Ethiopian airliner down in Africa

[The Bullwinkle]

Originally Posted by 73qanda

Because if it was a case of only a blocked pitot, the plane would have sat trimmed with a nose up bias and very few control inputs would have been required in order for the aircraft to fly away from the ground. It is thought by many ( waiting on report) that the MCAS system trimmed the stab nose down with no command from the pilots to do so, which would necessitate
1/ making more control inputs of greater magnitude in order to allow the aircraft to fly away from the ground
2/ recognising that the trim was running uncommanded
3/ preventing the trim from running uncommanded.

Two different scenarios really.

But it still doesn’t explain the unreliable airspeed.

[Interflug]

Originally Posted by The Bullwinkle

I’m still really confused as to why the MCAS is being held totally responsible for this crash.
Didn’t this flight and the Lion Air flight both have unreliable airspeed problems immediately after takeoff?
Can somebody please explain how the MCAS system can cause an unreliable airspeed indication after takeoff?
I still don’t understand why a blocked pitot isn’t even being discussed.
It’s my understanding that this was a morning flight (local time).
Could the pitot covers have been left on, or possibly a mud wasp got in there overnight?
At the end of the day, if the engines and the hydraulics were working, why weren’t two qualified pilots able to “fly” the aeroplane in apparently fine weather back to Addis Ababa?
I’m honestly confused as to why everybody seems to be jumping on the MCAS bandwagon!

developing on your train of thought, there is a lot of talking about the AoA sensor, but very little about the role of the ADIRU further down the signal chain.
I haven't followed every technically enlightening post here, but isn't for what we know by now the chain of events consistent with some kind of data corruption or software error in the ADIRU and it's related systems?
I wouldn’t rule out ADIRU problems, corrupted code or else.

[gearlever]

Originally Posted by The Bullwinkle

I’m still really confused as to why the MCAS is being held totally responsible for this crash.
Didn’t this flight and the Lion Air flight both have unreliable airspeed problems immediately after takeoff?
Can somebody please explain how the MCAS system can cause an unreliable airspeed indication after takeoff?
I still don’t understand why a blocked pitot isn’t even being discussed.
It’s my understanding that this was a morning flight (local time).
Could the pitot covers have been left on, or possibly a mud wasp got in there overnight?
At the end of the day, if the engines and the hydraulics were working, why weren’t two qualified pilots able to “fly” the aeroplane in apparently fine weather back to Addis Ababa?
I’m honestly confused as to why everybody seems to be jumping on the MCAS bandwagon!

FAA EAD 2018-32-51

An erroneous AOA input can cause some or all of the following indications and effects:

- Continuous or intermittent stick shaker on the affected side only.
- Minimum speed bar (red and black) on the affected side only.
- Increasing nose down control forces.
- IAS DISAGREE alert.
- ALT DISAGREE alert.
- AOA DISAGREE alert (if the option is installed).
- FEEL DIFF PRESS light.
- Autopilot may disengage.
- Inability to engage autopilot.

[Grummaniser]

Originally Posted by The Bullwinkle

But it still doesn’t explain the unreliable airspeed.

The AOA is used to calculate an adjustment to the Indicated Airspeed so a faulty AOA will cause the calculation to be unreliable thus causing an Unreliable Airspeed annunciation

[Loose rivets]

I think it was just mentioned that it modifies the static pressures. It would be easy to see why it required information about the angle of the airflow over the static plate.


EDIT - It's just occured to me that if the PF selected the left HS cutout switch in the hope of using the autopilot, he would have been * out of luck. I wonder if he knew those switches had different wiring on the MAX.

[wiedehopf]

Originally Posted by The Bullwinkle

I’m still really confused as to why the MCAS is being held totally responsible for this crash.
Didn’t this flight and the Lion Air flight both have unreliable airspeed problems immediately after takeoff?
Can somebody please explain how the MCAS system can cause an unreliable airspeed indication after takeoff?
I still don’t understand why a blocked pitot isn’t even being discussed.

I'm really curious why people don't look at the FDR graphs from the Lion Air flight.
They are readily available.
The airspeed difference between left and right side is about 10 to 20 knots.
The FDR readout is not consistent with a blocked pitot.

The pitot readout as well as the static port readout is corrected for AoA slightly, that is were the Airspeed disagree comes from.
This correction for AoA is small so even with the left AoA reading 21 degrees high the magnitude of the error on speed and altitude is there but both are still in the right ballpark.
I've attached the FDR readout from the first pdf report that was available on the Lion Air crash (with some annotations):


In this graph only the left side altitude readout is plotted, in the preliminary report you can see the difference to the side with working AoA:
https://www.flightradar24.com/blog/w...ary-Report.pdf

Note also that the 100-200 ft altitude dip on rotation is present on the Ethiopian ADSB data as well.
(That alone doesn't mean that there the AoA was misreading in that case as well but i makes it more likely in my opinion)

[A0283]

@ gums

US Congress hearing today shoulda had Sully. He knows about AoA, leading edge devices, underslung motors, crew coordination and such.

