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-   -   Indonesian aircraft missing off Jakarta (https://www.pprune.org/rumours-news/614857-indonesian-aircraft-missing-off-jakarta.html)

Vessbot 11th Nov 2018 16:03

I wonder if the omission of this system from the manual, along with other manual dumbing-downs that we've complained about, are an overreaction to the notion that "we're not learning how to build the plane" about requirements to learn useless minutiae. The pendulum has swung too far.

gums 11th Nov 2018 17:12

AHA!
They don't need to install a caution light, huh?
We can tell when the damned thing is working because the trim wheel moves all by itself!! Well, matbe, 'cause they say it is not part of the STS.
Gums wonders.. and this is getting scary.

Lonewolf_50 11th Nov 2018 17:38


Originally Posted by hum (Post 10307923)
Strangely my original posts have been deleted... I cannot imagine what the PPrune police objected to!

I was making a general point, AoA is fundamental to everything we do as pilots, We must insist that - especially in aircraft where it is measured and used to influence systems - it is displayed to pilots. We must also fundamentally change our philosophy whereby there is an obsession with speed alone from day 1 of flight training, Lift (=G) varies with both speed AND AoA.
When, as it seems in this tragic case, an important sensor gives an erroneous value, and assuming that value is displayed (which it was not apparently) a trained pilot will quickly separate truth from lies.

There was some discussion on this after AF 447, in re getting an AoA gage in the cockpit. (As I recall that system, one could get to a page somewhere in the displays that would show AoA, but there wasn't a gage in the cockpit in that model, nor in a number of (even most) other models ...) The industry apparently is of mixed opinion on that, for all that I agree with your core point.
I find that attitude to the AoA gage a little perverse: the various flight computers use and need AoA, there are AoA sensors, multiple, and yet somehow the pilots aren't deemed to need this information. (Nor is some space on the pilot displays allotted to it in many airliners, as I understand it).

Concours77 11th Nov 2018 18:19

Lonewolf 50

I don’t recall the drill exactly, but AoA can be calculated on the A330 using fpv (the “bird”). “Flight Path Vector” in the FD. ACARS recorded a crew attempt to disable the Computer PrimSEC 1. This would have allowed the FPV to appear, then some mental math subtracting bird from AH.

This explains the crew fiddling with the FDs, turning them on, when they were prohibited by the book.

It led to some harsh criticism of the crew, “trying to fly the FD”. They weren’t looking for airdata, but AoA. Wouldn’t we all?

as I recall

Vessbot 11th Nov 2018 18:38

AOA is the difference between bird vs. nose symbol, not bird vs. horizon

(and neither of those is counting the effect of wind)

gums 11th Nov 2018 18:42

Salute Concours!

Yep, if you have a fixed reference in the display, HUD or FD, the AoA is the difference between the "bird" and that reference WRT the longitudianl axis of the jet. Sure, the wing chordline versus the fuselage axis must be allowed for, but on some planes this is a fairly easy thing to do.

I flew HUD planes with great velocity vector displays and those were extremely useful for establishing glide path or descent angle or..... The AoA display was only constantly presented on the HUD or "indexers" or even a steam gauge in two of the jets ( about 1400 hours). The biggest use of the AoA, besides knowing you were approaching stall, was approach to landing. The approach AoA took into account your weight!!! So a quick, rough calculation we used in other planes was good, but that AoA was the real deal. E.g. "approach speed is 175 kias plus 5 knots for every 1,000 pounds above 3,000 pounds", I leave it to the reader to figure out what that plane was, heh heh. I was USAF, but the USN used AoA a whole lot more for landing on a carrier deck, and USAF was more "airspeed" on approach than AoA.

All that being said, use of AoA by the crew for most flight regimes requires some training and rationale. My vote would be what I used it for - approach speed and stall warning. Oh well, enuf philosophy.

Gums...

hec7or 11th Nov 2018 18:49

Pilots are encouraged to disconnect the automatics and fly manually if the system is doing something unfamiliar, in this case, by disconnecting the automatics, the MCAS is active.

So, in the event of an AoA sensor failure a "system" induced stab trim runaway occurs...which admittedly does have memory items.

When they design these safety enhancements, why don't they tell us?

The inhibited "stall" aural warning at high AoA during high AoA on AF447 is another example of design engineers assuming pilots will behave like monkeys, where the warning was inhibited at high AoA and activated when the crew reduced the AoA. Also the Colgan Air flight 3407 Dash-8 Q400 at Buffalo would in it's final stages have generated GPWS aural alerts concerning the glideslope deviation together with stall/low airspeed warnings, one warning requiring a pitch up and the other requiring a pitch down, which could overload a tired flightcrew.

