The National Transportation Safety Board did not travel to the scene of this accident.
Additional Participating Entities:
Federal Aviation Administration / Flight Standards District Office; Helena, Montana
Bureau d'Enquetes et d'Analyses; Paris
Investigation Docket- National Transportation Safety Board: https://dms.ntsb.gov/pubdms
Aviation Accident Factual Report - National Transportation Safety Board: https://app.ntsb.gov/pdf
Hat Creek Helicopters LLC: http://registry.faa.gov/N504WD
NTSB Identification: GAA15LA083
14 CFR Part 91: General Aviation
Accident occurred Monday, May 04, 2015 in Hamilton, MT
Aircraft: AIRBUS AS-350, registration: N504WD
Injuries: 2 Uninjured.
NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.
On May 4, 2015 about 1045 mountain daylight time, an Airbus AS-350 BA helicopter, N504WD, had a hard landing during a practice hovering autorotation at the Ravalli County Airport (6S5) in Hamilton, Montana. The flight instructor and the pilot receiving instruction were not injured. The helicopter sustained substantial damage. The helicopter was registered to Hat Creek Helicopters LLC, Hamilton, Montana, and operated by the flight instructor as a day, visual flight rules instruction flight under 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed at the time of the accident and no flight plan was filed. The flight originated from the Ravalli County Airport (6S5), Hamilton, Montana.
The flight instructor reported that during hovering autorotation training, the pilot receiving instruction pulled in too much collective pitch, the helicopter ballooned, and subsequently landed hard. The flight instructor reported that they performed several additional training maneuvers and then landed without further incident. A postflight inspection revealed substantial damage to the tailboom.
The flight instructor reported that it was difficult to control the throttle (fuel flow control lever - FFCL) due to its location, mounted on the pedestal between the front seats, necessitating the release of either the cyclic or collective control to manipulate the FFCL. Further, not having an idle detent, made the pilot/instructor vulnerable to inadvertently shutting down the engine while trying to manipulate the throttle for training or emergency purposes. He reported that the accident could have been prevented had the throttle been co-located on the collective as in other helicopter designs he'd flown, allowing the pilot/instructor to manipulate the throttle without relinquishing control of the collective or cyclic controls.
He further explained that seated in the left seat, he had to lean over the center console to the right, to control the FFCL. While in this position, he was unable to prevent the pilot on the controls from pulling too much collective, resulting in a drop in main rotor RPM as the helicopter ballooned. He added that he felt that if there was a governor or tail rotor problem, he would not be in position to handle the emergency while trying to manipulate three controls with two hands.
The Federal Aviation Administration (FAA) aviation safety inspector stated in the FAA Form 8020-23 Accident/Incident Report, (June 2015), "Due to the location of the throttle on the AS-350 BA and no idle detent, it is difficult for a flight instructor sitting in the left seat to manipulate the throttle."
The flight instructor verified that there were no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.
ADDITIONAL INFORMATION
Fuel Flow Control Lever Design
The accident helicopter had a control quadrant located on the floor between the pilot's seat (right) and left front seat. The quadrant was comprised of a rotor brake lever, a fuel flow control lever, and a fuel shut-off lever. The fuel flow control lever is the center lever. The lever's track is designed with three positions and two detents that require the pilot to pull the lever to the right to move out of the detent and move the lever forward or back. The lever is made of thin spring steel and is easily flexed to the right. The upper detent serves as the stop (fuel cut-off) and start position. The second detent serves as the flight position. This position automatically meters fuel to the engine based on power demands. When moved out of flight position detent and forward, the emergency range is entered.
Cockpit Restraint System Testing
During November 2015, the cockpit restraint system with a floor mounted FFCL was tested for compliance with 14 Code of Federal Regulation (CFR) Part 27 Airworthiness Standards: Normal Category Rotorcraft and FAA Advisory Circular (AC) 27-1B Certification of Normal Category Rotorcraft.
§27.777 Cockpit controls
Cockpit controls must be—
(a) Located to provide convenient operation and to prevent confusion and inadvertent operation; and
(b) Located and arranged with respect to the pilots' seats so that there is full and unrestricted movement of each control without interference from the cockpit structure or the pilot's clothing when pilots from 5'2" to 6'0" in height are seated.
Essential controls should be evaluated with the shoulder harness locked in the retracted position.
The following are examples of cockpit control issues which should be avoided:
(iv) Control/seat relationship which requires unusual pilot contortions at extreme control displacements.
(viii) Controls for accessories or equipment which require a two-handed operation.
(x) Essential controls which cannot be actuated during emergency conditions with the shoulder harness locked.
(xi) Throttle controls which can be inadvertently moved through idle to the cutoff position.
(xii) Switches, buttons, or other controls which can be inadvertently activated during routine cockpit activity including cockpit entry.
The tests were conducted by an FAA airworthiness inspector at the request of the National Transportation Safety Board (NTSB) investigator in charge (IIC).
The inspector tested the cockpit restraint system with personnel of varying heights. The four-point cockpit restraint system had an inertia reel with a manual lock. He reported that the test subjects reached the floor mounted control quadrant, but none were able to grab the controls with a full or partially closed hand. With the FFCL in the "flight" gate, the test subjects were able to reach it with the first segments of their fingers and fingertips. The same minimal accessibility was reported for the fuel shutoff lever. He also surmised that during a violent oscillation of a helicopter during a crash, it would be impossible to control the FFCL.
An example of a violent oscillation in a helicopter accident sequence can be viewed in the video footage of WPR16FA029 (Airbus AS-350 B3 accident).
