Loiter Crash


I have been flying my Finwing Penguin with a 3DR Pixhawk with uBlox GPS for the past 6 months or so. I typically start out in Manual, fly to a reasonable height, test out RTL, and then fly in FBWA/B as needed.

This past week I started testing out Loiter and had reasonable success initially. Unfortunately, my last flight I took off in Manual, flew up to 300 ft, and initiated a Loiter. The aircraft performed 2-3 nice circles but on one of the circles the aircraft banked very hard right, dropped the right wing and plummeted into earth (at about 58 mph) as I was fiddling with my Fatshark goggles (I caught the last second or so of the death spiral but was not able to pull the Penguin out of the dive).

Has anyone experienced anything similar and would anyone be able to assist in telling me why this occurred?

Attached is a log file from the crash. Any assistance would be greatly appreciated.

Finwing Penguin
3DR Pixhawk w/ uBlox GPS


Video of the crash:


This is just a guess from watching your video.

I’ve built three planes with APM and they all have a problem where when loitering in the wind, they lose altitude on the down-wind leg, that causes the APM to throttle up which gets it going very fast, then when they turn up-wind the balloon up, slow down and the cycle repeats.

I believe your plane got below the desired loiter altitude which caused it to throttle up, eventually to full which even caused a voltage sag. I’ll bet your penguin pushes the nose down at high throttle and the combination of the turn, full throttle and the pitch limits prevented it from ever getting out of its descent.

I’ve tried many things on my planes to try to prevent altitude loss in tight turns but have never really been successful.

Turning TECS_RLL2THR down doesn’t really seem to make it not apply massive throttle when it gets below the desired altitude.

Turning up “Pitch Max” makes it try to climb too aggressively on auto.

Reducing “Bank Max” will make it less likely to happen but also makes loiter in windy conditions very bad and auto performance much less crisp overall.

Turning FBW max speed down may help, I haven’t tried that.

I had the most hope for PTCH2SRV_RLL and spent a lot of time on it. Cranking that up will make the plane initially gain altitude in gentle turns, but sharp turns and loiter will still end up losing altitude.

I would love to know what’s needed to prevent the lost of altitude combined with high throttle when loitering, especially in windy conditions.

Luckily for me, my planes have never gotten as bad as yours. They normally lose about 20M of altitude on the down-wind leg then balloon back up on the up-wind one.

If your airspeed sensor is well calibrated the problem is less severe but still occurs.

Today I tried reducing the MAX FBW airspeed and I’m pretty sure that would have prevented your crash.

It will still apply power when the plane goes below the target altitude while loitering, but it eases off when it hits the max speed.

Thanks for the responses Noircogi, I will give this a shot and hope that the same thing does not happen again.

Do you believe that RTL would have the same response / problem as they are both automatic modes?

RTL will do one turn when it’s initiated, that could result in a small loss of altitude. It will then fly home and loiter at your RTH altitude which could possible get into the same state.

My speculation/suggestions are:

  • look into why it says “battery low” right before crash - did something get disconnected? did the ESC shut off?
  • check the APM logs for things like the radio signal (it could have gone into failsafe if you bumped your radio off)
  • I do see the mph values changing a lot, so agree there might be something to the wind causing altitude shift, but this is clearly a crash not just dip in altitude.

Loiter is VERY good in my experience, and seems to work fine on your plane, so this was likely a system error.

I checked your log, what type of batteries are you running ? As you / autopilot increases throttle your main voltage is really sagging, also the pixhawk supply voltage drops significantly at the same time, this happened as you started loiter and then towards the end… I suspect that your servos probably locked out either because the ESC had a brownout or the pixhawk.

Before you fly again use a newer pack if its old or run one that matches your discharge, so for 40 amps and 2000mah type battery you really need 30 - 40c