Ryanair B738 over Bay of Biscay on Oct 9th 2018, altitude deviation, IRS fault and associated system faults

Last Update: October 31, 2019 / 18:43:57 GMT/Zulu time

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Incident Facts

Date of incident
Oct 9, 2018

Classification
Report

Airline
Ryanair

Flight number
FR-6606

Departure
Porto, Portugal

Destination
Edinburgh, United Kingdom

Aircraft Registration
EI-GJT

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

A Ryanair Boeing 737-800, registration EI-GJT performing flight FR-6606 from Porto (Portugal) to Edinburgh,SC (UK) with 177 passengers and 6 crew, was enroute at FL360 over the Bay of Biscay about 260nm north of Porto with autopilot and autothrottle engaged when the aircraft began a slow and uncommanded pitch and climbed by about 600 feet, the speed decayed and fell below minimum maneouvering speed causing the autothrottle to activate minimum speed protection and increase thrust to about 95% N1 (the first officer recalled, he increased thrust too). The PFD showed a pitch angle of about 10 degrees though the crew perceived the actual pitch was lower than that. After the altitude warning sounded the captain disengaged both autopilot and autothrottle and returned the aircraft manually to the assigned flight level using stand by instruments. The attitude indicator in the captain's PFD slowely toppled soon showing 60 degrees of left bank with the aircraft wings level. The PFD of the first officer appeared to be working normally and in agreement with the stand by instruments. The crew consulted the flight crew operating manuals for according checklists but found no guidance. The crew attempted to engage the first officer's autopilot in LNAV mode, the autopilot did engage however performaed erratically, the crew switched to heading select mode. About 10 minutes later the right autopilot disconnected with a master caution indicating an IRS caution indication. The crew now noticed the left IRS channel was showing a fault. The crew worked the IRS fault checklist, switched the left IRS from NAV to ATT mode, the fault light extinguished, however, the erroneous attitude information continued. Checklists told to not use any autopilot. The crew thus manually flew the aircraft to Edinburgh given that the aircraft was controllable, the captain and first officer shared the flying task. Only in the last stages of cruise the first officer noticed some unexpected handling characteristics in roll. Nearing Edinburgh the crew declared PAN advising of the IRS failure and issues with the autopilot. The crew briefed their approach considering the IRS failure would affect other systems like speed trim system, the possibility of a stick shaker activation was a particular concern. During the approach the crew encountered a number of erroneous airspeed readings and windshear warnings, the autobrake system did not arm, the unexpected roll characteristics became more evident. The crew performed the ILS approach to Edinburgh's runway 24 for a safe landing about 105 minutes after the trouble began over the Bay of Biscayne.

The UK AAIB released their final bulletin concluding the probable causes of the serious incident were:

On EI-GJT, the IRS of the left ADIRU suffered a fault which led to an erroneous calculation of position. On G-FDZF, the ADIRU was contaminated by fluid ingress. In both cases, this resulted in the display of faulty attitude information on the commander’s PFD, and the supply of erroneous information to several aircraft systems. The problems were contained through manual flight and the use of standby instrumentation. Although action could have been taken to restore reliable attitude information to the commander’s PFD, such action was not directed from within the QRH. The training given to the crews discouraged them from acting unless directed by the QRH, so this lack of relevant information in the QRH contributed to the situation.

The AAIB reported following safety action was taken: "Following this incident, Boeing decided to amend the QRH checklist for irs fault. The reference to ATT mode would be removed and the checklist would direct crews to use the IRS Transfer Switch to supply relevant aircraft systems from the serviceable side."

The AAIB reported the next day another TUI Airways Boeing 737-800 registration G-FDZF encountered a similiar problem with the right hand IRS.

The AAIB analysed the flight of EI-GJT:

Analysis of the flight data showed that a longitudinal acceleration offset was induced in the left IRS. The resultant velocity error induced an IRS position error, initially along the track of the aircraft. As the flight progressed, the computed track remained constant, passing east of the north pole, while the computed heading changed. The angular difference between track and computed heading (ie the drift) increased until, after a period of approximately 20 minutes, it reached a value of 60° and triggered an irs fault caution.

As the position error increased, the commander’s flight instruments began to react in relation to a false position over the surface of the Earth, and he was presented with incorrect attitude information on his PFD. The erroneous attitude information caused the AP to climb the aircraft away from its assigned flight level. Pitch and roll comparator annunciations appeared on both PFDs. By comparing the information on the co-pilot’s PFD and the ISFD the crew were able to satisfy themselves that the faulty information was being displayed on the commander’s PFD. The AP and AT were disconnected, and the aircraft was recovered to level flight using standby instruments, cross-referenced against the co-pilot’s PFD.

During the event the crew believed that the AT may have malfunctioned because they observed the N1 reaching 95%. However, the flight data indicated that during the uncommanded climb the airspeed fell below minimum manoeuvre speed and the AT minimum speed protection activated.

