Transavia France B738 at Paris on Feb 8th 2018, IAS, AOA and ALT disagree
Last Update: November 16, 2020 / 11:36:53 GMT/Zulu time
The French BEA rated the occurrence an incident and opened an investigation. The BEA reported that a post flight inspection revealed an AoA probe had failed. The previous evening, while enroute on a ferry flight from Norwich,EN (UK) to Orly following maintenance in Norwich the AoA probe had encountered a first failure.
On Feb 28th 2018 the NTSB announced they have assigned an accredited representative for the state of design and manufacture of the aircraft and aircraft components to join the investigation led by the BEA.
On Nov 16th 2020 the BEA released their final report reporting, a similiar occurrence on a ferry flight of Feb 7th 2018 was included in the same report, and concluded the probable causes of both incidents were:
The following factors contributed to the dysfunction of the right AOA sensor and the display of the IAS DISAGREE, AOA DISAGREE and ALT DISAGREE alerts during the take-off of the flights of 7 and 8 February.
- The contamination of resolver 2 of the right AOA sensor by a solvent which led to a failure of the resolver and erroneous speed and altitude indications, followed by the AOA, IAS and ALT DISAGREE alerts. The investigation was not able to determine the cause of this contamination which seems to be an isolated case nor the reason why this defect, present since the installation of the sensor on the aeroplane, led to the activation of the alerts during a post maintenance flight. However, it is possible that the handling of the sensor during maintenance exacerbated the dysfunction without the technicians realising this.
- The technician working on the aeroplane between the two flights not using the FIM. Its use would have ensured that a more complete check was carried out, the failure would have probably been detected and the sensor replaced.
The BEA analysed:
The Boeing 737-800, registered F-GZHO, was delivered new to Transavia and put into service on 9 February 2015. It had logged 8,927 flight hours for 4,397 cycles on 7 February 2018 and had just come out of a C-type maintenance inspection carried out by KLM UK Engineering in Norwich (United Kingdom). One of the maintenance operations carried out during this inspection required the two AOA sensors to be manually turned by 30°. The technicians who had carried out this operation had not noticed any anomaly.
Transavia France indicated that this sensor (manufactured in 2014) had not been disassembled from the aeroplane since its delivery to the company.
The crew of the flight of 7 February contacted the company during the flight to report the failure. A technician, employed by a subcontractor of the company which Transavia used for the maintenance of its planes, worked on the plane during the night. He explained that the only information that he had was that recorded in the AFL in which the crew mentioned the three IAS DISAGREE, AOA DISAGREE and ALT DISAGREE alerts. He tried to obtain more details about the event from the prime contractor maintenance company but the latter was unable to give him further information.
He therefore complied with the AMM procedures concerning the inspection of AOA sensors, Pitot probes and static pressure ports and the stall warning and did not observe anything abnormal. He added that the AOA sensor moved freely. He explained that he did not consult the procedures in the Fault Isolation Manual (FIM) corresponding to each alert but that the actions that he carried out ultimately corresponded to what was required by the FIM. He nevertheless specified that the FIM procedure for the AOA DISAGREE alert requires the SMYDC(12) to be tested in order to check whether it has recorded a failure and that he had not thought to do this. The SMYDC test carried out the next day, at the end of the second flight, revealed error messages concerning an AOA sensor anomaly requiring its replacement.
The technician explained that an AOA sensor failure is usually accompanied by an Autoslat Fail alert and that this alert was not reported by the crew. He thought that the AOA sensor might have iced which could explain why he had not detected any problem. The aeroplane was therefore returned to service for the flight of the following day.
Transavia provided the BEA with all the flight data recorded by the aeroplane’s maintenance recorder (QAR) since March 2015.
The analysis of this flight data brought to light that the dysfunction of the right AOA sensor gradually evolved over time with the values changing in increasingly longer steps compared to the left AOA sensor where the changes are a lot more dynamic. The following figures show the evolution of the AOA sensor dysfunction between 2015 and 2018 based on the recordings of the values provided by the left and right AOA sensors.
