Singapore B738 at Kathmandu on May 6th 2022, tail scrape on departure

Last Update: May 8, 2023 / 18:22:42 GMT/Zulu time

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Incident Facts

Date of incident
May 6, 2022

Classification
Report

Airline
Singapore Airlines

Flight number
SQ-439

Departure
Kathmandu, Nepal

Destination
Singapore, Singapore

Aircraft Registration
9V-MGL

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

A Singapore Airlines Boeing 737-800, registration 9V-MGL performing flight SQ-439 from Kathmandu (Nepal) to Singapore (Singapore) with 165 passengers and 8 crew, was departing Kathmandu's runway 20 when the crew suspected a tail strike and levelled off at FL150 due to the high terrain around Kathmandu. The aircraft diverted to Kolkata (India) for a safe landing.

A post flight inspection confirmed the tail skid assembly had contacted the runway surface at Kathmandu, however, there was no damage to the aircraft's aft fuselage.

On May 8th 2023 Singapore's TSIB released their final report concluding the probable causes of the serious incident were:

- The tail strike occurred during the take-off at KTM airport was due to overrotation by the PF, coupled with a likely tailwind with a component of about 10 knots along the runway. The rotation pitch rate was, at times, greater than per second as recorded by the FDR. The pitch angle of 11.07 recorded also exceeded the 7 – 9 normal pitch angle range.

- The flight crew could have considered beginning the Tail Strike NNC after reaching about 10,500 feet and while the aircraft was still climbing to the MSA of 14,400 feet.

- The incident aircraft did not have a tail strike warning indication system installed. If such a warning indication system was available in the flight deck, it would have helped the flight crew make better decision as to whether to perform the Tail Strike NNC urgently.

- The operator’s recurrent training programme for LIPs/Captains did not include practising the skills relating to manipulating cabin pressurisation. The investigation team suspects that the PM might have been out of practice with the skills relating to manipulating cabin pressurisation.

- The operator did not consider, as part of its HIRM programme, the complexity of a tail strike situation at aerodromes at high elevations surrounded by high terrain and did not develop targeted guidance and training for its flight crew.

- In response to the Cabin Altitude Warning (CAW), the PM manually deployed the passenger oxygen masks as a precautionary measure without informing the PF. The operator did not have a requirement for standard callout for oxygen masks deployment.

The TSIB reported the crew consisted of two captains, one of them being a line instructor pilot (59, ATPL, 10,069 hours total, 4030 hours on type) occupying the right hand seat as well as being the pilot monitoring and the pilot in command, the captain under monitoring (48, ATPL, 12,088 hours total, 5200 hours on type) occupied he left hand seat and was pilot flying.

The aircraft departed with 75,477 kg gross mass which was below the maximum takeoff mass of 76,100kg.

The TSIB analysed:

During rotation of an aircraft at take-off, a high pitch rate will increase the likelihood of a tail strike. According to the aircraft manufacturer’s FCTM, when the proper rotation technique is followed, the resultant rotation rate will be between 2° and 3° per second. In this incident, the pitch rate was, at times, greater than 5° per second. As mentioned in paragraph 1.6.1 there was likely a tailwind with a component of about 10 knots along the runway during the aircraft take-off roll up to rotation. The high pitch rate together with the tailwind component during rotation probably eroded the tail-to-ground clearance margin and resulted in the tailskid contacting the runway.

As mentioned, there were several events involving pitch rate greater than 3° per second registered by the FDAP. Whether there was any likely tailwind component involved in these events, the high rotation pitch rate is of concern and the investigation team opined that the operator should re-emphasise to all its flight crew the proper rotation technique.

Performing Tail Strike Non-Normal Checklist (NNC)

A tail strike occurring when operating out of an aerodrome in high terrain areas, such as KTM airport, can be a major challenge to the flight crew. The aircraft manufacturer has a Tail Strike NNC and guidance in the FCTM. The Tail Strike NNC contains the action items to be taken when a tail strike is confirmed or suspected and the FCTM states that the Tail Strike NNC should be carried out without delay.

As mentioned, “without delay” means the flight crew should begin the Tail Strike NNC when the desired flight path and appropriate configuration are correctly established, which in this event would be at about 10,500 feet. The investigation team opined that the flight crew in this event could have considered beginning the Tail Strike NNC after reaching about 10,500 feet and while the aircraft was still climbing to the MSA of 14,400 feet.

