SAS and ATT mode
 

can anyone tell me the difference between the SAS mode and ATT mode in detail?

Thanx in advance.

SAS, short term stabilizazion.
ATT, long term stabilization and requirement for coupled operations.

Just like the 412.

Yes, tottigol has it right. ATT mode fully couples the system to the aircraft's gyros.

In SAS mode there is no "right way up" or heading datums, only stabilisation of aircraft attitude.

Its a little more complicated but essentially that. In SAS mode you can equate it like you, the pilot is the stabilization system. For example in the Jet Ranger, when you hover, you have to fine tune your cyclic and collective to keep the aircraft in a steady hover. This is SAS. Now let's add wind to the simple hover and you start to drift.To stop the drift and get back to your original hover position, you put cyclic into the wind and a little collective to compensate for the loss of lift and pedal since you added collective. This is ATT, or attitude retention. In aircraft with both SAS and ATT, if you only have SAS on and you move the controls, the aircraft will not hold the attitudes you have put the aircraft into. In ATT mode, you press your cyclic and collective trim releases put the helicopter in a nose up with a right bank and let go of the trim sync releases, the ATT mode will take those datums you set and try to hold the aircraft in that attitude (always to limits, and in some aircraft through limits, engine and/or torque). SAS won't do that for you. Now if we go one step forward and add flight director, then it changes those datum values to ones that you set through your cyclic and collective "hats". The ATT will achieve those new datums and the aircraft moves. Hopefully that helps. One could go on with describing the series and parallel actuators and there relationships, auto-trim, etc. but that is for the classroom, n'est pas?;)

In SAS mode suppose you are in a hover, thus the system is continuously monitoring aircraft attitude and pilots movements, if you don't move the cyclic but the aircraft moves a few degrees from it's previous pitch and roll attitude because of an outside interference like a gust of wind, then the computer says . . . . "hey the aircraft moved, but the pilot did not move the controls, so it must be an outside interference, so I will use my actuators to return the aircraft to the position where it was before" Voila! Stabilization!!

In ATT mode, suppose you are flying straight and level and to hold straight and level the aircraft attitude is 2 degrees nose down and wings level, as soon as you release the force trim button, that aircraft attitude is "photographed" and the computers thru the actuators will hold that 2 degrees nose down and wings level attitude, which will make the aircraft more or less hold that same flight regime, it will not hold Altitude or Heading like an autopilot, but at least it will keep it still for a few seconds, in ATT mode you can make minor adjustments thru the "chinese hat" say you bump it a couple of times, that will make the attitude increase a certain amount.

A lot of newbies (Specially Robinson Newbies) to the Agusta or Bell 412 for example will fly the aircraft and keep stirring the cyclic, making a lot of control inputs, minor adjustments to keep the aircraft steady, this drives the SAS computers crazy since while the pilot is stirring the cyclic, the computers can never really get to work on stabilizing the attitude, these pilots discover that if they just stop stirring the cyclic and realize that this new aircarft does all the small adjustments for you, the aircraft becomes rock solid.

Could we make it a bit simpler yet.....SAS is a "rate" controlled response and ATT uses an "attitude reference". (Thinking Sperry systems....)

SAS tries to keep the pitch, roll, and yaw movement rate equal to "zero". The system senses a rate of movement and applies a countering input to return the rate of movement to zero.

ATT tries to retain a particular attitude reference selected by the pilot. The system senses a displacement from the selected attitude and applies input to return the aircraft to the selected attitude.

SAS mode can be used with Force Trim selected "OFF" or "ON"

ATT mode requires Force Trim "ON".

In SAS mode, the Beep Trim control is inoperative, thus the pilot must manually reposition the cyclic for attitude changes.

In ATT mode, the pilot can either use the Beep Trim Switch to adjust the attitude reference, or reset the attitude reference by releasing the Force Trim momentarily and moving the cyclic to obtain the desired attitude, then set the attitude reference by releasing the Force Trim button.

Minor adjustments or temporary adjustments of attitude can be done by manually over-riding the Force Trim system once the new attitude reference is set.

By all means.....let the helicopter do as much of the flying as it can for you.

I flew with the Force Trim "ON" at all times....and used ATT almost continuouslly. I still have a big ol' callus on my right Thumb!

