In a report released today, the National Transportation Safety Board made the unusual determination that ambiguous directions from Anchorage Center were a factor in an accident that occurred last year and took the lives of ACE Air captain Jeff Day and co-pilot Neil Jensen.
Further, the NTSB cited the air traffic controller for other factors, including a failure to monitor the flight. This determination stemmed from inaction on the part of the controller as the crew descended the aircraft below the minimum published altitude for the approach. A 20-year search of the NTSB accident database found only one previous instance where air traffic control was cited as a factor in an Alaska accident. In that case, it was a brief ground collision that resulted in one minor injury.
The crash, which took place about 10 miles east of Aleknagik in Muklung Hills on March 8, 2013, was an all-cargo flight in a Beechcraft 1900. The NTSB found the primary cause to be the flight crew's "failure to maintain terrain clearance which resulted in controlled flight into terrain in instrument meteorological conditions."
ACE Air flight 51 was en route to Dillingham from King Salmon operating under instrument flight rules when it crashed at about 8:15 a.m. When flying at an altitude of 5,900 feet, the crew requested the RNAV/GPS runway 19 approach to Dillingham at 8:03 a.m. and then routing directly to a point on charts known as ZEDAG, the initial approach fix. At this point, Flight 51 was 30 miles from ZEDAG and the Anchorage Air Route Traffic Control Center radar controller responded:
Ace Air 51 cleared direct to the Dillingham Airport via direct ZEDAG ZEDAG transition. Maintain ah maintain at or above 2,000 until established on a published segment of the approach. Cleared RNAV runway one niner approach to Dillingham Airport. Remain this frequency.
Ace Air responded:
We'll stay with you. Cleared to ZEDAG transition for RNAV one nine approach into Dillingham. Maintain 2,000 until a published segment of the approach Ace Air 51.
Having obtained clearance to ZEDAG, Flight 51 began a descent from 5,600 feet. About six minutes later, while approaching ZEDAG, the crew asked to enter a holding pattern there so they could contact ground personnel at Dillingham and check on runway conditions. At this time, 8:09:31 a.m., the aircraft was at an altitude of about 2,200 feet. The controller responded: "Ace Air fifty one hold north of ZEDAG as published expect further clearance one eight zero zero upon your request."
Flight 51 responded: "Hold north of ZEDAG expect further clearance one eight zero zero. We're still checking on runway conditions Ace Air 51 thanks."
This was the aircraft's final transmission with the controller.
According to the published instrument approach procedure, the terminal arrival area minimum altitude when approaching ZEDAG from the southeast, as Flight 51 was doing, is 5,400 feet, and the published holding pattern at ZEDAG is 4,300 feet due to rising terrain in the area. As stated by the NTSB, "...the flight crewmember's acceptance of what they believed to be a clearance to 2,000 feet, their descent to that altitude, and their initiation of a hold at that altitude indicates a lack of awareness of the information contained on the published procedure...they should have noticed that the minimum safe altitude in the TAA (terminal arrival area) southeast of the IAF was 5,400 feet msl (mean sea level) and that the minimum altitude for the hold was 4,300 feet msl."
The report says that the air traffic controller's clearance was "ambiguous." Rather than instruct the pilot to "maintain at or above 2,000 feet," the NTSB states the controller should have instructed Flight 51 to "proceed direct to ZEDAG, enter the TAA at or above 5,400 feet, cleared RNAV runway 19 approach." The controller failed to specify the segment of the approach which should be flown at 2,000 feet and failed to notice the pilot's incorrect readback of the clearance in which he "indicated that he intended to 'maintain 2,000 feet' until established on the approach."
During a post-accident interview with the NTSB, the controller handling Flight 51 stated that "he did not expect the airplane to descend below 5,400 feet and that he did not notice when it did so. He stated he did not notice the airplane's actual altitude when the pilot requested holding at ZEDAG. He stated that, when he cleared the pilot to hold at ZEDAG 'as published,' he expected the pilot to climb the airplane to 4,300 feet msl as shown in the profile view of the approach procedure."
According to recorded data, as Flight 51 descended below the published altitude, the aircraft's trajectory generated minimum safe altitude warnings both visually and aurally on the controller's radar display. At 8:09:16 a flashing MSAW indicator appeared on the aircraft's data block on the display that continued until the end of the flight. According to the NTSB, the controller was not aware of such warnings nor did he issue any terrain conflict alerts or climb instructions to the flight crew as the MSAW warning continued to report.
The final cause for the crash of ACE Air Flight 51 includes contributing factors as "the flight crew's failure to correctly read back and interpret clearance altitudes issued by the air traffic controller, their failure to adhere to minimum altitudes depicted on the published instrument approach chart, and their failure to adhere to company checklists."
The factors also include "the air traffic controller's issuance of an ambiguous clearance to the flight crew, which resulted in the airplane's premature descent, his failure to address the pilot's incorrect read back of the assigned clearance altitudes, and his failure to monitor the flight and address the altitude violations and issue terrain-based safety alerts."
Contact Colleen Mondor at colleen(at)alaskadispatch.com.