An unlatched oil reservoir cap touched off a series of events that led to the 2011 crash of a Northern Thunderbird Air (NT Air) King Air 100 as its pilots turned back to Vancouver International Airport (CYVR) because of an oil leak, the Transportation Safety Board of Canada (TSB) says.
The two pilots were killed in the Oct. 27 crash, and all seven passengers were seriously injured. The airplane was destroyed.
The TSB said in its final report on the accident that, although considerable oil had leaked because of the unlatched cap, “enough remained to allow the engine and propeller to operate normally.”
The accident flight began around 1541 local time with takeoff from CYVR, where the airplane had been kept in a hangar overnight. It was inspected in the hangar by NT Air maintenance personnel, who added 1 L (1 qt) of oil to the left engine and signed off the overnight inspection as complete.
“It is likely that the oil cap was not secured or verified during completion of the overnight inspection,” the report said.
The captain arrived at the hangar at 1420 local time, and, about two minutes later, pulled the airplane from the hangar. The first officer arrived while the airplane was being fueled outside the hangar. Neither pilot conducted a complete preflight inspection, the report said.
After the engines were started, the airplane, being operated as a sub-charter for another carrier based at another location at the airport, was taxied to the other location to pick up the passengers.
“Before the flight, an oil puddle was discovered under the left engine after the aircraft was taxied to pick up the passengers,” the report said. “The crew was aware of this oil, but no further action was taken to determine the source. Overfilling the oil reservoir will cause oil to vent, but it could not be determined whether the captain thought that this was the cause.”
After a passenger briefing, the airplane took off on an instrument flight rules flight plan to Kelowna, British Columbia, about 175 nm (324 km) east. The captain was the pilot flying as the airplane climbed to 16,000 ft above sea level, and the first 15 minutes of the flight were described as uneventful. Then the crew determined that oil was leaking from the left engine. They requested and received clearance to return to CYVR, and then turned toward the airport and began the descent (Figure 1).
About five minutes later, they conducted the “Abnormal” checklist for low oil pressure, agreeing that they would fly the airplane normally “unless the oil pressure dropped below 40 pounds per square inch (psi), at which time they would follow the emergency checklist and single-engine procedures.”
The crew received clearance to intercept the localizer for a visual approach to Runway 26L, and later, 3.8 nm (7.0 km) from the runway, they received clearance to land. The initial approach proceeded without incident, the report said, but the last 45 seconds of the flight were marked by an increase in crew activity.
“The flaps were lowered to 60 percent,” the report added. “The ground proximity warning system (GPWS) announced the altitude above ground level in feet as ‘500.’ The speed was announced as ‘105 kt,’ then ‘VREF’1 (99 kt) and finally ‘VREF minus 5.’ There was a change in the propeller noise and an immediate aircraft upset. The aircraft yawed left, rolled about 80 degrees left and pitched nose-down about 50 degrees.”
The airplane crashed into a roadway outside the airport perimeter fence during a gap in traffic and fire broke out. Motorists who had been stopped at traffic lights helped several passengers out of the wreckage. Aircraft rescue and firefighting personnel from a fire station 700 m (2,300 ft) away arrived at the accident site within three minutes to rescue the seventh passenger. Air traffic control notified Vancouver Airport Authority firefighters, who arrived one minute later and worked with the local firefighters to extinguish the flames and pull the pilots from the wreckage.
Everyone in the airplane was taken by ambulance to a hospital; three people on the ground — two people in a car that was struck by the airplane and one cyclist who was almost hit — were examined by medical personnel at the scene and released.
13,876 Flight Hours
The captain of the accident airplane had an airline transport pilot license and 13,876 flight hours, including 7,200 hours in twin-engine turboprops similar to the accident airplane and 978 hours in type. In the 90 days before the accident, he had flown 184 hours, including 46 hours in type. Before reporting for duty, two hours before the accident, he had been off duty for 38 hours.
The first officer, with a commercial pilot license and 1,316 flight hours, had 85 hours in type; in the 90 days before the accident, he had flown 192 hours, including 65 hours in type. He also reported for duty two hours before the accident, after 20 hours off duty.
The accident airplane was manufactured in 1970 and had accumulated 26,993 hours of total airframe time. It was equipped with two Pratt & Whitney Canada (P&WC) PT6A-28 engines and had been certified, equipped and maintained as specified by relevant regulations and procedures.
However, the airplane had never been modified according to P&WC Service Bulletin (SB) 1506R2 — issued in 1995 and re-issued in 2010 — which recommended action to limit oil loss “in the event that the oil-filler cap is not properly installed in the locked position.” Compliance with the SB was not required by regulations.
The SB called for replacing the oil-filler tube with an oil-filler-tube valve assembly that had a ball-type check valve and replacing the oil quantity gauge with a “new or modified shortened oil-quantity gauge,” the report said.
Although Transport Canada (TC) did not elevate the SB’s recommendations to the mandatory force of an airworthiness directive, the agency issued Service Difficulty Advisory AV‑2006-08 in 2006, to recommend that operators comply with P&WC service information letters and SBs, “and following engine oil servicing, always double check to ensure that oil caps are properly fastened.”