Some good answers I think. To give a practical example that explains the previous posts, suppose you are flying along in straight and level at 100 kts and 2000 ft, with wings level and 0 degrees of pitch attitude, and encounter a sudden nose up disturbance (wake turbulence, for example.) The aircraft response will be as follows:

a) No SAS or ATT system - the aircraft will pitch nose up, and will continue to do so until a pilot input is made.

b) SAS system fitted - the SAS will sense the rate of attitude change and attempt to counter it, so eventually (quite quickly) the nose will not be rising or dropping. However, the will be a new pitch attitude, that will be higher than previously, and will lead to a deceleration and climb until the pilot adjusts the attitude himself.

c) ATT fitted - ATT systems have SAS as an integral part of their function, so the SAS will counter the pitch up, as described above, and then the ATT system will further adjust the aircraft attitude to wings level and zero degrees pitch. This pitch attitude will result in the same IAS - more or less - but the aircraft will have climbed and will now fly level at some height above the original altitude.

d) ATT with Upper modes - if you have a coupled autopilot with Altitude and IAS hold, you will get a combination of rate sensing, attitude sensing and external barometric data, all combining to give continued flight at the original datums.

Hope that makes sense!

IIRC a simple SAS takes the displacement information from the gyro, integrates it giving you displacement over time = rate and then opposes that rate using a feedback loop to modify the output. The stabilisation is therefore primarily achieved by the means of rate damping thus reducing pilot workload.

Our 4 axis autopilot has force sensing links in the control run so that the ASE computer can differentiate between pilot input and real error.

I think you are mixing two concepts together. A SAS would normally use a rate gyro which is giving the rate signal directly. A secondary input could be from an attitude gyro signal which the SAS computer differentiates to get rate (dθ/dt).

An ATT system would normally use an attitude gyro signal directly, but could use the rate gyro signal as a secondary input, which it then integrates to get attitude.

Some confusing information given earlier.
Both SAS and ATT require force trim to be on to work. If force trim is off, the SAS actuators will move the cockpit controls instead of the rotor head.
SAS typically will only damp out rates.
Force trim can be used to re-datum the attitude being held. But holding the force trim button down disables attitude hold.
If the stick is moved with the force trim engaged (and the force trim button is not held down), the helicopter will move, but return to the datum when the stick is returned to the original position (i.e. the forces on the stick are released).
If anyone wants a document on how to demonstrate the various modes to gain a better understanding of how these systems work, please pm me.

That might need some clarification Shawn - clearly it is not the case that an aircraft with the trim turned off (or FTR depressed) is no longer stabilised by the SAS.........I'm assuming you mean the pilot's grip becomes the force against which the SAS actuator works?

212man.....you saved me from getting confused.....otherwise the SAS light would not illuminate showing it is working (or at least selected) with the Force Trim selected off by the pilot's thumb.

Given the SAS system acutators only have about ten percent authority then it would follow the pilot's death grip on the cyclic would suffice to hold the thing against the action of the actuators as Shawn states.

Actually, the cyclic stick does not move in either mode even when the SAS Beep Trim is actuated.

212man - I clearly didn't remember correctly:ok: Now I think about it the Gazelle SAS used rate gyros as you describe and the Sea King AFCS uses differentiated attitude info to give rate damping. Doh!

Mark the Calendar Lads!

Strewth.....Crab admitted to an error!

He must be mellowing in his doterage.

Hi Shawn

On a number of Eurocopter aircraft, the series actuators are down stream of the 1st stage hydraulic boost (autopilot hydraulics) so that they are working against a hydraulic lock unless the pilot moves the stick. On these aircraft, SAS works with or without force trim turned on, with or without the pilot holding on to the cyclic (though not holding on to it in SAS mode is not a good idea for too long!). With autopilot hydraulics turned off, there is considerable resisitive force built into the unpressurised hydraulic pack - again the SAS actuators will move the head servos rather than feed back into the cyclic even if the pilot is not holding the cyclic.


As others have said, SAS is all about the system generating control inputs to resist rates of pitch, roll and yaw. Whether the input data is from rate gyros, FOG/laser gyros or differentiated attitude/heading doesn't really matter.

ATT is about the system generating control inputs to maintain a particular pitch and roll attitude, so that when the pilot releases the controls, the pitch and roll attitude revert to the memorised datums. Typically those datums can be adjusted to new values using the cyclic coolie hat and/or by temporarily releasing the stick trim (system sets the datums to the values current at the moment the trim is reengaged)

HC

Helicomparator:
Just goes to show that when you think you know nearly everything, something comes along to screw it all up. I guess the lesson is that you really need to know your system in order to know what is going to work when!
And at the risk of really confusing things, there is often a difference between what the R&D community calls an attitude hold, and what the rest of the world calls it.
Know your definitions clearly!

