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DH8D, Aalborg Denmark, 2007

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Summary
On 9 September 2007 the crew of an SAS Bombardier DHC8-400 approaching Aalborg were unable to lock the right MLG down and prepared accordingly. During the subsequent landing, the unlocked gear leg collapsed and the right engine propeller blades struck the runway. Two detached completely and penetrated the passenger cabin injuring one passenger. The Investigation found that the gear malfunction had been caused by severe corrosion of a critical connection and noted that no scheduled maintenance task included appropriate inspection. A Safety Recommendation to the EASA to review the design, certification and maintenance of the assembly involved was made.
Event Details
When September 2007
Actual or Potential
Event Type
Airworthiness, Loss of Control, Runway Excursion
Day/Night Day
Flight Conditions VMC
Flight Details
Aircraft BOMBARDIER Dash 8 Q400
Operator SAS
Domicile Sweden
Type of Flight Public Transport (Passenger)
Origin Copenhagen Airport, Kastrup
Intended Destination Aalborg Airport
Take off Commenced Yes
Flight Airborne Yes
Flight Completed No
Flight Phase Landing
LDG
Location - Airport
Airport Aalborg Airport
LOC
Tag(s) Significant Systems or Systems Control Failure
RE
Tag(s) Overrun on Landing,
Directional Control
EPR
Tag(s) MAYDAY declaration
CS
Tag(s) Evacuation on Cabin Crew initiative
Flight Crew Evacuation Command
AW
System(s) Landing Gear
Contributor(s) Inadequate Maintenance Schedule,
Inadequate Maintenance Inspection,
Component Fault in service,
Corrosion/Disbonding/Fatigue
Outcome
Damage or injury Yes
Aircraft damage Major
Injuries Few occupants
Causal Factor Group(s)
Group(s) Aircraft Technical
Safety Recommendation(s)
Group(s) Aircraft Airworthiness
Investigation Type
Type Independent

Description

On 9 September 2007 the crew of a Bombardier DHC8-400 (LN-RDK) being operated by SAS on a scheduled domestic passenger flight (214) from Copenhagen to Aalborg found during approach to destination that it was not possible to confirm that the right MLG was locked down. The alternate gear extension procedure did not resolve the fault and after declaring an emergency and holding to reduce the quantity of fuel on board, a landing was made in day Visual Meteorological Conditions (VMC) during which the right MLG collapsed, directional control was lost and a runway excursion followed. The runway surface was damaged by propeller blade impact and the right MLG, right engine and propeller and the aft lower fuselage of the aircraft all suffered major damage. Seven of the 73 occupants sustained minor injuries of which one was caused when hit by a propeller blade, one of two which penetrated the passenger cabin during the landing when the right wing dropped.

Investigation

An Investigation was carried out by the Danish AIB. Recorded data relevant to the Investigation was recovered from both the SSDFR and the SSCVR. It was noted that " the weather at the time was VMC and did not influence sequence of events".

It was noted that the flight had been uneventful until the flight crew selected the landing gear down during the approach at Aalborg and failed to get the expected 'down and locked' indication for the right MLG. ATC were informed and a go around to 2000 feet Altimeter Pressure Settings was commenced from 1100 feet QNH.

The QRH was consulted and the alternate landing gear extension procedure was followed. When the right MLG indication still showed it to be unsafe, a MAYDAY was declared. Further attempts to obtain a locked down indication were made but without success and the aircraft was put into a holding pattern in order to reduce the amount of fuel on board and to allow an opportunity for the aircraft commander to brief the passengers and for them to be prepared for an emergency landing. There were only seven unoccupied passenger seats and it was decided to use them to reseat those passengers in rows 6, 7 and 8 on the right hand side of the aircraft in order to get them away from the immediate vicinity of the right engine propeller.

A second approach to runway 26R was then commenced. As soon as the flaps were selected to 10°, the landing gear warning horn began to sound; this continued for the remainder of the flight. During the landing the left MLG contacted the runway just ahead of the right. The latter collapsed very soon after runway contact had occurred and as a result, the right wing and propeller blades (still turning under flight idle power) dropped into contact with the runway, resulting in substantial engine damage due to inertial overload to the forward section. Directional control was lost and the aircraft departed the runway to the right onto grass and came to stop on a heading of 340°.

All six right hand propeller blades struck the runway; three were found completely separated from the hub, the remains of the other three were found attached to the hub and as scattered debris. Two blades penetrated the passenger cabin structure, one completely via the window adjacent to seat 8F which then hit the opposite cabin wall above seat 8A and the other partially at seat 7F before becoming embedded in the adjacent fuselage structure. A passenger seated in 8C was hit by the first blade. It was concluded that the reseating of the passengers from the right hand side in rows 6, 7 and 8 on the initiative of the crew had reduced the number of passenger injuries.

Although the overall cabin structure remained intact, the passenger cabin suffered substantial damage and propeller-related debris was widespread. Visibility in the cabin was very poor after the accident due to dust and/or smoke and this hindered passengers seated in the middle of the cabin as they sought to find their way to the nearest emergency exit. Passengers also reported that the combination of daylight and dust and/or smoke not clear of the floor made the fluorescent tape on the aisle floor - intended to substitute for an emergency escape lighting system - difficult to see. When they left the flight deck, the pilots reported being unable to see the rear of the cabin because of the continuing effect of dust and/or smoke.

