After an uneventful domestic flight with 12 passengers from Denver International Airport towards Telluride Airport (both in the state of Colorado, USA), the crew of a Beechcraft 1900D was configuring the aircraft for a landing on runway 27.
The aircraft in its final position (source; avherald.com © Unknown)
The first officer was the pilot flying (PF) while the captain was the pilot monitoring for the flight.
The weather was good on this summer day;
Wind - 090º at 12 knots
Clouds - Broken at 10000 ft agl
Visibility - 10 miles
Temperature - 19ºC
Dew point - 9ºC
QNH = 30.43 inHg
While on the approach to runway 27, at 12.31 lt (local time) the crew selected the landing gear down. Both the nose gear and the right main landing gear indicated down and locked after a short while, however, both the green down and locked lights for the left main landing gear did not illuminate, while the red in transit light remained on. after which the captain started the "Failure of Landing Gear to Extend Normally" checklist. At 12.40 lt the "Landing Gear Manual Extension" checklist was initiated by the captain. Via Telluride operations the operators Maintenance Control Center was informed. After a short while Telluride ops relayed a message from dispatch, asking the crew to consider a diversion to Farmington (Minnesota). After discussing possibilities, taking fuel burn, safety concerns and fire fighting equipment in to consideration, the crew determined that Telluride was the best option. Dispatch was informed accordingly.
At 12.47 lt an attempt was made by the captain to manually pump the gear down according the checklist, the landing gear extension handle however would not move. Three minutes later the crew started "Gear Up Landing" checklist, followed by the "Planned Emergency Preparation" checklist, ATC was contacted, asking for fire fighters, and then briefed the passengers.
The accident aircraft with all three gears down and locked, at an unknown date and location (Source: Aviation-Safety.net ©Leslie Snelleman)
At 12.59 lt the captain initiated the "Landing with One Main Up or Unsafe" checklist and after preparing for landing, the crew conducted the approach and landing on runway 27. During the landing roll, as the airplane slowed through about 80 knots, the left main landing gear collapsed. The airplane stopped on runway 27 at 1309 and the crew initiated an evacuation. None of the occupants were injured during the evacuation.
Damage to the aircraft was substantial;
Left wing tip abrasion damage
Left inboard flap damaged
Left outboard flap damaged
Left nacelle damaged
Left aileron damaged (outboard 24")
Left vetral strake damaged
Left engine inboard mount fractured
Left engine fire wall damaged
Left engine damaged
Left propeller damaged
The accident was investigated by the National Transportation Safety Board (NTSB). Post-accident inspection indicated hydraulic fluid had leaked from a hole, which was normally plugged with a Lee plug, on the left main actuator between the primary and secondary extend ports. The Lee plug was missing and was not found. Examination of the actuator and of the hydraulic fluid quantity remaining indicated the Lee plug was in place during the in-flight attempts to extend the landing gear. The airplane landed with the gear not locked down and as the landing gear collapsed, the piston in the actuator forced hydraulic fluid back through the system increasing the landing gear actuator internal hydraulic pressure sufficiently to force the Lee plug out of position. The remaining fluid in the actuator leaked out of the hole until system pressure decreased to ambient pressure.
The probable cause of the accident was determined to be:
"The failure of the left main landing gear actuator internal lock to engage in the locked position for undetermined reasons."
The NTSB report in to the accident, on which this blog is based, is available for the readers' reference by clicking on the .pdf file below;