On the S76, I would fly a takeoff and landing with force trim off, SAS on, Cruise and descent was FT On, and some of the modes engaged (usually HDG and ALT hold).

Stability augmentation was definitely in operation with trim off, demonstrated by turning off both autopilots and watching it try to turn itself inside out above 110kt.

Same in the 412, trim off, SAS on - unless it was night time, :eek:when it was trim on, ATT on for takeoff and climbout.

Tiger, they confused you, didn't they?

SAS, short term stabilization, YOU fly the helicopter with or without force trim (a lot more fun).
ATT, long term stabilization and requirement for coupled operations, the helipilot computers fly the helicopter, the force trim HAS to be ON to couple the Flight Director.
ATT is the required mode for IFR flying.

Just like the 412, provided you are talking about a different helicopter, like the AW-139.:ok:

Why not fly in ATT mode all the time, with the force trim on?
The helicopter is much more 'stable', and it won't be any different when you turn the upper modes on. As for the control forces, in any helicopter I've flown (which is most of them), they aren't onerous. Learn how to fly with those forces - i.e. in the hover, having trimmed the stick, make adjustments without re-trimming by just holding the forces for the brief time you need to have the stick away from trim.
You can hover just as accurately as with SAS mode and FT off, and with a lot less control activity.
You have to learn how use the system, not just fly a new helicopter like it was a Jet Ranger or other light helicopter.
But the main point is to use the AFCS / autopilot the same way all the time, not just plug in ATT mode when you're IFR.
No fixed wing airline operation would allow their pilots to choose whether they used the yaw damper and autopilot or not. Why are helicopters any different?

Shawn,
one pilot's "fine control" is another pilot's "stick stirring" !

Personally, I'm with you and Saless on this one. I have never seen the point of turning the trim off.

Shawn - why? ..............
 

...........because they are helicopter pilots and are therefore 'onnery, awkward and self-opinionated and mostly don't operate in a disciplined environment.

A written SOP in helicopter company is scarcer than sunny day in Ireland - not including all those well behaved offshore guys in that statement of course!

G. :ok:

Turbulence, Autopilots
 

Hi all.
I've read a lot of interesting stuff in this thread about Autopilots, SAS and ATT modes. I've a little experience with French Autopilots, and with the older Bell Sperry types. (say about a thousand hours).:O
Most of these had an Autopilot disengage plate on the cyclic. There is another thread about whether to fly "through the AP" that is , against the cyclic force, after trimming the helicopter for level flight, or to use the hat to trim. (http://www.pprune.org/rotorheads/834...med-turns.html) It was a very interesting discussion, especially because I have had different instructors telling me to do the exact opposite of each other, depending on what they'd been used to flying. Those flying the Russian Mi-s would insist on using the FTR and the ones trained on the SeaKing in the UK would be ready to throw you out of the cockpit if you ever touched the FTR button on the cyclic. In the Russian APs, there was no 'coolie hat' trimmer, and the only way to change the attitude was by the use of the FTR. This was also essential when you needed to get the 'ball' in the centre.
I understand from the Trimmed Turns forum that the 'fly through' or not, has a lot to do also with the way the particular Autopilot is designed. Nick says in his post that " The type that Sikorsky generally builds uses a "fly through" concept, where the pilot can move the stick against trim, and some degree of smooth acceptance by the autopilot keeps things stable". I have had this experience in the French APs including the one in the Alouette III - it was called AP146, and did a pretty good job. The Bell , on the other hand, tended to react pretty badly if one ever tried to fly through the AP. In the Simulator, I had a high time Bell 412 pilot telling me to "Tap-Tap-Tap" when in a turn. It took me some time to understand that he was asking me to tap on the FTR switch on the cyclic in the turn - and all the time, I was flying against the pressures!
Another difference I notice is that most Autopilots are meant to be switched off when taxying on the ground. Any idea why some are not? In some of the Agusta models, there is no Quick Disengage plate on the cyclic, and one has to pull the switches into the off position. Couldn't get any satisfactory answer at the Factory too. I have seen a kind of Camel ride on a Bell 212 where the Pitch channel had a problem. Disengaging the SAS and re-engaging it without the Pitch channel worked wonders.

