On 6 May 2022, the crew of a Boeing 737-800 (9V-MGL) being operated by Singapore Airlines on a scheduled international passenger flight from Kathmandu to Singapore Changi became aware of a probable tail strike during takeoff in day VMC. The appropriate standard response to the suspected occurrence was delayed because of a perceived need to climb above the MSA before commencing it. Implementation of the response was then mismanaged before a procedurally required diversion - to Kolkata - was made without further event. There, it was found that the tail strike had not caused any structural damage to the aircraft’s aft fuselage.
A Serious Incident Investigation was carried out by the Singapore Transport Safety Investigation Bureau (TSIB). The FDR and the CVR were removed from the aircraft and relevant data were successfully downloaded from both recorders. It was noted that the flight was part of a ‘Station Check’ supervised by a Training Captain which the PF Captain required to satisfy a company requirement that such a Check must be conducted on any Captain who has not operated to Kathmandu at least once in a 12 month period if they are to remain current for such flights. The 59 year-old ‘Line Instructor Pilot’ in command occupying the right hand seat as PM had a total of 10,069 hours flying experience of which 4,030 hours were on type, all in command. He had been promoted to command in 2014 on the Airbus A320 before moving to the 737 fleet the following year. The PF Captain under supervision had a total of 12,088 hours flying experience of which 5,200 hours were on type.
During takeoff from runway 20 at Kathmandu, a ‘light thud’ was heard as the aircraft was rotated but no engine parameter anomalies were seen. The PM monitored cabin interphone communications and heard the aft cabin crew telling the SCCM about “a thud and scraping sound just before the aircraft lifted off”. Having concluded that a tail strike had probably occurred, he informed the PF who replied that he had not heard any abnormal noise. The aircraft did not have a tail strike warning indicator installed as part of the flight deck instrumentation.
It was not initially possible to determine if the cabin was pressurising normally but after completing the ‘After Takeoff Checklist’, the ‘Tail Strike Non-Normal Checklist’ (NNC) was not consulted after it was decided to defer this until the aircraft had climbed above the 14,400 feet en route MSA. As the climb continued, no indication of any pressurisation problem was seen and the bleed air source for the air conditioning packs was reconfigured from the APU to the engines at an altitude of 8,400 feet. Although the flight had been cleared to climb to 27,000 feet, ATC approval to level at 15,000 feet was sought and received and on reaching, a cabin altitude of 4,000 feet and rate of climb zero was observed and an aircraft systems check revealed nothing abnormal.
The flight was transferred to Kolkata ACC and clearance to continue at 15,000 feet was obtained. The Tail Strike NNC was then reviewed but its actions, which included depressurising the aircraft and landing at the nearest suitable airport, were deferred at that time pending a descent back to what was now a safe altitude of 10,000 feet. This was agreed in order to avoid triggering the Cabin Altitude Warning (CAW) when depressurising above 10,000 feet but having read the caution in the Tail Strike NNC stating that continued pressurisation may cause “further structural damage”, a PAN was declared and clearance to descend back to 10,000 was obtained.
In descent through approximately 12,000 feet, the PF called for the Tail Strike NNC and the PM selected cabin pressurisation from ‘AUTO’ to ‘MANUAL’ and momentarily selected the outflow valve (OFV) to open but to no apparent effect so repeated this after “around 20 seconds”. Between these two OFV selections, aft-stationed cabin crew called the PF to report “a loud wheezing sound” which he had “believed was the sound of air rushing out through the OFV opening” as expected. The PM did not recollect hearing this call and had therefore made a further (inappropriate) momentary opening of the OFV.
With the aircraft still above 10,000 feet, the CAW then activated and the crew completed the corresponding memory items of the required Checklist - donning oxygen masks, setting oxygen flow regulators to 100%, establishing crew communications and ensuring the cabin pressurisation mode selector was set to manual. Since they assessed that the cabin altitude was controllable, the OFV was not closed as per the CAW Checklist “because the flight crew’s intention was to open the OFV (fully) as required by the Tail Strike NNC”. The PM did manually deploy the passenger oxygen masks “as a precautionary measure” but he did not tell the PF that he was doing so and the latter only became aware of this when subsequently informed of it by the cabin crew.
Once at 10,000 feet, further descent to 5000 feet was obtained and when the CAW ceased below 6,000 feet, both pilots removed their oxygen masks. The PF obtained confirmation that there were no passenger injuries as a result of the depressurisation. Some ATC radio communication difficulties were encountered once at 5000 feet and a relayed request to climb back to 7000 feet was approved which restored direct communications. The PF informed ATC of the suspected tail strike and their intention to divert to the company nominated alternate of Kolkata and an overweight landing followed there without further event.
A post flight inspection found that the tailskid assembly skid shoe (see the illustration below) had been abraded by runway surface contact but there was no evidence of any damage to the aft fuselage structure. It was noted that the drag lever had not been compressed and part of the green band of the warning indicator was still visible.
The Tail Skid Assembly. [Reproduced from the Official Report]
Why it happened
FDR data showed that the pitch angle during rotation had reached 11.07° before the aircraft became fully airborne and that for most of the rotation, the pitch rate had been greater than the recommended 3° per second and at times more than 5° per second. It was noted that the TOW had been close to the applicable regulated takeoff weight (RTOW). The Boeing FCTM recommends a rotation pitch rate of between 2° and 3° per second which will normally achieve a pitch angle at lift off of between 7° and 9° - the nominal lift-off pitch angle for a Flap 1 takeoff is given as “around 8.5° with a tail clearance from the ground of about 33cm". It is noted that a tail strike will occur when the lift-off pitch angle exceeds 11° with the main landing gear wheels still on the runway and landing gear struts extended.
