MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

[bill fly]

Originally Posted by Smythe

Really, how certain are you that it functions that way? Because Boeing stated so? How many pilot reports show that turning OFF AP resolves the situation? Lets see, to disconnect MCAS, you disconnect AP trim?

AoA high? really, how high does the AoA have to be for the wing to stall? MCAS pushes the nose down 2.5 degrees at low speeds to prevent stall? That is a hell of a different AoA on final...

Aside from that issue, what about the rest of the argument by myself and others?

Been grounded about 100 days now...just for stick feel...

Words fail... That whole post once again disqualifies you from commenting on aviation matters.

Are you a troll?

[Fly Aiprt]

Originally Posted by yoko1

Please tell me you aren't being serious here. If you really feel we are that bereft of information, there really isn't any point in continuing this discussion.

Indeed.
Lots of speculation here - one way or the other - but accurate technical info is scarce.
As usual, when things are sorted out and information is made public, some opinions here will appear to be right, and others to be wrong, but in the mean time, who actually knows ?

[WHBM]

American Airlines CEO Douglas Parker says the Max grounding is now down to politics

https://www.cnbc.com/2019/06/23/amer...s-737-max.html

Extraordinary ...

[yoko1]

Originally Posted by mattler

And just because you want to believe otherwise, doesn't make it so. That line goes both ways....


.

The thing is, the word "stability" has a very particular meaning when it comes to aviation. It is a definition that anyone can look up and see if the data supports one's position. Part of the problem is that some posters like to use the word "unstable." That in itself is not correct usage. There are two types of stability - static and dynamic. Within these category, there are further qualifications of positive, neutral, and negative. When someone says the 737 is "unstable," then that implies that it demonstrates negative static or dynamic stability. There is no evidence in the record that supports this position.

That being said, "positive" stability is not a fixed value. An aircraft can demonstrate more or less positive stability according to how quickly it returns to its initial state after a perturbation. The Part 25 of the FAR's specify the minimum required stability performance for transport aircraft. The lightening of the control forces in the pitch axis at high AOA is an indication that the degree of positive static stability decreases, but that doesn't mean it goes negative. MCAS does increase the positive stability characteristics of the 737, but that is a very different thing from saying that the aircraft is "unstable" without it.

[Less Hair]

What possible legal consequences concerning certification would it have to call MCAS some "anti stall" device? It feels like all that spin doctoring concerning the fine print wording might somehow be legally motivated?

[mattler]

yoko1. Fair enough, but it would seem to me that if an aircraft requires less and less back pressure as its angle of attack continues to increase, then you're looking at some kind of instability in that particular part of the envelope. The statement made by the Boeing engineer that I quoted above also implies this much.

We definitely ought to be using correct terminology here though, so I'm 100% with you there. This subject matter is a big beyond the scope of the avg aviator, myself included.

[etudiant]

Originally Posted by WHBM

American Airlines CEO Douglas Parker says the Max grounding is now down to politics

https://www.cnbc.com/2019/06/23/amer...s-737-max.html

Extraordinary ...

Seems quite reasonable comment, that the FAA does not want to be alone in releasing the MAX.
Given that the initial FAA certification left issues that caused other regulators to ground the aircraft, there is a rebuilding trust issue for the FAA which imho is most important for the agency. The MAX schedule is not their primary concern.

[zzuf]

Originally Posted by yoko1

The thing is, the word "stability" has a very particular meaning when it comes to aviation. It is a definition that anyone can look up and see if the data supports one's position. Part of the problem is that some posters like to use the word "unstable." That in itself is not correct usage. There are two types of stability - static and dynamic. Within these category, there are further qualifications of positive, neutral, and negative. When someone says the 737 is "unstable," then that implies that it demonstrates negative static or dynamic stability. There is no evidence in the record that supports this position.

That being said, "positive" stability is not a fixed value. An aircraft can demonstrate more or less positive stability according to how quickly it returns to its initial state after a perturbation. The Part 25 of the FAR's specify the minimum required stability performance for transport aircraft. The lightening of the control forces in the pitch axis at high AOA is an indication that the degree of positive static stability decreases, but that doesn't mean it goes negative. MCAS does increase the positive stability characteristics of the 737, but that is a very different thing from saying that the aircraft is "unstable" without it.

How do you know that it is not related to manoeuvre stability?
After all MCAS = Manoeuvre Characteristics Augmentation system.

