Hello all-

I am fairly new to this fascinating hobby and looking for some help with ALT_HOLD. I have a QAV500 style  quad, 850 motors, 1045's props, FrSky TRF8, and MB1200XL sonar. Nothing fancy. No GPS, video, telemetry, or other RF equipment.

Sometimes ALT-HOLD works great, usually on my first few minutes of flying for the day. Then after some flight time the quad decides to change it's mind and either slowly descend and land, or endlessly rise until in desperation I put it back in STAB mode.

My first thoughts were that the sonar is reporting bad data, but the logs would indicate otherwise. Here is a typical log file which leaves me with more questions than answers.

Red=Sonar Alt

Green=Baro Alt

Blue = Throttle Out

White = Throttle In


You can see where I initially switched from STAB to ALT_HOLD at line #1500. From here all the way to line #4900 the quad 'more or less' maintained altitude. Not nearly as tight as I would expect, but reasonable. I was just hovering in the yard, not doing anything crazy, testing this feature. At line #4900the quad started to decent. At line #5100 I cranked up the throttle (still in ALT_HOLD) in an attempt to keep it aloft. At line #5200 it simply plopped itself on the ground.

What's going on? What am I doing wrong???

What should be the effect of the throttle stick when in ALT-HOLD mode? Should I be able to make manual adjustments to the "held" altitude?

Complete log attached.


2013-03-08 16-00 37.log

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  • Thanks to all who have tried to help me figure out what's up with ALT_HOLD. This hobby is like an onion... peeling back layer after layer, finding more and more smelly but deliciousness layers with eyes watering at every step. I have learned that log files are gold. Sure enough, my sonar on last Saturday's flight went berserk half way through, throwing ALT_HOLD into a tizzy.


    Tim - low battery? I have a cheap HC buzzer which tells me when the battery is ready to land. I have been trusting this to sensor to not let my battery go TOO low. Though I will admit to trying to fly just a speck more after the beeping starts. Performance usually sucks at this point. I'll have to pay more attention to this. Another layer.

    So I guess my next step is to disable sonar AND failsafe, and see how well the barometer handles ALT_HOLD. Can't wait for Saturday! I was hoping to do low altitude flying to learn, and that's why I thought sonar was a good idea in the first place. Since I am just learning and flying in a small open field, I would probably prefer a crash (disabled failsafe) over an unplanned trip to another county. Lots and lots of heavy woods around me, and I have a mental picture of my poor quad permanently parked high in an oak tree.

    Randy - Thanks for the video link. Another layer of the onion! My offsets appear to be OK.

    One step at a time. I'll let you know how things play out this weekend.



  • Scott & John-

    Thank you for your advice. I DO have a new Futaba 8FG transmitter coupled with a FrSky 8 channel receiver. I guess it is possible that the TX/RX hiccuped and kicked my quad into failsafe (and gently landed it). That would make sense. For now, failsafe is officially disabled.

    Now back to original topic... ALT_HOLD. I finally had a chance to get out and fly today to test out some of the great advice I have picked up here. Even got my GPS on for the first time to play with Loiter. So here is what happened today. My quad took off great... stable and happy! Switch to ALT_HOLD... works great! Switch to LOITER... works great! Happy day.

    I was playing around and having fun switching between modes, and about 4-5 minutes into the flight ALT_HOLD starts underdamping and bouncing up and down trying to hold altitude. LOITER mode has my quad heading south rather than holding position. STAB is the only way I can fly until the battery alarm tells me to pop in a fresh battery. Once again ALT_HOLD and LOITER work great for the first 4-5 minutes of flying with a fresh battery, then have issues for the remainder of the flight.

    Thinking back of my experience with this whole ALT_HOLD issue from the start, I think this is a smoking gun. ALT_HOLD has worked for me with a fresh battery, but degrades as the battery voltage drops.

    So why does ALT_HOLD performance degrade as my battery voltage drops?

    I am running APM2.5 with 3DR power module, 3600mAh 30C Turnigy battery, turnigy 2217 20T 860kv motors on a QAV500 copy cat home made frame.

    The only thing I can think of is that my motors are a little small for this hefty frame? And as the voltage drops, don't have what's needed to do the job? That's a far fetched guess.

    Any thoughts?


  • Moderator

    Looks like you had a failsafe event. Question is why. Can we see a shot of your logs including the ThrIN parameter at the time of the event?

  • Hey Rich,

    I am having the same problem. Please see my thread here. There are same good advises there. 

    I haven't gotten time to work on it yet but I will soon. I am still learning this board.


  • Several comments:

    The slightly poor altitude hold for me, was having the basic throttle P term set too low. The correction term then had to build a long way to actually get a throttle response, so it'd slowly climb or descend. As a human pilot I was moving the sticks much more than the altitude algorithm by default does, and so the low term didn't register....

    Then a quick glance at the logs shows the last mode selected as stabilise, just before it starts to descend. I think you must have flipped your switch?.

    In hold mode, a large movement, should give a slow climb/descent. When you get where you want, _slowly_ reduce the stick to the hover position. It takes quite a time to actually 'record' the new altitude you are now at, as the new default.

    Best Wishes

  • A few thoughts, probably not terribly useful but they are some troubleshooting steps anyway:

    Could you be running out of battery and going into failsafe?  I have failsafe turned off for this reason - I like the idea, but I don't like the idea of it just going RTL to another country if there is a bad GPS lock.

    Any chance the quad goes into Land or RTL mode for some other reason?  Are you able to see in the log that it is still in ALT_HOLD when it lands?

  • Sonar only is effective below about 20 feet, pretty low. Above that is the barometer. Also depends on type of flying. If your just moving slowly about in small area I'd expect pretty good hold if your zooming around doing big circuits the banking and leveling will naturally make the quad descend then correct etc sometimes several even 10s of feet or more if your flying control is erratic. 

    If your in ALT-HOLD your throttle will control the altitude but its rather sluggish with the preferred position being at the hover throttle near center position. I find it best to stablize a hover, click on ALT-HOLD then slowly get the quad moving forward using pitch and then using the yaw to turn but not flying real fast. With a good 30+ feet of altitude I can usually get going faster with enough "safety" altitude to allow for the quad to correct.

    Can you post a much bigger version of your chart its too small to really read it. 

  • Anybody with any thoughts on this problem? I'm still struggling to understand what;s going on.

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