NT Air had experienced no known problems involving unsecured oil-reservoir caps and did not implement the modification outlined in the SB or take alternative actions, the report said.
Engine Oil System
Each of the accident airplane’s engines had an oil reservoir with a capacity of 8.7 L (9.2 qt). Normally, there is a negligible amount of oil venting; if the oil reservoir has been overfilled, however, surplus oil leaks from the vent when the engine is operating.
Engine-oil pressure is maintained as long as there is oil in the system, but engine-oil temperature may increase, especially if the amount of engine oil decreases while the engine load is high, the report said, adding that temperature indications are not reliable if the oil has been depleted.
A post-accident examination of the wreckage found that the cap on the left engine oil reservoir was unlatched. Accident investigators found 0.8 L (0.8 qt) of oil in the left oil reservoir and about 7 L (7 qt) of oil in the right reservoir.
Founded in 1971
NT Air has operated since 1971 in British Columbia and the Yukon. At the time of the accident, the company had a fleet of Beechcraft 1900s, King Airs and Cessna Caravans that were used for scheduled flights, charter service, medical evacuations and corporate and cargo flights.
NT Air is an approved aircraft maintenance organization and performs maintenance on its own aircraft. Under maintenance-control procedures approved by TC, NT Air’s director of maintenance and quality manager review all SBs to determine their applicability to company aircraft. (As noted, implementation of applicable ADs is mandatory.)
“The overnight maintenance inspection checklists include ‘check engine oil level’ as one of their tasks,” the report said. “This task implies removing the oil reservoir cap and replacing it; however, there is no check for verifying the security of the cap. The cap is difficult to see when closing the engine cowling.
“The overnight inspection of the occurrence aircraft was performed by an apprentice aircraft maintenance engineer (AAME) … [who] had been working at NT Air for about six months and had carried out several hundred similar overnight inspections before without issue. The AAME was authorized to carry out this inspection without supervision; however, a licensed aircraft maintenance engineer (AME) was required to sign — and did sign — that the inspection was completed.”
Although it was not required, NT Air had implemented a safety management system (SMS); it had not been approved by TC.
“A fully functioning safety management process would be expected to rigorously challenge and validate any underlying assumptions and safety risks,” the report said. “The company’s SMS had not identified company occurrences of oil reservoir caps being left unlatched. The SB had not been assessed by the company’s SMS, nor had the company’s SMS identified any other mitigation of the risks associated with unlatched oil-reservoir caps.”
The company’s standard operating procedures (SOPs) suggest that the captain delegate the preflight inspection of the airplane. Nevertheless, the captain typically completed the preflight inspection, the report said, adding that in this case, the preflight inspection was incomplete.
The SOPs include no additional discussion of the inspection, but the pilot’s operating manual, issued by the manufacturer, includes this item among its preflight procedures: “Engine Oil — CHECK QUANTITY, CAP SECURE.”
The NT Air Quick Reference Handbook did not discuss engine oil leaks but its “Abnormal” checklist and “Emergency” checklist both referred to low oil pressure. In both cases, the checklists “suggest reducing power,” but neither “cautions the pilot about the effect on the minimum speed to be maintained when the engine power is reduced, the propeller is not feathered and asymmetrical thrust is applied,” the report said. The “Emergency” checklist, however, includes a statement that, during single-engine operations, all VREF speeds should be increased by 10 kt.
The company’s SOPs discuss the need for a stabilized approach after the airplane passes the final approach fix, as well as the criteria for a missed approach. However, the SOPs described limits that “would not trigger a missed approach until the aircraft has already exceeded the instrument approach standards set out in the preceding paragraphs of the SOPs,” the report said. In addition, the SOPs “lacked clear direction on how the aircraft was to be configured for the last 500 ft, or what to do if an approach is still unstable when 500 ft is reached, specifically in an abnormal situation.”
The report also noted that all training scenarios for single-engine operations make clear that “pilots have to feather the propeller. … All actions and data then assume that the failed engine propeller is feathered.”
During the accident flight, however, with the left engine at idle and the propeller not feathered, airspeed dropped below VREF and the pilots were unable to maintain control.
After the accident, NT Air issued an “online training center” communication warning pilots that specific low-power settings “may produce undesirable or uncontrollable yaw as airspeed decreases” and an SOP bulletin prescribing an airspeed of 130 kt until a King Air 100 is in final landing configuration, on final approach slope, the pilots have the airport in sight and the pilot flying says “target VREF,” the report said.
In addition, the report said that TC is working with the engine manufacturer “to improve implementation of … P&WC SB no. 1506R2 to mitigate the consequences of an unsecured oil filler cap. The implementation may include mandating the subject design change for this and other engines.”
This article is based on TSB Aviation Investigation Report A11P0149, “Loss of Control and Collision With Ground; Northern Thunderbird Air Inc.; Beechcraft King Air 100, C-GXRX; Vancouver International Airport, Richmond, British Columbia; 27 October 2011.” The report is available at <tsb.gc.ca>.
- The report defined VREF as “a landing reference speed based on the aircraft’s weight and configuration … found on the approach checklist.”