Doing so might result in the CAW triggering and automatic deployment of oxygen masks for the passengers, but the inconvenience of oxygen masks deployment should be manageable and acceptable, in view of the unknown risk of aircraft structural failure with the continued pressurisation of the aircraft.

Tail strike indication

The incident aircraft does not have a tail strike warning indication system. It is an option for operators who purchased the HUD system. The flight crew determined that the aircraft had probably experienced a tail strike basing on the cabin crew’s account. The investigation opined that if a tail strike warning indication was available in the flight deck, it would have helped the flight crew make better decision as to whether to perform the Tail Strike NNC urgently.

Operation of the outflow valve (OFV)

As mentioned, the PF requested the PM to carry out the actions required by the Tail Strike NNC and one of the action items was to depressurise the aircraft. Accordingly, the PM toggled the OFV switch manually to open the OFV after selecting the cabin pressurisation mode selector to MANUAL. The PF noticed that the OFV was operating, and the PF mentioned this to the PM. However, the PM did not see any appreciable movement on the OFV position indicator despite having toggled the OFV switch twice and had apparently not heard what the PF said to him.

The investigation team could not think of a reason why the OFV position indicator could not show any appreciable movement in view of the following:

- The PF had been informed by the cabin crew that there was a wheezing sound which the PF interpreted as a sign that the OFV was operating.

- Post-incident test on the manual operation of the OFV on the incident aircraft did not reveal any anomaly, and the problems described by the PM in paragraph 1.1.110 could not be replicated.

The PM said that he was about to try a third time in toggling the OFV switch when the CAW was triggered. This was despite the PM noticing that the cabin altitude was at about 6,000 feet after having toggled the OFV switch. The fact that the cabin altitude had changed from the initial 4,000 feet to 6,000 feet clearly showed that the OFV had responded to his inputs and was opening.

The investigation team suspected that the PM was too focused on checking the OFV position indicator and did not monitor the cabin altitude/differential pressure and cabin rate of climb indicators, and also did not register the PF saying that the OFV was opening.

The investigation team could only suspect that the PM might have been out of practice with the skills relating to manipulating cabin pressurisation. The investigation team noticed that since the PM’s conversion training to the B737 fleet as a captain in 2015, he did not practise manipulating cabin pressurisation.

Operator’s Hazard identification and Risk Mitigation (HIRM) programme

The operator stated that it is impossible for airport HIRM to address all nonnormal events that may happen and did not identify tail strike out of KTM airport as a hazard that would require additional mitigation over and above the Tail Strike NNC and was satisfied that its flight crew had been trained to perform the Tail Strike NNC tasks. The investigation team opined that tail strike out of KTM airport would represent a very challenging situation due to the uniqueness of the operating environment, i.e. aerodrome in the vicinity of high terrain, when the flight crew needed to clear the MSA and the aircraft needed to be depressurised, as required by the Tail Strike NNC, to avoid any further structural damage.

It is understood that the aircraft manufacturer’s FCTM and NNCs are developed to cover the majority of airline operations and each operator has to consider its unique operational specificities and adopt appropriate control measures. The operator should have considered, as part of its HIRM programme, the complexity of a tail strike situation at aerodromes at high elevations surrounded by high terrain such as KTM airport, and developed targeted guidance and training that will:

- help the flight crew decide when would be the best time to execute the Tail Strike NNC;

- remind the flight crew of the potential risk of aircraft structural failure with the continued pressurisation of the aircraft;

- and remind the flight crew to expect that the CAW could be triggered when performing the Tail Strike NNC at around or above 10,000 feet.

Standard callout for oxygen mask deployment

As mentioned, the PM, in response to the CAW, manually deployed the passenger oxygen masks as a precautionary measure. However, the PF was not aware that the passenger oxygen masks were deployed as the PM did not inform the PF in any way of his oxygen masks deployment action.

Keeping each other in the cockpit informed of the flight operational situation is a common safety strategy practised by flight crew and is important for ensuring that flight crew members have the same operational picture. This will allow them to better manage any abnormal situation that pops up.

Oxygen masks deployment, an irreversible action, is a significant event in flight operation. It is desirable for the operator to consider including oxygen masks deployment in the list of standard callout items.