Protecting against pilot error when switching to stabilize with the throttle down

I am adding some logic to my GCS to detect flight mode changes and then try to recover if a non-automatic mode like stabilize or loiter is used with the throttle too low by switching to guided or brake. (If there isn’t an RC, the throttle is automatically set to 50%.) Running against the simulator (SITL), I noticed that the copter will not switch into guided or brake if the mode is switched to stabilize with the throttle at minimum. The copter just plummets to the ground even if I start at 100 m or so. The only way I’ve seen to recover is to raise the throttle. If I use Loiter, the logic of switching to guided works fine. I don’t want to override the RC throttle since the pilot expects to fly it with the RC for this case.

I haven’t tried this on a real copter yet, but wondered if anybody knew why the flight controller won’t recover by switching immediately into guided if the mode is switch initially to stabilize with the throttle down?

Also, would this type of logic be useful in flight controller itself? Many pilots have crashed their copters by forgetting to raise the throttle before switching to loiter or stabilize.

It looks like temporarily overriding the throttle with a value of 1500 and then switching to guided mode from stabilize works in the simulator. Once I detect that the flight mode has changed to guided, I release the throttle override to allow the RC pilot to takeover with the RC.

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Should the flight controller reject a change to stabilize when the throttle is at a minimum when in a flying state?

The copter falls very quickly and can start to tumble if stabilize is selected and may not be recoverable before it hits the ground. Using the simulator, my GCS can’t prevent the copter from falling 1-3 meters before stabilizing, which is OK if the copter is high enough off the ground.

No. This is the best recovery mode for most pilots - it shouldn’t be
second-guessing the pilot.

It’s rather useful when you decide you must crash now

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It also causes a lot of inadvertent crashes when pilots forget to raise the throttle before switching modes. Center sprung throttle sticks help. Disarm motors can be assigned to an RC switch or ctrl-key in the GCS for emergency situations. I am also adding reminders to the pilot to raise the throttle if an RC is detected by the GCS

What are some other alternatives to mitigate a crash for this scenario?

One of the first things I learned when starting to fly copters is to leave the throttle in the ‘recovery’ position.
You crash one or two to begin with before this and it soon implants into the brain.
If it doesn’t then I would question whether you should be in control of a vehicle.

Spring loaded sticks will help.

But as someone who has to fly long missions I would object to anything getting in the way of instantly switching to Stabilise.
In an emergency of having to take manual control you only have a couple of seconds at most to react.
And it happens, especially when trying to do survey work in wind gusting to 56km/hr and having to take manual control because the parameters in auto don’t have the grunt to combat the wind.
And thats just one type of scenario.

And as @peterbarker points out, there are times when you have to make the abort flight decision and the very last thing you want in such a critical emergency is for something to get in the way.

For the pilots that can’t seem to get this into their head, try changing your mode selections to include AltHold prior to Stabilise. The control is similar and it would give you a chance to glance at the controls (and there isn’t always time to do that) to make sure your throttle is correct.

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Don’t have STABILIZE on a switch would seem to spring to mind :slight_smile:

Not having stabilize would be a good mitigation, but I can’t control the configuration of every user’s RC. Stabilize is a nice last resort mode in case the GPS goes wonky or your firmware was setup in super simple mode and you’re flying from a moving boat and can’t figure out why you can’t get the copter back to you in loiter mode (that actually happened to me, lesson learned).

I’m not advocating keeping the user from switching to stabilize during flight if the throttle is correct, just not if he has the throttle at the minimum. Unless you’re very high, you will not likely recover fast enough at 9.8 m/s/s . In two seconds you’ve fallen 20 meters or 66 feet.

I’ve accidentally crashed a couple of copters over the last 6 years forgetting to raise the throttle and I’ve had customers that destroyed $5-10K copters accidentally as well not counting the labor and parts to fix them.

Anyway, its a real issue and I imagine every single pilot on this forum has forgotten to do it at least once, especially when switching from an auto mode. I’ll continue to mitigate it on the GCS side with warnings and by raising the throttle automatically and switching back to guided or brake. Its a little dangerous to raise the throttle while in stabilize mode since the copter is falling quickly possibly tumbling even within 1-2 seconds so when the throttle kicks in, I’m not sure if the copter will actually level itself quickly. (It works in the simulator, but I haven’t done a lot of testing in the field since I don’t want to sacrifice a copter.)

Preventing the stabilize mode change if the throttle was down would be safer and then adding a switch on the RC to disarm the motors would be better than the current behavior.

Enable “motor interlock”.
The motors will not stop spinning, and the copter will not disarm.
And you have a switch to manually immediately kill the motors if you want to.

Why not handle it in the RC transmitter?
I am sure with the right mixing and probably using logical switches you could work out a mode switch that would not allow stabilise unless the throttle was raised.

Thanks. I’ll look into that. I had wondered what motor interlock was.

My question would be… Why would you have the throttle down if the copter is in the air? Once the copter is flying the throttle should be in the middle if you go into auto modes. If the user can’t remember that, there are bigger problems I would say.

It’s easy to forget to raise the throttle when you’re running consecutive auto or guided missions using a GCS and then switch to RC.

This IS where the spring loaded throttle idea came from, just this very question.
This way your users have to be deliberately holding the throttle down on the RC to get into the position of no throttle on mode change.

And just for the record I hate these spring loaded throttles, but, if that’s what you were trained on and are used to!

Been there, done that!
My “solution” was to set an alarm/haptic on my radio if the throttle was lowered below 25% threshhold. It doesn’t solve the problem but it does warn you if the throttle is moved to a risky level.

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While I do get it that it seems simpler to just sit at the ground station and click a button, I never train our customers to do that. Our procedures are to always perform the start of the flight with the remote, that is arm, switch to auto than raise the throttle. This only leaves an accidental bump of the throttle stick into question. This way we never had this switch to stab with low throttle happen.