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Indonesian aircraft missing off Jakarta

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Indonesian aircraft missing off Jakarta

Old 14th Nov 2018, 03:47
  #1141 (permalink)  
 
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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.
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Old 14th Nov 2018, 05:12
  #1142 (permalink)  
 
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Originally Posted by silverstrata View Post


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

The way I understand it from reading here:
On a normal flight you would expect the STS to trim ANU during acceleration as it is trying to keep the aircraft at the previous speed. The previous captain said the STS was trimming incorrectly, so different from normal, so AND. The MAX has a new MCAS system, that will trim AND if it senses a low speed situation. There were reports the AOA on the captain side was giving erroneous high AOA values, leading to stick shaker as well, if the MCAS used these faulty values it could have kept on trying to trim AND every time the pilots stopped using elevator trim (if the even tried to override the airplane trim inputs manually). This could have resulted in the stabilizer being full AND, and the elevators not having enough authority to keep the nose up. The aircraft trim input could be stopped by using the elevator trim or, more permanentely, by the cutout switches (apparently is has been confirmed this would stop the MCAS from trimming.

TLDR: I think the STS trim being wrong write up was "correct", because the STS will during acceleration trim ANU not AND.
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Old 14th Nov 2018, 05:22
  #1143 (permalink)  
 
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Originally Posted by FCeng84 View Post
Per 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.
Just to clarify, the MCAS function "moves the horizontal stabilizer trim upward at 0.27 degrees per second up to 2.5 degrees and 9.26 seconds at a time." So, each time the MCAS function runs, that's another 2.5 degrees.

Last edited by LaissezPasser; 14th Nov 2018 at 05:24. Reason: grammar
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Old 14th Nov 2018, 05:33
  #1144 (permalink)  
 
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The B737 FCOM states:

As airspeed decreases towards stall speed, the speed trim system trims the stabilizer nose down and enables trim above stickshaker AOA. With this trim schedule the pilot must pull more aft column to stall the airplane.
How does that differ from the operation of MCAS when approaching a stall? MCAS seems to be based on AOA while STS is based on IAS, but is there any other major difference, such as trim rate, duration, range of trim operation, etc? Would a pilot of a MAX aircraft who knew about MCAS handle the JT610 scenario any differently to one who did not?
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Old 14th Nov 2018, 05:53
  #1145 (permalink)  
 
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Originally Posted by BuzzBox View Post
The B737 FCOM states:
How does that differ from the operation of MCAS when approaching a stall? MCAS seems to be based on AOA while STS is based on IAS, but is there any other major difference, such as trim rate, duration, range of trim operation, etc? Would a pilot of a MAX aircraft who knew about MCAS handle the JT610 scenario any differently to one who did not?
Complete the Runaway Stabiliser NNC checklist memory items prior to the flaps being retracted. I just so happens that the first 2 items of the runaway checklist ALREADY covered by the Airspeed Unreliable NNC memory items.

Net result is actually very simple: Airspeed Unreliable NNC memory items, THEN stab trim cutoff switches.....CUTOFF

Originally Posted by JRBarrett​​​
I’m still very curious as to why a system that is designed to automatically change the aircraft configuration without pilot input (via stab trim) would act based on the output data of only one of the two AOA probes. The stick pusher, which also activates based on AOA, requires both probes to agree that a stall is imminent before it will trigger, specifically because AOA probes can and do malfunction.
$$$$ - Saved some development or certification costs. Rated De covered this succinctly in #1083
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Old 14th Nov 2018, 05:58
  #1146 (permalink)  
 
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Originally Posted by JRBarrett View Post

I assume that operation of the pusher is well covered in sim and classroom training, but it appears that Max flight crews have been kept completely in the dark about the MCAS system and what it does (until now).

