A lot to discuss and no real hard information.It can get confusing and counter-productive.We have the write up by commander of previous flight and the
ADSB data.
Airspeed unreliable and alt disagree shown after take off. STS was also running to the wrong direction, suspected because of speed difference. Identified that CAPT instrument was unreliable and handover control to FO. Continue NNC of Airspeed Unreliable and ALT disagree. Decide to continue flying to CGK at FL280, landed safely rwy 25L
Its not confirmed that this write up is genuine but we do have Lionair's own admission of a tech problem on previous flight.We can reasonably take it
as being a genuine(if illegal) leak.What does it tell us?a)They had a "single side" only UAS event b)the Captains side(ASI ALT) was corrupted with bad data c)control was handed over to the FO and flight continued with good side data d)STS operated during the identification phase of the UAS...STS only operates when AP is NOT engaged.Therefore CAPT was PF and he was flying manually during the UAS discovery.
What can we reasonably
infer from the write up?a)That the UAS was not detectable on the takeoff roll(otherwise they would have aborted).b)The ALT DISAGREE confirms disagreement between CAPT and FO altimeter following takeoff ie CAPT's altimeter stuck(or lagging) vs FO altimeter normal.Prime suspect static vent left side.The inference of (b) is supported by (a).Static vent blockage is not discoverable on takeoff roll.c)IAS DISAGREE not reported....CAPTs ASI would be underreading but they would have to climb/accelerate before discrepancy between the 2 ASIs triggers the IAS DISAGREE warning.Control is handed over to the FO before that trigger is activated.
UAS comes in all shapes and sizes.Total,partial,intermittent,insidious etc You can fly into icing in the cruise and lose all ASI data(ie zero...ram air blocked but drain holes unblocked)) with AP siren suddenly awakening you out of your slumber.You can be climbing through icing and get a steadily increasing ASI with an overspeed warning.If you pull back you can end up stalling.You can be descending through icing and get a steadily decreasing ASI followed by stick shaker.If you increase speed you can end up overspeeding,overstress the aircraft with loss of control.You can be in the cruise and ASI just freezes and be totally unaware that you have UAS(total pitot blockage ram air + drain holes) You can enter a hail storm,lose the radome and end up with UAS..And then there are the non-icing events....insects,covers left on etc.A whole myriad of possibilities..
Boeing gives you the UAS NNC.They have changed it recently in an attempt to cover the murky waters.They did this because the onset of UAS may go unnoticed and the automation may place the a/c into an unstable flight regime prior to identification of the need to run the NNC.The new checklist starts with the assumption that the a/c is in unstable flight.With flaps up you fly 4 degrees and set 75% N1,with flaps down you fly 10 degrees and set 80% N1.Its a compromise,a prescription to cover all possibilities.However,like all compromises,it isnt always the best corrective action.Its never wrong but isnt always the "best" thing to do.If a pitot static anomaly is detected after liftoff and it is total(both sides affected),you are already in a stable flight regime(takeoff thrust 15 degrees) and you know categorically that you have lost basic airspeed/altitude data and it isnt coming back(not icing but maintenance issue),then it is not only not the "best" thing to do but a "bad" thing to do.You dont want to go flying under these conditions.The air is your enemy now.
I think the Lionair undoubtedly had a UAS,it was either a repeat of the previous flights scenario but this time control was not handed over to the FO,or following maintenance work both static vents were now affected,they had an overspeed and lost control of the aircraft.It would not surprise me if it was done with the AP engaged right after takeoff.