The start of this dysfunction can be detected in the QAR data as early as March 2015, just a few weeks after the aeroplane had been delivered to Transavia. The dysfunction then becomes significantly more marked and is visible up to the last flight before the maintenance work in Norwich. There is then a clear change in the behaviour of the angle of attack values recorded during the flights of 7 and 8 February and the dysfunction is less visible.
The activation of the AOA DISAGREE alert cannot be checked using the QAR data of the occurrence flight: the AOA DISAGREE alert is generated by comparing the values measured by resolver 2 of each sensor whereas the recorded angle of attack value may come from resolver 1, depending on the SMYDC selection logic. It is therefore probable that the difference between the resolver 2s before 7 February was not sufficient to activate an alert and that this difference increased during the flight of 7 February although the difference between the resolver 1s had decreased.
The faulty sensor was removed from the aircraft and sent to KLM EM(16) in Amsterdam for tests and disassembly in the presence of the BEA. The examination found damage to several internal components. The gears between the two resolvers were damaged and at ambient temperature, the shaft of resolver 2 was blocked in rotation and was slightly deformed. At operating temperature, reached after the vane heater test, the two resolvers gave indications which differed from each other by around 100°.
Resolver 2 was sent to the manufacturer of the AOA sensor, Collins Aerospace, who coordinated an examination with its subcontractor, MOOG, the resolver supplier. The examination of the resolver blocked in rotation found the presence of a viscous and tacky substance, between its stator and rotor, which prevented the relative movement of these two parts.
The Collins and Transavia historical records concerning the two AOA sensors on the aircraft indicate that no maintenance actions were carried out after delivery of the units to Boeing in February 2015. Therefore, it is possible that the viscous substance was introduced during the manufacturing process. It was not possible to determine, however, if the viscous substance found within the resolver was present from when either the resolver or the sensor was manufactured or if it was caused by exposure to an external environment after delivery.
Examinations carried out by Collins Aerospace and MOOG found that this substance was composed primarily of epoxy resin, used in the manufacture of resolvers (winding) but contaminated with one or more other substances.
In the two incidents, the pilots did not immediately carry out the memory items. In both cases, they first tried to identify the side which was supplying the erroneous information and initially used this assessment to continue the flight with the automatic systems engaged.
They did not set the recommended pitch attitude and thrust parameters. They indicated that they knew the applicable procedure well but that during take-off, the selected thrust and the pitch attitude of around 15° allowed the aeroplane to climb safely. They did not think it was opportune to reduce the pitch attitude and thrust in a critical flight phase such as the initial climb. They added that during simulator exercises with respect to the Airspeed Unreliable procedure, certain company instructors recommended to pilots to keep the pitch attitude of 15° and the take-off thrust in initial climb.
The captain of the flight of 7 February indicated that neither he nor the first officer mentioned out loud the difference in speed that they had observed when 80 kt was called out. He had not considered the situation dangerous and had not envisaged rejecting the take-off. He added that the VMC conditions had facilitated the management of the occurrence and that he would have perhaps reacted differently in IMC conditions.
During the flight of 8 February, the pilots had had time to talk and the instructor and captain asked the pilot in training if he was happy to continue the take-off despite the difference of 10 kt. The latter indicated that in retrospect he would have been very willing to reject the take-off but that the way the question was asked pushed him to reply in the affirmative. He specified that having joined the company very recently, he did not feel comfortable about contradicting the instructor.
The instructor and captain explained that he had not had time to analyse the consequences of the difference in speeds displayed on the PFDs for the rest of the flight and that he had not considered the situation dangerous when the IAS DISAGREE was activated. In retrospect, he thought that it would have been preferable to reject the take-off. He specified that it was the first time that he had been confronted with the occurrence of a failure between 80 kt and V1.
This article is published under license from Avherald.com. © of text by Avherald.com.
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