Now, my question is this... You're flying VMC, good weather and all, with the basic stab ON, (no higher / upper modes or holds engaged), and you encounter some clear weather turbulence (say flying over hills and the like). The turbulence is a bit disconcerting, and of course sends the ball all over the place too. Would it be better to fly with the Force Trim off? This way, with hands-on flying, perhaps the ride would be smoother, in particular with the rudder control. Finer control of the attitude and roll in turbulence would be smoother by hand. My experience is that the ride did turn out smoother. Of course, it is a personal observation, and I may be only perceiving an improvement in ride quality!!

(Many high-time Bell pilots I understand, prefer to fly with the Force Trim off in all cases, unless of course in IFR.)

Any takes on this?

Weight on wheels logic cuts it out totally or puts it in SAS mode.

Sounds like a good way to induce an out of phase PIO! The mass of your arm will be subject to the turbulence forces and will become part of the control loop.

If the Ball remains in the bottom of the curved tube.....and never varies....how does one know it is working?

In the scenario you provided....rough air, VMC, and a bit of Turbulence....I would leave the Force Trim engaged, and if in SAS mode hand fly the machine. The autopilot will "dampen" the attitude variations but not return the aircraft to a preselected attitude.

If you use the ATT mode....the aircraft will return or attempt to return to the preselected attitude. You could fly through the autopilot and "assist" it in that task.

Shy of the ball lodging in one corner....I would not be too concerned about it moving about a bit as it is only indicating some Yaw or Roll deviations that result in short term untrimmed flight.

I am an advocate for using the Force Trim all the time....and living with the Calloused Thumb.....set it, click it, tap tap tap....however you must to get the desired result but let the Autopilot fly the machine as much as possible.

Each autopilot system is a bit different in design and function so one has to learn to use them to the best effect.

I also advocate using a bit of cyclic friction to assist in "smoothing" the ride. Sperry Helipilot systems on Bell 212/412's
required the use of a minimum friction setting to reduce cyclic oscillations caused by turbulence or maneuvering.

My experience on the 412 was that the ATT mode was easily flyable at all times, even hovering and touchdown. You had to get used to flying against the forces in the stick, which took a bit of adapting.
But every system is different!!
Learn what it is supposed to do, and then try to learn to use it to best advantage.
Anyone who wants a sample 'learn how your AFCS works' screed I wrote a while ago, please PM me. Good place to start, even if it's not exactly how your system works.

There are significant differences even within one product range from one manufacturer. With the AS332L Super Puma, when flying against the trim for short term over-rides (eg in turbulence), the aircraft was quite stable and there tended to be a fairly direct relationship between stick displacement and pitch or roll attitude. On the EC225 as soon as you operate against the spring trim it goes into a kind of SAS mode and is fairly unstable, especially when you add in the light cyclic forces and the non-linear forces that arise from the breakout force from the trimmed position. All in all it is not easy to fly against the springs in the cruise and I advocate using the cyclic trim release button (or more sensibly be fully coupled with upper modes!)

So I think the moral is, try the different methods and then stick with the one that works best for you on your particular machine.

HC

412 AP SAS/ATT
 

Shawn,

Please correct me if I am wrong but I believe that in the 412 with ATT Mode selected, the ATT Mode is disengaged whenever the Force Trim button is pressed and the AP reverts to what is known as "Modified SAS Mode" therefore you are actually not really flying in ATT Mode. Also earlier SASless said that the cyclic does not move when Beep Trim is operated in ATT Mode.
I believe that this also is not strictly correct and the cyclic does actually move to allow the control actuators to recenter.
In ATT Mode the cyclic and force trim are used to set the Attitude datum in the AHRS (or TARSYNS on older models) and effectively the cyclic position becomes an integral part of that set datum. Moving the cyclic against the force trim in ATT Mode although it works to move the flight controls is once again abusing the designed use IMHO!

Agree 100% - I constantly see people unecessarily screw up by fighting a system that they do not fully understand even after they have been flying it for ever!

But I wait to be corrected!

Trog

Trog:
My understanding of the 412 system is that in ATT mode, when you move the stick, you are temporarily disabling the system's ability to hold the datum attitude.
The way to see this is in the hover, with ATT engaged, just move and hold the stick slightly away from trim and see what happens. When the stick is returned to trim, the system should return (within reason, as it's not that tight a system) to the original datum.
So, in the hover, if everything is set up nicely, you can make minor corrections without using the Force Trim by merely flying against the forces for brief periods.
If you want more on how demon how the system works, PM me.

