SAS and ATT mode

[tiger 139]

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

Thanx in advance.

[tottigol]

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

Just like the 412.

[ShyTorque]

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.

[SARBlade]

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?;)

[BlenderPilot]

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.

[SASless]

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!

[212man]

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!

[[email protected]]

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.

[212man]

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.

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.

[Shawn Coyle]

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.

[212man]

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.

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?

[SASless]

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.

[[email protected]]

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!

[SASless]

Mark the Calendar Lads!

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

He must be mellowing in his doterage.

[HeliComparator]

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

[Shawn Coyle]

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!

[Ascend Charlie]

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.

[tottigol]

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:

[Shawn Coyle]

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?

[212man]

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.

[Geoffersincornwall]

...........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:

[PolarisPilot]

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/83405-trimmed-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?

[212man]

I notice is that most Autopilots are meant to be switched off when taxying on the ground. Any idea why some are not?

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

Would it be better to fly with the Force Trim off?

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.

[SASless]

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.

[Shawn Coyle]

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.

[HeliComparator]

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

[Troglodita]

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!!


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!


Learn what it is supposed to do, and then try to learn to use it to best advantage.



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

[Shawn Coyle]

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.

[Troglodita]

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

[Gomer Pylot]

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.

[griffothefog]

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:

[Darthvader5000]

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.

[[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.

[HeliMannUK]

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

This is some thread resurrection!

[luckyrat]

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 😳

[[email protected]]

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.

[Cornish Jack]

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 😳
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 ! ;)

[212man]

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.
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.

[PlasticCabDriver]

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.

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

[SASless]

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.

[Darthvader5000]

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.
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

[[email protected]]

The RAF Shawbury Gazelles had a SAS light but it didn’t really do anything. It was just a confidence light… So those three switches at the bottom of the console marked Pitch Roll and Yaw were for show too then............

[[email protected]]

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 Not so much an opposite force in pitch/roll but a rate damping effect between pilot control input and the AFCS output to the rotor to improve handling qualities.

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!!:ok:

[Ascend Charlie]

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

Well, that's weird, the S76 and B412s I have flown used SAS just to smooth out the hand flying (no attitude hold or any kind of other autopilot function) and no Force Trim on, and in ATT mode, (force trim must be on) it held the last attitude you had when you took your finger/thumb off the force trim release button.

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.

[ring gear]

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.

https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/2000x1504/dscn0713_copy_053df433e7487cabe2477b36248c62b57b946bb9.jpg
https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/1024x768/dscn1613_614c37c6d2eaa7b0ecbda5a5db95d8f01b00628a.jpg



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

[ShyTorque]

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

That depended on how hard you gripped the controls.

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.

[SASless]

By disengaging the SAS/APLT, all you have done is killed the "brains" which are easily fooled by outside disturbances.

That notion could be applied to a lot of helicopter piliots I have known in my career.

Ring Gear provides good information in his post.

[gipsymagpie]

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.

So I would start from scratch and dispel a few myths here. For one the autopilot does not "fly" the aircraft through the trim motors. I will explain.

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.

[[email protected]]

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

[gipsymagpie]

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.

[gipsymagpie]

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.

[megan]

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 movementIndeed it will, you'll find where the blade stops are on the head by the banging noise if deck movement is sufficient.

[[email protected]]

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). I would definitely recommend SAS rather than ATT for deck landings especially in difficult conditions. You can trim the aircraft to a decent hover before you move in but you want crisp control response to deal with the ship movement. On a flat calm day then ATT would work fine but for anything else I'd go for SAS - including at night.

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.

[gipsymagpie]

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.

[212man]

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?

[gipsymagpie]

True but the force gradient on 412 is quite steep in ATT so it only takes a small pressure to end up with stick jump.

[[email protected]]

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. In ATT mode, you can roll into a turn against spring pressure and then release and it will roll you out again - you can beeper trim it into the turn and it will stay there. I don't understand what you mean by 'doesn't recognise pilot input in ATT'.

