold the stick where you leave it, two, provide artificial feel (so the pilot has feedback about how far he is moving away controls away from trim). This may then give the pilot the option of temporarily removing the forces (force trim release) and/or beeping the trim position (beeper trim) and/or moving the control to a new position through against friction (typically in yaw or collective). Some aircraft have all three optkons, some less, some different for different axes.
All these various trim options do is change the neutral point. It so happens that the trim system is generally also a core part of the autopilot which is where we go next.
Now autopilots. While our SAS (with trim) is usuay good for VFR, for IFR there are stricter requirements about how much deviation a SAS actuator runaway is allowed to cause and the amount of deviation that can occur when the pilot is distracted. In some cases IFR is allowed with just SAS but it might come with a dual pilot limitation (so one of you will notice the huge deviation before it gets you!). There may also be a second series actuator (usually in the pitch axis) which is there to cancel out a runaway in the other one (again the EC135 pitch damper achieves this).
But if you want to do proper IFR you need more. Typically the minimum entry point is a heading and altitude hold but that's getting ahead of ourselves.
What we want next is for the attitude to stay exactly where we left it while we rifle through the Jeppesen or calculate a hold. We basically want a lower workload. We want attitude hold (ATT).
This needs more sensors (gyros or AHRS for each axis) plus magnetometers usually. Now the system can sense the outside world. It now senses the attitude of the aircraft, it compares this to what you wanted, then I commands the SAS actuators we already have to "stabilise" the aircraft around the new datum. Then it repeats, what you see is the aircraft bank or pitch to achieve/restore the chosen datum within limits set for how much attitude can be applied to do this. So what about the controls?
Remember the saturated SAS from earlier? This issue is now (usually) automated. The autopilot makes inputs through the SAS, but the autopilot constantly assesses the saturation of those actuators and automatically moves the controls through the trim actuator I mention above to recebtre the SAS actuator so it can do its thing. This function is called Autotrim on some aircraft.
So the autopilot always flies the aircraft through the SAS actuators not the controls (myth dispelled). The trim actuators can of course move the controls through their full range. If this ran away it would be catastrophic if these actuators acted quickly. So they don't. But combine short throw fast acting series actuators with a long throw slow acting actuator and you have the best of both worlds. Of note the trim actuators are mounted in parallel to the control run and so are sometimes called parallel actuators or rotary trim actuators or similar. Where are axis doesn't have a trim actuator, the pilot is typically required to move the control periodically to recentre the series actuator.
Now there's one problem with the aircraft holding the attitude. What if you want to turn. Some aircraft absolutely don't recognise the pilots inputs: the series actuator does its best but saturates. The pilot continues to fight the controls. The autopilot commands the trim actuator to move the control to get the SAS actuator back in the middle of its range but you're holding the control. The spring in the trim actuator then gradually builds up a force. You then release the control and BANG all that pent up force is released...rock and roll! The A109E does this.
However better aircraft always recognise the pilot makes an input and suspends the Autotrim function and attitude hold in that axis - effectively that axis drops into SAS (this may be called SCAS). It then seamlessly resumes ATT on release of the control. Pressing force trim release does the same but also removes the artificartificial
the autopilot can build on this underlying attitude system by building progressively more advanced (higher order) modes which use the underlying ones. So for heading hold, the autopilot commands a smooth series of attitude datum changes to achieve a nice turn. The navigation mode in turn commands a series of heading changes to achieve a nav route.
So deck landings. You want a stable airborne platform as possible to reduce your workload. Even if the ship is moving you want to stay stable relative to the world. Therefore ATT would seem the logical choice PROVIDED the system recognises pilot input. In this case SAS may be warranted but probably working against the trim (IE Force trim release not held down).
However immediately after landing the autopilot, ATT, SCAS, SAS needs to come off (or be automatically inhibited by weight on wheels/skids) otherwise the sensors I mentioned will sense attitude or rate change and attempt to damp the entire ship. The ship will win and the rotor disc will keep moving to try to damp the ships motion. Hello dynamic rollover.
