The Air Accident Investigation Unit (AAIU) of the Department of Transport today issued its final report into a Serious Incident involving Ryanair B737-8AS (msn 33632) registered EI-DYD which occurred at Dublin Airport on September 11 2008. The aircraft was operating a flight from Dublin to Stansted when the incident occurred. There were 6 crew and 148 passengers on board at the time.
The aircraft departure was delayed slightly due to a passenger on the inbound leg requiring wheelchair assistance. To expedite the departure, most of the passengers boarded the aircraft by the rear steps which resulted in a higher percentage of the aft seating being occupied. The subsequent weight distribution was within limits. The flight was the cockpit crew's fourth and final sector of the day with the aircraft Commander acting as Pilot flying (PF). For the departure, Flap 5 and Low Engine Thrust 22k were selected. During the take off roll the Tail Skid Shoe assembly which is mounted on the underside of the aft fuselage came in contact with the runway surface. The crew were aware of a slight bump on take off but were not sure what had occurred. With the aircraft in a stable climb and with the post take off checks complete the aircraft Commander handed control to the First Officer, thus allowing the Commander to investigate the cause of the bump. By the time the Commander had concluded that a tail strike had in fact occurred the aircraft was passing FL100. The Commander initially attempted to make contact with the Cabin Supervisor but she was involved in the post take off passenger announcements. The Commander then contacted the Nr. 2 Attendant who confirmed the tail strike at which point he assumed the role of PF and requested the NNC (Non Normal Checklist) for a Tail Strike on Take Off. The aircraft climbed to FL120 and levelled for 40 seconds before descending. As part of the NNC for a Tail Strike the pressurisation valve is opened. This action is designed to prevent the fuselage from pressurising in the event of a tail strike having caused structural airframe damage. In this instance the aircraft was below FL140, the altitude at which the oxygen masks would be deployed automatically in the event of a sudden loss of cabin opressure. With the loss of cabin pressure the "Cabin Altitude Warning Horn" sounded which requires the crew to execute the "Rapid Depressurisation" NNC, having first donned oxygen masks. The Cabin Supervisor, realising that the cabin was depressurising and that the oxygen masks had not deployed contacted the crew, initially by intercom and then by banging on the cabin door - the crew hadn't heard her initial call as they were donning their own masks at the time. Following this contact the crew manually deployed the masks.
In its subsequent investigation the AAIU inspected the Tail Skid Shoe assembly at the rear of the aircraft. The assembly is manufactured in such a way that it effectively operates as as a "Strike Meter" which measures the force of impact. A red and green decal is positioned on one side of the assembly skirt. The skirt assembly compresses under impact and depending on whether or not the green portion of the decal is still visible after impact the aircraft is deemed serviceable or requiring attention. In this case the green portion of the decal was still visible after the impact which would have meant that the aircraft could have returned to service had the oxygen mask deployment not taken place.
Picture of the Tail Skid Shoe assembly showing scoring from the tail strilke and the Red / Green Indicator decal showing Green. Picture courtesy of Air Accident Investigation Unit, Department of Transport.
When the masks were deployed, a total of 9 units failed to release in seats 6A, 6B, 6C, 11A, 11B, 11C, 16D, 16E, 16F. Some passengers were therefore required to move to other seats where the masks had deployed. Once the aircraft was on the ground and hangared it was found that all masks deployed correctly. Boeing has noted instances where masks have failed to deploy in the past. The root cause of such failures has been hard to pin down. In some instances incorrect stowage of the mask was found to be the cause but in the case of EI-DYD the root cause will never be known since the doors all subsequently operated as intended.
The passenger loading of the aircraft resulted in an aft centre of gravity, but this was within limits. The B737-800 and 900 series aircraft are more susceptible to tail strikes than the shorter variants due to their longer fuselage lengths. In this particular incident the tail strike was of a minor nature which would have required a return to land followed by a visual inspection.
This incident was a classic case where it wasn't the original event that caused the serious incident but rather the chain of subsequent events :-
- The crew elected to climb the aircraft when the nature of the problem was not known.
- Once the nature of the problem had been identified the Tail Strike NNC was incorrectly executed in that the aircraft was pressurised during the actioning of the checklist.
- The aircraft depressurised when the outflow valve was opened causing another NNC to be activated for a cabin altitude warning.
- The oxygen masks were only deployed when the matter was brought to the cockpit crew's attention by the Cabin Supervisor.
The AAIU report singles out the actions of the Cabin Supervisor as being significant and pivotal in the reactionary measures taken by the crew which brought about a satisfactory ending to the episode. Following this incident, Ryanair implemented a number of changes to operating and training procedures to ensure that an incident of this nature cannot reoccur.
The AAIU report contains two safety Recommendations :
1. That the Operator should include a tail strike specific module in its training for cabin crew and flight crew.
2. That the Operator should review its training material in relation to the deployment of Passenger Oxygen Systems and the duties of cabin crew in the event of cabin depressurisation.
In its response, Ryanair has confirmed that it accepts the recommendations of the report and that it has put in place measures to implement those recommendations.