Clearly the safety systems require a rethink

rideforever 11th Nov 2018 18:55


Originally Posted by hec7or (Post 10308421)
Pilots are encouraged to disconnect the automatics and fly manually if the system is doing something unfamiliar, in this case, by disconnecting the automatics, the MCAS is active.

Should they not also disconnect STS and therefore MCAS ? during such a situation ?

Concours77 11th Nov 2018 19:03


Originally Posted by hec7or (Post 10308421)
Pilots are encouraged to disconnect the automatics and fly manually if the system is doing something unfamiliar, in this case, by disconnecting the automatics, the MCAS is active.

So, in the event of an AoA sensor failure a "system" induced stab trim runaway occurs...which admittedly does have memory items.

When they design these safety enhancements, why don't they tell us?

The inhibited "stall" aural warning at high AoA during high AoA on AF447 is another example of design engineers assuming pilots will behave like monkeys, where the warning was inhibited at high AoA and activated when the crew reduced the AoA. Also the Colgan Air flight 3407 Dash-8 Q400 at Buffalo would in it's final stages have generated GPWS aural alerts concerning the glideslope deviation together with stall/low airspeed warnings, one warning requiring a pitch up and the other requiring a pitch down, which could overload a tired flightcrew.

Clearly the safety systems require a rethink

On Colgan, the bugs were set high, the Stall Warn activated with twenty knots left of room. Pilot pulled, eventually wrestling with the pusher all the way down. GPWS? He was pulling with all his might as it was. Did he pull harder with GPWS command? Dunno.

CurtainTwitcher 11th Nov 2018 19:24


Originally Posted by hec7or (Post 10308421)
So, in the event of an AoA sensor failure a "system" induced stab trim runaway occurs...which admittedly does have memory items.

Unfortunately this MCAS intervention is not a classic "runaway stabiliser", it stops for 5 seconds after trim application, then activates again. You could see how a crew dealing with an unreliable airspeed could end up with the MCAS progressively ratcheting in forward trim, sporadically counteracted by pilot back trim, or even holding a significant backstick force against the MCAS.

Imagine dealing with what you believe is the "bigger problem" (UA) and the elevator & trim act in a unexpected manner (due to the undocumented MCAS), that is what this crew probably faced. Would any of us have recognised we actually had a flight control system problem?

Ultimately, is is possible that the actual cause of this accident will turn out to be an undocumented flight control issue.

hec7or 11th Nov 2018 19:32

"Should they not also disconnect STS and therefore MCAS ? during such a situation ?"

of course, but initially only the AP/AT would be disconnected in response to the airspeed disagree, the auto/main trim switch selection would come when you recognise the trim runaway, assuming you are not distracted by airspeed limit alerts and tactile warnings, ironically it would appear that the STS/MCAS is only active once the AP is disconnected.

"Unfortunately this MCAS intervention is not a classic "runaway stabiliser"

agreed, so leaving the AP/AT engaged will give you the wrong pitch attitude and thrust setting, disconnecting the AP will give you an intermittent stab trim runaway.

Both easily controllable once memory items have been actioned, but you have to recognise it first as a multiple failure.

The point is, most pilots would assume that disconnecting the AP/AT would give you full manual control of the aircraft.

Clearly it doesn't.

Smott999 11th Nov 2018 20:03


Originally Posted by hec7or (Post 10308456)
"Should they not also disconnect STS and therefore MCAS ? during such a situation ?"

of course, but initially only the AP/AT would be disconnected in response to the airspeed disagree, the auto/main trim switch selection would come when you recognise the trim runaway, assuming you are not distracted by airspeed limit alerts and tactile warnings, ironically it would appear that the STS/MCAS is only active once the AP is disconnected.

"Unfortunately this MCAS intervention is not a classic "runaway stabiliser"

agreed, so leaving the AP/AT engaged will give you the wrong pitch attitude and thrust setting, disconnecting the AP will give you an intermittent stab trim runaway.

Both easily controllable once memory items have been actioned, but you have to recognise it first as a multiple failure.

The point is, most pilots would assume that disconnecting the AP/AT would give you full manual control of the aircraft.

Clearly it doesn't.

..,but did it in the NG? Wasnít Hal / STS still enabled? Just not at mercy of bad AoA?

CurtainTwitcher 11th Nov 2018 20:23


Originally Posted by hec7or (Post 10308456)
"Unfortunately this MCAS intervention is not a classic "runaway stabiliser"

agreed, so leaving the AP/AT engaged will give you the wrong pitch attitude and thrust setting, disconnecting the AP will give you an intermittent stab trim runaway.