NTSB report MIA07TA017 (AS-350 BA) reported that the pilot removed his hand from the collective to manipulate the FFCL during an emergency. The pilot had slowly advanced the FFCL to the flight gate detent, when he observed a sudden spike in torque and then heard the engine begin to rev rapidly. The helicopter started to shake violently and bounce on the ground. He attempted to close the fuel shutoff valve; however, the collective control rose each time he released it, when attempting to close the FFCL, lifting the helicopter off the ground.
NTSB report CEN11FA599 (AS-350 B2) reported in the Party Submission on the design of the FFCL that, "Because of this design, the pilot must remove his hand from the collective flight control to alter the engine RPM. Additionally, the floor mounted fuel flow lever does not have an "idle detent" that would normally provide a pilot with a tactile indication or a physical stop at the engine idle point. Without the tactile feedback or physical "stop" it is possible to inadvertently reduce engine RPM below idle and potentially cause the engine to shut down. Due to the design limitation, the aircraft manufacturer has a restriction when conducting engine failure training.
A08A0007 (AS-350 BA, Canada) discussed the FFCL having no idle detent position, "Because there is no physical stop between the flight detent and the stop detent, it is possible that the FFCL was inadvertently set at or accidently moved to a position that caused the engine to spool down."
SL 2013/11 (AS-350 BA, Norway) reported that the FFCL having no idle detent position, "Thus there is a risk of an inadvertent shut-down of the engine while reducing the FFCL to set the engine at idle."
The FAA published Safety Alert for Operators (SAFO) 16006 in June 2016. This SAFO discusses the location of the hydraulic switch for the Bell UH-1 series helicopter and showed that in a recent accident, due to the aircraft configuration, the pilot was forced to remove his hand from the collective, place it on the cyclic so he could reach across with his right hand to shut off the hydraulic switch. This action of removing the pilot's hand from the collective control, might have created a situation where control of the helicopter was compromised.
Governor Failure Procedure – Excessive Fuel Flow Rate
An internal FAA memorandum (April 2010) from the Systems Safety and Analysis Branch (AAL-240) to the Recommendations and Analysis Division (AAI-200) discusses the emergency procedures for a governor failure (excessive fuel flow rate) and states in part:
The emergency procedure requires the pilot to release the collective control while flying to modulate fuel flow with the fuel flow control lever. When a collective input is made the fuel lever would have to be readjusted. Modulating fuel in the manual governor mode while flying without collective input at low level and maneuvering would be nearly impossible.
14 CFR Part 27 Airworthiness Standards: Normal Category Rotorcraft prescribes airworthiness standards for the issue of type certificates, and changes to those certificates, for normal category rotorcraft with maximum weights of 7,000 pounds or less and nine or less passenger seats. Additional information on the human factors aspect of cockpit control design can be found in the FAA Human Factors Division report DOT/FAA/TC-13/44 Human Factors Considerations in the Design and Evaluation of Flight Deck Displays and Controls (2013).
§27.771 Pilot compartment
For each pilot compartment—
(b) If there is provision for a second pilot, the rotorcraft must be controllable with equal safety from either pilot seat; and
§27.777 Cockpit controls
Cockpit controls must be—
(a) Located to provide convenient operation and to prevent confusion and inadvertent operation; and
(b) Located and arranged with respect to the pilots' seats so that there is full and unrestricted movement of each control without interference from the cockpit structure or the pilot's clothing when pilots from 5'2" to 6'0" in height are seated.
Essential controls should be evaluated with the shoulder harness locked in the retracted position.
As background, the following are examples of cockpit control issues which should be avoided:
(iv) Control/seat relationship which requires unusual pilot contortions at extreme control displacements.
(viii) Controls for accessories or equipment which require a two-handed operation.
(x) Essential controls which cannot be actuated during emergency conditions with the shoulder harness locked.
(xi) Throttle controls which can be inadvertently moved through idle to the cutoff position.
(xii) Switches, buttons, or other controls which can be inadvertently activated during routine cockpit activity including cockpit entry.
§27.1143 Engine controls
(d) If a power control incorporates a fuel shutoff feature, the control must have a means to prevent the inadvertent movement of the control into the shutoff position. The means must—
(1) Have a positive lock or stop at the idle position; and
(2) Require a separate and distinct operation to place the control in the shutoff position.
(4) If throttle controls incorporate a fuel shut-off feature, a means should be provided to prevent inadvertent movement to the shut-off position. This means should--
(i) Provide a positive lock or stop at the idle position. An idle detent (mechanical or electrical/mechanical such as solenoid) is an accepted arrangement.
(ii) Require a separate and distinct operation to place the control in the shut-off position. Separate action (switch or button) to displace the idle stop or distinct offsets in throttle motion to allow movement from the idle stop to shutoff are accepted arrangements.
In its 2006 study of the aircraft certification process, the NTSB made two recommendations to the FAA concerning human/aircraft interaction issues in the certification of aircraft. Safety Recommendation A-06-37 asked the FAA to — amend the advisory materials associated with 14…[CFR] 25.1309 to include consideration of structural failures and human/airplane system interaction failures in the assessment of safety-critical systems.
Safety Recommendation A-06-38 asked the FAA to… require a program for the monitoring and ongoing assessment of safety-critical systems throughout the life cycle of the airplane… Once in place, use this program to validate that the underlying assumptions made during design and type certification about safety-critical systems are consistent with operational experience, lessons learned, and new knowledge.
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