This Safety Hearing was the 1st in a series of at least 2 hearings. It was announced, and confirmed during this 1st hearing, that Boeing will be invited to the 2nd hearing. Date as yet unknown.
So at this point in time we have two international safety investigations on Lion and Ethiopian, a series of Congressional hearings, FBI investigation (I am not sure if this is technical support or indeed a criminal investigation), and two DoT IG investigations on the 737, the first referring to Southwest engine problems and maintance (perhaps including looking into the insertion of acquired aircraft) and the second on the MAX (which reports to the DoT and parallel to Congress). Not to forget the NTSB safety investigation into the Southwest engine fatality case. They all may add some pieces and answers to the puzzle that we are discussing here.

[Golf-Sierra]

It seems like there is an element of clash between western and eastern culture here. I believe in the previous flight problems with airspeed discrepancy and the stick shaker coming on one side were experienced. And the runaway trim condition was also experienced. Yet - the flight continued to the destination. Is this what would likely happen in Europe or would the pilots elect to make an emergency landing in such circumstances?

I find it hard to believe the crew were not aware what the correct course of action was when they saw there is something blatantly wrong with the automatics in the flight control system. Switch it off! The CVR will explain everything once it is published. They were either trying to troubleshoot the problem and/or some conflict (save face/pull rank) occurred in the cabin.


Golf Sierra

[Fzz]

Originally Posted by Loose rivets

The 26th bit data error giving ~20 degrees on the AoA signal is compelling. On pressure detections, I'm biased towards the static system.

I'm not convinced by the 26th bit error theory. If you look at the AoA data from the previous flight in Fig 7 on page 16 of the preliminary report, the AoA disagreement is there from the start, but it increases significantly around rotation. This isn't really consistent with it being a constant 22 degree error.

https://www.flightradar24.com/blog/w...ary-Report.pdf

[Bleve]

Originally Posted by The Bullwinkle

I’m still really confused as to why the MCAS is being held totally responsible for this crash.
Didn’t this flight and the Lion Air flight both have unreliable airspeed problems immediately after takeoff?
...
I’m honestly confused as to why everybody seems to be jumping on the MCAS bandwagon!

In the aftermath of the AF447 accident, Boeing (and Airbus) substantially changed the Unreliable Airspeed non-normal checklist (NNC). The message that has since been repeatably reinforced is that if there are ANY abnormalities related to airspeed, pilots should NOT make any assumptions as to what indication is right and what is wrong. They should simply run the Unreliable Airpeed NNC methodically until the correct indications are determined. So a stick shaker activating after takeoff (due to a faulty AOA signal) will be treated (and reported to ATC) as 'Unreliable Airspeed'.

The accident scenario that we are discussing might run something like this:
- At rotation stick shaker activates (due to a faulty AOA signal).
- You see normal speed indications.
- Stick shaker at normal indicated airspeeds is NOT logically consistent. Post AF447, that is an Unreliable Airspeed event. (You are NOT jumping to any conclusions at this stage that the IAS is correct and the Stick Shaker is wrong).
- Climb to a safe height and begin the Unreliable Airspeed NNC.
- Report to ATC that you have Unreliable Airspeed (because that is the NNC that you are now working through).
- Manually set a known power and attitude for level flight (the NNC requires the autopilot to be disconnected).
- Unbeknownst to you, MCAS has now been activated because of the faulty AOA signal.
- Your efforts to trim the aircraft for straight and level flight are proving difficult (because of the hidden MCAS intervention). Confusion increases because you can't even establish straight and level flight to continue the NNC.
- You are soon overwhelmed.

[DaveReidUK]

Originally Posted by Fzz

I'm not convinced by the 26th bit error theory. If you look at the AoA data from the previous flight in Fig 7 on page 16 of the preliminary report, the AoA disagreement is there from the start, but it increases significantly around rotation. This isn't really consistent with it being a constant 22 degree error.

And the converse is true of the accident flight - the AoA disagreement reduces around rotation. I don't buy the bad bit theory either.

[gums]

Salute!

And the converse is true of the accident flight - the AoA disagreement reduces around rotation. I don't buy the bad bit theory either.

Ditto, Dave. And we still don't know where the AoA signal comes from. Being "digital" on the FDR, somebody has to be digitizing it unless the sensor outputs digital bits and that does not seem to be the case. My point being that the incorrect AoA could have come from the ADRIU and not the sensor itself.

