AUTO will reduce altitude until it hits terrain if ARSPD_ENABLE=1

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Thanks to Chris Anderson, the problem is narrowed down to the summery above.
Please see: 
http://www.diydrones.com/forum/topics/help-auto-loses-altitude-below-set-altitud
e-sets-negative-pitch

Short - if airspeed sensor is enabled, the aircraft assumes negative pitch 
until it would hit the ground.
The same aircraft - same conditions - will follow plan perfectly at proper 
altitude if airspeed sensor is disabled.

Original issue reported on code.google.com by andre.kj...@gmail.com on 19 Mar 2012 at 5:51

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Another interesting log:
Min Max FBW speed is 8 and 20m/s for this flight.  - airspeed enabled for 
entire flight.  (APM rebooted since any change)
Please see RTL at log position 67.11% to 70%  it switches off motor, points 
nose down, and would hit water if I did not took over.

The rest of the flight proves that airspeed is pretty close to GPS speed, as it 
should (there were almost no wind today)

hope you can make something useful of it.
.

Original comment by andre.kj...@gmail.com on 21 Mar 2012 at 6:41

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Attached config
FBW_MIN for the last flight was 9m/s - still - the plane was at minimum flight 
speed, and below set altitude, and nosed down instead of increasing throttle 
and going up.

Original comment by andre.kj...@gmail.com on 21 Mar 2012 at 6:48

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removed some I values - reset one P to default.
Flew then in strong wind,  motor is used now and then, on/off suddenly, no 
smooth throttle operation. often *very* slow ground speed, due to wind.

Original comment by andre.kj...@gmail.com on 22 Mar 2012 at 11:03

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I'm facing the same problem.
APM 1.0 - firmware 2.5 ( tested also at 2.33 and 2.34)
MAX THROTTLE = 100%
CRUISE SPEED = 12 m/s

The airplane doesn't get to the defined altitude at the waypoints and very 
often decreases altitude constantly at a 5-10 degrees descent rate.

Original comment by mpsou...@gmail.com on 5 Aug 2012 at 4:55

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Similar problem:
With a 9Kg model, APM 1, arduplane, 2.33, 2.34 and 2.5. The model was flying 
very well until a nose tip during takeoff. Small damages. We fix the nose. 
After that with same APM Parameters its altitude decreases.
We turned off arduplane 2.5 AIRPEED USAGE(=0). The model them is flying very 
well.
We noticed a high noisy ASP value after the crash, even when the model still in 
the ground.

A noisy ASP value will impact the arduplane altitude control algorithm? 
regards

Original comment by omarach...@gmail.com on 7 Aug 2012 at 1:28