Warning! Don't Try This At Home Kids (radio fail safe test disaster)

I've been building and flying drones since October of last year without any major incidents, at least not until today.  I have a Flamewheel 450 running a pixhawk FMU a Frsky Taranis transmitter and a Frsky Delta 8 receiver - it works great and I have lots of flight hours on it.  I have tested the radio fail safe on the ground several times and it all seems to work as advertised.  So is says to myself, I wonder if the radio fail safe really works in the air?  Seems like a reasonable thing to test.  Not that I would every get far enough away to loose radio contact given the power of the Taranis and the new FAA regulations on VLS flight.  I've been out 500 meters with no problems, so hey, why not give it a try - Bad Idea! 

I understand exactly what happened - now.  In fact, I had one of those "Oh Crap" moments of awareness as by beloved copter was free falling from 20 feet up with the motors off.  Here's what happened. 

First, let me say that the Pixhawk did EXACTLY what it was advertised to do in a radio fail safe situation.  So no problem there.  The Frsky Taranis is an awesome transmitter for sure, but it has this feature on power up where it checks that all the switches are in the down position and the throttle is off before it will turn on the RF.   I was flying it Position Hold mode (middle mode switch position) when I started the test by turning the transmitter off.  Sure enough, the copter went into RTL mode and started to come home.  Imagine my joy.  Everything would have been fine except I tried to regain control by turning the transmitter back on (you know what happens next don't you).  The transmitter started complaining, verbally, that the switches were in the wrong position and the throttle was not down.  Again, exactly what it is advertised to do.  So, I pulled the throttle down and put the mode switch back to it's down position which was Stabilize mode.  That was the fatal move.  The receiver woke back up and sent the new mode to the Pixhawk which forced it out of RTL mode (as advertised) and put it into Stabilize mode (as I commanded) with the throttle off. 

The rest is history.  The bill of materials for my 20 foot free fall was not that bad considering.

  1 3DR telemetry radio; 2 props; 1 motor boom; and 1 GPS mast.  

A bunch of other stuff came apart, but it was designed to do that so no harm.  This would make an interesting study in energy distribution in random events.  Pieces were spread out all over the place with the furthest piece, my RunCam 2, landing a good 15 feet from the crash site.  The good news is it was still running.

Bottom line - DO NOT live test the radio fail safe mode with a Frsky Taranis transmitter - Crap!

  Doug

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At least don't turn the transmitter back on if you do.

Ive been there and done that too,  and with a Flamewheel F450, fortunately from only about 5 feet up and not with a Taranis radio.

Once you've done that once, you tend to pre-think the consequences a bit more and the second time you are ready for it and can get the switches set right and are ready to throttle up the moment it arms.

Still not something that ever feels comfortable.

Best regards,

Gary

You should be able to press a button on the Taranis to skip past the "Your switches are in the wrong position" warning.

Personally, I think this should be tested.  But do it with the copter hovering at 1 foot.

And for good reason.  I actually just tested this last week on one of mine.  After turning off the Tx, it just sat there.  Apparently the FS Rx isn't programmed right.

Hard luck.

I almost had a similar disaster recently, not FS-related, but my first vibration-induced uncontrolled vertical flyaway, in Loiter mode. My first instinct was to drop the throttle to slow the climb, then, slightly panicked - to switch back to stabilize. It obeyed, but throttle was now near-zero, and down she plummeted.

I did respond at the last second with throttle, and salvaged a hard landing instead of full-on splat.

I don't use Taranis, but my transmitter does the same switch-check on start-up. Mine doesn't apply to the rotary switches, though, and that's where I have my flight modes.

I agree with rehearsing in my head what I'm going to do, with head and hands, if something goes wrong in any test, and then proceeding on the assumption that something is about to go wrong. I'm always pleasantly surprised when it doesn't.

It makes for a nervous flyer, but hopefully a safer one. 

Exactly for this reason I am only turning off OpenLRS Tx module and keep radio running (I have power switch on my version). Basically as I understand if you really loose RF signal right thing to do is not fiddle any switches or throttle an just focus on getting signal back (get with radio to elevated spot, correct antenna or just wait). When signal comes up craft (plane or copter) continues in RTH unless you switch it to other mode.

Btw in my 9x with Er9x I can disable this switch check and I have it disabled. As you can not probably turn off just RF module, it may be good idea to turn check off.

Thanks everyone for your feedback and comments.  I agree with Kolin, and realize now that the best thing I could have done was leave the transmitter off and let the copter land itself.   That won't happen again.

 Regards,

   Doug

This is a very timely blog. Thanks Doug, for making me do a triple check.

I was going to test this today for the first time ever in 6 years of drones.

Last night I was going through the motions as my Futaba 12fg also does the switch and throttle checks. Just as well, because I had forgotten my radio also asks for confirmation that you really really do want to transmit even after setting the stick and switches correctly.

More importantly, my rx wasn't set right even though I thought I had set the failsafes.

I didn't want want to turn off the checks because I also fly nitro helis and have burnt out a clutch in the early days when I started at half throttle with a radio that didn't care. Very lucky I always hold the head when starting.

This blog got me curious and I now know that the Futaba 12fg can turn off the checks on a per model basis, I just assumed it was system wide. I guess this would be the same for all the upper end Futaba radios. The setting can be found in the "Linkage" menu.

FrSky please take note and include the ability to disable checks on a per model basis in the Horus ( really want one when they come out). I can disable each of the following checks individually.

Condition (flight modes)

Thr cut

Thr position

Thr hold

Idle down

Motor

Airbrake

Snap roll

I always check Failsafe with the copter on the floor, I turned off the radio on new builds and looks in Mission planner if mode change to RTL and not risk anything but I read and saw many people checking in this risk way; is my method not reliable in a thrue failsafe case? ....If I have to turn off my radio with my guy flying I enter in panic :O

In openTX you can turn any switch and throttle warning off for each separate model.

Always test at low altitude (with some margin to allow for altitude changes).

When possible above tall grass and in a large open area (not in a small backyard as many people do on youtube).

My Tunrnigy 9XR does the same thing. You just press EXIT when the message pops up and it works fine.

Now I also did some stupid things in moments of panic...

i turn all those checks off for every model. You can also selectively turn them on, and as stated before, press any button on around your screen and it will cancel the warning.

Thanks again for the inputs.  I turned off the throttle switch warning and the flight mode switch as well.  That should take care of it should I ever feel the urge to try this gain - not any time soon.  I should have my F450 back in the air in a few days. 

  Regards,

    Doug

Doug, 

You have a couple of options with the Taranis radio:

1.) After turning on the transmitter with the throttle above zero you should be prompted for "Press any key to skip".  This will allow you to continue with whatever switch positions and throttle state you had before turning the transmitter off.

2.) In model setup on page 2, "Throttle State" and "Switch Positions" can be turned off.  I personally prefer to leave these enabled, so I am sure everything is as it should be on transmitter startup - no surprises.  

Consider making your default mode PosHold with all switches in the default startup position.  I use SD as my mode switch (PosHold,Circle, Stabilize), and SA (AltHold, Break) with SF as RTL.

Don't forget that your receiver has a fail-safe mode as well which can be managed from page 2 of your model setup.  I always set "Failsafe mode" to "no pulses" for APM, to make absolutely sure the FC knows it has lost communication and needs to initiate an RTL - or whatever you have setup as failsafe.

Good luck.


Rich

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