I retract. We don't know for sure that the shop was calibrating sensors incorrectly. However, given how badly things were being done at Xtra, and the specific deficiencies regarding the AOA sensor calibration, there's a very good chance that is what went wrong.
What specific deficiencies were identified? Any source for "how badly things were being done at Xtra"??
aerolimani wrote:My interpretation of page 89-91:
If you haven't already, you should read the sections in the JT610 report, starting on pages 89, 202, and 227.
They were only able to show the possibility of an offset being inserted into the calibration procedure. They did not conclude that Xtra Aerospace actually did make that mistake, and make that mistake on the accident sensor.
They also state they witnessed a Xtra employee perform a calibration of identical AOA sensor, and concluded it was done satisfactorily (at least they don’t state any issues with what was done):
“An Xtra Aerospace technician performed tests 3.A through 3.E from the CMM Revision 8 Testing and Fault Isolation section. The tests performed were the Insulation Resistance test, Vane Friction test, Heater Current test, and Alignment Accuracy test. For this demonstration, the technician used a North Atlantic 8810A Angle Position Indicator (API) to measure the 0861FL1 resolver outputs. A Peak Electronics SRI-201B API (“Peak API”) in “relative” mode, is used to read the synchro connected to the vane zero/indexing test stand (Note the Peak API could also be used for measuring the AOA resolver outputs, but for this demonstration the North Atlantic 8810A was used). The CMM specifies that resolver angles should be measured using a North Atlantic Model 8810A Angle Position Indicator (API), but includes the note “Equivalent substitutes may be used.” The Peak SRI-201B is not listed in the CMM.”
Then they go into the Xtra maintenance history to determine what equipment was used during the time frame the crash AOA sensor was repaired by Xtra. They don’t state which of the equipment in the list was used during the crash sensor calibration, other than the Peak SRI-201B was on the list.
“Xtra Aerospace utilized several pieces of test equipment to complete repair and evaluations on the AOA sensor that were not specified in the CMM Revision 8. Xtra Aerospace instead utilized the following equipment in service at the time of repair of S/N 14488:
• Peak Electronics SRI-201B (Model 7724-00-2) (Peak API) (quantity 3); and
• North Atlantic 8810A (quantity 1).
Finally, the report states that, while the Peak SRI-201B is not listed in the CMM, the FAA Flight Standards District Office (FSDO) accepted its use based on Xtra’s engineering test equipment equivalency report.
The report states that they observed Xtra using the Peak API in relative mode for the vane indexing/zero fixture setup:
“A Peak Electronics SRI-201B API (“Peak API”) in “relative” mode, is used to read the synchro connected to the vane zero/indexing test stand (Note the Peak API could also be used for measuring the AOA resolver outputs, but for this demonstration the North Atlantic 8810A was used).
However, the 25 deg offset calibration scenario they demonstrated, used the Peak API for measuring resolver outputs in relative mode. But the FAA investigation stated they only observed the Peak API being used during the vane indexing/zero setup. So absolutely no impact on any of this IMO.
“The exemplar AOA unit was secured in the vane zero fixture, with both resolver gears disengaged from the main gear but positioned to 45°. The REL/ABS switch on the Peak API was positioned to ABS. Resolver 2 was then rotated until the display on the Peak API displayed 25° (Resolver 1 was not adjusted; its position remained at 45°). With the Peak API connected to Resolver 2, the REL/ABS switch was moved to the REL position at which time the Peak API display changed to indicate approximately 0°.
More importantly, Xtra had no documentation of how to conduct the test using their nonstandard equipment; something supposedly required in the FAA authorization of alternate test equipment. While it is circumstantial, I think it's worthwhile to note that the FAA chose to shut down Xtra, and not simply fine them and monitor them for a while.
The FAA and Boeing are motivated to find the AOA defective rather than a production issue with installing ADIRU assemblies on Max planes in Renton. Further, the FAA shutdown the repair shop a full year after the crashes, and just days before Boeing's CEO was to testify to Congress.
Let's also be clear that the report says most likely improperly calibrated. It's not imprecise, as you seem to indicate.
The KNKT needs to justify where and how they come to their conclusions. If they aren't 100% sure about something, they should at least state other possible scenarios like an issue with the ADIRU, for example.My interpretation of page 202, 227
KNKT statement in section 2.6 is unfounded:
“The Xtra Aerospace utilized the Angle Position Indicator (API) Peak Model SRI- 201B (Model 7724-00-2) for test and calibration repair of the accident AOA sensor, part number 0861FL1 serial number 14488.
The investigation by the FAA described on page 89-91 only says that the Peak API was on the list of capable test equipment, of which an approved North Atlantic API was also listed as well.
The rest of KNKT statements in this section are only guesses and hypotheticals about what caused the 21 deg offset in the crash DFDR data, ending with:
“This immediate 21° delta indicated that the AOA sensor was most likely improperly calibrated at Xtra Aerospace.
Page 227 are KNKT’s recommendations for Xtra Aerospace operating manuals, based only on KNKT’s hypotheticals.