Boeing 737 Max Recertification Testing - Finally.
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Simply put:
Even though there should/could be a way to reduce takeoff thrust in the event of unreliable airspeed, or low altitude level off, leading to full thrust at altitude capture, this isn't part of the autothrottle logic. Take off thrust will still be the AT mode unless SPD (speed) has been selected on the MCP panel in the event of a "low altitude" capture or for other reasons as stated in the initial report. The takeoff situation with ET and the AT at full thrust with all the other caution/warnings and MCAS activation were the precursor to this tragic accident.
Boeing has "memory items" for flight with unreliable airspeed which is covered during transition and UPRT training.
A difference of 5 knots between Captain and First Officer airspeed tape on the PFD (primary flight display) triggers "IAS disagree" on both PFD's!
Autopilot (if engaged).......................................Disengage
Autothrottle (if engaged)....................................Disengage
F/D switches (both)................................................OFF
Set the following gear up pitch attitude and thrust:
Flaps extended.................................... 10° and 80% N1
Flaps up........................................... 4° and 75% N1
Even though there should/could be a way to reduce takeoff thrust in the event of unreliable airspeed, or low altitude level off, leading to full thrust at altitude capture, this isn't part of the autothrottle logic. Take off thrust will still be the AT mode unless SPD (speed) has been selected on the MCP panel in the event of a "low altitude" capture or for other reasons as stated in the initial report. The takeoff situation with ET and the AT at full thrust with all the other caution/warnings and MCAS activation were the precursor to this tragic accident.
Boeing has "memory items" for flight with unreliable airspeed which is covered during transition and UPRT training.
A difference of 5 knots between Captain and First Officer airspeed tape on the PFD (primary flight display) triggers "IAS disagree" on both PFD's!
Autopilot (if engaged).......................................Disengage
Autothrottle (if engaged)....................................Disengage
F/D switches (both)................................................OFF
Set the following gear up pitch attitude and thrust:
Flaps extended.................................... 10° and 80% N1
Flaps up........................................... 4° and 75% N1
Last edited by 568; 8th Mar 2021 at 22:25. Reason: format
https://theaircurrent.com/industry-s...n-737-max-787/
...didn't watch...but seems he's sticking his nose into a prickly pear...
...didn't watch...but seems he's sticking his nose into a prickly pear...
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568
The Airspeed Unreliable procedure is not necessarily the first thing you do when you get an IAS Disagree alert.
The condition for the procedure is: Airspeed or Mach indications are suspected to be unreliable.
The objective of the procedure is: To identify a reliable airspeed indication, if possible, or to use the Flight With Unreliable Airspeed table in the Performance Inflight chapter for the remainder of the flight.
According Boeing items which might indicate unreliable airspeed are:
•Difference between captain and first officer airspeed indications
•Speed/altitude information not consistent with pitch attitude and thrust setting
•Blank or fluctuating airspeed indication
•Continuous or intermittent stick shaker
•IAS DISAGREE alert
•ALT DISAGREE alert
•AOA DISAGREE alert
•SPD failure flag
•SPD LIM failure flag
•Erroneous minimum speed bars
•Erroneous maximum speed bars
•Airspeed low aural
•Airspeed low PFD indications on one side
•Overspeed warning
•Erroneous Flight Director (FD) pitch command bar
•Radome damage or loss
If the AOA DISAGREE alert is shown, one or more of the following flight deck effects can occur:
•Erroneous Pitch Limit Indicator (PLI)
•Windshear alerts
•Autoslat operation
The Airspeed Unreliable procedure is not necessarily the first thing you do when you get an IAS Disagree alert.
The condition for the procedure is: Airspeed or Mach indications are suspected to be unreliable.
The objective of the procedure is: To identify a reliable airspeed indication, if possible, or to use the Flight With Unreliable Airspeed table in the Performance Inflight chapter for the remainder of the flight.