There’s an old saying about the word “assume”, and there is no ‘pusher’ on any 737 model. Including the MAX8.
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Old 14th Nov 2018, 06:12
  #1147 (permalink)  
 
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Pilots were kept in the dark. One has to ask why? What reason would you have to not disclose an additional system? https://www.seattletimes.com/busines...ion-air-crash/

One question worth asking is does this additional system affect the type certificate? If the answer to that is yes then it opens a whole new can of worms.
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Old 14th Nov 2018, 06:22
  #1148 (permalink)  
 
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Now that the new MCAS is firmly implicated, I wonder why it was implicated on the MAX. One possible reason was that it was considered to be required by transport airworthiness certification via section 25. But looking through section 25 it is hard to find which exact requirement. The full regs can be easily googled (can't post as am probationary)

The previous requirement for STS is fairly easy to understand, as section 25.173 calls for "(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots". Effectively the certification standard demands the aircraft emulate (some!) characteristics of a theoretical longitudinally stable aircraft which would have:
a) COG significantly ahead of the Neutral point
b) generally have the trimmed horiz stab flying at significantly lower AoA than the wing (probably to the point of downward lift at the tail)

However for modern air transports, which are desired to have optimal efficiency, it should be best to operate at near neutral longitudinal stability (i.e. just stable). In this configuration the tail is generating very little lift in either direction and hence very little drag. "Manual flight" is really "Simulated longitudinally stable manual flight necessary to conform to 25.173", which is confusing because as other posters have noted, this is really a less-automated mode instead of a manual mode. It's not so relevant to this thread anymore, but the STS seems more regulatory than useful really...changing the stab trim position via STS does not really create a more longitudinally stable aircraft and the pilot is likely to respond by removing the trim input anyway to avoid unwanted altitude excursions. This leaves him back with a slightly stable aircraft which would probably cause fatigue if pilots were expected to fly whole sectors in "manual mode", but of course they are not.

So why MCAS? It is not a speed stability system, it appears to be a beefed up stall prevention system. If the MAX COG has crept further back, it's stability margin has decreased but I don't see why an antistall stab trimmer becomes a good idea or a regulatory requirement. Looking through the stall/stall warning sections (25.201, 25.203, 25.207) there is a preference for aggressive pitch down at stall which may have something to do with it:The airplane is considered stalled when the behavior of the airplane gives the pilot a clear and distinctive indication of an acceptable nature that the airplane is stalled. Acceptable indications of a stall, occurring either individually or in combination, are—
(1) A nose-down pitch that cannot be readily arrested;
(2) Buffeting, of a magnitude and severity that is a strong and effective deterrent to further speed reduction; or
(3) The pitch control reaches the aft stop and no further increase in pitch attitude occurs when the control is held full aft for a short time before recovery is initiated. If anyone has a better understanding of how MCAS is dictated by section 25 airworthiness would be very interesting to hear.
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Old 14th Nov 2018, 07:09
  #1149 (permalink)  
 
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Originally Posted by BuzzBox View Post
The B737 FCOM states:



How does that differ from the operation of MCAS when approaching a stall? MCAS seems to be based on AOA while STS is based on IAS, but is there any other major difference, such as trim rate, duration, range of trim operation, etc? Would a pilot of a MAX aircraft who knew about MCAS handle the JT610 scenario any differently to one who did not?
Explanation of sorts here: https://theaircurrent.com/aviation-s...em-mcas-jt610/
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Old 14th Nov 2018, 07:47
  #1150 (permalink)  
 
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I stand by the theory that this is not MAX specific and that Boeing have been caught out on a technicality only(albeit a very embarassing one).
Its difficult to theorize without any data but I am guessing STS doesnt actually exist on the MAX but has been replaced by the "enhanced" MCAS,which actually performs the same or very similar function(s).The accident report will have to use the new name MCAS and since no pilots actually have any FCOM data on that system but only on its presumably very similar predecessor, STS,its a difficult but mandatory admission for Boeing to make before the investigation continues.