412 SAS/ATT
 

Shawn,

I agree that you are temporarily removing the systems ability to maintain attitude by moving the cyclic and do not disagree for a second that it is entirely possible to accurately hover or manoeuvre using this method.

My contention is that the ATT system works by extending and contracting actuators which need something to push against to do their job of passing that movement to the flight controls.

The way that you are temporarily removing the ATT part of the system is to move the bit that the actuators push against i.e the cyclic held in place by the force trim! As you move the cyclic (without pressing the trim release and selecting Modified SAS Mode) you are not disabling the input from the AHRS and the actuators are still moving but now have nothing to push against so end up in an incorrect position - perhaps explaining why the cyclic/flight controls are in a less than perfect position (not the original datum) when you release them

Not sure that is what Mr. Sperry originally intended which is why he fitted a trim release button.

Trog

I flew the 412 in ATT mode all the time. If you need to change the attitude, just press the FTR briefly. You don't want ATT mode engaged on the ground, since you can get interesting results, but I kept it on from just before liftoff until at flat pitch after landing. In the S76, I just moved the cyclic as necessary, without worrying about pressing any buttons. That's one reason the S76 is more fun to fly, you don't have to worry about tiring your thumb out. I've never met a single pilot who would get in a 412 if (s)he had a choice of an S76.

I had a numpty TRE in the Gulf that used to insist that in ATT mode (during IF check ride) I should desist in using the FTR button to make minor attitude adjustments and only use the chinese hat..... :ugh:

The AHRS loved that and IIRC moved the cyclic back to its original datum. To compound the problem the CH was as slow as a snail, so you were constantly chasing the datum :cool:

click,click,click, every trip :ok:

what about on a floater
 

Hi,
For the folks out here who land on floaters ( ships) , ,on touch down, do you revert to the SAS mode from ATT, or do you switch the APs OFF. This is for the 412. I ask because on the Seaking which I flew in the Navy, we'd select STAB OFF on touch down on decks, if we were going to be there for a while.

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.

When weight on wheels is detected a lot of systems disengage on the S92. Including the ATT functions.

This is some thread resurrection!

You lot don’t know you are born. As an Army Air Corps Gazelle pilot in the 1980’s the only thing that SAS meant was Special Air Service! It was just the pilot versus the helicopter in our poor Army.
Apparently the RAF would place their Gazelles U/S if the sas was not functional 😳

I taught on both the HT3 (RAF Gazelle) and the AH 1 (Army Gazelle) - SAS and stick trim were good to have but not essential it is just a great aircraft to fly.

Hnnnh - the RAF (with Whirlwinds) got their STAB and ATT from friction increase or clamping cyclic between knees or holding collective with left knee ... difficult to do both at the same time !! :cool:

...this could descend into a Python sketch ! ;)

I never flew th B412 but did fly B212s, with three different AFCS systems, and our SOP was to 'turn off' for every landing - onshore or offshore. It was using the right 'pinky' on a cyclic button as I recall.

The RAF Shawbury Gazelles had a SAS light but it didn’t really do anything. It was just a confidence light….

I flew the same 212's as did 212man.....and my memory tells me the following.

The SFENA system was rarely serviceable thus was of not much value. When bored you could try to determine if it was working or not or what part was or was not.....but other thantaat it was useless.

The Bell SCAS system was designed by Bell for use in the AH1-G Huey Cobra....which I do not recall seeing installed in our fleet of 212's ....but I used it at other places...was pretty good stuff for what it was....reliable and easy to use.

The Sperry System was excellent kit...reliable..easty to use and the only three of the systems that could incorporate a Flight Directer and was the same on the 212 and 412.

At the Operations we flew the same aircraft, 212man and I, we did not have the luxury of the Flight Director as they had been removed or not installed for those contracts.

Due to my background flying US Army Chinooks where the Force Trim system was ON all the time....I was quite happy to us ATT Mode all the time and use the Cyclic Trim Button to reposition the Cyclic.

I engaged it just prior to TakeOff and went to SAS mode after landing....or turned it off all together if it was the final landing if shutting down,

When landing on Moving Decks....I turned the AFCS OFF.

My view was this....as the AFCS could assist in maintaining a constant attitude....then why not use it. Set the attitude you desire/need....release the Force Trim Button....thus setting the datum...and let the helicopter do its part.

The other lesson learned was never buy SFENA.