Originally Posted by
Rananim
I concur but.....
We're going round in circles here,data is minimal,no engineering input.
It doesnt have to be STS,it could be AP STAB trim.Whats important is that something gave them unwanted trim based on faulty ADIRU data.Now that something can be STS or AP STAB trim.Thats assuming unwanted "incorrect"(ie incorrect for the reality they were facing) trim is the golden nugget here.Talk of elevator feel providing the incorrect sensitivity for the high speed has been sparse here.I prefer the trim theory as it would explain the sudden loss of control more readily IMO.We can only work on deduction here so lets try that.
Previous flight commander reports ALT IAS disagree and STS trimming "wrong way"------->we deduce UAS undiscoverable on takeoff---->we deduce he was PF and he flew manually during UAS event (STS was operating) before disabling AP stab trim and giving control to FO------->Flight returns to Jakarta below RVSM airspace with FO flying manually.
Accident flight....Commander reads and signs tech log----->we deduce he knows of the previous UAS event----->we deduce further he will surely be stop minded if ASI disagree on takeoff----->takeoff is not aborted------>therefore same fault occurs on accident flight as previous flight------>possible static vent problem------->not discoverable until after liftoff------>Capts ALT lags or sticks------>Capts ASI underreads in climb.
We know from previous UAS accidents crews want to throw it back to the autopilot.Understandable but highly undesirable.What if the accident Captain engages AP B(good side) after takeoff in an effort to give himself time to think?What will happen?AP B will use FCC B to control pitch roll up to MCP selected altitude of 5000 right?FCC B is being fed with right side ADIRU data which we are assuming is good.Things dont go well and control is lost....why?AP stab trim is being controlled by FCC A(bad side data).FCC A commands stab trim/STS signal on power up.Result------->overspeed with AP stab trimming nose down.The crew are confused and disoriented(by low AS on Capts side)and distracting warnings, and dont retard thrust,possibly dont retract flaps.They ask ATC for speed readout and return to Jakarta.AP captures 5000 with difficulty and pitch oscillations,soon after sudden loss of control.
....you theorize Captain realizes poss faulty AS on his side, and engages AP B (good side AS) ...yet AP Stab Trim is still being controlled by the (bad side) FCC A.
What should Captain have done to get Stab Trim looking at B?
As a mere SLF freight it seems odd that Cap would recognize faulty airspeed on his A side, and take action to flip the AP to B side....yet still allow the AP Stab Trim (which I infer is separate somehow) to continue to receive input from the faulty A side...?
Perhaps this is a common mistake? All thoughts appreciated and thanks again for the informative forum.