However,we know that the previous commander recognized the unwanted insidious trim and actually noted it in the tech log as STS input simply because he had never heard of MCAS.The NG AMM clearly describes how STS will trim to the stops under certain conditions if the pilot doesnt intervene.
The fact that it hasnt happened on the NG doesnt mean it cant; rather,it just means the specific sequence hasnt lined up yet,or if it has it was controlled by the crew.Suppose 610 hadnt crashed (for example if the AoA failure had been right side on 610,then there wouldnt have been any trim down as FCC A would have been controlling...1st flight of day) and Lionair maintenance got it right on the 5th attempt and finally fixed the fault.Would there be an AD?..............
Its those holes in the swiss cheese again that have to line up for an accident to occur.

Point being,that whether we choose to call it STS or MCAS,unwanted insidious trim contributed to this accident. Insidious because its not actually a runaway.Insidious because it can be overriden ..Insidious because the AP stab trim motor is quieter and slower.Insidious because the noise of the continuous stick shaker was highly distracting.
I see that the press are out in fulll force proclaiming Boeing causes air disaster...well,they may well be in for a bit of trouble, but this accident was avoidable and the crew wouldnt need to know the difference(s) between STS/MCAS to do so.The previous flight proves that.
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Old 14th Nov 2018, 07:54
  #1151 (permalink)  
 
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I love the way that, after 1000+ posts, we're back at Aerodynamics 101.
Originally Posted by Small cog View Post
Originally Posted by Concours77 View Post
No airplane can fly with zero AoA.
Wow! I don’t see a lot of aircraft falling from the sky. Could you be incorrect?

No, not incorrect, just badly worded.
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Old 14th Nov 2018, 08:54
  #1152 (permalink)  
 
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Originally Posted by Concours77
No airplane can fly with zero AoA.
I love the way that, after 1000+ posts, we're back at Aerodynamics 101. [
No, not incorrect, just badly worded.
Badly worded, and certainly not correct. Clear need to define terms and check textbooks.
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Old 14th Nov 2018, 09:25
  #1153 (permalink)  
 
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Originally Posted by Rananim View Post
I stand by the theory that this is not MAX specific and that Boeing have been caught out on a technicality only(albeit a very embarassing one).
Its difficult to theorize without any data but I am guessing STS doesnt actually exist on the MAX but has been replaced by the "enhanced" MCAS,which actually performs the same or very similar function(s).The accident report will have to use the new name MCAS and since no pilots actually have any FCOM data on that system but only on its presumably very similar predecessor, STS,its a difficult but mandatory admission for Boeing to make before the investigation continues.

However,we know that the previous commander recognized the unwanted insidious trim and actually noted it in the tech log as STS input simply because he had never heard of MCAS.The NG AMM clearly describes how STS will trim to the stops under certain conditions if the pilot doesnt intervene.
The fact that it hasnt happened on the NG doesnt mean it cant; rather,it just means the specific sequence hasnt lined up yet,or if it has it was controlled by the crew.Suppose 610 hadnt crashed (for example if the AoA failure had been right side on 610,then there wouldnt have been any trim down as FCC A would have been controlling...1st flight of day) and Lionair maintenance got it right on the 5th attempt and finally fixed the fault.Would there be an AD?..............
Its those holes in the swiss cheese again that have to line up for an accident to occur.

Point being,that whether we choose to call it STS or MCAS,unwanted insidious trim contributed to this accident. Insidious because its not actually a runaway.Insidious because it can be overriden ..Insidious because the AP stab trim motor is quieter and slower.Insidious because the noise of the continuous stick shaker was highly distracting.
I see that the press are out in fulll force proclaiming Boeing causes air disaster...well,they may well be in for a bit of trouble, but this accident was avoidable and the crew wouldnt need to know the difference(s) between STS/MCAS to do so.The previous flight proves that.
finally a post making sense!
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Old 14th Nov 2018, 09:34
  #1154 (permalink)  
 
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Originally Posted by Rananim View Post
I stand by the theory that this is not MAX specific and that Boeing have been caught out on a technicality only(albeit a very embarassing one).
Its difficult to theorize without any data but I am guessing STS doesnt actually exist on the MAX but has been replaced by the "enhanced" MCAS,which actually performs the same or very similar function(s).The accident report will have to use the new name MCAS and since no pilots actually have any FCOM data on that system but only on its presumably very similar predecessor, STS,its a difficult but mandatory admission for Boeing to make before the investigation continues.

