Indonesian aircraft missing off Jakarta
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Look who's reading into things now...
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From reading this thread and the Boeing alert, it seems that a big part of the issue is that the crew was not aware of MCAS. The writeup for STS "trimming the wrong way" may very well have been written by a captain that thoroughly understood STS and was aware of which way it should normally trim....but what he was seeing was the undocumented MCAS feature trimming .
Ok, I did not know the Max has a new stall assistance system. As mentioned before, the STS normally trims down after take off, so it would be operating in the same manner as the new stall system. So again the tech log entry about STS appears incorrect.
And why did Boeing incorportate a stall assistance system that trims, instead of using a stick pusher? This seems to go against the normal rules for flying. You fly the aircraft with the control column, not the trimmer.
A stick-pusher acts on the control columm, to increase speed, and withdraws its influence instantaneously to allow you to recover from the dive. A stall trimmer will trim you down, but leave that nose-down pressure on and impede your recovery from the dive.
A stick pusher is flying the control column, in the normal manner; while a stall trimmer is flying the trimmer. Is this how the new stall system works? I cannot find details on-line.
Silver
From reading this thread and the Boeing alert, it seems that a big part of the issue is that the crew was not aware of MCAS. The writeup for STS "trimming the wrong way" may very well have been written by a captain that thoroughly understood STS and was aware of which way it should normally trim....but what he was seeing was the undocumented MCAS feature trimming .
Ok, I did not know the Max has a new stall assistance system. As mentioned before, the STS normally trims down after take off, so it would be operating in the same manner as the new stall system. So again the tech log entry about STS appears incorrect.
And why did Boeing incorportate a stall assistance system that trims, instead of using a stick pusher? This seems to go against the normal rules for flying. You fly the aircraft with the control column, not the trimmer.
A stick-pusher acts on the control columm, to increase speed, and withdraws its influence instantaneously to allow you to recover from the dive. A stall trimmer will trim you down, but leave that nose-down pressure on and impede your recovery from the dive.
A stick pusher is flying the control column, in the normal manner; while a stall trimmer is flying the trimmer. Is this how the new stall system works? I cannot find details on-line.
Silver
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I do not have access to a 737 Max AMM, so do not know the post-installation functional checks required after replacing a faulty probe. Most probes contain a guide pin which inserts into a matching hole in the fuselage to insure that the probe body is physically oriented correctly in relation to the airframe, but on all aircraft models I work on, installing a new probe also requires a post-install rigging check to insure that the output position data is correct. Typically this is done with a rigging adapter that attaches to the probe and fuselage, containing a calibrated angle scale and pointer, and a mount to attach a calibrated digital protractor. The check usually involves rotating the probe to specific angles, while checking the generated data to insure it is accurate.
IF the 737 Max requires a rigging check when an AOA probe is replaced, the question is: did the Lion Air engineers perform one, or did they just replace the probe and sign it off? A rigging check would have revealed any inaccuracy in either the new or old probes. If no rigging check is required in the AMM procedure for replacing an AOA probe, then that is on Boeing, not Lion Air.
IF the 737 Max requires a rigging check when an AOA probe is replaced, the question is: did the Lion Air engineers perform one, or did they just replace the probe and sign it off? A rigging check would have revealed any inaccuracy in either the new or old probes. If no rigging check is required in the AMM procedure for replacing an AOA probe, then that is on Boeing, not Lion Air.
I don't have MAX AMM so don't know if that is different - if it is, then that itself could be a source of error (particularly if the difference isn't documented...).
It doesn't look easy to misfit it by 20 degrees (which was apparently the disagreement) - 8 mounting holes (so 45 apart), plus indexing holes. However, there are two compatible parts, one Rosemount one Conrac?, only one calibrator but an adapter is required for one type of vane. Sometimes when things don't fit people use a hammer as an adapter, I can't imagine that on an aoa vane but never say never.
Lots of questions - did they fit it properly, did they test it properly, did the have all the correct test kit, is the MAX different in some way (and did they know), and also, was the sensor replaced more than once (to cause the previous problems as well) with same error made each time.
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No airplane can fly with zero AoA. Let’s don’t do Bernoulli. If the centerline of chord is the same as the fuselage longitudinal line, and the fuselage level, the airplane will sink. The angle of incidence of the wing allows less nose up when close to level. It is “baked in” AoA. No?
I agree - the subject is sufficiently complicated without introducing Daniel Bernoulli! The above is correct in the case of the x-axis of the fuselage being coincident with that of the wing chord but Vessbot's was wondering why a wing AoA (disregarding wash-out) necessarily has to be related to the angle of incidence of the fuselage. The answer is, of course, that it doesn't.
