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B738, vicinity London Stansted UK, 2011

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Summary
On 13 March 2011, a Turkish Airlines’ Boeing 737-800 stopped climb shortly after take off after misreading the SID chart. After levelling at 450 feet agl, it continued following the lateral part of the SID only until ATC re-iterated the requirement to climb after resolving a temporary loss of contact due to an un-instructed premature frequency change. It was found that the crew had received but apparently not responded to an EGPWS ‘PULL UP’ Warning. It was concluded that there was an opportunity to improve the clarity of UK SID charts to aid pilots with limited English language skills.
Event Details
When March 2011
Actual or Potential
Event Type
Controlled Flight Into Terrain (CFIT), Human Factors
Day/Night Day
Flight Conditions VMC
Flight Details
Aircraft BOEING 737-800
Operator Turkish Airlines
Domicile Turkey
Type of Flight Public Transport (Passenger)
Origin Stansted
Actual Destination Ankara/Esenboğa International Airport
Take off Commenced Yes
Flight Airborne Yes
Flight Completed Yes
Flight Phase Take Off
TOF
CFIT
Tag(s) Into terrain,
Vertical navigation error,
IFR flight plan
HF
Tag(s) Data use error,
Procedural non compliance
Safety Net Mitigations
Malfunction of Relevant Safety Net No
TAWS Available but ineffective
Outcome
Damage or injury No
Causal Factor Group(s)
Group(s) Aircraft Operation
Safety Recommendation(s)
Group(s) Air Traffic Management
Investigation Type
Type Independent

Description

On 13 March 2011, a Boeing 737-800 being operated by Turkish Airlines on a passenger flight from London Stansted to Ankara stopped climb prematurely in day Visual Meteorological Conditions (VMC) soon after take from runway 22 on a CLN 8R SID. After earlier noting a normal take off and initial climb, the TWR controller subsequently observed the aircraft in a steep nose down attitude before it levelled off and began the prescribed SID left turn at that level. Although a frequency change to departure had not been given, the TWR controller was unable to make contact with the aircraft and advised the departure controller accordingly. The latter was eventually able to re-iterate the need to climb and the rest of the SID was flown normally.

Investigation

An Investigation was carried out by the UK AAIB. It was found that the reason for the failure to climb was the misreading of the SID procedure chart by the flight crew. Such misreading was also found to be the reason for a premature change of frequency to departure which had prevented TWR from making contact with the aircraft when the unexpected flight path was observed, since this UK standard procedure was re-iterated on the SID chart. The Jeppesen Chart used by the crew is reproduced below as included in the Investigation Report.

Clacton 8R SID (reproduced from the Final Report)

It was advised that the First Officer had been PF and was making his first visit to London Stansted and noted that although the aircraft commander was experienced on the aircraft type, he was on only his third visit to any UK airport. It was determined that prior to take off the First Officer had selected 800 feet as the stop altitude on the MCP and then, soon after take off, had engaged the AP, at which point the aircraft had pitched nose down reaching a maximum altitude of approximately 1050 feet QNH before descending to 800 feet QNH - equivalent to approximately 450 feet agl. Upon being alerted by the TWR controller to loss of contact with the aircraft and its abnormal flight path, the departure controller called the aircraft to request its passing and cleared altitude and after a repeated request, the aircraft was understood to have responded with “now at 8800 feet” - contrary to the radar display reading of 800 feet. A request for confirmation of “8800 feet” produced a response of “altitude eight hundred six sixty now”. A further request to confirm climbing to 4000 feet, the applicable SID stop altitude, received the response “no climbing 4000” and the mode ‘S’ data display showed a change from 800 feet to 3200 feet and then to 4000 feet during the dialogue. The remainder of the SID was flown normally.

It was found that the setting of 800 feet on the MCP had followed an interpretation by the pilots of the chart instruction to make an “initial climb straight ahead to 850 ft” in conjunction with the following two instructions (see chart) as a requirement to level at 850 feet until ATC further vertical clearance was issued with 800 feet being the nearest available setting which would not exceed 850 feet.

The Investigation subsequently learned that the crew had received Terrain Avoidance and Warning System (TAWS) Alerts of ‘Don’t Sink’ followed by the EGPWS Hard Warning ‘PULL UP’ during the incident but the relative timing of these activations could not be established in the absence of FDR/QAR or EGPWS NVM data and there was no mention of any response.

A review of the context for the event noted that the International Civil Aviation Organisation (ICAO) minimum height for commencing a turn on a SID of 120 metres (equivalent to 394 feet) is modified in the UK to 500 feet aal. However, it was found that there were detail variations in how UK AIP SID charts included this requirement in respect of whether it was included as a QNH or aal figure.

It was considered that it would be possible for pilot whose first language was not English to misinterpret an instruction in the form ‘Initial climb straight ahead to xxx feet’ as meaning that a level off at that altitude was required. It was also considered that an instruction ‘Do not climb above SID levels until instructed by ATC’ was open to interpretation as a need to obtain positive ATC clearance before climbing above any level specified in a SID.

Safety Action taken by ANSP NATS since the incident to harmonise London TMA SID instructions in respect of minimum turn altitude to read “no turns below….” was noted.

The formal Conclusion of the Investigation was that:

“The pilots caused the aircraft to level incorrectly at 800 ft aal because they misinterpreted the information written in the general information section of the departure chart. This information originated from the UK AIP. The pilots’ misinterpretation of the information was reinforced by the previous experience of the crew.”

One Safety Recommendation was made as a result of the Investigation as follows:

  • that the Civil Aviation Authority should ensure that the vertical profile information included within the general information section of all SIDs published in the UK AIP is unambiguous and that the wording used is consistent across all UK SIDs. [2011-089]

The Final Report AAIB Bulletin: 2/2012 EW/C2011/03/05 was published on 9 February 2012.

Further Reading