I certainly remember the beeper trim being too slow for precise hover work That's why you would use SAS mode for that.

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.

[SASless]

I suppose if one takes off the boxing gloves things go much smoother.

It makes for much ore sensitivity.

[gipsymagpie]

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.

[albatross]

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.

[[email protected]]

The recognise bit is sometimes called fly through mode on some types (eg airbus) ah yes, familiar with that terminology from the 139.

i think 'spirited' angles of bank would have to be in excess of 30 degrees and much more than you would use IFR but I take your point. The Sea King Mk 3 was very noticeable when the ASE saturated in roll (no ATT mode on Mk 3) giving an exaggerated control response but that was effectively going from SAS mode to no mode instead of ATT to SAS.

Agreed, jumping from a 350 into a 412 requires a very different stick trim technique.

[Sir Korsky]

This is the first decent thread on here for a while - even though that it's been resuscitated. Good stuff folks and keep up the input.

[[email protected]]

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. Yes a lot of 'autopilots' do that - the 412, the Sea King and the Dauphin for starters. :ok: always entertaining when a student forgets...

[megan]

Never ever experienced or heard of the "jump", never ever used the coolie hat other than to set a ROD, flew the 76 and 412 coupled en route and reverted to trim off when hand flying, VFR operation, cough, cough.

[SASless]

Due to my time teaching in Simulators....76 and 212/412.....It was plain that far too many Helicopter Pilots did not understand the Sperry System or in some cases have any clue as to how to fly instruments. They perhaps understood procedures and things that could be learned by rote memory.

In my Sim....if you failed to brief for the Missed Approach...it was dead certain you would not see the airport as with a click of a a button I made it dissapear from the visual display.

How many times did I watch two well qualified and experienced pilots kill themselves upon realizing they had to actually perform a Missed Approach.

They punched the Go Around Button and started scrambling to read the Approach Chart to figure out what to do.....and did not adjust the Collective to apply Climb power.

The pitch attitude changed....the VSI showed a climb....and their noses dropped as they both studied the Approach Chart....and some point the airspeed dropped to a point where the Autopilot threw up its digital hands and surrendered to Allah's Will.....and the pilots died a virtual death.

At that point...we broke for Coffee....I handed them a video cartridge and asked them to watch it while they had their Coffee and I would join them in a few minutes.

In variably the later sessions went much more smoothly as the teaching point. had been made.....the Simulator Training that their employer was spending lots of money was so they could learn....and improve on their skill levels.

We used to set the aircraft up for an ILS Approach...with the "Upper Modes" armed.....and we watched the aircraft fly right down the ILS without a bit of deviation....with an engine shutdown and a fire warning light showing on the other and several other Caution Lights illuminating with the Master Caution Flashing each time.

Teaching point....the machine can do it....why can you not?

Perhaps if you let the machine do it....and stay in the Loop by executing command authority over the machine....the machine would help you in flying the machine.

But.....you had to understand the machine before you could get full use of. it....and not cause yourself a problem.

Sometimes adherence to a set procedure but not fully understanding the aircraft systems could cause you serious problems.

Some Operators came to us thinking their Standard Procedures were very carefully thought out....but were not.

We had the advantage of seeing many different approaches to the same problems/issues plus having direct access to the Manufacturer's experts.

I sense some of that in this thread.

Sometimes one has to sort out the wheat from the chaff.

[gipsymagpie]

Ah, the joys of Go Around mode on a three-axis (ie no collective) autopilot where there is no speed protection. A certain recipe for unexpected death in an IFR go around for the unpracticed. The really annoying bit is that generally the go around mode is linked to vertical speed. So your left hand is not actually controlling the rate of climb but rather your airspeed. And naturally of course your focus at low altitude IMC is on the altimeter and not the airspeed. You see the initial pitch up from the GA and then you get onto other things and miss the speed washing off .

I personally teach (on a three axis system) that you've got four options (GA (lethal), ALT ACQUIRE (not much better), IAS and reverting to ATT). The latter seems the most natural (power directly related to going up) but IAS works the best overall for a 135 at least.

