Originally Posted by
ApolloHeli
Curious to read about the accidents you're referring to?
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Continuing the accident discussion you inquire about...
Historically, the manufacturer has a pattern of generating incomplete solutions for correcting flawed systems. How long did it take the factory to incorporate a simple guard over the ACCU TEST switch to prevent inadvertent activation? The ACCU TEST switch removes stored hydraulic pressure in the tail rotor yaw-compensator, significantly increasing the forces required to move the pedals. Incidentally, in many aircraft the ACCU TEST switch is adjacent or very near the landing/taxi light switches. And for years, the ACCU TEST switch was referred to as HYD TEST, which probably contributed to operator confusion. The manufacturer used both labels throughout the flight manual for many years.
See this accident report,
and this report.
When the factory started to offer a dual-hydraulic option, it continued to obscure switch labels, compounding the confusion. The dual version removed the 3 accumulators on the main rotor servos, yet retained the yaw-load compensator in the tail. Thus, the function of the ACCU TEST switch was altered, but to this day, it retains the ambiguous label.
At the same time, the HYD CUTOFF (“yaw servo hydraulic switch, is also called the hydraulic pressure switch or hydraulic cut off switch in various Airbus Helicopters rotorcraft flight manuals” – FAA’s statement) switch on the collective head also changed function since it was now impossible to actually “turn off” the hydraulics. Further contributing to the confusion, is that the physical switch and their locations are identical to the single-hydraulic version.
In addition, the RFM was (and still is) written from the perspective of a single-hydraulic configuration. All of the dual-hydraulic information is contained in a brief supplemental chapter inserted into the back of the RFM. The supplement highlights changes to the operating procedures which must be incorporated into the manual’s runup, shut down and emergency procedures. Imagine flying NVGs on an EMS flight and asking the nurse, an FAA required crew member for off-airport NVG landings (?) to find the dual-hydraulic emergency procedure so you could review it?
Next followed a series of incidents/accidents in which it was believed pilots made procedural failures, by attempting takeoff with incorrect switch positions. These inappropriate switch configurations, and resultant loss of control were never alluded to in the RFM. Further exacerbating these flaws was the fact that the cockpit lacked a warning indication of the potentially grave misconfiguration.
See this accident report,
and this report the author is aware of additional related incidents which didn’t result in the creation of an NTSB report.
What followed next, was a failed attempt to address the design short-comings by publishing a series of communiques describing the functions of the dual-hydraulic system and the inadequate established procedures for preventing incorrect switch configuration. The result was the somewhat predictable, yet horrific accident at Frisco, Colorado.
See this accident report.
Oddly, the immediate response to this accident was to change the runup and shutdown procedures via an emergency service bulletin. The altered procedures mandated a confusing series of page swaps to the RFM, which had the pilot now perform the runup check during shutdown. This has the extremely odd and useless consequence of performing a check of a critical system after the flight is completed. Subsequent revisions to the manual left operators confused as to it's proper arrangement. A follow-on “pen and ink” change to the RFM included the addition of the almost laughable statement, “The yaw servo hydraulic switch (collective switch) must be in the “ON” (forward) position before takeoff.” A command to a pilot which seemingly
forbids him/her from making a switch configuration mistake!
The factory then modified the warning system to alert the pilot of misconfigured switches. However, the modification was only installed on new aircraft and offered for optional purchase to install on existing aircraft. The manufacturer finally designed a new mono-stable ACCU TEST switch which would prevent the incorrect switch configuration, and thus the heartbreaking saga of a human-factors nightmare is closed.