Jetstar Japan A320 enroute on Jul 9th 2016, unreliable airspeeds
Last Update: September 20, 2018 / 19:26:39 GMT/Zulu time
Japan's TSB have opened an investigation into the occurrence.
The airline reported the occurrence was rated a serious incident and is being investigated.
Japan's Ministry of Transport reported the indicated air speeds varied between 270 and 60 knots.
On Oct 19th 2016 the US NTSB reported they have appointed an accredited representative representing the state of design and manufacture of the Pitot Static Probes to join the investigation led by the JTSB.
On Sep 20th 2018 the JTSB released their final report concluding the probable cause of the serious incident was:
It is probable that this serious incident occurred because the icing occurred in the Pitot tube when the aircraft was flying at an altitude of 37,000ft, which led to the temporary failure of airspeed indication on the Captain's side and Co-Pilot's side.
It is somewhat likely that the icing of the Pitot tube occurred because the aircraft flew in an ice crystal area that was existing in the vicinity of a cumulonimbus that grew to a high altitude.
The JTSB reported no damage was found on the pitot probes in a visual inspection after landing. Tests and detailed inspection found a bend on pitot tube #2, there was no anomaly on pitot systems #1 and #3.
The JTSB analysed with respect to the weather:
it is highly probable that there was a severe rainfall phenomenon in the area from West Japan to the Pacific Ocean side of East Japan where this serious incident occurred, caused by warm and humid air from the south, flowing towards the front and low pressure area. Also, as convective system became active due to the influx of warm and humid air, it is highly probable that a cumulonimbus with echo peak altitude ranging from 12 to 14 km (from 39,370 to 45,931 ft) was generated. Furthermore, it is highly probable that from the forecasted significant weather described in 2.6.1 (2), that there were ordinary turbulence and icing, rain, lightning strikes and cumulonimbus in the adverse weather area (in the clouds).
The Captain and Co-Pilot state that they heard the sound like as rain drops hitting the cockpit window at -44°C, with the airborne weather radar echo in the green air space; but in general, the limit for naturally occurring water to remain a liquid under supercooled conditions is considered to be around -40°C. Therefore it is somewhat likely that the melting of ice crystals on the cockpit window, which is electrically heated to prevent icing.
In addition, as described in 2.1.2 by the Captain and Co-Pilot, as well as the information on ice crystals in 2.9.2, as there were no traces of icing on the wings and the ice indicator of the Aircraft after hearing the sound like as raindrops hitting the window, it is somewhat likely that the aircraft encountered ice crystals existing near or above the center of the cumulonimbus, which formed due to highly active growth existing in the weather front. It is probable that the temporary unreliable airspeed indication on the Captain’s side and the Co-Pilot’s side due to icing of the No. 1 and No. 2 Pitot tubes, while encountering ice crystals.
The JTSB analysed that the bend most likely did not exist until after the visual inspection after landing back, it was thus likely the bend occurred while the pitot tubes were sent to the manufacturer for further analysis.
The JTSB analysed that the ADR #2 was detected erroneous first, the values were below 60 knots and below the median values. It is likely ELAC determined ADR#2 as faulty and set an according flag. The speed data were taken from ADR #1 as result. Subsequently ADR #1 also dropped below 50 knots, a NAV ADR DISAGREE message was generated likely because ADR #2 was identified erroneous and the data of ADR#1 and ADR#3 became inconsistent. The captain reported the AoA was indicated at 2.5 degrees nose up, the thrust was set to CLIMB thrust in order to continue operating the aircraft safely in the present conditions. About 50 seconds after the onset of the trouble the altimeter changed from 37500 to 36900 feet, which was not consistent with AoA and engine power settings, it is thus likely the altitude drop was the result of static pressure source switching and the recovery of both ADR #1 and ADR #2. Another 50 seconds later the captain was able to engage autopilot and autothrust again.
With respect to possible ice crystals the JTSB analysed:
It is highly probable that the Captain and the Co-Pilot had been flying through a route in the airspace where the echo intensity was relatively weak; but on the way to Narita International Airport it was necessary to cross the weather front and it is somewhat likely that the aircraft may have encountered ice crystals; however, since the ice crystals are very small, it is probable that it was difficult for the Captain to directly detect them with the airborne weather radar. In order to predict and avoid similar events as much as possible, it is desirable for the Company to educate the relevant people in the Company, using the FCTM provided by the manufacturer.
In addition, the aircraft may have had the unreliable airspeed indication on the Captain's and Co-Pilot's side while encountering ice crystals, but in the short period of time from the occurrence of this event to the recovery of the unreliable airspeed indication, it is probable that the Captain and the Co-Pilot carried out the appropriate countermeasures so as to continue to fly the aircraft safely.
Similarly to this case, when an event occurs where ice crystals are encountered and the unreliable airspeed indication continues, after stabilizing the aircraft by referring to the QRH related to the unreliable airspeed indication, as it is probable that it is important for safety reasons to escape from the existing ice crystal area early with the aircraft attitude and thrust maintained, it is desirable to call attention to the countermeasures in accordance with QRH to flight crewmembers.
This article is published under license from Avherald.com. © of text by Avherald.com.
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