On the topic of workload in the go around, due to having access to a decent FMS/GPS (GTN750) I teach to use the GPS coupling for the missed approach and just wait until your in the (automated) hold before even thinking about what's next. Needs practice though

[gipsymagpie]

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.
The H145 Helionix AFCS has protections against inadvertent coupled entry into autorotation (and so by inference protection from NR split off). Also has a handy marker on the otherwise diabolical FLI that shows where NR split off will occur (the FLI is a collective pitch gauge so zero pitch is not zero torque) The latest version (for D3) also has protection from droopping the NR when OEI. I presume there are similar protections on similar Airbus Helionix types.

They joy of punching in heading on an early Sperry without centering the heading bug first was exhilarating too.

The heading bug defaulting to North on early EC135 led to the loss of a Scottish Police aircraft (not Clutha). I hope to high heaven any future autopilot won't have this "feature".

[gipsymagpie]

Never ever experienced or heard of the "jump", never ever used the coolie hat other than to set a ROD, flew the 76 and 412 coupled en route and reverted to trim off when hand flying, VFR operation, cough, cough.
Here's a proper description of stick jump from Eric Fitzpatricks book on flight test in the chapter about flight control characteristics:

Where a force trim release button is fitted the sudden release of the force will oftencause the control to ‘jump’ as the pilot changes the amount of force he is applying. In the case of the cyclic this is known as stick jump. To the pilot this can become irritating, as it is not possible to make small changes to the datum trim position. To test control jump, the control is displaced from the trim position, the release operated and the reaction of the control noted. The amount of displacement used for this test is a realistic amount that an operational pilot might employ before operating the release. After ground tests, an airborne assessment is made to determine if any control oscillations cause an undesirable aircraft response.

The Bell 412s I flew needed a microscopic amount of cyclic displacement before the dreaded stick jump. A109E not much better

[[email protected]]

It is no more than a mild irritation though and becomes almost imperceptible when you become accustomed to it.

Personally, I never fly stick trim off, even ASE out as I much prefer a 'datum' position to work around rather than a floppy stick. It helps prevent overcontrolling, especially IMC.

I was flying a Wessex yesterday which has no ASE at the moment and with stick trim on you can at least relax your grip on the cyclic a little.

[SASless]

Gipsy,

One can always lead with power.....when the decision is made to go around you invariably want to fly up....adjust collective to climb power....adjust the pitch attitude either by means of the GA Button or as you suggest....manually using force trim on and ATT Mode.

After you are established in a climb....the rest of procedure can be dealt with.

A note....the canned Missed Approach is fine if that is the same as the clearance you were provided as there can be non-standard instructions given.

Now some Sky God shall come along and want to argue which action should be "first"....but either pitch or collective is fine...but for sure the collective must be adjusted to climb power or all the other actions will not matter much.

It worked in the Huey with no SAS or AFCS....and every other dinosaur aircraft.

[PlasticCabDriver]

Ah, the joys of Go Around mode on a three-axis (ie no collective) autopilot where there is no speed protection. A certain recipe for unexpected death in an IFR go around for the unpracticed. The really annoying bit is that generally the go around mode is linked to vertical speed. So your left hand is not actually controlling the rate of climb but rather your airspeed. And naturally of course your focus at low altitude IMC is on the altimeter and not the airspeed. You see the initial pitch up from the GA and then you get onto other things and miss the speed washing off .

I personally teach (on a three axis system) that you've got four options (GA (lethal), ALT ACQUIRE (not much better), IAS and reverting to ATT). The latter seems the most natural (power directly related to going up) but IAS works the best overall for a 135 at least.

