AoA Vane must have Failed, the Boeing Fix is In, Senate Grills FAA

New information confirms that MAX is similar to the NG, that the AoA analog interface is connected to two different computers, the Stall Management Yaw Damper (SMYD) and Air Data Inertial Reference Unit ADIRU) (which supplies the Flight Control Computer (FCC)). The SMYD uses AoA for Stall Warning and is evident by activation of Stick Shaker. FCC hosts MCAS, and if AoA from ADIRU too high it can trigger MCAS. The significance is that both SMYD and FCC responded to AoA large bias on JT043/JT610, and therefore, the AoA sensor must have been producing that erroneous output. The AoA sensor is what failed.

Boeing has confirmed that AoA disagree Alert will become basic, that AoA disagree will inhibit MCAS from triggering, and that MCAS cannot issue more than one trim command without AoA recovery. 

The Senate grilled FAA acting Administrator Dan Elwell over excessive delegations and allowing Boeing to sell "safety critical items". Elwell stumbled on some key points, notably describing the 737 MAX as Fly-By-Wire and that the aft column cutout switch can stop MCAS trim commands.

Both Boeing and the FAA continue to insist that MCAS "runaway" is easy to detect and simple to respond to, but neither of these claims align with observations.

How Did MCAS Get Here and What Hurdles Remain?

The Seattle Times reported that the development of MCAS was limited to features that would not jeopardize differences training, including any new warning light.

“A single point of failure is an absolute no-no,” said one former Boeing engineer who worked on the MAX"

"Rick Ludtke, a former Boeing engineer who worked on designing the interfaces on the MAX’s flight deck, said managers mandated that any differences from the previous 737 had to be small enough that they wouldn’t trigger the need for pilots to undergo new simulator training."

"He said that if the group had built the MCAS in a way that would depend on two sensors, and would shut the system off if one fails, he thinks the company would have needed to install an alert in the cockpit to make the pilots aware that the safety system was off. And if that happens, Ludtke said, the pilots would potentially need training on the new alert and the underlying system. That could mean simulator time, which was off the table." 

"Matt Menza, a former Boeing pilot who worked on the MAX, said that during flight testing of planes ready for delivery, he wasn’t aware of any events that indicated a problem with the stall warning or the MCAS system. But he said an ideal system would have been built on two angle-of-attack probes, so that a single bad value wouldn’t cause problems. Menza and two other pilots who have worked on the MAX said they were unaware that the system used only one AOA probe."

REVISED 27 March 10am - ADDED AC120-53B FSB discussion

Taking the Next Steps while Awaiting on the Preliminary Report from ET302

Little new information has been released offering any specific detail on the events regarding ET302, except vague statements of similarities to JT610. It has been reported that a flight observer on JT043 identified the stabilizer cutout as a corrective measure. Further, it has been reported that the crew on JT610 were calm and methodical in trying to identify the source of their difficulty, and that control was lost shortly after the First Officer took over. This last point confirmed my thoughts from examining the flight data back in November, where it looked like the column forces had been transferred just prior to the final MCAS trim commands.

Ethiopian ET302 similarities to Lion Air JT610

Reports from Ethiopian investigators have implicated the same Angle of Attack (AoA) sensor malfunction that was observed on Lion Air. Lion Air captain AoA sensor read about 22 degrees higher than the First Officer AoA sensor (a large bias error). Initial assessment of Lion Air AoA failure modes did not reveal any obvious electrical malfunction that could create the bias. The simplest explanation was that the AoA vane had been bent, causing a gross aerodynamic offset in the readings. If ET302 encountered the exact same offset, with the likelihood of it being bent exactly the same way not being conceivable, some other factor must be in play. For example, the ARINC 429 representation of AoA uses two's complement fraction binary notation (BNR). It is interesting to note that bit 26 represents 22.5 degrees which would be the bit "flipping" between the Captain and F/O AoA values (all other bits would match).  Is it possible that the ARINC 429 word is getting corrupted (software defect)?  If the ET302 offset was something like 20 or 24, this theory falls apart.

With this in mind, what are the issues left to restore 737 MAX airworthiness?

What have we learned this week?

The crash of Ethiopian ET302 brought a tragic beginning to what must be one of the worst weeks in aviation, ending with the grounding of the Boeing 737 MAX. What does Lion Air JT043 and JT610 teach us? Did we, as an industry do everything we should after JT610? Accusations of impropriety levied at Boeing and the FAA seemingly are always on the ready. Adding to the week was an update on Atlas 5Y3591 which at first seemed to be one thing, but with a slight wording change thankfully seems to be another altogether.

Comparing Ethiopian ET302 to Lion Air JT043, JT610

Did Ethiopian ET302 succumb to the same situation as Lion Air JT610? The FDR and CVR data is being processed as I write this, with a public release in some form expected by Monday March 18. Aviation authorities have grounded the Boeing 737 Max before receiving a report from this recorder data. The only information to the public is from commercial ADS-B data brokers, notably Flightradar24.com, whose recording is incomplete. Aireon has shared space-based ADS-B data with several parties, but not to the public. Canada and the US make reference to the Aireon data as a factor in their assessment, with the claim that there were sufficient similarities to be concerned.

Atlas Air 5Y3591 Dove because of Nose-Down Elevator Command

The NTSB has revealed that Atlas Air 767 Freighter flight 5Y3591 dove based on nose-down elevator command. The nose was pushed over 49 degrees nose down. The engines were brought up to full thrust. There was no stall warning that preceding these actions. The dive reached 430 knots before impacting. Pitch attitude rose from -49 deg. to -20 deg. in the dive.

Exploring Rain Fade in an Extreme Rain Fall Zone

Rain Fade is a factor with Ku-band and Ka-band satellite communications.  Rainfall rate is measured in mm/hour. The occurrence of significant Rain Fade events is scaled by the rainfall rate and its duration. Generally these events are transient as the rain cloud moves through. The path of interest is only the line from the satcom terminal to the servicing satellite. Only while the rain cloud is in the way do problems occur.

Panama is in a very severe rainfall rate region and will create frequent issues with Ka band service operating below about 15,000 feet. Ku should operate through these scenarios with less disruption. It is prominent in the region along with the Brazilian rain forests and significant as a hub airport. 

5Y3591 Stabilizer Trim Actuator appears to be Nose Down

When KHOU took a video tour of the collected wreckage, they included a frame of the stabilizer trim actuator (the jackscrew).

Limiting RF exposure to Humans in Close Proximity to Satellite Transmitters

Satellite radios produce powerful Radio Frequency (RF) emissions. A method is provided for limiting the RF exposure to humans that are in close proximity to satellite transmitter antennas by establishing radial keep-out zones.