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On the other hand to do it via stabilizer trim is a lot simpler, you've already got all the hardware you need installed, the jack screw and it's actuator. About all it takes is adding some code to the software for the control computer.* So, there you go, no new parts, no additional weight, and no regulator looking over your shoulder saying "Whoa, you just added a mechanical system that applies control inputs to the primary control system. Lets take a look at all the specs for such systems"
* I'm kinda speculating on that, details are still a little fuzzy on what exactly the MCAS 'is" but so far it doesn't seem that there are any physical parts to it that aren't already there and used by other systems.
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Why? Because there are numerous "cruise conditions", even on the same flight. How so? The required AOA depends on a variable that is constantly changing: weight. Do you set "best performance" for the conditions at top of climb? Half way to destination? Just before descent? With a completely full passenger load? With a partial passenger load? With full tanks? Partially filled tanks? There are multiple variables that drive weight.
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Why? Because there are numerous "cruise conditions", even on the same flight. How so? The required AOA depends on a variable that is constantly changing: weight. Do you set "best performance" for the conditions at top of climb? Half way to destination? Just before descent? With a completely full passenger load? With a partial passenger load? With full tanks? Partially filled tanks? There are multiple variables that drive weight.
But the way I read FCeng's post (maybe I misread, or he'll elaborate) is that even in the original baseline design, which would be reflected in the average of the day to day, it's advantageous to have the fuselage pitched up. That's what I'm questioning.
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A lot has been said today in the media about pilots not knowing about the new system and sure enough I can’t find anything about MCAS in the manuals. However, the max and NG FCOM both state that the speed trim system will trim the stabiliser nose down as the airspeed approaches the stall speed. Neither alludes to what the exact inputs are to this system (eg. AoA vanes), nor that it can erroneously occur if x y z fails but the actual function of the system that is suspected to have caused the problem isn’t new to the 737 max? Or have I missed something?
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Based on an old 737 AMM and zero personal hands-on experience, it looks like 737 has something very similar. AMM refers to a "calibrator" which is to used in system performance test after installation. Stall warning system is to be tested with various flap settings rotating the vane until shaker triggers.
I don't have MAX AMM so don't know if that is different - if it is, then that itself could be a source of error (particularly if the difference isn't documented...).
It doesn't look easy to misfit it by 20 degrees (which was apparently the disagreement) - 8 mounting holes (so 45 apart), plus indexing holes. However, there are two compatible parts, one Rosemount one Conrac?, only one calibrator but an adapter is required for one type of vane. Sometimes when things don't fit people use a hammer as an adapter, I can't imagine that on an aoa vane but never say never.
Lots of questions - did they fit it properly, did they test it properly, did the have all the correct test kit, is the MAX different in some way (and did they know), and also, was the sensor replaced more than once (to cause the previous problems as well) with same error made each time.
I don't have MAX AMM so don't know if that is different - if it is, then that itself could be a source of error (particularly if the difference isn't documented...).
It doesn't look easy to misfit it by 20 degrees (which was apparently the disagreement) - 8 mounting holes (so 45 apart), plus indexing holes. However, there are two compatible parts, one Rosemount one Conrac?, only one calibrator but an adapter is required for one type of vane. Sometimes when things don't fit people use a hammer as an adapter, I can't imagine that on an aoa vane but never say never.
Lots of questions - did they fit it properly, did they test it properly, did the have all the correct test kit, is the MAX different in some way (and did they know), and also, was the sensor replaced more than once (to cause the previous problems as well) with same error made each time.
Just don't forget to take the kit off for the heater checks....
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Whole thing should be very scary to mortal pilots, and I am very upset with Boeing, as I always thot that made very good planes and they were pilot-friendly.
Boeing once were a company that understood the inherent risks. They were a manufacturing company, their emphasis was rightly on risk minimisation.
Now it is simply another corporate entity, driven by MBA and financial statement manipulation. These idiots know little of the industry they financially engineer and care less, the entire focus is the next quarter.
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Single Point Failure
It’s interesting that such a single point system failure may have cause the stab trimming and that differences wise it’s not contained in the training if media reports are correct. Something to look at when updating similar aircraft types and operating aircraft with a common flight deck but updated systems.
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Some Google Fu reveals a document called "OPERATIONAL EVALUATION REPORT – BOEING 737" "ORIGINAL – JANUARY 10, 2018" from ANAC (Brazilian civil aviation authority). In "Appendix 2 – OPERATOR DIFFERENCE REQUIREMENTS (ODR) TABLES" MCAS is listed as "No" on both flight characteristics (FLT CHAR) and change of procedures (PROC CHNG).