However,we know that the previous commander recognized the unwanted insidious trim and actually noted it in the tech log as STS input simply because he had never heard of MCAS.The NG AMM clearly describes how STS will trim to the stops under certain conditions if the pilot doesnt intervene.
The fact that it hasnt happened on the NG doesnt mean it cant; rather,it just means the specific sequence hasnt lined up yet,or if it has it was controlled by the crew.Suppose 610 hadnt crashed (for example if the AoA failure had been right side on 610,then there wouldnt have been any trim down as FCC A would have been controlling...1st flight of day) and Lionair maintenance got it right on the 5th attempt and finally fixed the fault.Would there be an AD?..............
Its those holes in the swiss cheese again that have to line up for an accident to occur.

Point being,that whether we choose to call it STS or MCAS,unwanted insidious trim contributed to this accident. Insidious because its not actually a runaway.Insidious because it can be overriden ..Insidious because the AP stab trim motor is quieter and slower.Insidious because the noise of the continuous stick shaker was highly distracting.
I see that the press are out in fulll force proclaiming Boeing causes air disaster...well,they may well be in for a bit of trouble, but this accident was avoidable and the crew wouldnt need to know the difference(s) between STS/MCAS to do so.The previous flight proves that.
Spot on. You nailed it
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Old 14th Nov 2018, 09:36
  #1155 (permalink)  
 
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Originally Posted by Rananim View Post
I stand by the theory that this is not MAX specific and that Boeing have been caught out on a technicality only(albeit a very embarassing one).
Its difficult to theorize without any data but I am guessing STS doesnt actually exist on the MAX but has been replaced by the "enhanced" MCAS,which actually performs the same or very similar function(s).The accident report will have to use the new name MCAS and since no pilots actually have any FCOM data on that system but only on its presumably very similar predecessor, STS,its a difficult but mandatory admission for Boeing to make before the investigation continues.

However,we know that the previous commander recognized the unwanted insidious trim and actually noted it in the tech log as STS input simply because he had never heard of MCAS.The NG AMM clearly describes how STS will trim to the stops under certain conditions if the pilot doesnt intervene.
The fact that it hasnt happened on the NG doesnt mean it cant; rather,it just means the specific sequence hasnt lined up yet,or if it has it was controlled by the crew.Suppose 610 hadnt crashed (for example if the AoA failure had been right side on 610,then there wouldnt have been any trim down as FCC A would have been controlling...1st flight of day) and Lionair maintenance got it right on the 5th attempt and finally fixed the fault.Would there be an AD?..............
Its those holes in the swiss cheese again that have to line up for an accident to occur.

Point being,that whether we choose to call it STS or MCAS,unwanted insidious trim contributed to this accident. Insidious because its not actually a runaway.Insidious because it can be overriden ..Insidious because the AP stab trim motor is quieter and slower.Insidious because the noise of the continuous stick shaker was highly distracting.
I see that the press are out in fulll force proclaiming Boeing causes air disaster...well,they may well be in for a bit of trouble, but this accident was avoidable and the crew wouldnt need to know the difference(s) between STS/MCAS to do so.The previous flight proves that.
As nearly as I can tell, you theory is that Boeing just changed the name from STS to MCAS and that whatever happened to the Lion airplane could happen to other 737 model prior to the MAX . There's a lot of evidence which suggests that this theory is fanciful. Not the least of whcih is the fact that Boeing, after realizing that they had a serious problem that was killing entire planeloads of Boeing passengers at a whack, and after careful consideration, issued a Manual bulletin, and subsequent Emergency Airworthiness Directive which describes the problem as being exclusive to the 737 MAX models, and not applicable to 737NG or prior. Even given that Boeing hasn't behaved admirably leading up to the accident, It is very difficult to imagine the folks at Boeing, after realizing what likely led to the accident, deciding: "Hey, even though we realize there's a serious problem, and even though this problem could just as likely cause an NG to crash through the same sequence of events, we're going to publicly pretend that this is limited to MAX models only. Point being, Boeing probably had pretty sound reasons for saying that the problem was exclusive to the 737 MAX -8 and -9, and not including the NG and prior models.
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Old 14th Nov 2018, 09:44
  #1156 (permalink)  
 
