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Many dual-hydraulic AS350 series helicopters still lack a modification intended to help prevent pilots from taking off without hydraulic assistance to the tail rotor. Airbus Helicopters Photo
One year ago, Airbus Helicopters issued a safety notice to operators of dual-hydraulic AS350/AS355 series helicopters warning of the potential for pilots to inadvertently take off without hydraulic assistance to the tail rotor.
Now, the problem described in the safety notice is central to lawsuits filed on behalf of flight nurses David Repsher and Matthew Bowe, survivors of last month’s fatal Flight For Life helicopter crash in Frisco, Colo.
Repsher and Bowe were seriously injured — and pilot Patrick Mahany was killed — in the July 3 crash of the Airbus Helicopters AS350 B3e (now called H125) operated by Air Methods Corp. According to a preliminary report by the National Transportation Safety Board (NTSB), the aircraft began rotating counterclockwise immediately after takeoff from the Summit Medical Center helipad before descending into a nearby parking lot. A post-impact fire destroyed the aircraft and severely burned Repsher, who was on fire as he extricated himself from the helicopter.
The NTSB’s preliminary report gives no indication as to the cause of the accident, and its final investigation report is not expected for many months. However, complaints filed on July 31 by attorneys for Repsher and Bowe claim, among other things, that Airbus Helicopters was negligent in its design of the AS350 tail rotor system, and that Air Methods was negligent for failing to adopt a design change recommended by Airbus for dual-hydraulic AS350 helicopters.
“Defendant Air Methods breached its duty of care and was negligent for failure to follow safety-critical warnings, instructions, and directions from Airbus relating to proper and safe operation of the helicopter, resulting in takeoff of the subject helicopter without hydraulic assistance to the tail rotor,” the complaints state.
Airbus Helicopters issued Safety Information Notice No. 2776-S-29 last August after receiving reports of pilots taking off with their collective-mounted yaw servo isolation switches in the “OFF” position, likely because they omitted a step in their pre-takeoff hydraulic checks. In AS350 and AS355 series helicopters, this can result in significantly increased pedal loads that may be incorrectly perceived by the pilot as a tail rotor failure.
According to a spokesperson for the manufacturer’s U.S. subsidiary, Airbus Helicopters Inc., Airbus has received formal reports of two such events that did not result in injuries: one confirmed case involving an AS350 in the U.S., and one possible case involving an AS355 outside the U.S. “Please be aware Airbus Helicopters cannot confirm or comment on any potentially responsive information received in pending Accident Investigation Board investigations,” the spokesperson noted.
Safety Information Notice No. 2776-S-29 described a modification designed to address the problem, 07.4622, which incorporates dedicated amber “HYDR” caution lights for each hydraulic circuit. It also integrates a flashing mode on the “HYDR 2” light when the collective-mounted yaw servo hydraulic isolation switch is in the “OFF” position — providing pilots with a prominent visual reminder to restore the switch to “ON” before flight.
The modification has been incorporated on new-production AS350 B3e (now called H125) helicopters for the past year, and is available as a kit for retrofit on older aircraft via service bulletin. Yet relatively few operators have installed it. According to Airbus Helicopters Inc., as of July 28, U.S. customers had ordered 54 kits for modification 07.4622. With 204 aircraft in the U.S. fleet eligible for the modification, that means at least 150 aircraft have not been retrofitted (information for the global fleet was not immediately available).
The expense of the modification is borne by the operator. According to Airbus Helicopters Inc., the cost of the kit for modification 07.4622 is approximately US$2,250, and the list price for a new caution and warning panel (CWP) to accommodate the modification is $4,597.74. Discounts are possible for larger orders.
As previously reported in Vertical, the difficulty in controlling yaw without hydraulic assistance on AS350 helicopters is related to the high zero pitch return moment of the increasingly large aft tail rotor blade surface area on B1 and higher models. On these aircraft, a yaw load compensator is installed in parallel with the tail rotor servo, providing the load assistance required for an average pilot to control the aircraft in the event of a hydraulic system failure.