In respect of the uncontrolled depressurisation, the Boeing FCOM was noted to include a caution that “Switch actuation to the manual mode causes an immediate response by the outflow valve. Full range of motion of the outflow valve can take up to 20 seconds”. The Investigation performed a ground test on the manual operation of the OFV on the incident aircraft and found that it was responding instantaneously to any manual input with no anomalies.
A review of both pilots’ recurrent training records found that manual pressurisation control was only covered in discussion and computer-based training and did not involve Line Instructor Pilots physically manipulating the manual pressurisation control and monitoring the OFV position indicator, cabin altitude/cabin differential pressure and cabin rate of climb indicators.
In respect of the decision to delay actioning the Tail Strike NNC “without delay” if appropriate, Boeing advised the Investigation that “without delay” meant “when the desired flight path and appropriate configuration are correctly established, which in this event would have been at about 10,500 feet”. The aircraft operator advised that it had “not seen the need for specific or additional guidance” on Tail Strike when operating out of airports that have surrounding high terrain because a corresponding NNC and adequate guidance in the FCTM were available.
It was found that the aircraft operator’s OFDM programme was configured to detect exceedance of a rotation pitch rate of 4° per second based on the recommended 2°- 3° second but no such exceedances had been registered with no change when it was reduced to 3° per second. It was eventually found that the exceedance duration had been set to 2 seconds and it was reduced to 1 second after which several rotation pitch rate events above 3° per second were identified. Three of these - including the flight under investigation - had a rotation pitch rate of more than 5° per second and two had a pitch angle of more than 9° per second.
During the Investigation, the aircraft operator stated that “it is impossible for its airport ‘Hazard Identification and Risk Management' (HIRM) Programme to address all non normal events that may happen and did not identify tail strike out of Kathmandu as a hazard that would require additional mitigation”. The Investigation took the view that such a tail strike could represent a very challenging situation due to the uniqueness of the departure operating environment and the possibility of (further) structural damage.
The Conclusions of the Investigation were (in summary) formally documented as follows:
- The tail strike occurred due to over-rotation by the PF coupled with a likely tailwind component of about 10 knots. The rotation pitch rate was at times greater than 5° per second and the maximum recorded pitch angle of 11.07° exceeded the normal 7° - 9° pitch angle range.
- The flight crew could have considered beginning the Tail Strike NNC after reaching about 10,500 feet whilst the aircraft was still climbing to the 14,400 feet MSA.
- The aircraft involved did not have a flight deck tail strike warning indication system installed. Had such a system been available, it would have helped the flight crew make a better decision as to whether to perform the Tail Strike NNC urgently.
- The aircraft operator’s recurrent training programme for Line Instructor Pilots/Captains did not include practising the skills relating to manipulating cabin pressurisation. It was suspected that the aircraft commander may have been out of practice in respect of skills relating to manipulating cabin pressurisation.
- The aircraft operator’s airport ‘HIRM’ programme did not consider the complexity of a tail strike situation at aerodromes at high elevations surrounded by high terrain and targeted guidance and training for flight crew had not been developed.
- In response to the Cabin Altitude Warning, the aircraft commander acting as PM manually deployed the passenger oxygen masks as a precautionary measure without informing the PF. The aircraft operator did not have a requirement for standard callout for oxygen masks deployment.
Safety Action taken by Singapore Airlines as a result of the investigated event was noted to have included changing the threshold for detection of rotation pitch rate in the OFDM system.
Seven Safety Recommendations were made as a result of the Investigation findings as follows:
- that Singapore Airlines re-emphasise to all its flight crew the proper rotation technique to prevent tail strike. [TSIB RA2023-001]
- that Singapore Airlines consider providing targeted guidance and training for aerodromes at high elevations surrounded by high terrain that will help the flight crew decide when would be the best time to execute the Tail Strike Non-Normal Checklist. [TSIB RA2023-002]
- that Singapore Airlines consider providing targeted guidance and training for aerodromes at high elevations surrounded by high terrain that will remind the flight crew of the potential risk of aircraft structural failure with the continued pressurisation of the aircraft. [TSIB RA2023-003]
- that Singapore Airlines consider providing targeted guidance and training for aerodromes at high elevations surrounded by high terrain that will remind the flight crew that the Cabin Altitude Warning could be triggered after completing the Tail Strike Non Normal Checklist at around or above 10,000 feet. [TSIB RA2023-004]
- that Singapore Airlines include the practising of the skills relating to manipulating cabin pressurisation in the recurrent training for Line Instructor Pilots/Captains. [TSIB RA2023-005]
- that Singapore Airlines include oxygen mask deployment in the list of standard callout items. [TSIB RA2023-006]
- that Boeing consider making the tail strike warning indication a standard feature on the flight deck to better help flight crew decide as to whether to urgently perform the Tail Strike Non Normal Checklist in the event of a confirmed or suspected tail strike situation. [TSIB RA2023-007]
The Final Report was published on 2 May 2023.