For those interested in following the document trail that lurkingpax has added to, the original document is here:
OPERATIONAL EVALUATION REPORT THE BOEING COMPANY BOEING 737
Archived copy OPERATIONAL EVALUATION REPORT THE BOEING COMPANY BOEING 737
Page 18 is the only listing for the MCAS
Appendix 2 – OPERATOR DIFFERENCE REQUIREMENTS (ODR) TABLES This Design Differences tables, from the Boeing 737-800 to the Boeing 737-8, were proposed by The Boeing Company and validated by ANAC. They list the minimum differences levels operators must use to conduct differences training, checking and currency of flightcrew members
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How’s this for arrogance and obfuscation! Boeing and FAA are “evaluating” the “need” for a software update and updating training procedures. So 189 deaths are not sufficiently compelling?!
Boeing, U.S. Regulator Weigh Software Fix on 737 Max After Crash
https://www.bloomberg.com/news/articles/2018-11-13/boeing-u-s-regulator-weigh-software-fix-on-737-max-after-crash
Boeing, U.S. Regulator Weigh Software Fix on 737 Max After Crash
https://www.bloomberg.com/news/articles/2018-11-13/boeing-u-s-regulator-weigh-software-fix-on-737-max-after-crash
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original #978 (permalink)
Just to clarify & simplify the above statement. The MCAS is active when:
From the extremely brief description of the MCAS, we can possibly conclude:
Originally Posted by Halfnut
MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control computer using input data from sensors and other airplane systems.
The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers. The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.
The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers. The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.
- Flaps UP
- High Angle of Attack signal (real or spurious)
- Flaps extended
- Reduced AoA
- Pilot Electric trim
- Stabilizer trim cutout switches to cutout - Does this prevent the MCAS operating.
From the extremely brief description of the MCAS, we can possibly conclude:
- JT 610 retracted flaps to the clean configuration (MCAS becomes active)
- Low mach number, therefore maximum rate of horizontal stabilizer of 0.27 degrees per second, and a forward deflection to the limit with a spurious AoA input.
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A couple of thoughts on cruise deck angle and maximum performance (i.e., L/D):
1. At a given airplane configuration (read flap/gear position and payload/fuel distribution) the deck angle that generates lift = 1g will be a function of Mach number and airspeed. You can fly faster at a lower deck angle or slower at a higher deck angle. As fuel is burned during the flight, the lift required to balance weight will go down and the deck angle at a given speed will reduce. This is partially compensated during a flight by increasing altitude along a constant Mach line that yields lower impact pressure and thus less lift. All of this is factored into the airplane design.
2. While I am not an aerodynamicist and may be going out on a limb here, I believe that a configuration with the body at a slight positive deck angle will realize greater incremental benefit from the small amount of lift generated by the body vs the slight increase in body drag going from zero AOA to a small positive AOA. It is quite clear that negative AOA for the body would be a loser as it would result in increased drag and decreased lift.
3. Another variable in the mix is engine angle with respect to both the fuselage (left/right) and wing (up/down). Some of that is based on inlet flow distortion due to the forebody and considerations for engine out characteristics, but I believe that pitch/lift trim for cruise also factors in.
Standing by for someone with more aero background than me to comment.
1. At a given airplane configuration (read flap/gear position and payload/fuel distribution) the deck angle that generates lift = 1g will be a function of Mach number and airspeed. You can fly faster at a lower deck angle or slower at a higher deck angle. As fuel is burned during the flight, the lift required to balance weight will go down and the deck angle at a given speed will reduce. This is partially compensated during a flight by increasing altitude along a constant Mach line that yields lower impact pressure and thus less lift. All of this is factored into the airplane design.
2. While I am not an aerodynamicist and may be going out on a limb here, I believe that a configuration with the body at a slight positive deck angle will realize greater incremental benefit from the small amount of lift generated by the body vs the slight increase in body drag going from zero AOA to a small positive AOA. It is quite clear that negative AOA for the body would be a loser as it would result in increased drag and decreased lift.
3. Another variable in the mix is engine angle with respect to both the fuselage (left/right) and wing (up/down). Some of that is based on inlet flow distortion due to the forebody and considerations for engine out characteristics, but I believe that pitch/lift trim for cruise also factors in.
Standing by for someone with more aero background than me to comment.
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Not an aerodynamicist either but the fuselage most definitely does have a lift coefficient (extreme example: SR-71). Would love it if someone with deep experience could add some commentary here.
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Per https://twitter.com/jonostrower the 737 MAX MCAS system runs the stabilizer at 0.27 deg/sec regardless of speed/mach. The amount of nose down stab motion provided by MCAS is listed as "up to 2.5 degrees. MCAS uses the stab trim motors and is disabled via selecting stab cutout.