The damaged right engine and fuselage side showing one propeller blade which had penetrated the passenger cabin. (Reproduced from the Official Report)

An emergency evacuation was initiated by the Cabin Crew ahead of the same command from the flight deck. All occupants were out within 72 seconds of the aircraft coming to a stop on the grass, comfortably within the certification requirement of 90 seconds. It was noted that the commander had chosen to make the 'brace' call to the passengers himself and to do so 10 seconds rather than the usual one minute before the expected touchdown. This was done to reduce the chance that some passengers might look up during the landing on the basis that "passengers could hold their breath and tighten their muscles for 10 seconds but hardly for one minute". It was reported that the passengers had all responded as instructed 10 seconds before the landing. The Investigation noted that this alternative procedure had "prolonged the time of situation awareness" for the passengers and considered that "this new procedure should be open for discussion."

Video evidence showed that during the ground roll/slide, sparks and flames had appeared briefly when the aft right fuselage made contact with the runway but ceased when the aircraft skidded off into the grass. A momentary fire also occurred in the right engine following a major engine case separation as the propeller struck the runway but this too went out by itself before the aircraft came to rest on the grass. The airport Rescue and Fire Fighting Services, who had been in position prior to the landing, had arrived at the aircraft half a minute after it came to rest. They had seen smoke coming from the right engine and had briefly applied foam to reduce the possibility of fire until the smoke quickly ceased.

The passenger cabin showing part of one propeller blade. (Reproduced from the Official Report)
The passenger cabin showing another propeller blade lying across the aisle. (Reproduced from the Official Report)
The passenger cabin showing the same propeller blade across the aisle from above the seal from the window it entered through can be seen around it. (Reproduced from the Official Report)

A review of the formalised abnormal procedure responses available to the pilots was carried out. It was noted that the crew did not refer to the generic Quick Reference Handbook (QRH) checklist which dealt with an emergency landing with both engines operating. However, in respect of the specific malfunction encountered, an "unsafe landing gear", it was found that neither the QRH nor the manufacturer’s Aircraft Flight Manual (AFM) contained a procedure - although the AFM did contain a checklist covering landing gear malfunctioning. The fact that in this type of aircraft, the consequences of an MLG collapse on landing could be significant meant that this finding was of concern. It was also found that:

  • The QRH had only one condition as header in the alternate landing gear extension checklist., the presence of a LDG GEAR INOP caution light. However, the AFM had several such conditions in Appendix H.
  • Both the QRH and the AFM assumed that the Alternate Landing Gear Extension procedure would be successful.
  • Neither the QRH nor the AFM referred to an appropriate checklist if this procedure was unsuccessful.
  • Neither the QRH nor the AFM made any mention of shutting down the engine on the side with the unsafe MLG leg.
  • Neither the QRH nor the AFM contained procedures in the “Landing Gear” chapter to reseat passengers at the affected side.

In respect of the direct loss of airworthiness that was discovered to have been the immediate Cause of the Accident, the Investigation found that:

  • Due to severe corrosion of the threaded connection between the right MLG retraction/extension actuator piston rod and rod end, the separation of the actuator piston rod and rod end caused the malfunctioning of the right MLG.
  • When selecting the landing gear to the down position, the landing gear was released from the landing gear up-lock hook.
  • Due to the separation of the rod end from the actuator piston, the right MLG extended in an undamped free fall condition.
  • The kinetic energy (generated) caused the failure of the stabiliser brace link joint lugs.
  • This failure rendered the stabiliser brace incapable of safely locking the right MLG in down position."

Five Causal Factors were identified:

  • There were no specified inspection tasks for inspection of the MLG retraction/extension actuator and rod end either in the Maintenance Review Board report or in the Maintenance Requirement Manual (AMM) in so far as “L”, “A” and “C” checks.
  • The right and left MLG retraction/extension actuator piston and rod end were made of noble martensitic stainless steel and the less noble 4340 steel material, respectively.
  • Severe corrosion in the threaded connection between the right MLG actuator rod and rod end.
  • Separation of the right MLG retraction/extension actuator from the actuator piston rod end.
  • The failure of the right MLG stabiliser joint lugs.

The SAS maintenance organisation subsequently undertook a DHC8-400 fleet inspection and found that 26 out of 40 MLG retraction/extension actuator rod ends inspected had loose jam nuts.

Safety Action taken as a result of the accident findings included the issue by Transport Canada on 12 September 2007 of an Emergency Airworthiness Directive applicable to all DHC8-400 aircraft requiring that before further flight "a Detailed Visual Inspection (DVI) be must be accomplished on the Main Landing Gear (MLG) system, the MLG Retract Actuator Jam Nut and the MLG Retract Actuator and corrective actions (taken) as necessary". EASA replicated the essential details of this AD for compliance by European Operators in an Emergency AD issued the following day. Revisions to the detail in these initial ADs requiring that inspections similar to the initial one must be undertaken at prescribed intervals "until terminating action becomes available" were subsequently issued by Transport Canada and EASA.

Two Safety Recommendations were made as a result of the Investigation as follows:

  • that the European Aviation Safety Agency (EASA) should review the design, the certification and the maintenance program of the MLG retraction/extension actuator and rod end. [REK-01-2009]
  • that the European Aviation Safety Agency (EASA) should review the landing gear abnormal and emergency procedures contained in the manufacturer’s Airplane Flight Manual and Quick Reference Handbook. [REK-02-2009]

The Final Report was published early in 2009.


Further reading