Description

On 19 June 2009 a Boeing 767-300 being operated by LOT Polish Airlines on a scheduled passenger flight from Chicago O’Hare to Warsaw was in the cruise at FL330 in night Instrument Meteorological Conditions (IMC) when the Captain's air speed indicator and altimeter suddenly displayed increased readings and an overspeed warning occurred. The crew responded by commencing a climb and reducing thrust which led to a continuous stall warning until descent to a level 5000ft below the previous cruise level had been made and thrust increased. A diversion to Toronto was made but whilst in the hold whilst in a hold to burn off fuel, thrust was reduced to idle with the AP engaged and ALT hold selected and the stall warning warning system was again activated. Recovery from this then led to a climb above the cleared level and a loss of separation occurred with another aircraft that responded to its TCAS RA to avoid the conflict.

Investigation

An Investigation was carried out by the Canadian TSB. Flight Data Recorder (FDR) data was available for the investigation but the 30 minute Cockpit Voice Recorder (CVR) was not stopped after the in flight events or after landing and was of no use to the Investigation.

It was found that the Captain had accumulated 19,000 total flying hours including 8000 hours on type and the First Officer had accumulated 7000 total flying hours including 1800 hours on type. Both pilots had completed their most recent unreliable airspeed training 15 months previously. The aircraft commander had been in the role of PF and was occupying the left hand pilot seat with the autopilot (AP) and auto throttles (A/T) engaged. Suddenly, over a period of 5 seconds, the left hand airspeed indicator (ASI) and the left hand altimeter displays had simultaneously increased (from 276 to 320 knots and from 33000 to 33450 feet respectively). To re-capture altitude, the AP commanded a pitch down of approximately 2 degrees and an over speed warning activated. The PF response was to retard the throttles to idle and leave the AP engaged. The AP commanded a further pitch down and then a pitch up of around 8 degrees. The over speed warning remained on for about 40 seconds. The PF then disconnected the AP and initiated a manual climb with 12 degrees pitch up and the thrust still at idle. A second over speed warning corresponding to a false high reading on the left hand altimeter occurred. Having reached approximately FL354, a descent began. At FL347 with the over speed warning still active, stick shaker activation occurred and remained active for nearly 2 minutes with no crew response until descending through FL300 when the PF increased thrust. Within 9 seconds, the stick shaker stopped and shortly afterwards the fluctuations on the left hand ASI stopped. The aircraft continued its descent to FL297. Throughout this event, the First Officer’s ASI registered normally.

The flight crew advised ATC that they wished to divert to Toronto because they had experienced an over speed and had problems maintaining altitude but did not declare an emergency. No further anomalies with the aircraft or its systems were encountered during the remainder of the flight. However, whilst in a hold at 10,000ft to burn off fuel, the thrust was reduced to idle with the AP engaged and ALT hold selected. As the aircraft slowed, the AP commanded an increasing pitch to maintain level flight until, when the pitch reached 7.6 degrees nose up, the stick shaker activated again. As the aircraft descended to approximately 9600 feet, the flight crew manually increased thrust (a maximum of 111% was recorded on the FDR) and the aircraft began to climb. Passing 9860 feet, the AP was disconnected and the aircraft exceeded its cleared altitude and lost separation with another aircraft that responded to a TCAS RA to avoid the conflict.

After landing at Toronto, the flight crew filled out an aircraft technical report. When the aircraft was inspected, no structural damage was found and no faults were identified in the air data system and the aircraft was released to service without needing any rectification.

The Investigation was appraised of a similar transient fault on the same aircraft a month later on the opposite sector. In this event, the autopilot was disengaged and the aircraft was flown manually. The flight crew noticed a discrepancy between the reading on the left hand ASI and that on the right hand ASI and the Standby ASI. The over speed warning ceased and ASI indications returned to normal when the Captain changed his selected Air Data Computer (ADC) from normal to alternate. Unlike the 19 June 2009 occurrence, the aircraft did not pitch nose-up and there were no stick shaker activations.

The Investigation noted that The Boeing 767 FCTM states that when correcting for an over speed during cruise at high altitude, flight crews must avoid reducing thrust to idle because this causes the engines to accelerate slowly back to cruise and may result in over-controlling the airspeed and a loss of altitude. However, in both events, the immediate reaction of the LOT crews to the over speed warning was to reduce the throttles to flight idle.

In the event of a suspected unreliable ASI, the LOT Quick Reference Handbook (QRH) guides the flight crew to cross-check the Captain’s and First Officer’s ASI indications against the Standby ASI and states that a main instrument differing by more than 15 knots from the standby reading should be considered unreliable. If the reliable airspeed data source can be determined, the flight crew should select the reliable source (i.e. the other ADC).

Tests conducted on the suspect ADC on the aircraft involved in these events disclosed a fault within the circuitry which would have led to the observed malfunction.

The Investigation, noted with concern that the lack of any CVR data had precluded any analysis of crew decisions, actions or overall CRM during the incident flight. Although the right hand ASI did not display the same erroneous over speed information as the left hand one, it could not be determined when or if the First Officer became aware of the conflicting ASI indications, or if any such awareness was communicated to the PF. Since the ‘AIRSPEED UNRELIABLE’ checklist was not completed and the ASIs were not compared as required, the Investigation considered it probable that airspeed anomaly was not noticed.

Some minor but pertinent differences between the EICAS messages generated on the incident aircraft and other aircraft in the LOT fleet were noted, as was some ambiguity in the FCOM amendment process.

The Investigation made the following “Findings as to Causes and Contributing Factors”:

• There was a fault within the phase locked loop (PLL) circuitry of the ADC which

resulted in sudden and erroneous airspeed and altitude indications on the Captain’s instruments.

• . The readings on the Captain’s instruments were not compared to those on the First Officer’s or the Standby instruments. Consequently, the crew believed the Captain’s instruments to be correct and made control inputs that resulted in significant altitude and airspeed deviations.

The Investigation also made the following “Findings as to Risk”

• LOT Polish Airlines initial and recurrent flight training syllabus does not include practical training for an over speed warning event. Consequently, flight crews may respond improperly and exacerbate the situation.

• Although revision 5 of the Boeing SB 767-34A0332 requires changes to chapters of the FCOM, it does not specify what the changes should be. Therefore some manuals may not be properly amended, thereby increasing the risk of crews being ill-informed of the status of the aircraft they operate.

• The LOT Polish Airlines FCOM incorrectly states that the IAS DISAGREE and ALT DISAGREE EICAS messages will not be displayed on the occurrence aircraft during an unreliable airspeed incident. This increases the risk of a crew misidentifying a problem.

• The installation of CVRs with less than 2 hours of recording capacity creates the risk that relevant information will not be available to accident investigators and that significant safety issues may not be identified.

• During the initial examination and disassembly of the ADC, it was noted that there was a large build-up of dust and dirt inside the unit, which could cause an increase in the internal temperature.

And a further Finding was that:

• In the hold, with thrust at idle, the flight crew did not monitor the airspeed. In an attempt to maintain altitude, the autopilot increased the angle of attack until the stick shaker activated. During the recovery, the crew allowed the aircraft to climb through the flight’s cleared altitude, resulting in a loss of separation

The Final Report of the Investigation was authorised for release on 8 March 2011. It contains a transcription of the relevant flight data and also briefly notes Safety Action taken by LOT. No Safety Recommendations were made as a result of the Investigation.

Further Reading

• Loss of Control
• Unreliable Airspeed Indications
• Altimetry System Error
• Loss of Separation
• ACAS Resolution and Traffic Advisories