I hope that answers your question. Happy to relate it to a particular type or variant of needed.
But if you want to do proper IFR you need more. Typically the minimum entry point is a heading and altitude hold but that's getting ahead of ourselves.
What we want next is for the attitude to stay exactly where we left it while we rifle through the Jeppesen or calculate a hold. We basically want a lower workload. We want attitude hold (ATT).
This needs more sensors (gyros or AHRS for each axis) plus magnetometers usually. Now the system can sense the outside world. It now senses the attitude of the aircraft, it compares this to what you wanted, then I commands the SAS actuators we already have to "stabilise" the aircraft around the new datum. Then it repeats, what you see is the aircraft bank or pitch to achieve/restore the chosen datum within limits set for how much attitude can be applied to do this. So what about the controls?
Remember the saturated SAS from earlier? This issue is now (usually) automated. The autopilot makes inputs through the SAS, but the autopilot constantly assesses the saturation of those actuators and automatically moves the controls through the trim actuator I mention above to recebtre the SAS actuator so it can do its thing. This function is called Autotrim on some aircraft.
So the autopilot always flies the aircraft through the SAS actuators not the controls (myth dispelled). The trim actuators can of course move the controls through their full range. If this ran away it would be catastrophic if these actuators acted quickly. So they don't. But combine short throw fast acting series actuators with a long throw slow acting actuator and you have the best of both worlds. Of note the trim actuators are mounted in parallel to the control run and so are sometimes called parallel actuators or rotary trim actuators or similar. Where are axis doesn't have a trim actuator, the pilot is typically required to move the control periodically to recentre the series actuator.
Now there's one problem with the aircraft holding the attitude. What if you want to turn. Some aircraft absolutely don't recognise the pilots inputs: the series actuator does its best but saturates. The pilot continues to fight the controls. The autopilot commands the trim actuator to move the control to get the SAS actuator back in the middle of its range but you're holding the control. The spring in the trim actuator then gradually builds up a force. You then release the control and BANG all that pent up force is released...rock and roll! The A109E does this.
However better aircraft always recognise the pilot makes an input and suspends the Autotrim function and attitude hold in that axis - effectively that axis drops into SAS (this may be called SCAS). It then seamlessly resumes ATT on release of the control. Pressing force trim release does the same but also removes the artificartificial
the autopilot can build on this underlying attitude system by building progressively more advanced (higher order) modes which use the underlying ones. So for heading hold, the autopilot commands a smooth series of attitude datum changes to achieve a nice turn. The navigation mode in turn commands a series of heading changes to achieve a nav route.
So deck landings. You want a stable airborne platform as possible to reduce your workload. Even if the ship is moving you want to stay stable relative to the world. Therefore ATT would seem the logical choice PROVIDED the system recognises pilot input. In this case SAS may be warranted but probably working against the trim (IE Force trim release not held down).
However immediately after landing the autopilot, ATT, SCAS, SAS needs to come off (or be automatically inhibited by weight on wheels/skids) otherwise the sensors I mentioned will sense attitude or rate change and attempt to damp the entire ship. The ship will win and the rotor disc will keep moving to try to damp the ships motion. Hello dynamic rollover.
I hope that answers your question. Happy to relate it to a particular type or variant of needed.
the tip path plane moves just under SAS inputs without your stick moving. It is enough to take heads off when manoeuvring under the disc with any sort of deck movement
So deck landings. You want a stable airborne platform as possible to reduce your workload. Even if the ship is moving you want to stay stable relative to the world. Therefore ATT would seem the logical choice PROVIDED the system recognises pilot input. In this case SAS may be warranted but probably working against the trim (IE Force trim release not held down).
The question was about the 412 and the ATT mode, while you can fly through it easily enough, isn't as good for any form of general handling as SAS mode.
For all the military-type training we did on the 412, ATT was only used for IFR - everything else, including SAR work, was in SAS mode.
Pretty sure the 412 doesn't recognise pilot input in ATT, same as 109. I certainly remember the beeper trim being too slow for precise hover work and the force trim release causing way too much stick jump.
