Fairchild SA227-AC N425MA accident near Denver, Colorado

Probable cause

The overstress failure of the left main landing gear lower drag brace links during takeoff which resulted in a runway excursion during landing due to the left main landing gear collapsing.

NTSB analysis

Key Lime Air flight 143 experienced a runway excursion while attempting to land with an unsafe left main landing gear (LMLG) indication at Denver International Airport (DEN), Denver, Colorado. The unscheduled domestic cargo flight was operated under the provisions of Title 14 Code of Federal Regulations Part 135 from North Platte Regional Airport (LBF), North Platte, Nebraska to Centennial Airport (APA), Denver, Colorado. The airplane sustained substantial damage and the pilot was uninjured.

According to the pilot, after takeoff from runway 30 at LBF he observed the LMLG would not retract and the LMLG indicating light illuminated red, indicating the LMLG did not retract. Both the nose and right main landing gear (RMLG) indicting lights had extinguished. After completing the “after takeoff” checklist, he selected the landing gear down position and the LMLG continued to indicate unsafe, while the nose and RMLG lights had green safe indicating lights.

The pilot raised the landing gear handle to the up position and the LMLG remained down and indicated unsafe. The pilot decided to divert to DEN, due to the longer runways, and communicated with company maintenance and dispatch to troubleshoot the anomaly. The troubleshooting included cycling the landing gear handle multiple times, attempting the gravity drop procedures, and switching the electrical buses, but the anomaly persisted.

While descending between 10,000 ft and 8,000 ft, the pilot observed a right hydraulic pressure warning light illuminate and that the hydraulic pressure had begun to fluctuate between 0 and about 1,000 psi. Subsequently, the left hydraulic pressure warning light illuminated. The pilot requested priority handling and air traffic control declared an emergency for the flight. The pilot completed the emergency gear extension checklist and the hydraulic system failure checklist and was vectored for the instrument landing system (ILS) approach to runway 35R.

Initially, the pilot coordinated with air traffic control to conduct a low approach and fly over of the runway, to allow ground personnel to visually inspect for extended landing gear. However, the airplane began to accumulate ice aft of the deicing boots and the pilot stated the flight controls became sluggish. While turning on final approach the pilot received a clearance for the ILS runway 35R approach and subsequently visually acquired the runway.

Due to increasing concerns with icing, the pilot elected to not conduct a fly over and elected to land. The airplane touched down on the RMLG and as the left wing dropped the pilot shut down the left engine and feathered the propeller. As the airplane speed decreased the pilot was unable to maintain directional control and the airplane departed on the left side of the runway. The pilot shut down the airplane and exited without further incident. After deplaning the pilot observed the LMLG doors open and the gear folded aft.

Figure 1. Accident airplane. (Source: DEN Department of Aviation)

Postaccident inspection revealed that the LMLG lower drag brace links were fractured which caused the LMLG to collapse aft at landing. Damage was observed to the left propeller, left nacelle, LMLG, and left wing. The damage to the left wing consists of two punctures in the lower wing skin, cracking of the lower wing skin, and deformation of one of the lower wing stringers.

The inboard and outboard lower drag brace links were found fractured at the lower trunnion pin lug attaching them to the LMLG strut. Both the inboard and outboard lugs were fully fractured, with the separated inboard lug section, inboard link, and outboard link remaining with the airplane. The outboard lug section was found at LBF, consistent with its failure at takeoff.

Photographs of the fracture surfaces showed significant abrasion damage to two of the surfaces consistent with contact with the runway on landing. The unabraded facture surfaces exhibited a dull grainy angular appearance consistent with overstress separation. There was no evidence of fatigue or corrosion.

Maintenance personnel tested the hydraulic system and flight control system, per the aircraft maintenance manual (AMM). No faults were found during the test.

Text from the NTSB. Open the NTSB docket for DCA26LA151 for the full record.