[yoko1]

Originally Posted by Less Hair

What possible legal consequences concerning certification would it have to call MCAS some "anti stall" device? It feels like all that spin doctoring concerning the fine print wording might somehow be legally motivated?

The phrase "anti-stall" falls into the much the same category as "unstable." It is not a defined term for the purposes of the aircraft certification. While we can use it generically for any system or device that helps in the prevention of or recovery from stalls, it does not correspond to any particular certification requirement. There are requirements for stall warning devices and handling characteristics approaching and during a stall that are spelled out in detail in FAR Part 25, and it is those requirements that Boeing will point to when they explain why MCAS was needed.

In particular, FAR Part 25 make frequent references for the requirement that "the stick force curve must have a stable slope" in various flight regimes and that approaching a stall the "longitudinal control force must be positive up to and throughout the stall." The requirement for a "stable slope" in particular is what drove the need for MCAS.

[yoko1]

Originally Posted by zzuf

How do you know that it is not related to manoeuvre stability?

Define "manoeuvre stability" as it applies to Part 25 certification requirements and I might better be able to address your question.

[DaveReidUK]

Originally Posted by WHBM

American Airlines CEO Douglas Parker says the Max grounding is now down to politics

https://www.cnbc.com/2019/06/23/amer...s-737-max.html

Extraordinary ...

In the sense that the FAA will almost certainly feel the need to get EASA on side before lifting the grounding, then yes, it's down to politics.

[billybone]

Originally Posted by 568

If you fly an "RA" TCAS event, then the autopilot and Auto throttle should be disengaged. TCAS "TA" is to alert crew of other "traffic" but is not deemed to be a threat.TCAS "RA" maneuvers are generally flown with "smooth" control inputs for pitch up or down depending on the course of action. These events are not subject to high or low pitch angles in general, so MCAS probably wouldn't be a factor.

Thank you ! I realize that normally TCAS only provides up/down altitude change alerts supposedly at a range/time sufficient to avoid collision. As I understand it- MCAS only works/engages if/when AP is off, flaps up, etc. And that normally, the AOA sensor provides input to AP. But the disconnect/removal of the yoke/stick switch which has been standard for decades such that a pull or push in opposition to stab/trim input under a TCAS maneuver which would normally cut electric power to stabilizer might be a surprise to many. Seems to me if HAL( MCAS) is/was G limited by design - but no longer is - then a conflict between pilot and HAL is probable ??

[WHBM]

Originally Posted by DaveReidUK

In the sense that the FAA will almost certainly feel the need to get EASA on side before lifting the grounding, then yes, it's down to politics.

That is indeed correct and appropriate. However there's a subtext there that some political manoeuvring will occur between foreign certification authorities and that of the US.

[SLF3]

Have Boeing formally submitted ‘the fix’ to the FAA for approval?

[armchairpilot94116]

The Max is turning out to be the worst debacle that Boeing has ever faced. I can't think of a worst one.

I'm rooting for Boeing to get it all sorted out !

https://www.sfgate.com/technology/bu...h-14039499.php

[bnt]

Boeing are definitely working on the planes: according to a footage shot by the King 5 News channel in Seattle, Boeing is even using parts of the employee parking lots to house them.

[bill fly]

Originally Posted by bnt

Boeing are definitely working on the planes: according to a footage shot by the King 5 News channel in Seattle, Boeing is even using parts of the employee parking lots to house them.

Interesting pic - and the lifting influence of the Leap engines can be better appreciated from that angle.

Could be though, that they have been letting the employees use the odd Max at weekends...

[yoko1]

Originally Posted by billybone

Thank you ! I realize that normally TCAS only provides up/down altitude change alerts supposedly at a range/time sufficient to avoid collision. As I understand it- MCAS only works/engages if/when AP is off, flaps up, etc. And that normally, the AOA sensor provides input to AP. But the disconnect/removal of the yoke/stick switch which has been standard for decades such that a pull or push in opposition to stab/trim input under a TCAS maneuver which would normally cut electric power to stabilizer might be a surprise to many. Seems to me if HAL( MCAS) is/was G limited by design - but no longer is - then a conflict between pilot and HAL is probable ??

I'm having a hard time trying to make out what you are asking here, however let me throw out a few things out there that might help clarify.

MCAS only works with A/P off.

AOA provides an input into several different systems. That has not changed. However, I don't believe there is a direct input from the AOA sensor to the autopilot, but rather feeds indirectly through the Air Data Inertial Reference Unit (ADIRU) and the Stall Management Yaw Damper (SMYD).

