This past week the National Transportation Safety Board (NTSB) published a report about the July 3, 2015 crash of an Airbus AS350B3e conducting Helicopter Air Ambulance (HAA) operations in Colorado. As seen in video at the base helipad, the aircraft yaws left while skids are on the ground and then continues to yaw left as the helicopter becomes airborne and goes out of camera view. The helicopter reappears as it crashes behind the site, and a post-crash fire develops almost instantly. What happened? What actions could the pilot have taken?
The Tail Rotor Control System
The first thing we should discuss is the AS350B3e tail rotor hydraulic control system. We are going to keep this simple.
The main rotor is controlled by two hydraulic systems (upper and lower, or left-hand and right-hand, respectively) for redundancy. However, the tail rotor design does not incorporate a redundant hydraulic system, and merely uses hydraulic pressure from one of the main hydraulic systems (the lower/right-hand one).1 They did include a backup system called a “yaw load compensator” that basically is an accumulator that stores some hydraulic pressure to allow the pilot to continue to use the pedals for a period in the event of failure. There are other helicopters that use a similar configuration.
As part of the preflight checklist the yaw load compensator is turned OFF and the accumulator depleted, which results in harder-to-move pedals, as they are not using hydraulics at this point. Then, the system is restored and the compensator is turned back ON. In the standard configuration, there is no light or audible indication to the pilot that the system is OFF: one must rely on following the checklist and verifying that the switch is returned to the ON position. There is a non-mandatory Airbus Helicopters’ Service Bulletin (AS350-67.00.64) that advises the installation of a system that adds a light to indicate this condition, however.
Needless to say, if you try to begin flight with the helicopter and this system OFF or in a failed state, you’ll have a very difficult time controlling the yaw of the helicopter complicated by not having any aerodynamic assistance from the vertical stabilizer in forward flight.
The NTSB determined that “the pilot most likely did not return the yaw servo hydraulic switch to its ON position before takeoff, resulting in no hydraulic pressure in both the tail rotor servo control and the yaw load compensator accumulator, a lack of hydraulic boost to the pedals, and significantly increased pedal loads.”2 They also indicated a second factor in the accident: the lack of a hover check.
Every time we pick the helicopter up into a hover we have a moment to confirm that the controls feel normal and that the helicopter sounds/vibrates normally. It takes just seconds to do, and if we find an issue, we can just set right back down in place. It is taught in basic flight instruction, applied in most companies’ training, established in many operators’ standardized procedures, and encouraged by the Federal Aviation Administration (FAA).
The FAA released a Safety Alert for Operators (SAFO) on this very subject not too long ago, which I, and many other instructors, passed along to student/fellow pilots or even promoted through social media. The FAA indicated that a “review of helicopter incidents and accidents over the past five years has identified several accidents where a loss-of-control (LOC) was encountered immediately after liftoff while light on the skids/gear, or from other issues caused by missed checklist items.”3 Clearly this accident applies.
The recommended steps listed in the SAFO are:
- Always ensure the area you are taking off from is sufficient for the conditions and the capabilities of the aircraft, as well as free and clear of debris that could pose a hazard to an aircraft.
- Using strict discipline and without compromise, pilots should ALWAYS USE an APPROPRIATE CHECKLIST to ensure the helicopter is properly configured for takeoff.
- Unless prohibited by environmental conditions such as the possibility of whiteout, brownout, etc., always perform a hover check prior to takeoff. If a takeoff from the surface is required, perform the hover check, land, and then depart from the surface, taking the aircraft’s performance into consideration.
- When performing a vertical takeoff, raise the helicopter vertically from the surface to a normal hovering altitude (2 to 3 feet) with minimal lateral or longitudinal movement maintaining a constant heading. If at any time during initial collective pull the helicopter does not appear to be stabilized, ABORT the takeoff by smoothly reducing the collective.
These steps are easy to implement in our routine, and still allow for the ability to conduct a no-hover take off if the situation calls for it.
Who’s to blame here? Airbus for their system design or lack of indicators? The pilot for not following the checklist and not following a policy-required/commonly-recommended hover check?
I’m not here to make that call.
Regardless of who’s to technically blame, we, as pilots-in-command, are still responsible for the safe operation of the helicopter. While placing the blame is part of what goes on in accident investigations, blame is irrelevant to us when we are seriously injured or dead from something we could have prevented. We have checklists: use them properly. We never have to take off immediately: don’t rush it. Stay ahead of the helicopter.
This accident, and the others that have come before it, emphasize the value of following checklists and taking a few seconds for a hover check. If something goes wrong in the hover check or while we are in the process of pulling pitch from the ground: land and assess!
- Airbus Helicopters. [August 21, 2014]. Safety Information Notice 2776-S-29: Hydraulic Power. [PDF]. Retrieved March 29, 2017 from http://airbushelicoptersinc.com/images/safety/2776-S-29-Rev-0-EN.pdf
- National Transportation Safety Board. [March 28, 2017]. Loss of Control at Takeoff, Air Methods Corporation, Airbus Helicopters AS350 B3e, N390LG Frisco, Colorado, July 3, 2015, NTSB/AAR-17/01 [PDF]. Retrieved March 29, 2017 from https://www.ntsb.gov/news/events/Documents/2017_frisco_BMG_abstract.pdf
- Federal Aviation Administration. [November 15, 2016]. Safety Alert For Operators (SAFO): Helicopter Stabilized Hover Checks Before Departure. [PDF]. Retrieved March 29, 2017 from https://www.faa.gov/other_visit/aviation_industry/airline_operators/airline_safety/safo/all_safos/media/2016/SAFO16016.pdf
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