Jetstar B788 near Darwin on Dec 21st 2015, airspeed fluctuations and difficulties to maintain altitude

Last Update: June 14, 2017 / 14:28:24 GMT/Zulu time

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Incident Facts

Date of incident
Dec 21, 2015

Classification
Incident

Flight number
JQ-7

Aircraft Registration
VH-VKE

ICAO Type Designator
B788

A Jetstar Boeing 787-800, registration VH-VKE performing flight JQ-7 from Melbourne,VI (Australia) to Singapore (Singapore), was enroute at FL400 over the Timor Sea about 150nm west of Darwin,NT (Australia) when the crew decided to divert to Darwin reporting an instrument problem. The aircraft landed safely on Darwin's runway 29 about 70 minutes later.

A passenger understood some anti-ice system was involved, another passenger understood a sensor had failed.

The airline reported that following turbulence the crew noticed a technical problem with one of the instruments and opted to divert to Darwin. The aircraft is going to undergo maintenance over night, the flight is being continued on Tuesday morning and is estimated to arrive in Singapore with a delay of 20 hours.

The occurrence aircraft was able to continue the flight as JQ-7D after about 22 hours on the ground and reached Singapore with a delay of 24 hours.

Australia's Transportation Safety Board (ATSB) rated the occurrence a serious incident and opened an investigation reporting, that the crew encountered airspeed fluctuations and difficulties to maintain the assigned altitude followed by faulty flight instruments. The crew dumped fuel and diverted to Darwin for a safe landing. The ATSB is reading out the black boxes in order to "determine the factors that contributed to the airspeed fluctuations". The final report is estimated for December 2016.

On Jun 14th 2017 the ATSB released their final report concluding the probable causes of the serious incident were:

Contributing factors

- The aircraft entered a region of high ice water content which caused icing of the three independent pitot sources and anomalous airspeed indications.

- Short duration anomalous airspeed indications caused the flight control computer to transition to secondary mode. In secondary mode, the autopilot and some auto-flight protections were unavailable to the flight crew.

The ATSB summarized the sequence of events:

On 21 December 2015, a Boeing 787-8 (B787) aircraft, registered VH-VKE, departed Melbourne, Australia on a scheduled passenger transport service to Singapore. The flight, operating as Jetstar Flight 07, departed at 0140 UTC (1240 local time)1. At 0550 UTC, the aircraft was in cruise, being operated at flight level2 (FL) 400. The first officer was the pilot flying (PF) and autopilot and autothrottle were engaged.

At the date of this occurrence an airworthiness directive (AD/B787/2013-24-01) relating to reducing the risk of engine icing was in effect, requiring avoidance of ice crystal icing3 conditions. As such, weather information with relevant en route icing conditions was used by the operator for the purposes of flight planning. In accordance with the AD, the crew manoeuvred around any observed weather conditions that had potential to cause icing to the engines.

Approaching waypoint CURLY, about 250 NM north of Darwin, Australia, the crew reported that a green coloured4 area, with magenta5 patches appeared ahead on their weather radar. This area was too close to avoid.

As a precaution the crew activated the seat-belt light. The aircraft entered an area of light turbulence which then increased to moderate. Concurrently with the increase in turbulence the crew noticed erratic airspeed indications on both PFDs, the autopilot disconnected and multiple EICAS6 messages were displayed including AIRSPEED UNRELIABLE and FLIGHT CONTROL MODE. The FLIGHT CONTROL MODE annunciation indicated that the aircraft flight control system had reverted to secondary mode.

Shortly after the event, the captain took over the pilot flying role and the crew conducted the AIRSPEED UNRELIABLE checklist. The captain reported that this was a high workload situation and effective communication and coordination with the first officer greatly assisted the procedure.

The crew maintained an airspeed by setting a pitch angle and thrust level, as indicated in the applicable quick reference handbook table. Through comparison with this table, the crew were also able to identify the most accurate airspeed indication. The crew reported that at this stage it appeared that all indications had returned to normal.

The aircraft was 4 hours into an 8 hour journey, latched in secondary mode and could only be flown manually. Based on this, a decision was made to divert to Darwin, Northern Territory.

After jettisoning fuel to reduce the aircraft weight to maximum landing weight, the crew requested a straight-in approach and made an uneventful landing at Darwin airport.

The ATSB analysed:

Reasons for erratic airspeed indications

The weather analysis provided by the aircraft manufacturer’s meteorological specialists found that the erratic airspeed values likely resulted from ice crystal icing of the pitot-static systems. In this instance the icing affected the voted airspeed values for approximately 17 seconds.

Although there are three independent pitot-static systems for determining computed airspeed, adverse environmental conditions, as encountered during this flight, can affect all three simultaneously. On the B787, the synthetic AOA speed allows an independent source of airspeed as a comparison in order to validate the voted airspeed.

Once the airspeed values returned to normal, the ongoing flight was affected by the reversion and latching of the flight control computer to secondary mode.

Secondary mode reversion

A drop in recorded airspeed to, or below, a threshold level is referred to as a No Computed Data (NCD) state. Throughout the recorded airspeed fluctuations, there were two occasions where the value of voted airspeed went NCD for a sufficient time to revert the flight control system to secondary mode.

Boeing has taken action to revise the software in the flight control computer related to the secondary mode reversion. This revision has increased the time required in an NCD state to revert to secondary mode.
Incident Facts

Date of incident
Dec 21, 2015

Classification
Incident

Flight number
JQ-7

Aircraft Registration
VH-VKE

ICAO Type Designator
B788

This article is published under license from Avherald.com. © of text by Avherald.com.
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