Commercial, Public Service, Safety

NTSB: Aircraft Design Probable Cause to 2015 EMS AS350 Crash

By S.L. Fuller | March 28, 2017

An Air Methods Airbus Helicopters AS350 B3e falls out of the sky on July 3, 2015, while taking off on a public relations mission in Frisco, Colorado. Image courtesy of the NTSB

An Air Methods Airbus Helicopters AS350 B3e falls out of the sky on July 3, 2015, while taking off on a public relations mission in Frisco, Colorado. Image courtesy of the NTSB

In determining the probable cause of the July 2015 crash of an Air Methods Airbus Helicopters AS350 B3e, the NTSB approved a different version than the investigation staff originally submitted. The NTSB staff — and Acting Chairman T. Bella Dinh-Zarr, member Christopher Hart, member Robert Sumwalt and member Earl Weener — met on March 28 to discuss the case of the helicopter that crashed in Frisco, Colorado. The pilot was fatally injured and both flight nurses were seriously injured. The helicopter was on a public relations mission at the time.

The facts of the case were generally not disputed. Security camera footage showed the helicopter begin yawing left (north) about 20 seconds into the flight. No footage was shown detailing what happened while at peak altitude, which a witness had put at some 100 feet. But the impact into a parked recreational vehicle in an adjacent parking lot and post-crash fire were caught on tape. An NTSB presentation explained that the dual-hydraulic system was not functioning correctly during flight, most likely because the pilot failed to move the position of the yaw servo hydraulic switch from “off” to “on.” In a dual-hydraulic AS350 B3e, operating procedures instruct the pilot to move the switch to the “off” position during pre-flight hydraulic check. Before takeoff, the pilot is then to move the switch back to the “on” position. It is widely known that failing to do so could lead to an accident. However, if the system is off, there is no mandated alerting mechanism to communicate that to the pilot. Therefore, the NTSB staff proposed this probable cause:

Advertisement

"The National Transportation Safety Board determines that the probable cause of this accident was the pilot's failure to reset the yaw servo hydraulic switch to its correct position during the pre-flight hydraulic check, which resulted in a lack of hydraulic boost to the pedal controls, high pedal forces and a subsequent loss of control after takeoff."

The NTSB said that contributing to the accident were two things: "the lack of salient alerting to the pilot of the absence of hydraulic boost to the pedal controls, and the resulting high pedal loads"; and "the pilot's failure to perform a hover check after liftoff, which would have alerted him to a pedal control anomaly at an altitude that could have allowed him to safely land the helicopter. Contributing to the severity of the injuries was the helicopter's fuel system, which was not crash-resistant, and facilitated a fuel-fed post-crash fire."

However, Sumwalt did not find this probable cause sufficient. An expert in the area of human error, he felt the emphasis should be moved from the pilot to the design of the aircraft. Sumwalt proposed an amendment that determined the probable cause as: "pre-flighting hydraulic check, which depleted hydraulic pressure in the tail rotor hydraulic circuit"; and "the lack of a salient alert to the pilot if hydraulic pressure was not restored before takeoff. Such alerting might have cued the pilot to his omission of the last step of the preflight hydraulic check, which resulted in a lack of hydraulic boost to the pedal controls, high pedal forces and a subsequent loss of control after takeoff."

His proposal continued: "Contributing to the accident was the pilot's failure to perform a hover check after liftoff, which would have alerted him to a pedal control anomaly at an altitude that could have allow him to safely land the helicopter. Contributing to the severity of the injuries was the helicopter's fuel system, which was not crash resistant and facilitated a fuel-fed post-crash fire."

Weener, who was at Boeing for 24 years as an engineer-turned-executive, disagreed with Sumwalt.

“This proposal kind of flips things around,” Weener said. “The proper design of a vehicle like this is not only the configuration of the hardware, but it is also the operating procedures by which it is operated. Both of those were certified … I think there's a hazard if you take responsibility for operating according to the certified procedures. Basically in this case, what we're saying is we're faulting the design before we're faulting the procedures.”

One main argument presented was that there have been other cases where a dual-hydraulic system has failed in the same or a similar way. Although operating procedures outline how to avoid a mishap, it is not hard to see how an error could occur.

“This pilot did not do what he was supposed to do,” Sumwalt said. “But if we dig deeper — and that's what really bothers me about the original probable cause is that it is saying that the pilot screwed up. Well, we do believe that the pilot did not restore the switch. However, I think it was a trap. To put the pilot as the primary factor of the probable cause is just wrong, because it points to the last person who made the last mistake. I think we need to dig deeper to get to the root cause of this.”

And the root cause Sumwalt suggested was a “poorly implemented preflight procedure.” To find a point of negotiation, Hart suggested some different language that might appease both Sumwalt and Weener. His suggestion proved acceptable, as the amendment passed unanimously. The official probable cause for the July 3, 2015, crash was read as:

The National Transportation Safety Board determines that the probable cause of this accident was Airbus Helicopters dual hydraulic AS350 B3e helicopter’s

1.) Preflight hydraulic check, which depleted hydraulic pressure in the tail rotor hydraulic circuit and

2.) Lack of salient alerting to the pilot that hydraulic pressure was not restored before takeoff. Such alerting might have cued the pilot to his failure to reset the yaw servo hydraulic switch to its correct position during the preflight hydraulic check, which resulted in a lack of hydraulic boost to the pedal control, high pedal forces and a subsequent loss of control after takeoff.

Contributing to the accident was the pilot’s failure to do a hover check after liftoff, which would have alerted him to a pedal control anomaly at an altitude that could have allowed him to safely land the helicopter. Contributing to the severity of the injuries was the helicopter’s fuel system, which was not crash-resistant and facilitated a fuel-fed post-crash fire.

Receive the latest rotorcraft news right to your inbox


Curated By Logo