When you refer to the "removal of the yoke/stick switch," I think you are talking about the control column cutout switches that prevent trimming in the opposite direction that the control column is displaced (that is, stops nose down trim when pulling back, stops nose up trim when pushing forward). This feature has not been removed and still functions as designed except that these switches are bypassed for MCAS activation (and only for MCAS activation).​​​​​​

Any maneuvers flown in response to a TCAS alert would be hand flown, but how the plane reacts would really be no different from any other hand flown maneuver.

Keep in mind that the MCAS software has already been changed to prevent the type of erroneous activation that occurred in the accident aircraft. The only time then that one would see MCAS activate during a TCAS event would be if the pilot mishandled the procedure and brought the aircraft close to a stall. In that case, you want MCAS to activate!

Let's please remember that last point. When the 737MAX approaches a stall, we want the MCAS to activate just like we want a stick pusher to activate on aircraft that are so equipped. We just don't want it to activate when it is not needed.

[BDAttitude]

Originally Posted by yoko1

The phrase "anti-stall" falls into the much the same category as "unstable." It is not a defined term for the purposes of the aircraft certification. While we can use it generically for any system or device that helps in the prevention of or recovery from stalls, it does not correspond to any particular certification requirement. There are requirements for stall warning devices and handling characteristics approaching and during a stall that are spelled out in detail in FAR Part 25, and it is those requirements that Boeing will point to when they explain why MCAS was needed.

In particular, FAR Part 25 make frequent references for the requirement that "the stick force curve must have a stable slope" in various flight regimes and that approaching a stall the "longitudinal control force must be positive up to and throughout the stall." The requirement for a "stable slope" in particular is what drove the need for MCAS.

From BA - I assume you are either directly or indirectly on their payroll:

As airspeed varies from a trimmed condition, the column force required to maintain a new speed (without re-trimming) is a measure of static longitudinal stability. For any conventional airplane, the location of the CG has the strongest influence on static longitudinal stability. For a statically stable airplane the required column force, as speed varies from the trimmed condition, is less at an aft CG than it is at a forward CG. The minimum average gradient allowed by U.S. Federal Aviation Administration FAR Part 25 is one pound for each six knots. As the CG moves aft (BDAttitude: or center of lift moves forward), it reaches a point where the stick force per knot drops to zero, then reverses. This location is called the neutral point.The difference between the actual CG location and the neutral point is called the static margin. With a CG forward of the neutral point, an airplane has a positive static margin and positive static longitudinal stability. At a CG aft of the neutral point, an airplane has a negative static margin, is statically unstable, and requires some form of augmentation to be flown with an acceptable workload.

So I will not call it "unstable" but "smaller than required or negative static margin hence not stable enough to be certifyable".

Never the less it is a stability problem not a handling beauty contest as you would like to spin it.

[yoko1]

Originally Posted by BDAttitude

So I will not call it "unstable" but "smaller than required or negative static margin hence not stable enough to be certifyable".

Thanks for the link, a lot of good information there. However, this document does not specifically address the 737. It does reference flight control augmentation on the MD-11 and 777, but I believe those aircraft actively manage c.g. for cruise performance and hence reduces the longitudinal stability.

The text you need to focus on is this:

The minimum average gradient allowed by U.S. Federal Aviation Administration FAR Part 25 is one pound for each six knots. As the CG moves aft, it reaches a point where the stick force per knot drops to zero, then reverses.

The reported performance of the 737MAX approaching a stall is one of stick forces decreasing, but never stick forces reversing. Thus we can say that the longitudinal stability decreases at high AOA, but it does not go negative. MCAS does improve the longitudinal stability at high AOA, but the 737MAX still demonstrates positive static stability in this area of the envelope.

​​​​​​There's probably another important point worth mentioning here. No one is claiming that the MAX doesn't need MCAS to meet the stability requirements of the FAR's. Well guess what? The 737NG doesn't meet the stability requirement without its Speed Trim System. I also suspect that just about every swept wing commercial transport doesn't meet the stability requirements without a yaw damper. Having a stability augmentation system (SAS) is nothing new in commercial aircraft - they've literally been in use for decades. It is worth noting, however, that the FAR's also place requirements on these same aircraft in case of failure of a SAS, and those requirements would basically preclude negative stability characteristics in absence of the SAS.