Late motor-racing champion Niki Lauda will be associated not just with his airline entrepreneurship but with an investigation which resulted in scrutiny of design assumptions, and the adequacy of testing and certification methods, similar to that now being given to the Boeing 737 Max.

His airline, Lauda Air, suffered the catastrophic loss of a Boeing 767-300ER over Thailand in May 1991 after the thrust-reverser of its left-hand Pratt & Whitney PW4000 engine deployed in flight.

Aerodynamic forces resulted in the horizontal stabiliser and vertical fin separating, as the aircraft entered a high-speed descent from which the crew was unable to recover. The 767 completely broke up, in a matter of seconds, and none of the 223 occupants survived.

Lauda himself described the 767 accident as the “worst time” of his life, more so than his infamous German Grand Prix accident 15 years earlier, in an interview with UK newspaper The Observer in 2006.

“My first reaction was to say that, if my company is responsible for this, then we have to stop running the airline immediately,” he said. Lauda criticised Boeing’s reaction and, as a qualified commercial pilot, insisted the airframer give him the chance to fly simulations of the accident scenario.

While the specific cause of the uncommanded deployment could not be determined, the inquiry focused on the reasons why the aircraft was unable to cope with the reverser activation.

Thrust-reverser system certification by the US FAA required the mechanism to produce no more than idle thrust should it deploy in flight.

The FAA criteria also stated that the aircraft “must be shown by analysis or test, or both” to be capable of continuing safety flight and landing “under any possible position of the thrust-reverser”.

Its policy was to require a flight demonstration of in-flight thrust reversal, supported by a controllability analysis across other parts of the flight envelope.

But the inquiry found that such demonstrations were usually carried out at relatively low airspeeds with the engine at idle.

“It was generally believed that slowing the airplane during approach and landing would reduce airplane control surface authority thereby constituting a critical condition from a controllability standpoint,” it stated.

“It was also generally believed that the higher speed conditions would involve higher control surface authority, since the engine thrust was reduced to idle, and airplane controllability could be appropriately analysed.”

Several unintended in-flight reverser deployments on Boeing 747s at moderate and high speeds – with no apparent controllability problems – had reinforced this thinking.

Although the Lauda aircraft was PW4000-powered, Boeing’s demonstration of in-flight deployment on the 767 featured a Pratt & Whitney JT9D-equipped aircraft.

The demonstration took place at 10,000ft, at a speed of 220kt, and the engine remained in idle reverse thrust for the approach and landing.

“Controllability at other portions of the flight envelope was substantiated by an analysis prepared by the manufacturer and accepted by the FAA,” says the inquiry.

Although the 767 met the criteria for certification in 1981, the inquiry expressed scepticism as to whether the FAA’s rules were sufficient and fit for purpose.

“The circumstance of this accident…bring into question the adequacy or interpretation of the FAA requirements and the demonstration [and] analyses that were required,” it said.

“This accident indicates that changes in certification philosophy are necessary.”

Investigators pointed out that the Lauda 767’s reverser had not been restored to the forward thrust position before impact and were unable to determine whether it could have been.

Simulations of the accident also indicated that the aircraft was unable to maintain controlled flight if full wheel and full rudder were not applied within 4-6s of the reverser’s deployment.

“The consideration given to high-speed in-flight thrust reverser deployment during design and certification was not verified by flight or windtunnel testing and appears to be inadequate,” the inquiry added.

Analysis of the crash found that assumptions over the positive effects of high-speed flight on aircraft controllability – which had driven the belief that low speeds were more critical for reverser testing – had not taken into account the effects of reduced lift caused by a combination of the reverser plume or engine inlet spillage.

These effects resulted in 25% loss of lift across the wing and, for a crew caught by surprise, recovery was highly unlikely.

“Future controllability assessments should include comprehensive validation of all relevant assumptions made in the area of controllability,” the inquiry said.

“This is particularly important for the generation of twin-engine airplanes with wing-mounted high-bypass engines.”

Loss of the Lauda Air aircraft prompted a review of design philosophy and certification by a industry and government task force, as well as a redesign of the thrust-reverser system for PW4000s on the 767, resulting in fleet modification by early 1992.

This reverser redesign, said the inquiry, “should effectively prevent in-flight deployment even after multiple failures”.

Source: Cirium Dashboard