According Boeing items which might indicate unreliable airspeed are:
•Difference between captain and first officer airspeed indications
•Speed/altitude information not consistent with pitch attitude and thrust setting
•Blank or fluctuating airspeed indication
•Continuous or intermittent stick shaker
•IAS DISAGREE alert
•ALT DISAGREE alert
•AOA DISAGREE alert
•SPD failure flag
•SPD LIM failure flag
•Erroneous minimum speed bars
•Erroneous maximum speed bars
•Airspeed low aural
•Airspeed low PFD indications on one side
•Overspeed warning
•Erroneous Flight Director (FD) pitch command bar
•Radome damage or loss
If the AOA DISAGREE alert is shown, one or more of the following flight deck effects can occur:
•Erroneous Pitch Limit Indicator (PLI)
•Windshear alerts
•Autoslat operation
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Agreed, but I wrote that an IAS Disagree triggers when a 5 knot difference is suspected between Captain and FO PFD's but didn't imply that the airspeed unreliable QRH should be used first.
But, if you read the QRH for IAS disagree, the next step is to read the airspeed unreliable QRH!
But, if you read the QRH for IAS disagree, the next step is to read the airspeed unreliable QRH!
Last edited by 568; 5th May 2021 at 16:22. Reason: text
As suggested previously, when encountering the longitudinal stability issue, Boeing could have completed aerodynamic mods to the design to remove the anomaly, which resulted from the nacelle adding lift to the wing/nacelle structure that was more than anticipated. That alone would have suggested that an angle grinder should be taken to the strakes to trim their nose hair a shade. Instead, they came up with a neat trick to change a high speed design that had questionable redundancy on trigger events to being low speed too by removing one of the two trigger conditions which gave a single point of failure as a matter of certainty. As AOA probes have a fairly modest MTBF in use, that wasn't a great concept.
Here is a set of charts that show the effect of having strakes or not, which would have been a relatively minor change to the aircraft. As the engines are inboard, it is a matter of certainty that reducing the section CLmax proximate to the nacelle would have ended up in an improvement in the stick force/g. Being judicious, the effect to Vs1g would have been quite modest, and surely, please surely the OEM noted that the stall speed was curiously lower with their design than expected, otherwise, they need a serious boot in the bottom of their trousers for being myopic.
Here is a set of charts that show the effect of having strakes or not, which would have been a relatively minor change to the aircraft. As the engines are inboard, it is a matter of certainty that reducing the section CLmax proximate to the nacelle would have ended up in an improvement in the stick force/g. Being judicious, the effect to Vs1g would have been quite modest, and surely, please surely the OEM noted that the stall speed was curiously lower with their design than expected, otherwise, they need a serious boot in the bottom of their trousers for being myopic.
PM me and I can send you that research paper, it is recent, but it shows what I previously commented on 2 years ago. It isn't from TBC, but it should have been obvious to anyone who had more than a passing knowledge in aerodynamics. Heck, had anyone bothered to sit down with a cup of coffee and have a cogitation on the subject that should have been obvious. I alluded to this when MCAS became publicly disclosed, you may recall; it would have been a few quick test flights to optimize, and corrections to the performance to remove the surprise benefit that the nacelles would have certainly given to Vs1g, at the apparent surprise cost of non-linear longitudinal stability. That's what I would have recommended, but that opportunity was lost through the way things work in external certification in August 2016. Anyway, long stab in this case was a simple fix had anyone bothered to ask outside of the cloistered workshop. Institutions become parodies of themselves, with diminishing returns over time; it is the nature of systems without external input and reviews. Sucks to be us, I guess.
P.S.: While the image and graphs show CL and CD, it should be clear to anyone involved in the art that lowering the CLmax at high alpha for an inboard section of a swept wing will increase the nose down pitching moment of the full wing directly, and will result in a reduced down wash in the affected area proximate to the nacelle/wing flow field that will result in a reduction in the CL of the tail plane for part span in the wake of the wing, which would directly have improved the stick force/g of the aircraft. Quick guess, the effect on stall speed would be less than 1.5% change, and that would have been able to be optimised. Thats about a ~2kt Vs1g change, which is... about... maybe 100' of TODR, or 200lbs of payload loss, rough order.
FWIW, the guys that did the CFD did a nice job, seems to be void of artifacts, and shows some flow structures that are pleasing to see properly modeled.
Last edited by fdr; 21st Sep 2022 at 12:55. Reason: P.S.:
I am guessing that the nacelle strakes would have added drag and thus marginally reduced the fuel savings of the MAX over the NG. Since everything at Boeing is about the money that alone would be enough for senior management to tell staff to find some software cludge that would allow them to smoke it past the FAA...