On the topic of workload in the go around, due to having access to a decent FMS/GPS (GTN750) I teach to use the GPS coupling for the missed approach and just wait until your in the (automated) hold before even thinking about what's next. Needs practice though

Aircraft like a 175 is 4-axis pretty much all the time, but failures like collective trim fail etc will lead to a 3 axis approach (often happens in the sim, wonder why…?), so we try to mitigate in the brief: “on Go Around I will use the GA mode but will be controlling the IAS manually with collective, please pay particular attention to the IAS and prompt if it is reducing” or words to that effect. Helps to be 2 pilot crew of course.

[gipsymagpie]

Gipsy,

One can always lead with power.....when the decision is made to go around you invariably want to fly up....adjust collective to climb power....adjust the pitch attitude either by means of the GA Button or as you suggest....manually using force trim on and ATT Mode.

After you are established in a climb....the rest of procedure can be dealt with.

A note....the canned Missed Approach is fine if that is the same as the clearance you were provided as there can be non-standard instructions given.

Now some Sky God shall come along and want to argue which action should be "first"....but either pitch or collective is fine...but for sure the collective must be adjusted to climb power or all the other actions will not matter much.

It worked in the Huey with no SAS or AFCS....and every other dinosaur aircraft.
Interesting thought but actually from a coupled 3 axis approach (eg ILS) it very much does have to be upper mode change first, power second.

If you pull power while still coupled on the approach, you merrily accelerate towards the ground. "PITCH MODE->POWER is very much the mantra we have to use. I have seen some spectacular collisions with the runway when power is pulled first, the pilot language deteriorates to ever stronger expletives, then they fail to step back through the automation gracefully (ie they fight the modes).

It may not work or be necessary for some AFCS installations (particularly those where collective always controls vertical modes) but in earlier Airbus 3 axis machines or later machines in 3 axis mode, the mantra is essential in my opinion.

But this is really thread creep. Back to SAS and ATT!

[SASless]

Gipsy.....what airspeed do you use for an ILS Approach?

One single set speed for all approaches or do you use different speeds for different conditions?

How do those approach speeds compare to Vbroc and Vy airspeeds for your aircraft?

We also have to consider all of the possible ways an ILS Approach may be flown...hand flown using raw data right on up to by means of a full capability four axis autopilot system.

That can bring you back to your desire to talk only of SAS/ATT.....is one mode better than the other for a raw data approach or while using an uncoupled FD?

[[email protected]]

It may not be strictly on topic but is a valuable topic of conversation.

A common theme seems to be that not enough pilots seem to know their autopilots as well as they should - if you're not sure exactly what is going to happen when you push a button - then don't do it or find out before you do.

On a 4-axis AP you have more straightforward options providing everything stays coupled - GA or ALTA would be my preferred options to initiate the Go around.

With 3-axis or hand flying, as long as what you do gets you going safely away from the ground, does it really matter?

I imagine most pilots fly ILS at speeds well above Vy/Vbroc (same thing different terminology) so a speed reduction is probably needed at some stage - especially if you are OEI.

[gipsymagpie]

Gipsy.....what airspeed do you use for an ILS Approach?

One single set speed for all approaches or do you use different speeds for different conditions?

How do those approach speeds compare to Vbroc and Vy airspeeds for your aircraft?

We also have to consider all of the possible ways an ILS Approach may be flown...hand flown using raw data right on up to by means of a full capability four axis autopilot system.

That can bring you back to your desire to talk only of SAS/ATT.....is one mode better than the other for a raw data approach or while using an uncoupled FD?
I don't really need to get back on topic. I am just concious this is a good resource and I would love to change the topic title to "SAS, ATT and other AFCS animals"

Anyway, we fly approaches at 100 kts except one particular type where an aircraft limitation holds us to 90kts. The speed is chosen to maximise speed (commercial need), minimises drift from wind and maximises stability without going up into Cat B minima. Nowhere near Vy. Climb out is different though.

Sadly the flying of approaches in a predominantly VFR operation is not easy to practice. There is generally little opportunity to practice when operational flying and every other proficiency check has to have a manually flown ILS in it. So people might fly one ILS automated in 12 months. So building confidence in the various options is difficult. I make everyone fly one either in recurrent training or in test. The whole enforcement of a manually flown 3D approach every rating revalidation is a bit daft. For a 4 axis multiple redundant AFCS there are quite a few failures in a row (or one very sneaky failure that the TRE knows about).

