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Air accidents investigation into Boeing 737 that experienced several malfunctions

Christian Fernsby |
Shortly after reaching cruise at FL360 the commander’s attitude indicator malfunctioned affecting numerous aircraft systems, and the aircraft climbed 600 ft en route from Porto to Edinburgh on October 9, 2018.

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After a significant time delay an irs caution was displayed.

The Quick Reference Handbook (QRH) was followed by the crew and the left Air Data Inertial Reference Unit (ADIRU) was put into ATT mode.

The left Primary Flight Display (PFD) continued to display erroneous attitude information to the pilot, and other systems were also affected.

The aircraft was flown manually to Edinburgh where it landed safely.

The left Inertial Reference System (IRS) suffered a transient fault in one of its accelerometers which led to an erroneous calculation of position.

False position information led to the incorrect attitude information on the commander’s PFD, and the autopilot (AP) responded by initiating a slow climb.

The flight was scheduled from Porto, Portugal, to Edinburgh with 6 crew and 177 passengers.

The crew had operated the sector into Porto and there had been no technical issues with the aircraft prior to departure for Edinburgh.

The crew conducted turnaround procedures for the IRS in accordance with the operator’s procedures.

The aircraft left the stand at 1953 hrs and took off at 2000 hrs.

The departure and climb were uneventful and the aircraft established in cruise flight at FL360.

Shortly after crossing into the Brest Flight Information Region (FIR), the aircraft began a slow and un-commanded pitch up.

The aircraft left its planned cruise altitude and climbed to approximately FL366.

As the speed decayed below the minimum manoeuvring speed, the autothrottle minimum speed protection activated increasing the N1 (thrust) to approximately 95% (although the co-pilot also recalled increasing power).

The commander’s PFD initially indicated a pitch attitude of around 10° nose-up, though the crew’s perception was that the actual aircraft attitude was lower than this.

The altitude warning sounded indicating a deviation from the selected flight level.

The commander deselected the AP and autothrottle (AT), and the aircraft was recovered to level flight manually using standby instruments.

The initial pitch indication on the commander’s PFD was followed by a slow ‘topple’ in roll with the attitude indicator (AI) showing around 60° left angle of bank.

The yellow pitch, roll and flight path vector (FPV) comparator annunciations appeared on both crew PFDs1 .

It appeared that the co-pilot’s flight instruments were serviceable, however.

During the recovery, the aircraft descended to FL357 and then returned to its assigned cruise altitude of FL360.

The crew discussed the failed attitude indication and the comparator annunciations.

They consulted the QRH for a checklist related to these indications but found no guidance.

They also consulted the Flight Crew Operating Manual (FCOM) but again found nothing of value to assist in resolving the issue.

They then engaged the AP on the co-pilot’s side.

There are checklists in the QRH for ‘Display Failure’, but the symptoms covered by these are significantly different from those encountered by the crew.

They tried to select the AP lateral navigation mode (LNAV) but while the AP would engage in this mode it did not operate correctly, so the crew engaged the heading select mode (HDG SEL).

Due to the thrust changes made by the AT during the event, the cause of which the crew felt was unresolved, the crew elected to continue using manual thrust.

After approximately 10 minutes, AP B disconnected and the Master Caution System illuminated with an irs caution.

This drew the crew’s attention to a left irs fault indication on the IRS Mode Select Unit (MSU).

The MSU is in the overhead avionics panel and, as it is out of direct crew view, caution lights in the system trigger the Master Caution System.

The commander took control of the aircraft and directed the co-pilot to action the QRH checklist.

The crew then completed the ‘IRS Fault’ checklist as directed by the QRH.

The checklist has several steps and decision points which the crew discussed and completed.

The IRS had been correctly set to nav for the flight, but the checklist now called for the crew to select att on the MSU, which they did.

The objective of selecting att is to recover limited ADIRU operation following an inflight power loss or certain ADIRU fault conditions.

Selecting ATT mode resets the local vertical reference.

Navigation data (position, groundspeed, track and wind information) is not available in ATT and is removed from the ARINC 429 data bus.

The decision point which follows this action in the checklist is based solely on whether the fault light clears: if it does, the QRH checklist ends with the advice to not select either AP.

In this case, on selecting att the fault light cleared but the IRS continued to provide erroneous attitude data to the pilot’s PFD due to a faulty accelerometer.

The crew continued in manual flight, sharing the flying task.

They considered diverting to an alternate airport but, given the aircraft was controllable, elected to continue to the planned destination of Edinburgh.

During the later stages of the cruise, the co-pilot noticed some unexpected handling characteristics in roll.

For the arrival into Edinburgh the weather was a moderate south-westerly wind, gusting to 30 kt, with good visibility and no cloud below 3,000 ft.

As the aircraft neared Edinburgh, the crew declared a PAN to Scottish ATC and informed them that there were issues with the aircraft’s APs.

During their preparations for the approach, the crew considered that the failed IRS would influence other systems and they prepared for this during their approach brief.

There was a failure of a single channel of the speed trim system, and the possibility of stick shaker activation was of particular concern.

During the approach, they encountered erroneous airspeed and windshear warnings, and the autobrake system would not arm.

The roll issues noted by the co-pilot were more evident, but the crew were able to control them satisfactorily and made an ILS approach in good weather conditions.

The landing was uneventful, using manual braking, after which the aircraft taxied to the stand where the passengers disembarked.

In the initial stages of the event, the crew were presented with several unusual indications and warnings.

The attitude information displayed on the commander’s PFD was erroneous and faulty data was supplied to AP A which was engaged.

The AP responded to the faulty data and the aircraft’s flightpath was disturbed.

After the crew disengaged the AP and recovered the aircraft to manual flight at the allocated flight level, yellow comparator annunciations appeared on each pilot’s PFD.

Aware there were no QRH memory items for PFD comparator annunciations, the crew consulted the aircraft QRH for related checklists; there were none.

The crew considered the possibility of using other checklists but discounted this because their training discouraged the use of QRH checklists except in response to relevant associated warnings.

When the crew completed the ‘IRS Fault’ checklist, they were aware that they had not recovered the system.

However, they had the aircraft under control and were able to maintain its flight path adequately using the co-pilot’s PFD and the commander’s Integrated Standby Flight Display (ISFD).

If the irs fault light clears, the QRH checklist reaches an endpoint.

They felt that the QRH discouraged any other actions and that this philosophy was also emphasised by the operator’s training department.

The ‘IRS Fault’ checklist directs selection of ATT mode on the failed system.

Once this has been done the QRH directs that neither AP should be engaged, so the crew flew the aircraft manually.

Due to the lack of an AP the aircraft was no longer compliant with Reduced Vertical Separation Minima (RVSM) regulations and the crew was required to declare this to ATC.

Given the unusual nature of the failure and their confusion over the QRH checklist outcome, the crew did not do so.

Nevertheless, after the initial upset the aircraft remained within the RVSM performance limits for the remainder of the flight.

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.

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.

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