It would be a confidence builder if that included a drag-polar comparison to show the amount of fuel penalty and, of course, the reason for the investigation, the Cm vs Alpha chart. Edit: that Cm vs Alpha chart should also be transposed to Alpha vs Stick force.
Last edited by MechEngr; 21st Sep 2022 at 17:03.
This is nearly identical to the solution that the Speed Trim System (STS) provides with no notable problems. The first crew to encounter the adverse behavior commented that they thought STS was running backwards. Notable is that MCAS operated exactly as it was specified to operate - the ADIRU did not. To my way of thinking MCAS exposed a number of really bad decisions that should have been corrected decades earlier, including suppliers who were able to supply mis-calibrated AoA sensors due to a single switch on a multifunction test box (crash 1.)
I am guessing that the nacelle strakes would have added drag and thus marginally reduced the fuel savings of the MAX over the NG. Since everything at Boeing is about the money that alone would be enough for senior management to tell staff to find some software cludge that would allow them to smoke it past the FAA...
Note similarity of vane on the nacelle on the B737NG below:
Instead of curing an aerodynamic problem with common sense and using aerodynamics, the operators got Heath Robinson's repurposed "all things to all people" speed trim system.
The manufacturer is a collective of individually competent and dedicated professionals, working within the lines of communications that unfortunately appear to act to inhibit cross discipline questioning, resulting in lost opportunities. The lack of inquisitive input to issues gives inevitable results; to a carpenter, every problem is cured with a hammer, etc. Our regulatory requirements lead towards the current structure as a natural consequence, not just in aviation, IMHO.
Given the problem that was encountered, as I stated over 2 years ago on the public disclosure of MCAS as a system, the suggested solution would have been to reduce the size of the nacelle vane; the fundamental issue being the nacelles generating excessive Cm component from excessive CL at high AOA. That the nacelles would be causing excessive CL compared to the earlier design was stated by me in an appropriate forum in Aug 2016, but what had been done to fix that was never disclosed. It is improbable that the system architecture would ever have been disclosed which would have been necessary to ask what on earth they were smoking in Renton with the system architecture.
Would urge that the OEM implement specialist team(s) of generalists to assess systems and designs unfettered by discipline divisions. FWIW, QA per sew is not the solution, they have their place, in assessing system integrity (of course, victimizing your QA Inspectors is hardly reasonable and comes with it's own set of inevitable, forseeable consequences) But QA itself doesn't evaluate assumptions against common sense. What "Critical Design Reviews" are essentially supposed to be but are not.
Last edited by fdr; 21st Sep 2022 at 20:58.
This is nearly identical to the solution that the Speed Trim System (STS) provides with no notable problems. The first crew to encounter the adverse behavior commented that they thought STS was running backwards. Notable is that MCAS operated exactly as it was specified to operate - the ADIRU did not. To my way of thinking MCAS exposed a number of really bad decisions that should have been corrected decades earlier, including suppliers who were able to supply mis-calibrated AoA sensors due to a single switch on a multifunction test box (crash 1.)
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So the FAA wrote a letter to Boeing that Boeing needs to provide certain materials. It's not Boeing waiting for the FAA. It's the FAA waiting for Boeing.
That is proprietary information, not sure TBC would be dishing that out, but there is reasonableness that comes from renting a MAX sim and doing a QTG flyout. The margins permitted for an approved simulator are quite large but will give some semblance of the polar, at least within normal envelope, and will give a fair answer on control loads, and derivatives. Getting hands on the QTG cards themselves is probably going to be frowned on, they are still proprietary information. The sims aren't the M-CAB or the real thing. The airline operators have accurate information from the parameters that are being pulled for the QAR data for monitoring, W/delta is a pain from line data, you need a stack of it to get a statistically significant performance set unless you have some exact data points from the OPM and digital performance DB.
I am guessing that the nacelle strakes would have added drag and thus marginally reduced the fuel savings of the MAX over the NG. Since everything at Boeing is about the money that alone would be enough for senior management to tell staff to find some software cludge that would allow them to smoke it past the FAA...