[gipsymagpie]

It may not be strictly on topic but is a valuable topic of conversation.

A common theme seems to be that not enough pilots seem to know their autopilots as well as they should - if you're not sure exactly what is going to happen when you push a button - then don't do it or find out before you do.

On a 4-axis AP you have more straightforward options providing everything stays coupled - GA or ALTA would be my preferred options to initiate the Go around.

With 3-axis or hand flying, as long as what you do gets you going safely away from the ground, does it really matter?

I imagine most pilots fly ILS at speeds well above Vy/Vbroc (same thing different terminology) so a speed reduction is probably needed at some stage - especially if you are OEI.
I agree. 4 axis is always GA for us. It does exactly what you want.

You're right it doesn't matter which way you go around but given how infrequently outside your six month check ride you actually do a go around, it needs to be slick as it's high workload. With that in mind I highlight all the options but offer my preference. But I make sure the pilot briefs how he is going to automate his go around at the pre approach brief. Then they have a vague chance of doing the right things . I also push that automation (eg FMS or Garmin) are used to guide the lateral navigation in the go around. That is as long as it's a standard missed approach which is in the database, is not a missed approach which is just climb on a heading and is not non-standard.

[SASless]

There is much benefit to going outside of just the SAS/ATT questions posed originally....so I am not being critical of anything you have said in that regard....as you and Crab both brought up good points of interest.

Technology is both a blessing and a curse.....as it adds new layers of benefit and problems both.

The key as always is knowing as much as you can about it all.

We can look to our fixed winged friends who almost very last words were something akin to "What is it doing now?" referring to the the AFCS.

Personally, I would prefer to see this discussion continue and broaden its scope as I am of the opinion it would be a nice change.

[JohnDixson]

Could not agree more with SAS, Crab,GM. Ther were a couple of factors in play here, historically ( maybe more than a couple? ). A lot of the “old heads “ considered SAS. AFCS etc as unnecessary, expensive, claptrap. Then, as those features appeared anyway, the ground school teaching syllabi and flight manual information was purposefully dumbed down to the extent that pilots understanding of how the systems worked was impossible. “ Oh, the pilots don’t need to know that “ was a phrase I heard, not infrequently. But they do, and if the subject is presented properly, it can be made both understandable and interesting. Just an opinion.

[gipsymagpie]

Airbus has recently started reacting to the demand from the coal face for more information about the AFCS. There are some excellent Flight Operations Briefing Notes (FOBN) for the AFCS on Helionix types. It's the manual that should have come with the aircraft in the first place. However we have still had to put in a form you can actually deliver in a classroom and we still have to add stuff from the FLM, the original (mostly rubbish) training manuals and things we have found out ourselves.

I remember on a slightly earlier variant of the airbus AFCS on a different type we found that there was an entirely undocumented sub mode of the AFCS that led to the roll channel adding a roll boost command to assist with rolling wings level when the stick was centred. Unfortunately in certain conditions the stick was close to centre when at significant angles of bank...the undocumented mode led to a gremlin seeming to grab the stick and roll you level. Took ages to work out what was going on!

Basically manufacturers should be putting as much as possible out in the documentation - well done Airbus for the FOBN (within the bounds of commercial sensibilities of course).

[[email protected]]

It seems to me like manufacturers love to load their AFCS with so many extras, maybe to try and out-spec the competition that they forget that a good AFCS does all the basics well through a simple, uncomplicated interface.

A pilot on a high pressure IMC go around shouldn't need to remember umpteen modes and sub modes, engagement parameters etc - if it says GA on the button, it should do exactly that and it should work seamlessly with the FMS.

Unfortunately I get the impression that engineers design AFCS/FMS forgetting that pilots need to be able to use it easily.