The crew consulted the Boeing QRH for information and actions related to the pitch and roll comparator annunciations, but it includes no such actions. They also searched the FCOM, which explains the purpose of the comparator annunciations but offers no advice on restoring instrument capability. This caused some confusion in the crew, but they were reluctant to take any other action because their training discouraged them from doing so. As a result, although a solution to restore attitude information on the PFD was available – selecting a different data source for the PFD – the actions required were not taken because they were not directed to be taken by the QRH. Consequently, the commander’s PFD displayed erroneous information for the rest of the flight, and the comparator annunciator indications remained on both PFDs. These displays are the pilots’ primary attitude reference and the information on them is crucial for safe flight. Both pilots were faced with significant distractions on these primary instruments for the remainder of the flight.

When the irs fault light illuminated, the crew selected ATT mode as directed by the QRH. The fault light cleared but the commander’s PFD continued to provide erroneous information. This was because the ATT mode uses the same ring laser gyros and accelerometers as the NAV mode and so the system remained affected by accelerometer anomalies.

Following this incident, Boeing decided to amend the QRH checklist for irs fault. The reference to ATT mode would be removed and the checklist would direct crews to use the IRS Transfer Switch to supply relevant systems from the serviceable side.

The crew’s consideration of the fault allowed them to predict and prepare for the influence of the faulty attitude information on other systems. Accordingly, they were prepared for a number of erroneous indications which arose during the rest of the flight. The crew felt that there was some degradation of the aircraft’s handling in manual flight, particularly in roll. The failed left ADIRU continued to supply information to the yaw damper computer for the remainder of the flight. As the yaw damper was periodically deflecting the rudder to the maximum extent allowed by the system, the roll issue felt by the crew was a secondary effect of the erroneous yaw damper action.

With respect to G-FDZF and the checklists the AAIB analysed:

The crew of G-FDZF faced similar problems to those experienced by the crew of EI-EGT: erroneous attitude information and failure of several systems. They recognised that the problem lay with the IRS but, like the crew of EI-EGT, they did not carry out any corrective action as there was no relevant guidance in the QRH.

QRH guidance

In both cases discussed in this report there was a significant period between the first symptoms of faulty attitude information and the appearance of the irs fault indication. Shortly after the attitude information failed, pitch and roll comparator annunciations appeared on both PFDs. While these flags indicate a failure, they do not decisively indicate where it lies. Pilots must use standby instruments to determine where the failure is and, if necessary, recover to the correct attitude through manual flight. Selecting a different source for the faulty PFD would remove the flags and restore valid attitude information on both pilots’ PFDs, although it would lead to a reduction in redundancy because all PFD attitude information would be from a single source. Information is available in the FCOM to aid crew understanding, but because of the expressed philosophy in the QRH discouraging troubleshooting, and the training discouraging the use of QRH checklists except in response to relevant associated warnings, it is unlikely crews will act unless specifically directed to do so by the QRH checklist.

In these events, the failure occurred in VMC and straight and level flight and the outcome was benign. However, the PFD is a primary instrument which dominates a pilot’s display panel, and a failed attitude display presents a powerful disorientating stimulus to the relevant pilot. The comparator annunciation appears simultaneously in both PFDs and, if no action is taken, can remain as a significant distraction for the remainder of the flight. In manoeuvring flight it could be unclear where the failure lay, and the presence of the failed display would continue to constitute a disorientating factor.

Boeing decided to amend the QRH checklist for irs fault but this would not address the situation where there was faulty attitude information but no IRS caution message. Therefore, the following Safety Recommendation is made:

Safety Recommendation 2019-012:
It is recommended that Boeing Commercial Aircraft amend the Boeing 737 Quick Reference Handbook to include a non-normal checklist for situations when pitch and roll comparator annunciations appear on the attitude display.
Aircraft Registration Data
Registration mark
EI-GJT
Country of Registration
Ireland
Date of Registration
Pefjpmnndqe Subscribe to unlock
Manufacturer
THE BOEING COMPANY
Aircraft Model / Type
BOEING 737-800
ICAO Aircraft Type
B738
Year of Manufacture
Iljp Subscribe to unlock
Serial Number
EqlAn Subscribe to unlock
Maximum Take off Mass (MTOM) [kg]
Mjfhd Subscribe to unlock
Engine Count
P Subscribe to unlock
Engine
impnfenfeiApi Subscribe to unlock
Main Owner
AndemjgAjllhAienhiffbknAiAf flpkfppejAhlhmijf gcmdblcf gielj cAl Subscribe to unlock
Incident Facts

Date of incident
Oct 9, 2018

Classification
Report

Airline
Ryanair

Flight number
FR-6606

Departure
Porto, Portugal

Destination
Edinburgh, United Kingdom

Aircraft Registration
EI-GJT

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

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