No, unless the downdraught was more than the climb capability.
Altitude hold is a feedback loop.
Does it occur in climb or descend as well ?
I have just edited my post above with the graphics. I had earlier posted data from the EGKA-EGJA flight but actually the video was done on the return flight which was FL070. I also corrected the GPS data for the 160ft error (SIRF-2 GPS).
Does it occur in climb or descend as well ?
I don’t think you would see it then. It is really slow – much slower than a typical climb or descent.
another thing you could do next time when the problem occurs switch from Altitude hold to Pitch attitude mode.
When this is also unstable its most likely the pitch servo or att-gyro
Or maybe your static hose from the AP to the static systems is blocked (sharp bend)
Yes; an excellent idea to try the PIT mode.
More plots from the same flight…
The climb looks smooth but one would probably not see anything on that
Looking at the 3000ft portion (Jersey Radar kept us at 3000ft and would have probably forgotten…) which is exactly 3 mins long at 3000ft ALT hold
That shows something similar going on.
The interesting thing is that I have had 3 or four pitch servo burnouts over the same spot in France, near Le Mans. Details here
If the problem occurs in climb and descend as well, it really doesn’t leave much possibilities, does it ?
I had it as well, a 2-500fpm amplitude around the pitch set by CWS, or by the up/down button; I also first thougt it to be the FD, but it wasn’t.
Wonder what makes your pitch servos burn out so quick.
A late update on this:
Apparently there is a SYDEREC installation where I had the 3 or 4 pitch servo burnouts in France. This is a low frequency communications system operated by the French military.
Peter, the SYDEREC squadron was only established in September 2010. And chances are you’d have seen their balloons if they were indeed to blame for roasting your servos when you flew by :-)
Interesting. I don’t remember who told me.
There is a military installation down there, shown on the map.
I wouldn’t spot something down below if you paid me for it. I just follow the GPS 
A Syderec balloon is huge and it trails about 2km of cable… can’t really miss it. Plus anytime they “fly” there’s a Restricted Area around it.
There was a capability gap between 2001 when the C160 Astarte of 1/59 Bigorre sq were decommissioned and 2010 when the Syderec captive balloon squadron was commissioned. The land-based primary system is satellite-based and is further up and west in Brittany.