[gipsymagpie]

It seems to me like manufacturers love to load their AFCS with so many extras, maybe to try and out-spec the competition that they forget that a good AFCS does all the basics well through a simple, uncomplicated interface.

A pilot on a high pressure IMC go around shouldn't need to remember umpteen modes and sub modes, engagement parameters etc - if it says GA on the button, it should do exactly that and it should work seamlessly with the FMS.

Unfortunately I get the impression that engineers design AFCS/FMS forgetting that pilots need to be able to use it easily.
Not so much the new AFCS. The Helionix GA is exactly what you want - maintains track, initiates a climb to the preset altitude, holds airspeed, manages NR in the event of OEI and initiates the missed approach in the FMS. Scarily dull. The older AP2000 was awful...it initiated a climb but just held the trimmed roll attitude even if that happened to be 30. Nasty.

[TeeS]

Hi Gipsy
totally agree with how excellent the autopilot is with helionix but I think the Autopilot is still essentially an AP2000. Happy to be corrected.
Cheers
TeeS

Ah, having woken up and reminded myself not to respond on social media when I have a large G & T in hand, I've re-read the last line of your post and realise I probably misread it, sorry!
This, from the Airbus FOBN, is what I was thinking about:
The BK117 D-2 automatic flight control system (AFCS) is an evolution of the so called APM 2000/2010 AFCS, which was fitted on the EC225, EC135, BK117 C-2, AS365 N3+, etc. The main difference between these autopilots is the physical implementation on the aircraft. The APM 2000 was implemented in a dedicated autopilot computer; the Helionix AFCS is embedded in a dedicated software partition inside the AMCs. In addition, various new functions (such as GTC/GTC.H) and improvements were implemented in the Helionix AFCS.
TeeS

[gipsymagpie]

Hi Gipsy
totally agree with how excellent the autopilot is with helionix but I think the Autopilot is still essentially an AP2000. Happy to be corrected.
Cheers
TeeS

Ah, having woken up and reminded myself not to respond on social media when I have a large G & T in hand, I've re-read the last line of your post and realise I probably misread it, sorry!
This, from the Airbus FOBN, is what I was thinking about:
The BK117 D-2 automatic flight control system (AFCS) is an evolution of the so called APM 2000/2010 AFCS, which was fitted on the EC225, EC135, BK117 C-2, AS365 N3+, etc. The main difference between these autopilots is the physical implementation on the aircraft. The APM 2000 was implemented in a dedicated autopilot computer; the Helionix AFCS is embedded in a dedicated software partition inside the AMCs. In addition, various new functions (such as GTC/GTC.H) and improvements were implemented in the Helionix AFCS.
TeeS
There are many versions of the APM2000 type AP - huge difference for example between the 3 and 4 axis versions of notionally the same thing. I was just appalled when moving from a 4 to 3 axis aircraft at the poor quality of the GA on the 3 axis systems (eg lack of wings level function) and more appalled that most established crews thought it did level the wings (the virtual wings, we are rotary drivers here)!

[[email protected]]

Thanks GM, I have heard that the Helionix is very good so some pilots must have been involved in its development :)

[sycamore]

Crab,PM for you...

[SASless]

As we are talking helicopters and not fixed wing aircraft....during off airport operations....how does one employ the maximum benefits of the latest technology.....say in EMS Accident Scene landings and take offs?

Have the approving authorities been keeping up with the approval to use use these Gucci Kitted helicopters to their fullest capability?

Or is that the bigger hurtle to get over in implementing such use?

[212man]

Not so much the new AFCS. The Helionix GA is exactly what you want - maintains track, initiates a climb to the preset altitude, holds airspeed, manages NR in the event of OEI and initiates the missed approach in the FMS. Scarily dull. The older AP2000 was awful...it initiated a climb but just held the trimmed roll attitude even if that happened to be 30. Nasty.
It's been a while since I used it (17 years), but I'm pretty sure that in the EC155 the GA function levels the wings. I think it also automatically brought in the 4th axis, if in 3 axis when engaged, to give Vy and 750 fpm