Gums....

[bsieker]

Originally Posted by The Bullwinkle

Can somebody please explain how the MCAS system can cause an unreliable airspeed indication after takeoff?

It doesn't, and I don't see that anyone has implied such. There is, however, a plausible common cause to both MCAS activation and IAS disagree: an erroneous AoA value by one sensor. AoA is used for calculating a correction to static pressure, and both ADIRUs independently calculate airspeed from their respective values for static and total (pitot) pressure. Since the static pressure has been adjusted for AoA, both ADIRUs will come up with different values for indicated airspeed, which will trigger an "IAS disagree" warning, which is a trigger for the unreliable airspeed procedure.

I still don’t understand why a blocked pitot isn’t even being discussed.

Because it doesn't fit the known FDR traces from the Lion Air accident. And from all we have heard, there are huge similarities between the Ethiopian and the Indonesian accident, which even Boeing has acknowledged and led them to recommend grounding the type ("out of an abundance of caution").

Could the pitot covers have been left on, or possibly a mud wasp got in there overnight?

From all we know, very very unlikely.

At the end of the day, if the engines and the hydraulics were working, why weren’t two qualified pilots able to “fly” the aeroplane in apparently fine weather back to Addis Ababa?

That is the 300-million-dollar question, isn't it? Probably because an insidious system did underhanded things, unknown to the crew, which were preoccupied with the Ureliable Airspeed procedures and distracted by a stick shaker. In end, nose-down trim overwhelmed the pilots' ability to control the aircraft.

I’m honestly confused as to why everybody seems to be jumping on the MCAS bandwagon!

Because all symptoms, and all data we know so far are consistent with an inadvertent MCAS activation, and are not consistent with most other plausible explanations.

I would think that Boeing would be the first, if there was any doubt that the Ethiopian accident was very similar to Lion Air, to vehemently deny any MCAS involvement.

Bernd

[Rananim]

I saw the hearing and found it interesting.Cruz asks Erwell towards the end if its true
that there was no information on MCAS provided to pilots.Erwell fumbles a bit and
then responds that the errant MCAS intervention is runaway trim and that is already
covered in the QRH under the Runaway Stabilizer NNC.Boeings AD in
November says the same thing.I think the Committee's investigation will center on this
argument plus the single sensor issue and the controversial decision
to have MAX added to a 1967 certificate as amended type with flimsy
self-administered ipad training.
I think the AoA presentation on the PFD is a red herring but agree that the
dispatch light for AoA disagree is a critical safety feature and should be fitted as
standard equipment.
The stick shaker in my view is still the golden nugget here as it blocks
the auditory channel of both pilots,masking trim runaway and leading
to startle factor for some crews.

[Loose rivets]

My mind's wide open on the D to A error, though I'll remain interested.

Fzz, one post shortly before yours shows the AoA diverging from a commons zero with distinct slopes.

Ethiopian airliner down in Africa

I don't know what to make of that, inasmuch as vanes are flopped anywhere but zero while bouncing along the ground. If t=0 is at some point during the acceleration run then how come they are both zero? Rhetorical question.

[Fzz]

Originally Posted by Loose rivets

Fzz, one post shortly before yours shows the AoA diverging from a commons zero with distinct slopes.

Ignoring the very left hand side of the graph, it looks to me like neither AoA indicator reads zero until the airspeed increases. This isn't surprising - without airspeed, we'd not expect them to read the same. After airspeed increases, the right AoA settles to zero and the left settles to 20 degrees. Thereafter, the left maintains a constant offset. The information from the previous flight seemed more interesting, as there the offset was not constant - it changed around rotation. If the problem was a fixed offset due to a stuck bit, we'd expect the offset to be constant after the aircraft has airspeed.

What happens at the very left hand side? I've no idea - but I wouldn't necessarily expect the AoA to read zero with no airspeed.

Edit: I think I was reading the previous flight graph wrong. That one also seems to settle into a constant offset as soon as the aircraft has airspeed, rather than around rotation. In which case this does not rule out a digital error.

[netstruggler]

26bits gives a resolution of approx 1 part in 70 million. What on an aircraft needs to be measured to that resolution?

[safetypee]

netstruggler, not a measurement, but the required system integrity?
Two aircraft crashing within months with the same indications involving a supposedly high integrity system - probably a probability of 1 : 70m

[Loose rivets]

netstruggler
Fair comment. It might be the approved chips just happen to be available/standardized on, but it wouldn't explain how one bit error would be such a large proportion of the full scale deflection.

[safetypee]

Loose rivets, et al, see:-
https://www.satcom.guru/2019/03/ethi...s-to-lion.html