Last year’s crash of an FDR-equipped helicopter has U.S. investigators keen for a closer look at design standards and practices, which could mean major changes down the road for OEMs and operators.
Shortly after 1230 local time on Aug. 10, 2005, the crew of a CopterLine S-76+ prepared for their fifth takeoff of the day from the City Hall heliport in the Estonia capital of Tallinn.
At 1239, they lifted off, the 41-year-old pilot-in-command flying from the right seat. The 12 passengers on board were headed to Helsinki, about 43 nm (80 km) to the north. The flight normally takes 18 min.
The pilot took off on a heading of 110 deg. in a light rain, with winds from 110 deg. at 14 kt. He then turned left to 355 deg., accelerated and climbed to 1,500 ft. at 130 kt. The crew then prepared to climb to 2,000 ft. or higher. The pilot told the 56-year-old co-pilot he was going to add power.
Five seconds later, the collective was raised energetically. The cyclic was pulled aft half of its maximum travel, then pushed full forward. The pilot made an exclamation. A warning horn sounded. Within 1.5 sec., vertical acceleration went from +1g to +3g. The aircraft’s pitch reached +40 deg. and kept rising. The Sikorsky rolled 40 deg. to the left and kept rolling in that direction. Heading decreased from 355 to 320 deg. The left turn continued for 4 sec., until the heading reached 250 deg.
Airspeed dropped. The aircraft had climbed to 1,700 ft. and stayed there for 10 sec. The pilot-in-command transmitted "May Day" three times, weakly.
"The tail has gone?" the co-pilot asked. The tail rotor, in fact, continued to turn, as did both engines, which were producing the necessary power and torque.
A right turn began and continued for 13 full revolutions. The aircraft pitched and rolled. The pilots worked the controls, but could not recover. Then, 37 sec. after the collective had been raised, the aircraft hit the surface of Tallinn Bay on a heading of 360 deg.
It sank in 10 sec. in nearly 150 ft. of water. Everyone on board died.
The only reason such details of this accident are known is because the S-76C+ was equipped with a flight data recorder–in this case, a Penny+Giles solid-state combined cockpit voice and flight data recorder.
According to the U.S. National Transportation Safety Board, which under ICAO rules is assisting the Estonian Aircraft Accident Investigation Commission probe of the crash, the accident was the first "involving a Sikorsky helicopter or a NTSB helicopter investigation in which an FDR was onboard." That has U.S. accident investigators both angry and intrigued.
They are angry because the U.S. FAA in 2003 exempted several helicopter models from its requirement that they be equipped with flight data recorders. In a letter to Era Helicopters, an operator of such aircraft, the FAA said exempting the helicopters from the FDR requirement "would be in the public interest and would not adversely affect safety."
The NTSB disagreed. "Because the information that investigators learn from FDR data can help prevent accidents and incidents from recurring," it wrote to the FAA at the time, "the lack of FDRs aboard helicopters undoubtedly affects safety."
On March 23, 2004, an Era S-76A++ crashed into the Gulf of Mexico 70 nm south-southeast of Galveston, Texas. Visual meteorological conditions existed, but it was a dark night with very few external visual cues. The aircraft was transporting eight workers to a drilling ship. They and the two pilots perished in the crash. With no FDR on board the aircraft and its CVR failing because of an improper installation, NTSB investigators were left with few clues to determine whether the crash involved factors that might still be lying in wait for other unsuspecting helicopter crews. The NTSB determined the aircraft probably crashed because the flight crew failed to identify and arrest the helicopter’s descent for undetermined reasons, which resulted in controlled flight into the water. It called on the FAA to require all U.S.-registered turbine-powered helicopters certificated to carry at least 6 passengers to be equipped with a terrain awareness and warning system (TAWS).
"A terrain warning system would have given the pilots enough time to arrest their descent and save the lives of all aboard," NTSB Acting Chairman Mark V. Rosenker said. "It is well past time for the benefits from these standard safety devices to be made available to passengers on helicopter transports as they are on fixed-wing planes. More than 2 million passengers are carried on Gulf of Mexico oil industry operations alone."
Whether that is what actually caused the crash, and whether TAWS will help prevent a recurrence, only time will tell, because an FDR could not.
In The Dark on Rotorcraft
NTSB investigators are intrigued because while they have spent decades using FDR clues to delve into the strengths and flaws of fixed-wing aircraft design and operational practices, they are largely in the dark about such matters in the rotorcraft world. The CopterLine accident, and the wealth of information from its FDR, awoke them to that ignorance.
There is a good prospect that they will have more data from helicopter accidents to mine, if not because of FAA diligence then because more operators are pursuing safety and operational improvement measures that rely on "black boxes." That can only bring greater scrutiny of rotorcraft design premises and operational practices.
In the CopterLine crash, the FDR helped investigators isolate indications that one of the hydraulic actuators that control the S-76’s main rotor blades may have extended uncommanded during flight, resulting in the loss of control. Examination of its actuators found "a number of serious discrepancies" within the forward actuator "that are potential sources of uncommanded movement."
On the S-76C, three hydraulic actuators (forward, aft, and lateral) control the main rotor blades. Each features a side-by-side dual design in which two independent hydraulic systems power each side of an actuator.
During post-accident testing, investigators reported, the accident helicopter’s forward actuator failed a manufacturer’s acceptance test.
Subsequent disassembly of the actuator found large pieces of copper/aluminum plasma coating had flaked off the piston.
Also, the piston head and balance tube seals had excessive wear and pieces of the piston plasma coating were embedded in the seals and control valve, all of which contributed to internal hydraulic fluid leakage. In addition, pieces of plasma coating had blocked one of the return ports in the control valve and numerous pieces of plasma coating were found throughout the actuator.