Hi guys.
Don't worry, many AS355 pilots who bring problems to me are just as confused with the good old 'Trim Release' functions.
You ask a popular question, one that I get asked and have to demonstrate several time a year from operators, newbies, CPL's & ATPL (Yes.... including TRI & TRE's!!!
).
So OK, lets try and sort this one out, here goes,
The AS355 autopilot system is the SFIM 85T31 The Flight Director is a factory option and for the F2 & N's the Failure Monitoring Unit is also an option to satisfy UK & EASA (JAR OPS 3) IFR certification requirements.
Firstly this sytem
DOES have automatic trim. It is operational all of the time and it's purpose is to sychronise (Keep Centre) the Primary AFCS (Automatic Flight Control System) servos. The position of which is displayed on the 'Galvinometers' on the instrument panel. The Galvinometers should be centred at all times! and indicate the position of the relevant servo.
If they do not centre then the AFCS is Un Serviceable. This is normally due to a failed trim system (Fuse is a good one because the airframes guy has put the wrong size fuse in
). However it is usually due to flight crew not knowing what the trim release switch does and that they have inadvertantly selected it to OFF
. It could also be due to a stuck servo (Nothing that a tap with a hammer by your friendly local airframes engineer won't sort out. Well, at least for a few flying hours or so....)
Warning !! if you see the Galvinometers
permanently out of centre with the AFCS ON and particularily with the AFCS OFF, then DO NOT FLY...
ground it! This is because in a nut shell, you will not have the correct primary flight control range of movement required. Particularily look out for a failure in pitch as you may not have sufficient primary flight control movement available to achieve a full flare needed for Auto's or a fast run on landing / Engine Off flare. Thats what the auto trim looks after for you and thats why the Galvonomiters are there for. By the time you learn this, it may have already of spoiled your day!
The
trim release effectively switches the autotrim OFF (By removing the power to the trim Motor).
This switch is there for emergencies only!,
ie AFCS Trim Runaway. In this situation you push the trim release switch and this will stop an AFCS Trim Runaway. Unfotunately, It is on the preflight check list to check that the Trim Release switch is selected to ON. The poblem is that many pilots do not know whether ON is with the switch In or Out. And some...just don't check it! You would logically think that it it is IN. But, the good old French and Logic, well they kinda do not go together. It should be, would you believe, Out ie. flush with the others and hence the confusion
.
May be, one day AS355 flight crew will consist of a pilot and his dog.
The dog will be there to bite the pilot for touching the 'Trim Release' Switch'
.
OK so may be we want to put in a large and fast longterm attitude change. The auto trim will not cope with this. So for this we can temporarily switch the trim off (Known to the veterans as 'Floppy Stick') via the momentry push 'Cyclic Trim Release' button. Set your new attitude, release the 'Cyclic Trim Release' button and Bingo,.... Servos are sychronised and the aircraft now maintains it's new attutude
.
Now for the 'Fly Through'. OK, so we have the AFCS engaged and we want to take evasive action around the microlight (the one without a transponder that didn't ping the TCAS) that we have just got a little bit too close for comfort to
because we were too busy heads in messing around with the portable satnav aerial connector that just vibrated out due to the tracking and balance issue that we have been carrying since the annual..... well guys at this point, the force gradient spings and sensor in the trim motor knows you are fighting it ...so guess what...yep, it automatically switches off the autotrim until you are ready to carry on sorting out the Garmin! Clever stuff ayyy
And finally...Turn & Slip... (Back to basics on the old theory of flight here...
) The SFIM 85T31 takes care of this just the same way as you should ....through the pedals...ie Yaw
not attitude. It has a Yaw channel. This should be engaged. Again the preflight check list covers this, you engane Pitch, Roll & Yaw. If your Green Yaw Light will not illuminate when you press it, ... then It's broke!
So in summary for the type training guys.....
The Avionics Blokes AS355 AFCS Type Tips
1) The pupose of the Trim Release switch is for Emergency Disengagement of the trim motors.
If you have a suspected AFCS trim motor runaway. Push the Trim Release Button on the LHS Ohd switch panel.
2) Don't fly around with 'Floppy Stick'
(Trim Disengaged ie'TRIM RELEASE' Depressed
3) Don't fly around with Galvonomiters out of centre
4) If you operate the cyclic Trim Release or 'Fly Through' then
MONITOR the GALVOS. you need to look after the servo
synchronisation during these modes of operation.
5) But seriously,
Always follow the Check List, SOPS and operate iaw with
the Flight Manual.
and you shouldn't go wrong....
And finally, well, pllease
always refer to your TRI for a demonstration and confirmation of the above. They do take the trouble to go the extra mile and find out this stuff and are constantly asking me awkward questions.
Hope this sorts out the confusion in some earlier posts.
Love and best wishes
...... your friendly local Avionics Bloke