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Originally Posted by hans brinker View Post
The way I understand it from reading here:
On a normal flight you would expect the STS to trim ANU during acceleration as it is trying to keep the aircraft at the previous speed. TLDR: I think the STS trim being wrong write up was "correct", because the STS will during acceleration trim ANU not AND.
The STS normally trims nose down after take-off.
It is safer that way....

Silver
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Old 14th Nov 2018, 09:55
  #1157 (permalink)  
 
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Fuselage contributing to lift

The wing lift is associated with drag, called lift-dependent drag or 'induced' drag. It is a function of the wing aspect ratio and the spanwise lift distribution. To minimize induced drag you want to have a high aspect ratio and ideally an elliptical spanwise lift distribution. The drop-off of lift near the tip causes the tip trailing vortices.

If the lift dropped to zero at the wing roots you would effectively have two wings with half the aspect ratio and twice the induced drag as the complete wing, and 'tip' vortices at the tips and wing roots.

That doesn't happen. The spanwise lift distribution is continuous across the fuselage except perhaps a slight dip. In other words, the fuselage contributes to the overall lift.

Last edited by Gysbreght; 14th Nov 2018 at 09:57. Reason: correction of typographical error
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Old 14th Nov 2018, 10:04
  #1158 (permalink)  
 
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Originally Posted by Halfnut View Post

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.


That seems a lot of trim to me. Try flying at 5,000 and 250 kt, and then mis-trimming forward by 2.5 degrees. You will be struggling.

Strikes me that Boeing designed this stall-trim system assuming that the aircraft would be slow and approaching the stall, when 2.5 degrees of trim would be acceptable. But at 250 kt in a normal climb, 2.5 degrees of trim becomes a liability. And if it gives you another 2.5 degrees of trim, you are uncontrollable.

Sounds like Boeing made this system on the cheap. The reason aircraft have stick pushers, is because the push is instantaneous, on the stick, and its withdrawal is instantaneous too - so you can pull out of the dive. But if you ‘push’ via a trim input, you cannot easily undo that trim - and so you cannot easily recover from the dive. And as a basic flying principle, we always fly with the stick, and trim out the stick pressure. You never fly with the trimmer, which is what this system is trying to do.

Furthermore, an erroneous stall-trim input at 250 kt (due faulty AoA indication) will give huge stick forces that the system did not intend - and perhaps the designer and test pilots never considered. A faulty stick push can be easily overcome with stick pressure - it was designed that way, and haveing already got the T-shirt on that one, the aircraft is easily flyable. But if a system gives you 2.5 of trim, and then another 2.5 of trim, you are going down, because the stab-trim is much more powerful than the elevator. Did anyone ever test this...?

Silver

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Old 14th Nov 2018, 10:09
  #1159 (permalink)  
 
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- degrees
- units
- % MAC

Where does a 737 jock read "degrees" of the STAB ?
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Old 14th Nov 2018, 10:14
  #1160 (permalink)  
 
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Originally Posted by DaveReidUK View Post
That's true, though of course most wings, being cambered, do still produce some lift at 0° AoA.
I wish you would all stop this angle of attack nonsense. You are all confusing fuselage pitch-angle (on the PFD) and the wing angle (which can be changed with the rigging angle). Of course you can fly an aircraft with zero fuselage angle, as long as the wing still has a positive rigging angle, to maintain a positive wing angle of attack. Which is how most aircraft are built.

The reason most aircraft fly with 2.5 degrees pitch up, is that we do not fly at max speed. If you flew at MMO plus 3%, which is the max testing design speed, you would probably find that the fuselage had a zero pitch angle. (A negative fuselage angle would induce some efficiency and stability problems.)

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