An illustration of the pre-takeoff hydraulic test sequence on a dual hydraulic AS350 B3 (from left to right). 1: The collective-mounted yaw servo isolation switch is placed to the “OFF” (aft) position. Loads on pedals remain manageable. 2: The ACCU TST button is depressed (light on button illuminates). Loads on pedals are high. 3: The ACCU TEST button is released (light on button extinguishes). Pilot should turn yaw servo isolation switch to the “ON” (forward) position and confirm no loads are felt on pedals. Elan Head Photos
If an actual loss of hydraulic pressure occurs, a check valve in the system closes, trapping the necessary hydraulic pressure within the yaw load compensator. On dual-hydraulic AS350 and AS355 models, this is simulated during pre-takeoff checks by turning the collective-mounted yaw servo isolation switch to the “OFF” position. This cuts off the flow of hydraulic fluid to the tail rotor circuit, allowing the pilot to confirm that the check valve is functioning properly and that the yaw load compensator keeps pedal loads manageable. (During these checks, because of the dual hydraulics, all main rotor servos will continue to receive hydraulic assistance.)
The pilot then presses the “ACCU TEST” button “ON”, which releases trapped hydraulic pressure through a solenoid valve. The pilot verifies correct operation of the solenoid valve by confirming that the pedals go to a zero-thrust position, and that significantly more force is required to move them from that position. When the “ACCU TEST” button is depressed, a light on the button illuminates to alert the pilot to its position.
Releasing the “ACCU TEST” button extinguishes the light on the button and closes the yaw load compensator solenoid valve. However, the system remains unpressurized until the collective-mounted hydraulic isolation switch is returned to the “ON” position, restoring the flow of hydraulic fluid to the tail rotor circuit. The pilot should then confirm restoration of full hydraulic assistance by verifying that no loads are felt on the pedals.
If the pilot inadvertently skips this step — whether due to haste, a lack of understanding of the system, or both — there is no illumination on the CWP to remind him or her of the omission unless the aircraft has been equipped with mod 07.4622. Meanwhile, loads on the cyclic and collective will remain normal — also providing no reminder to the pilot that the yaw servo isolation switch remains “OFF”.
The complaints filed in the Flight For Life crash contend that “Air Methods breached its duty of care and was negligent for failure to follow and adopt the recommended design change by Airbus for the dual hydraulics tail rotor system that would create a caution light on the caution/warning panel (CWP) in the event that the hydraulic activation switch was not engaged prior to takeoff.”
They further claim that Airbus Helicopters “designed, manufactured, and supplied an unsafe and unreasonably dangerous tail rotor system which is uncontrollable in the event of a failure” and that Airbus “failed to retrofit, recall, or otherwise modify the dual hydraulic circuitry so as to adequately warn pilots with either caution lights on the caution/warning panel (CWP) or auditory warning whenever the yaw servo hydraulic switch is not timely or properly activated prior to takeoff of the helicopter.”
The complaints accuse Airbus Helicopters of failing to provide “adequate instruction and warnings to operators and pilots” regarding operation of the tail rotor hydraulics system, and the differences that exist for pre-flight hydraulic checks between dual and single hydraulic aircraft. However, the complaints do not specifically question the adequacy of Air Methods’ own training for its pilots.
Peter Rietz, a lawyer for the plaintiffs, declined to answer questions from Vertical. Air Methods also refrained from comment on the case, citing the ongoing investigation by the NTSB and Federal Aviation Administration, and respect for “the legal process.”
Because of litigation surrounding the Flight For Life case, Air Methods also declined to comment on a similar incident involving an AS350 B3 in Hamilton, Texas, last June. Air Methods confirmed that there had been a hard landing with no injuries, but did not answer specific questions regarding the cause of the incident or how it had been addressed through the company’s safety management system. Since August 2014, when Vertical originally contacted Air Methods on the subject, the company has also refused to say how it responded to Safety Information Notice No. 2776-S-29.
PHI Air Medical, another operator that has experienced a recent accident involving an AS350, likewise declined to comment for this story due to the ongoing investigation into the accident.