THE US FEDERAL Aviation Administration is to take more action on the Robinson R22 and R44 light helicopters, but has rejected another National Transportation Safety Board (NTSB) call to have the machines grounded.

Following more accidents involving in-flight break-up, the NTSB says that "...until the causes of the accidents...are determined and appropriate flight- envelope restrictions and operating limitations are defined, the FAA should prohibit further flight [of the R22 and R44]".

The FAA says that the light helicopter, made in Torrance, California, suffers from "mast-bumping", but the manufacturer blames inexperienced pilots.

FAA associate administrator for regulation and certification, Tony Broderick, says: "People can look forward to rapid implementation of NTSB recommendations."

The matter is pending and a "...special certification review is still under way", says Broderick, who indicates that grounding is not being contemplated. "There are things we can adopt immediately, yet avoid grounding the helicopters - but, obviously, we can't do the research overnight," he says.

He adds that the biggest issue is to find out what is causing the problems and what can be done to correct them. "There have been a number of fatal accidents that resulted from mast-bumping. I'm sure there are good technical reasons," Broderick says.

The NTSB says that recent in-flight break-ups (see box) are believed to have occurred while the helicopters were being operated at speeds well within the operating envelope.

Robinson Helicopter believes that critically low main-rotor speed is contributing to the stall and divergence of the main-rotor blades, but the NTSB is concerned that qualified pilots seem unable to apply corrective action.

In 1994, the FAA formed a panel to research the R22's record, but ignored the NTSB's urging to restrict R22 flight operations, pending completion of the special-research group's work.

The FAA insists that the aircraft comply with certification criteria for light helicopters which incorporate "teetering"-rotor designs, but the NTSB is concerned that the unique design of the R22 and R44 rotor system may result in flight characteristics inadequately addressed by the certification.

"Of special concern to the Safety Board are the effects that turbulence may have on the main-rotor control system and ergonomic factors relative to the interaction between the pilots through the unique teetering cyclic-control systems in the R44 and R22 helicopters," says the NTSB.


Catalogue of disasters

2 April, 1994: R44 crashed in Germany, killing passenger and pilot with 110h on the R22, but performing first unsupervised R44 flight following 5h instruction. Main-rotor blades struck fuselage.

8 June, 1994: R22 broke apart in the UK, killing instructor and student. Instructor had 8,400h including 5,200h on type. Student had air-transport pilot's licence and 4,000h, including 22h on type.

28 September, 1994: R22 crashed in North Carolina, USA after in-flight separation of tailboom. Pilot had 790h with 305h on type.

8 December, 1994: R44 crashed in Germany, killing instructor, who had nearly 3,000 rotary hours, including 123h on type, and student, with helicopter commercial-pilot's licence. Main-rotor blades struck cockpit and tailboom.

27 December, 1994: R22 crashed near Zurich, Switzerland. Pilot and passenger killed when tailboom parts separated from the helicopter. Pilot experience not known.

Source: Flight International