Surely it only jumps if you press the FTR while holding the cyclic against the force?
Pretty sure the 412 doesn't recognise pilot input in ATT, same as 109. I certainly remember the beeper trim being too slow for precise hover work and the force trim release causing way too much stick jump.
I certainly remember the beeper trim being too slow for precise hover work
If you use the stick trim properly - ie press it before moving the cyclic - you don't get the stick jump. Keep ATT mode for the cruise and IFR.
I suppose if one takes off the boxing gloves things go much smoother.
It makes for much ore sensitivity.
It makes for much ore sensitivity.
Ah. The recognise bit is sometimes called fly through mode on some types (eg airbus). The system has force sensing in the controls and it automatically changes mode from ATT to SAS in the appropriate axis while you are effectively overriding on the controls. On A109E and I seem to remember 412, when you make a control input with ATT engaged, the system never leaves ATT. The SAS/series actuators strive to re-capture the original datum against this interference. If you are using moderate angles of bank, they don't quite saturate so you're alright. However, at more spiritered angles of bank, the actuators can saturate thus can no longer stabilise in that axis. Suddenly no stabilisation at all in that axis and the flying task gets a step change in difficultly harder. Fly in the cruise and IFR in turns against trim and as you say it works fine. Try to fly low level tactically and it's horrible.
The stick jump in 412 is nigh on impossible to stop. It is the merest of pressure (even the grip you hold on the cyclic whilst rotating your thumb on the FTR that can cause it). With practice it gets less but if you put yourself in the shoes of a pilot used to say a squirrel, initially at least they'll be fighting stick jump on the 412 - and wearing holes in the thumbs of their pussers flying gloves I seem to remember but it's four years (exactly I think) since I was in one so not got one to hand to describe in technicolour.
The stick jump in 412 is nigh on impossible to stop. It is the merest of pressure (even the grip you hold on the cyclic whilst rotating your thumb on the FTR that can cause it). With practice it gets less but if you put yourself in the shoes of a pilot used to say a squirrel, initially at least they'll be fighting stick jump on the 412 - and wearing holes in the thumbs of their pussers flying gloves I seem to remember but it's four years (exactly I think) since I was in one so not got one to hand to describe in technicolour.
One trap in the autopilot of a pointy twin I used to fly:
When fully coupled in Nav/heading, Airspeed and Altitude. If flying in turbulence the collective can, to correct an altitude excursion in an updraft, drive the collective fully down leading to very high Rotor RPM. So you went down a level of automation and flew in Nav/heading and either Airspeed OR Altitude but not both. The autopilot does not give any protection for high Rotor RPM. It was the first thing I was warned about when I got my check on type. In fact I don’t know of any autopilot that does.
They joy of punching in heading on an early Sperry without centering the heading bug first was exhilarating too. Also if you were on a heading of say 090 and ATC told you to turn right to a heading of 300 you would turn the bug right and the aircraft would start to turn right but if you spun the heading bug too fast to 300 and the aircraft heading had not yet passed 120 Mr. autopilot would decide it would be quicker to turn left to 300 and reverse the turn. The Sperry wanted to turn the fastest way to the selected heading.
When fully coupled in Nav/heading, Airspeed and Altitude. If flying in turbulence the collective can, to correct an altitude excursion in an updraft, drive the collective fully down leading to very high Rotor RPM. So you went down a level of automation and flew in Nav/heading and either Airspeed OR Altitude but not both. The autopilot does not give any protection for high Rotor RPM. It was the first thing I was warned about when I got my check on type. In fact I don’t know of any autopilot that does.
They joy of punching in heading on an early Sperry without centering the heading bug first was exhilarating too. Also if you were on a heading of say 090 and ATC told you to turn right to a heading of 300 you would turn the bug right and the aircraft would start to turn right but if you spun the heading bug too fast to 300 and the aircraft heading had not yet passed 120 Mr. autopilot would decide it would be quicker to turn left to 300 and reverse the turn. The Sperry wanted to turn the fastest way to the selected heading.
Last edited by albatross; 1st Aug 2022 at 19:53.