Both easily controllable once memory items have been actioned, but you have to recognise it first as a multiple failure.

That is exactly the point, the recognition of the actual problem is very subtle due to the intermit nature of the MCAS being disabled for 5 seconds after manual trim. The first step in the UA checklist is disconnect the autopilot...

Initially just setting the attitude & thrust will require lots of manual trimming, but once it is right, only then does the MCAS starts to intervene. Now the PM gets out the QRH performance and starts giving you target attitudes and thrust for you weight while you are trying to fly level at 5000 (Boeing provide this exact altitude for Flight with UA), it takes quite some time to let the speed stabilize the speed Straight and Level(S&L) for the crosscheck and to work out the erroneous indication(s).
However just flying S&L for the crosscheck is complicated by the MCAS/ pilot trim couple. As I said in an earlier post, it is possible that all three IAS indications were actually valid and in agreement, further confusing the issue.

JPJP 11th Nov 2018 21:59

737 AOA Indication
 

Regarding the AOA indication on the 737NG and MAX. It’s an ‘optional extra’ positioned at the top right on the PFD. The Flight Path Vector (FPV) is standard equipment and deselectable.

737 AOA Indicator.

https://cimg2.ibsrv.net/gimg/pprune....3e98b11bd.jpeg


gums 11th Nov 2018 22:27

Salute, JP!

That's an excellent AoA indicator you have shown. Has the critical areas of interest clearly shown.

IMHO, there's no need to monitor AoA second to second unless on approach or doing sone serious maneuvers down low and slow.

Where the indicator helps is when other thing are happening and that AoA could be a big player.

Gums...

Longtimer 12th Nov 2018 00:01

What was different
 
With a lot of 737 Max8s flying, you have to wonder what was different about this bird? If an overall design problem, then why has there not been more incidents? If with only that bird, perhaps new suppliers or even a manufacturing flaw.

PPRuNe Towers 12th Nov 2018 00:46

It is a good display Gums but as an extra cost option only - even on BBJs. Do I recognise it? Yes.

Have I flown it? Yes but only in the sim. Never seen one out in the wild.

Rob

CurtainTwitcher 12th Nov 2018 01:14


Originally Posted by Longtimer (Post 10308642)
With a lot of 737 Max8s flying, you have to wonder what was different about this bird? If an overall design problem, then why has there not been more incidents? If with only that bird, perhaps new suppliers or even a manufacturing flaw.

Not necessarily, it is entirely conceivable that a design flaw has been lurking there since day one of commercial operation. It just takes a specific sequence to cause an accident. I mentioned earlier the Lindy Effect, which is an observation that the longer something lasts, the higher the probability that it will continue to last.

The 737 STS design is very old, the MCAS is not. The STS dataset is enormous, it is well proven technology that has survived the test of time through huge numbers of potential failure modes. The MCAS is a new technology, with only 220 aircraft in service, it has a much smaller reliability dataset in the real world compared to the STS system. There haven't been a large enough sequences of failure modes to truely test the design of the MCAS, compared with the STS.

This is about a series of probability paths (multiple events in a unique sequence) that lead to an adverse outcome, STS has proven itself to be a solid design over an enormous range of these paths, and we can probably conclude it is unlikely to have a significant design flaw based on time in service. MCAS has not been around for long, and it appears that a unique series of events has demonstrated a flawed design early on in the service life.

CaptainMongo 12th Nov 2018 02:26

AOA is great except when it isnít.

Multiple air carrier incidents and accidents have been caused by a triggering event which was an inaccurate AOA input.

I, to, am a big fan of AOA, having flown the -16 on the back side of her years, but if the the sensors providing AOA are corrupted, AOA is worse than worthless, it is dangerous,

gums 12th Nov 2018 02:52

Good point, Mongo, about corruption

OTOH, the big thing is that the Viper FLCS was gee command in pitch and rate command in roll. So the Aoa inputs were focused upon "limiter" functions and did not actively move control surfaces about nor affect your control wheel or stick "feel".
We had a troop fly the Viper for about ten minutes with no AoA and damaged pitot because a big pelican busted the radome off. The rate sensors and stby gains let him fly for those ten minutes, but then it got too hard and poof!

Airbus is closest FLCS we see that compares with the Viper. And it does not provide tactile feedback to the control sticks last I checked. And the bus is aero stable and not as "sensitive' to small changes in gee or AoA.

Gums ....


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